Nuclear Batteries Report

CHAPTER 1INTRODUCTION

1.1 NUCLEAR BATTERYA burgeoning need exists today for small, compact, reliable, lightweight and self-contained rugged power supplies to provide electrical power in such applications as electric automobiles, homes, industrial, agricultural, recreational, remote monitoring systems, spacecraft and deep-sea probes. Radar, advanced communication satellites and especially high technology weapon platforms will require much larger power source than todays power systems can deliver. For the very high power applications, nuclear reactors appear to be the answer. However, for intermediate power range, 10 to 100 kilowatts (kW), the nuclear reactor presents formidable technical problems. Because of the short and unpredictable lifespan of chemical batteries, however, regular replacements would be required to keep these devices humming. Also, enough chemical fuel to provide 100 kW for any significant period of time would be too heavy and bulky for practical use. Fuel cells and solar cells require little maintenance, and the latter need plenty of sun.Thus the demand to exploit the radioactive energy has become inevitably high. Several methods have been developed for conversion of radioactive energy released during the decay of natural radioactive elements into electrical energy. A grapefruit-sized radioisotope thermo- electric generator that utilized heat produced from alpha particles emitted as plutonium-238 decay was developed during the early 1950s. Since then the nuclear has taken a significant consideration in the energy source of future. Also, with the advancement of the technology the requirement for the lasting energy sources has been increased to a great extent. The solution to the long term energy source is, of course, the nuclear batteries with a life span measured in decades and has the potential to be nearly 200 times more efficient than the currently used ordinary batteries. These incredibly long-lasting batteries are still in the theoretical and developmental stage of existence, but they promise to provide clean, safe, almost endless energy. Unlike conventional nuclear power generating devices, these power cells do not rely on a nuclear reaction or chemical process do not produce radioactive waste products. The nuclear battery technology is geared towards applications where power is needed in inaccessible places or under extreme conditions. The researchers envision its uses in pacemakers and other medical devices that would otherwise require surgery to repair or replace. Additionally, deep-space probes and deep-sea sensors, which are beyond the reach of repair, would benefit from such technology. In the near future this technology is said to make its way into commonly used day to day products like mobile and laptops and even the smallest of the devices used at home. Surely these are the batteries of the near future. Nuclear batteries run off of the continuous radioactive decay of certain elements. These incredibly long-lasting batteries are still in the theoretical and developmental stage of existence, but they promise to provide clean, safe, almost endless energy. They have been designed for personal use as well as for civil engineering, aeronautics, and medical treatments.The almost magical production of electricity in nuclear batteries is made possible by the process of betavoltaics. Through this technology, the electrons that radioactive isotopes regularly lose due to decay can be harnessed and directed into a stream of electricity. A semiconductor, possibly made from silicon, catches the flying electrons and directs them into a steady power source. Even a small amount of radioactive material will provide a charge for a very long time before it expires. Some people want to develop nuclear batteries to solve the pesky problem of your cell phone running out of juice just as you were writing down an important address. But other researchers see the potential for nuclear batteries to power things in situations where a battery really needs to last a long time because there is no way to replace it. They suggest applications such as pacemakers or other implants, detectors to be dropped in the bottom of an ocean or sealed deep within a bridge. Perhaps interstellar flights could be powered by a series of batteries each lasting several decad. These batteries can extract energy from radioactive isotopes in a number of ways. Some rely on thermal energy. As isotopes break down, they produce heat, which an atomic battery can harness to make electricity. The heat can also be useful on devices like spacecraft, which need a source of warmth to keep scientific instruments in a safe temperature range. In the deep cold of space, components would quickly freeze without heating, but expending energy on heat could cause the equipment to run out of power, so atomic batteries provide both heat and power to resolve this problem. Other devices rely on non-thermal methods of energy generation. The most common method takes advantage of beta particle emission to create electricity. This atomic battery design is known as a betavoltaic design, and is quite safe for use around people, because beta particles cannot penetrate human skin. They are much weaker than the more dangerous gamma particles that can be a concern with some radioactive isotopes. It is also potentially possible to convert the decay directly into kinetic energy for use to move mechanical components of a device. Experimental atomic battery projects have shown how this application could be useful for some medical devices and other equipment. The life of the battery depends on the characteristics of the isotope used to make it, but could be a decade or more. This can meet the needs of many devices, providing a stable energy supply for an extended period of time.

1.2 HISTORICAL DEVELOPMENTSThe idea of nuclear battery was introduced in the beginning of 1950, and was patented on March 3rd, 1959 to tracer lab. Even though the idea was given more than 30 years before, no significant progress was made on the subject because the yield was very less. A radio isotope electric power system developed by inventor Paul Brown was a scientific breakthrough in nuclear power. Browns first prototype power cell produced 100,000 times as much energy per gram of strontium -90(the energy source) than the most powerful thermal battery yet in existence. The magnetic energy emitted by the alpha and beta particles inherent in nuclear material. Alpha and beta particles are produced by the radioactive decay of certain naturally occurring and man made nuclear material (radio nuclides). The electric charges of the alpha and beta particles have been captured and converted to electricity for existing nuclear batteries, but the amount of power generated from such batteries has been very small. Alpha and beta particles also posses kinetic energy, by successive collisions of the particles with air molecules or other molecules. The bulk of the R &D of nuclear batteries in the past has been concerned with this heat energy which is readily observable and measurable. The magnetic energy given off by alpha and beta particles is several orders of magnitude greater than the kinetic energy or the direct electric energy produced by these same particles. However, the myriads of tiny magnetic fields existing at any time cannot be individually recognized or measured. This energy is not captured locally in nature to produce heat or mechanical effects, but instead the energy escapes undetected. Brown invented an approach to organize these magnetic fields so that the great amounts of otherwise unobservable energy could be harnessed. The first cell constructed (that melted the wire components) employed the most powerful source known, radium-226, as the energy source. The main drawback of Mr. Browns prototype was its low efficiency, and the reason for that was when the radioactive material decays, many of the electrons lost from the semiconductor material. With the enhancement of more regular pitting and introduction better fuels the nuclear batteries are thought to be the next generation batteries and there is hardly any doubt that these batteries will be available in stores within another decade.1.3 TYPES OF NUCLEAR BATTERIES These batteries can extract energy from radioactive isotopes in a number of ways. Some rely on thermal energy. As isotopes break down, they produce heat, which an atomic battery can harness to make electricity. The heat can also be useful on devices like spacecraft, which need a source of warmth to keep scientific instruments in a safe temperature range. In the deep cold of space, components would quickly freeze without heating, but expending energy on heat could cause the equipment to run out of power, so atomic batteries provide both heat and power to resolve this problem. Other devices rely on non-thermal methods of energy generation. The most common method takes advantage of beta particle emission to create electricity. This atomic battery design is known as a betavoltaic design, and is quite safe for use around people, because beta particles cannot penetrate human skin. They are much weaker than the more dangerous gamma particles that can be a concern with some radioactive isotopes.

Fig.2.1 Types of Nuclear/Atomic batteries

1.3.1 RADIOISOTOPE GENERATORSA thermoelectric converter uses thermocouples. Each thermocouple is formed from two wires of different metals (or other materials). A temperature gradient along the length of each wire produces a voltage gradient from one end of the wire to the other; but the different materials produce different voltages per degree of temperature difference. By connecting the wires at one end, heating that end but cooling the other end, a usable, but small (millivolts), voltage is generated between the unconnected wire ends. In practice, many are connected in series to generate a larger voltage from the same heat source, as heat flows from the hot ends to the cold ends. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier density and charge can be adjusted in semiconductor materials such as bismuth telluride and silicon germanium to achieve much higher conversion efficiencies.The alkali-metal thermal to electric converter (AMTEC) is an electrochemical system which is based on the electrolyte used in the molten salt battery|sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline -alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900 - 1300 K and a low pressure region containing a condenser for liquid sodium at 400 - 700 K. Efficiency of AMTEC cells has reached 16% in the laboratory and is predicted to approach 20%. A Stirling engine driven by the temperature difference produced by a radioisotope. New developments have led to the creation of a more efficient version, known as an Advanced Sterling Radioisotope Generator.

1.3.1.1 THERMAL CONVERTERA thermionic converter consists of a hot electrode which thermionically emits electrons over a space charge barrier to a cooler electrode, producing a useful power output. Caesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface ionization) to neutralize the electron space charge. A thermoelectric converter uses thermocouples. Each thermocouple is formed from two wires of different metals (or other materials). A temperature gradient along the length of each wire produces a voltage gradient from one end of the wire to the other; but the different materials produce different voltages per degree of temperature difference. By connecting the wires at one end, heating that end but cooling the other end, a usable, but small (millivolts), voltage is generated between the unconnected wire ends. In practice, many are connected in series to generate a larger voltage from the same heat source, as heat flows from the hot ends to the cold ends. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier density and charge can be adjusted in semiconductor materials such as bismuth telluride and silicon germanium to achieve much higher conversion efficiencies.

THERMOPHOTOVOLTAIC CELL Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light (rather than Visible spectrum visible light) emitted by a hot surface, into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric couples and can be overlaid on thermoelectric couples, potentially doubling efficiency. The University of Houston Thermophotovoltaic TPV Radioisotope Power Conversion Technology development effort is aiming at combining thermophotovoltaic cells concurrently with [[thermocouple]]s to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.

ALKALI-METAL THERMAL TO ELECTRIC CONVERTERThe alkali-metal thermal to electric converter (AMTEC) is an electrochemical system which is based on the electrolyte used in the Molten salt battery sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline -alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900 - 1300 K and a low pressure region containing a condenser for liquid sodium at 400 - 700 K. Efficiency of AMTEC cells has reached 16% in the laboratory and is predicted to approach 20%.

STIRLING RADIOISOTROPE GENERATOR A Stirling engine driven by the temperature difference produced by a radioisotope. New developments have led to the creation of a more efficient version, known as an Advanced Stirling Radioisotope Generator.

1.3.1.1.1 THERMIONIC CONVERTEAthermionic converterconsists of a hot electrode whichthermionically emitselectronsover a potential energy barrier to a cooler electrode, producing a useful electric power output.Caesiumvapor is used to optimize the electrodework functionsand provide anionsupply (bysurface ionizationorelectron impact ionizationin a plasma) to neutralize the electronspace charge. From a physical electronic viewpoint, thermionic energy conversion is the direct production ofelectric powerfromheatby thermionic electron emission. From athermodynamic view point,it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the interelectrodeplasma. The resulting current, typically severalamperesper square centimetre of emitter surface, delivers electrical power to a load at a typical potential difference of 0.51 volt and thermal efficiency of 520%, depending on the emitter temperature (15002000 K) and mode of operation. The scientific aspects of thermionic energy conversion primarily concern the fields ofsurface physicsandplasma physics. The electrode surface properties determine the magnitude ofelectron emissioncurrent andelectric potentialat the electrode surfaces, and the plasma properties determine the transport of electron current from the emitter to the collector. All practical thermionic converters to date employ caesium vapor between the electrodes, which determines both the surface and plasma properties. Caesium is employed because it is the most easily ionized of all stable elements. The surface property of primary interest is thework function which is the barrier that limits electron emission current from the surface and essentially is theheat of vaporizationof electrons from the surface. The work function is determined primarily by a layer of caesium atoms adsorbed on the electrode surfaces.[7]The properties of the interelectrode plasma are determined by the mode of operation of the thermionic converter.[8]In the ignited (or arc) mode the plasma is maintained via ionization internally by hot plasma electrons (~ 3300 K); in the unignited mode the plasma is maintained via injection of externally-produced positive ions into a cold plasma; in the hybrid mode the plasma is maintained by ions from a hot-plasma interelectrode region transferred into a cold-plasma interelectrode region

1.3.1.1.2 RADIOISOTOPE THERMOELECTRIC GENERATORA thermoelectric converter usesthermocouples. Each thermocouple is formed from two wires of different metals (or other materials). A temperature gradient along the length of each wire produces a voltage gradient from one end of the wire to the other; but the different materials produce different voltages per degree of temperature difference. By connecting the wires at one end, heating that end but cooling the other end, a usable, but small (millivolts), voltage is generated between the unconnected wire ends.

Fig 2.2 RTG used on theCassini probe

In practice, many are connected in series to generate a larger voltage from the same heat source, as heat flows from the hot ends to the cold ends. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier density and charge can be adjusted in semiconductor materials such as bismuth telluride and silicon germanium to achieve much higher conversion efficiencies.The design of an RTG as shown in fig2.2 is simple by the standards ofnuclear technology: the main component is a sturdy container of a radioactive material (the fuel).Thermocouplesare placed in the walls of the container, with the outer end of each thermocouple connected to aheat sink. Radioactive decay of the fuel produces heat which flows through the thermocouples to the heat sink, generating electricity in the process. A thermocouple is athermoelectricdevice that convertsthermal energydirectly intoelectrical energyusing theSee beck effect. It is made of two kinds of metal (or semiconductors) that can both conduct electricity. They are connected to each other in a closed loop. If the two junctions are at differenttemperatures, an electric current will flow in the loop.1.3.1.1.3 CRITERIAThe radioactive material used in RTGs must have several characteristics:1. It should produce high energy radiation. Energy release per decay is proportional to power production permole.Alpha decaysin general release about 10 times as much energy as thebeta decayof strontium-90 or cesium-137.2. Itshalf-lifemust be so long that it will release energy at a relatively constant rate for a reasonable amount of time. The amount of energy released per time (power) of a given quantity is inversely proportional to half-life. An isotope with twice the half-life and the same energy per decay will release power at half the rate permole. Typical half-lives forradioisotopesused in RTGs are therefore several decades, althoughisotopeswith shorter half-lives could be used for specialized applications.For spaceflight use, the fuel must produce a large amount of power permassandvolume(density). Density and weight are not as important for terrestrial use unless size is also restricted. Thedecay energycan be calculated if the energy of radioactive radiation or the mass loss before and after radioactive decay is known.

1.3.1.1.4 SELECTION OF ISOTOPESThe first two criteria limit the number of possible fuels to fewer than 30 atomic isotopeswithin the entiretable of nuclides.Plutonium-238,curium-244andstrontium-90are the most often cited candidate isotopes, but other such isotopes aspolonium-210,promethium-147,caesium-137,cerium-144,ruthenium-106,cobalt-60,curium-242,americium-241 andthuliumisotopes have also been studied.238Pu,90SrPlutonium-238has the lowest shielding requirements and longest half-life; its power output is 0.54 kilowatts per kilogram. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25mm ofleadshieldingto block the radiation.238Pu (the best of these three) needs less than 2.5mm, and in many cases no shielding is needed in a238Pu RTG, as the casing itself is adequate.238Pu has become the most widely used fuel for RTGs, in the form ofplutonium (IV) oxide(PuO2).238Pu has a half-life of 87.7 years, reasonable power density, and exceptionally low gamma and neutron radiation levels.Strontium-90also requires little shielding, as it decays by emission, with negligible emission. While its half life of 28.8 years is much shorter than that of238Pu, it also has a much lower decay energy. Thus its power density is only 0.46 kilowatts per kilogram. Because the energy output is lower it reaches lower temperatures than238Pu, which results in lower RTG efficiency.90Sr is a high yield waste product of nuclear fission and is available in large quantities at a low price.210PoSome prototype RTGs, first built in 1958 by the US Atomic Energy Commission, have usedpolonium-210. This isotope provides phenomenal power density because of its highradioactive activity, but has limited use because of its very short half-life of 138 days. A kilogram of pure210Po in the form of a cube would be about 48mm (about 2inches) on a side and emit about140kW.242Cm,244Cm,241AmCurium-242and curium-244 have also been studied as well, but require heavy shielding for gamma and neutron radiation produced fromspontaneous fission.Americium-241is a potential candidate isotope with a longer half-life than238Pu:241Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the power density of241Am is only 1/4 that of238Pu, and241Am produces more penetrating radiation through decay chain products than238Pu and needs about 18mm of lead shielding. Even so, its shielding requirements in an RTG are the second lowest of all possible isotopes: only238Pu requires less. With a current global shortageof238Pu, a closer look is being given to241Am.

1.3.1.1.5 LIFE SPANMost RTGs use238Pu, which decays with a half-life of 87.7 years. RTGs using this material will therefore diminish in power output by 10.51/87.74= 0.787% of their capacity per year. 23 years after production, such an RTG will have decreased in power by 16.6%, i.e. providing 83.4% of its initial output. Thus, with a starting capacity of 470W, after 23 years it would have a capacity of 392W. However, the bi-metallic thermocouples used to convertthermal energyintoelectrical energy degrade as well; at the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315W for Voyager 1 and to 319W for Voyager 2. Therefore in early 2001, the RTGs were working at about 67% of their original capacity instead of the expected 83.4%.

Fig. 2.3 90Sr-powered Soviet RTGs in dilapidated and vandalized conditionThis life span was of particular importance during theGalileomission. Originally intended to launch in 1986, it was delayed by theSpace Shuttle Challengeraccident. Because of this unforeseen event, the probe had to sit in storage for 4 years before launching in 1989. Subsequently, its RTGs had decayed somewhat, necessitating replanning the power budget for the mission.1.3.1.1.6 EFFICIENCYRTGs use thermoelectric couples or "thermocouples" to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 37%. Thermoelectric materials in space missions to date have included silicongermanium alloys, lead telluride and tellurides of antimony, germanium and silver (TAGS). Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.Athermionic converter an energy conversion device which relies on the principle ofthermionicemission can achieve efficiencies between 1020%, but requires higher temperatures than those at which standard RTGs run. Some prototype210Po RTGs have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these unfeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes.Thermophotovoltaic cellswork by the same principles as aphotovoltaic cell, except that they convertinfraredlight emitted by a hot surface rather than visible light into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and can be overlaid on top of thermocouples, potentially doubling efficiency. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%,[13]but have not been tested with actual radioisotopes. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells and silicon thermocouples degrade faster than thermocouples, especially in the presence of ionizing radiation.Dynamic generators can provide power at more than 4 times the conversion efficiency of RTGs. NASA and DOE have been developing a next-generation radioisotope-fueled power source called theStirling Radioisotope Generator(SRG) that uses free-pistonStirling enginescoupled to linear alternators to convert heat to electricity. SRG prototypes demonstrated an average efficiency of 23%. Greater efficiency can be achieved by increasing the temperature ratio between the hot and cold ends of the generator. The use of non-contacting moving parts, non-degradingflexural bearings, and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibration can be eliminated as a concern by implementation of dynamic balancing or use of dual-opposed piston movement. Potential applications of a Stirling radioisotope power system include exploration and science missions to deep-space, Mars, and the Moon. The increased efficiency of the SRG may be demonstrated by a theoretical comparison of thermodynamic properties, as follows. These calculations are simplified and do not account for the decay of thermal power input due to the long half-life of the radioisotopes used in these generators. The assumptions for this analysis include that both systems are operating at steady state under the conditions observed in experimental procedures (see table below for values used). Both generators can be simplified to heat engines to be able to compare their current efficiencies to their corresponding Carnot efficiencies. The system is assumed to be the components, apart from the heat source and heat sink. The thermal efficiency, denoted th, is given by:

Where primes ( ' ) denote the time derivative.From a general form of the First Law of Thermodynamics, in rate form:

Assuming the system is operating at steady state and,

th, then, can be calculated to be 110W / 2000W = 5.5% (or 140W / 500W = 28% for the SRG). Additionally, the Second Law efficiency, denoted II, is given by:

Where th,revis the Carnot efficiency, given by:

In which Theat sinkis the external temperature (which has been measured to be 510K for the MMRTG (Multi-Mission RTG) and 363K for the SRG) and Theat sourceis the temperature of the MMRTG, assumed 823K (1123K for the SRG). This yields a Second Law efficiency of 14.46% for the MMRTG (or 41.37% for the SRG).

1.3.1.1.7 RADIOACTIVE CONTAMINATIONRTGs pose a risk ofradioactive contamination: if the container holding the fuel leaks, the radioactive material may contaminate the environment. For spacecraft, the main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy. However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the CassiniHuygens probe launched in 1997 estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its 3 RTGs (or from its 129radioisotope heater units) during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million.[19]If an accident which had the potential to cause contamination occurred during the launch phases (such as the spacecraft failing to reach orbit), the probability of contamination actually being caused by the RTGs was estimated at about 1 in 10.[20]In any event, the launch was successful and CassiniHuygens reachedSaturn.Theplutonium-238used in these RTGs has ahalf-lifeof 87.74 years, in contrast to the 24,110 year half-life ofplutonium-239used innuclear weaponsandreactors. A consequence of the shorter half-life is that plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. 17.3curies(640GBq)/gcompared to 0.063 curies (2.3GBq)/g). For instance, 3.6kgof plutonium-238 undergoes the same number of radioactive decays per second as 1 tonne of plutonium-239. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same,plutonium-238 is around 275 times more toxic by weight than plutonium-239. The alpha radiation emitted by either isotope will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk is theskeleton, the surface of which is likely to absorb the isotope, and theliver, where the isotope will collect and become concentrated.There have been several known accidents involving RTG-powered spacecraft:1. The first one was a launch failure on 21 April 1964 in which the U.S.Transit-5BN-3navigation satellite failed to achieve orbit and burnt up on re-entry north ofMadagascar. The 17,000Ci (630TBq) plutonium metal fuel in itsSNAP-9a RTG was injected into the atmosphere over the Southern Hemisphere where it burnt up, and traces of plutonium-238 were detected in the area a few months later.2. The second was the Nimbus B-1 weather satellite whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Launched from theVandenberg Air Force Base, its SNAP-19 RTG containing relatively inertplutonium dioxidewas recovered intact from the seabed in theSanta Barbara Channelfive months later and no environmental contamination was detected.3. In 1969 the launch of the firstLunokhodlunar rover mission failed, spreadingpolonium 210over a large area of Russia. 4. The failure of theApollo 13mission in April 1970 meant that theLunar Modulereentered the atmosphere carrying an RTG and burnt up overFiji. It carried a SNAP-27 RTG containing 44,500Ci (1,650TBq) of plutonium dioxide which survived reentry into the Earth's atmosphere intact, as it was designed to do, the trajectory being arranged so that it would plunge into 69 kilometers of water in theTonga trenchin thePacific Ocean. The absence of plutonium-238 contamination in atmospheric and seawater sampling confirmed the assumption that the cask is intact on the seabed. The cask is expected to contain the fuel for at least 10 half-lives (i.e. 870 years). The US Department of Energy has conducted seawater tests and determined that the graphite casing, which was designed to withstand reentry, is stable and no release of plutonium should occur. Subsequent investigations have found no increase in the natural background radiation in the area. The Apollo 13 accident represents an extreme scenario because of the high re-entry velocities of the craft returning fromcis-lunar space(the region between Earth's atmosphere and the Moon). This accident has served to validate the design of later-generation RTGs as highly safe.

Fig. 2.4 A SNAP-27 RTG deployed by the astronauts of Apollo 14 identical to the one lost in the reentry of Apollo 13To minimize the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer ofiridiummetal and encased in high-strengthgraphiteblocks. These two materials are corrosion- and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the Earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highlyinsoluble. The most recent accident involving a spacecraft RTG was the failure of the RussianMars 96probe launch on 16 November 1996. The two RTGs onboard carried in total 200g of plutonium and are assumed to have survived reentry as they were designed to do. They are thought to now lie somewhere in a northeast-southwest running oval 320km long by 80km wide which is centred 32km east ofIquique,Chile.ManyBeta-MRTGs produced by the Soviet Union to powerlighthousesandbeaconshave becomeorphaned sourcesof radiation. Several of these units have been illegally dismantled for scrap metal resulting in the complete exposure of theSr-90source, fallen into the ocean, or have defective shielding due to poor design or physical damage. TheUS Department of Defensecooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used byterroriststo construct adirty bomb. 28 U.S. space missions have safely flown radioisotope energy sources since 1961.

1.3.1.2 NON THERMAL CONVERTERNon-thermal converters extract a fraction of thenuclear energyas it is being degraded into heat. Their outputs are not functions of temperature differences as are thermoelectric and thermionic converters. Non-thermal generators can be grouped into three classes. Non-thermal converters extract a fraction of the Nuclear binding energy|nuclear energy as it is being degraded into heat. Their outputs are not functions of temperature differences as are thermoelectric and thermionic converters. Non-thermal generators can be grouped into three classes.

1.3.1.2.1 DIRECT CHARGE GENERATORIn the first type, the primary generator consists of a [[capacitor]] which is charged by the current of charged particles from a radioactive layer deposited on one of the electrodes. Spacing can be either vacuum or dielectric. Negatively charged beta particles or positively charged alpha particles, positrons or Fission products fission fragments may be utilized. Although this form of nuclear-electric generator dates back to 1913, few applications have been found in the past for the extremely low currents and inconveniently high voltages provided by direct charging generators. Oscillator/transformer systems are employed to reduce the voltages, then rectifiers are used to transform the AC power back to direct current. English physicist Henry Moseley|H.G.J. Moseley constructed the first of these. Moseleys apparatus consisted of a glass globe [[silver]]ed on the inside with a radium emitter mounted on the tip of a wire at the center. The charged particles from the radium created a flow of electricity as they moved quickly from the radium to the inside surface of the sphere. As late as 1945 the Moseley model guided other efforts to build experimental batteries generating electricity from the emissions of radioactive elements.

1.3.1.2.2 BETAVOLTAICSBetavoltaics are generators of electrical current, in effect a form of battery, which use energy from a radioactive source emitting beta particles (electrons). A common source used is the hydrogen isotope, tritium. Unlike most nuclear power sources, which use nuclear radiation to generate heat, which then generates electricity (thermoelectric and thermionic sources), betavoltaics use a non-thermal conversion process, using a semiconductor p-n junction. Betavoltaics are particularly well-suited to low-power electrical applications where long life of the energy source is needed, such as implantable medical devices or military and space applications.

1.3.1.2.3 ALPHAVOLTAICSAlphavoltaic power sources are devices that use a semiconductor junction to produce electrical particle from energetic alpha particles.

1.3.1.2.4 OPTOELECTRICAn optolectric nuclear battery has also been proposed by researchers of the Kurchatov Institute in Moscow. A beta-emitter (such as technetium-99) would stimulate an excimer mixture, and the light would power a solar cell|photocel. The battery would consist of an excimer mixture of argon,xenon in a pressure vessel with an internal mirrored surface, finely-divided Tc-99, and an intermittent Ultrasound ultrasonic stirrer, illuminating a photocell with a bandgap tuned for the excimer. The advantage of this design is that precision electrode assemblies are not needed, and most beta particles escape the finely-divided bulk material to contribute to the battery's net power. Anopto-electric nuclear batteryis a device that convertsnuclear energyintolight, which it then uses to generateelectrical energy. Abeta-emittersuch astechnetium-99orstrontium-90is suspended in agasorliquidcontainingluminescentgas molecules of theexcimertype, constituting a "dust plasma." This permits a nearly lossless emission of betaelectronsfrom the emitting dust particles. The electrons thenexcitethe gases whose excimer line is selected for the conversion of theradioactivityinto a surrounding photovoltaiclayer such that a lightweight, low-pressure, high-efficiency batterycan be realised. Thesenuclidesare relatively low-costradioactive wastefromnuclear power reactors. The diameter of the dust particles is so small (a few micrometers) that the electrons from thebeta decayleave the dust particles nearly without loss. The surrounding weaklyionized plasmaconsists of gases or gas mixtures (such askrypton,argon, andxenon) with excimer lines such that a considerable amount of the energy of the beta electrons is converted into this light. The surrounding walls contain photovoltaic layers with wideforbidden zonesas e.g.diamondwhich convert the optical energy generated from the radiation into electrical energy.

1.3.1.2.5 RECIPROCATING ELECTROMECHANICAL ATOMIC BATTERYElectromechanical atomic batteries use the buildup of charge between two plates to pull one bendable plate towards the other, until the two plates touch, discharge, equalizing the electrostatic buildup, and spring back. The mechanical motion produced can be used to produce electricity through flexing of a material or through a linear generator. Mill watts of power are produced in pulses depending on the charge rate, in some cases multiple times per second (35 Hz). A piezoelectric cantilever is mounted directly above a base of theradioactive isotopenickel-63. All of the radiation emitted as themillicurie-level nickel-63 thin film decays is in the form ofbeta radiation, which consists ofelectrons. As the cantilever accumulates the emitted electrons, it builds up a negative charge at the same time that the isotope film becomes positively charged. The beta particles essentially transfer electronic charge from thethin filmto the cantilever. The opposite charges cause the cantilever to bend toward the isotope film. Just as the cantilever touches the thin-film isotope, the charge jumps the gap. That permits current to flow back onto the isotope, equalizing the charge and resetting the cantilever. As long as the isotope isdecaying- a process that can last for decades - the tiny cantilever will continue its up-and-down motion. As the cantilever directly generates electricity when deformed, a charge pulse is released each time the cantilever cycles.Radioactive isotopes can continue to release energy over periods ranging from weeks to decades. The half-life of nickel-63, for example, is over 100 years. Thus, a battery using this isotope might continue to supply useful energy for at least half that time. Researchers have demonstrated devices with about 7% efficiency with high frequencies of 120Hzto low-frequency (every three hours) self-reciprocating actuators.1.3.1.2.3 INDIRECT CHARGE GENERATORIndirect conversion typically involves two steps of conversion. The radioactive decay consisting of either alpha or beta particles is impinged on some radio luminescent material like phosphor to produce photons and then is collected using photodiodes or 'solar cells'. Optimization has to be done on the structure of the photo diode, the phosphor filling, the method to collect photons and the placement of the radioactive material. Also, the optical properties of the conversion processes need to be matched. At best, we can expect an overall efficiency of 2% at 3.5V open circuit voltage. Theoretically this efficiency can be 25%.

CHAPTER 2ENERGY PRODUCTION MECHANISM

2.1 BETAVOLTAICSBetavoltaics is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Betavoltaics are generators of electrical current, in effect a form of a battery, which use energy from a radioactive source emitting beta particles (electrons). The functioning of a betavoltaics device is somewhat similar to a solar panel, which converts photons (light) into electric current.Betavoltaic technique uses a silicon wafer to capture electrons emitted by a radioactive gas, such as tritium. It is similar to the mechanics of converting sunlight into electricity in a solar panel. The flat silicon wafer is coated with a diode material to create a potential barrier. The radiation absorbed in the vicinity of and potential barrier like a p-n junction or a metal-semiconductor contact would generate separate electron-hole pairs which in turn flow in an electric circuit due to the voltaic effect. Of course, this occurs to a varying degree in different materials and geometries. A pictorial representation of a basic Betavoltaic conversion as shown in figure 1. Electrode A (P-region) has a positive potential while electrode B (N-region) is negative with the potential difference provided by me conventional means.

Fig.2.1 Betavoltaic conversion

The junction between the two electrodes is comprised of a suitably ionisable medium exposed to decay particles emitted from a radioactive source. The energy conversion mechanism for this arrangement involves energy flow in different stages show in figure 2.2.

Fig.2.2 Mechanism of energy flow Stage 1 Before the radioactive source is introduced, a difference in potential between to electrodes is provided by a conventional means. An electric load RL is connected across the electrodes A and B. Although a potential difference exists, no current flows through the load RL because the electrical forces are in equilibrium and no energy comes out of the system. We shall call this ground state E0.Stage 2 Next, we introduce the radioactive source, say a beta emitter, to the system. Now, the energy of the beta particle Eb generates electron-hole pair in the junction by imparting kinetic energy which knocks electrons out of the neutral atoms. This amount of energy E1, is known as the ionization potential of the junction.Stage 3 Further the beta particle imparts an amount of energy in excess of ionization potential. This additional energy raises the electron energy to an elevated level E2. Of course the beta [particle dose not impart its energy to a single ion pair, but a single beta particle will generate as many as thousands of electron- hole pairs. The total number of ions per unit volume of the junction is dependent upon the junction material.Stage 4 Next, the electric field present in the junction acts on the ions and drives the electrons into electrode A. the electrons collected in electrode A together with the electron deficiency of electrode B establishes Fermi voltage between the electrodes. Naturally, the electrons in electrode A seek to give up their energy and go back to their ground state (law of entropy).

Stage 5 The Fermi voltage derives electrons from the electrode A through the load where they give up their energy in accordance with conventional electrical theory. A voltage drop occurs across the load as the electrons give an amount of energy E3. Then the amount of energy available to be removed from the system is E3= Eb - E1 L1-L2 Where L1 is the converter loss and L2 is the loss in the electrical circuit.Stage 6The electrons, after passing to the load have an amount of energy E4.from the load, the electrons are then driven into the electrode B where it is allowed to recombine with a junction ion, releasing the recombination energy E4 in the form of heat this completes the circuit and the electron has returned to its original ground state.The end result is that the radioactive source acts as a constant current generator. Then the energy balance equation can be written as E0=Eb E1 E3-L1-L2Until now betavoltaics has been unable to match solar-cell efficiency. The reason is simple: when the gas decays, its electrons shoot out in all directions. Many of them are lost. A new Betavoltaic device using porous silicone diodes was proposed to increase their efficiency. The flat silicon surface, where the electrons are captured and converted to a current, and turned into a 3- dimensional surface by adding deep pits. Each pit is about 1 micron wide. That is four hundred-thousandths of an inch. They are more than 40 microns deep. When the radioactive gas occupies these pits, it creates the maximum opportunity for harnessing the reaction.

2.2 DIRECT CHARGING GENERATORSIn this type, the primary generator consists of a high Q LC tank circuit as shown in figure 2.3. The energy imparted to radioactive decay products during the spontaneous disintegrations of radioactive material is utilized to sustain and amplify the oscillations in the high-Q LC tank circuit the circuit inductance comprises a coil wound on a core composed of radioactive nuclides connected in series with the primary winding of a power transformer.

Fig.2.3 LC equivalent resonant circuit

The core is fabricated from a mixture of three radioactive materials which decay primarily by alpha emission and provides a greater flux of radioactive decay products than the equivalent amount of single radioactive nuclei. Equitant circuit of the direct charging generator as shown in the figure 3.An LCR circuit 1 is comprised of a capacitor 3, inductor file, transformer T primary winding 9 and resistance 11 connected in series. It is assumed that the electrical conductors connecting the various circuit elements and forming the inductor file and primary winding 9 are perfect conductors; i.e., no DC resistance. Resistor 11 is a lump resistance equivalent to total DC resistance of the actual circuit components and conductors. The inductor 5 is wound on a core 7 which is composed of a mixture of radioactive elements decaying primarily by alpha particle emission. When the current flows in electrical circuit, energy is dissipated or lost in the form of heat. Thus, when oscillations are induced in an LCR circuit, the oscillations will gradually damp out due to the loss of energy in the circuit unless energy is continuously added to the circuit to sustain the oscillations. In the LCR circuit shown in figure 3, a portion of the energy imparted to the decay products such as alpha particles. During the radioactive decay of the materials inductor core 7 is introduced into the circuit 1, when the decay products are absorbed by the conductor which forms inductor 5. Once oscillations have been induced in the LCR circuit 1, the energy absorbed by the inductor 5 form the radioactive decay of the core7 material will sustain the oscillations as long as the amount of energy absorbed is equal to the amount of energy dissipated in the ohmic resistance of the circuit 1.If the absorbed energy is greater than the amount of energy lost through ohmic heating, the oscillations will be amplified. This excess energy can be delivered to a load 17 connected across the transformer T secondary winding 13. The process involved in the conversion of the energy released by the spontaneous disintegration of a radioactive material into electrical energy are numerous and complex. Materials that are naturally radioactive, decay by the emission of either an alpha particle or a beta particle and gamma rays may accompany either process. Radioactive materials that decay primarily by alpha particle emission are preferred as inductor core 7 materials. Alpha particles are emitted a very high speeds, in the order of 1.6*107 meters per second (m/s) and consequently have very high kinetic energy. Alpha particles emitted in radium, for example, decays are found to consist of two groups, those with a kinetic energy of 48.79*105 electron volts (eV) and those having energy of 46.95*105 electron volts. This kinetic energy must be dissipated when the alpha particles are absorbed by the conductor forming inductor 5. During the absorption process, each alpha particle will collide with one or more atoms in the conductor knocking electron from their orbits and imparting some kinetic energy to the electrons. This results in increase number of conduction electrons in the conductor there by increasing its conductivity.Since the alpha particle is a positively charged ion, while the alpha particle is moving it will have an associated magnetic field. When the alpha particle is stopped by the conductor, the magnetic field will collapse thereby inducing a pulse of current in the conductor producing a net increase in the current flowing in the circuit 1. Also, there will be additional electrons stripped from orbit due to ionization reduced by the positively charged alpha particles.

Fig. 2.4 constructed nuclear battery

Referring to figure 2.4, the nuclear battery is constructed in a cylindrical configuration. Inductor 5 is constructed of copper wire wound in a single layer around the radioactive core 7. Decay products, such as alpha particles, are emitted radially outward from the core 7 as indicated by arrows 2 to be absorbed by the copper conductor forming inductor 5. Eight transformers are arranged in a circular pattern to form a cylinder concentric with and surrounding inductor 5. The transformers have primary windings 9a-9h connected in series which are then connected in series with inductor 5 and capacitor 3 to form an LCR circuit. The central core 7, inductor5 and the eight transformers 15 are positioned within a cylindrical shaped container 19. Copper wire is wound in a single layer on the outside wall and the inside wall of cylinder 19 to form windings 23 and21 respectively. The transformers 15, secondary windings 13a-13h and windings 21 and 23 are connected in series to output terminals 25 and 27. The configuration of inductor 5 is designed to ensure maximum eradication of the copper conductor by the radioactive core source 7. The cylindrical configuration of the power transformer ensures maximum transformer efficiency with minimum magnetic flux leakages.

2.3 OPTOELECTRICSAn optoelectric nuclear battery has been proposed by researchers of the kurchatov institute in Moscow. A beta emitter such as technetium-99 are strontium-90 is suspended in a gas or liquid containing luminescent gas molecules of the exciter type, constituting dust plasma. This permits a nearly lossless emission of beta electrons from the emitting dust particles for excitation of the gases whose exciter line is selected for the conversion of the radioactivity into a surrounding photovoltaic layer such that a comparably light weight low pressure, high efficiency battery can be realized. These nuclides are low cost radioactive of nuclear power reactors. The diameter of the dust particles is so small (few micrometers) that the electrons from the beta decay leave the dust particles nearly without loss. The surrounding weakly ionized plasma consists of gases or gas mixtures (e.g. krypton, argon, xenon) with exciter lines, such that a considerable amount of the energy of the beta electrons is converted into this light the surrounding walls contain photovoltaic layers with wide forbidden zones as egg. Diamond which converts the optical energy generated from the radiation into electric energy.The battery would consist of an exciter of argon, xenon, or krypton (or a mixture of two or three of them) in a pressure vessel with an internal mirrored surface, finely-ground radioisotope and an intermittent ultrasonic stirrer, illuminating photocell with a band gap tuned for the exciter. When the electrons of the beta active nuclides (e.g. krypton-85 or argon-39) are excited, in the narrow exciter band at a minimum thermal losses, the radiations so obtained is converted into electricity in a high band gap photovoltaic layer (e.g. in a p-n diode) very efficiently the electric power per weight compared with existing radionuclide batteries can then be increased by a factor 10 to 50 and more. If the pressure-vessel is carbon fiber / epoxy the weight to power ratio is said to be comparable to an air breathing engine with fuel tanks. The advantage of this design is that precision electrode assemblies are not needed and most beta particles escape the finely-divided bulk material to contribute to the batteries net power. The disadvantage consists in the high price of the radionuclide and in the high pressure of up to 10MPa (100bar) and more for the gas that requires an expensive and heavy container.

CHAPTER 3FUEL CONSIDERATIONS AND ENERGY PRODUCTION

3.1 FUEL CONSIDERATIONSAny radioisotope in the form of a solid that gives off alpha or beta particles can be utilized in the nuclear battery. The first cell constructed (that melted the wire components) employed the most powerful source known, radium-226, as the energy source. However, radium-226 gives rise through decay to the daughter product bismuth-214, which gives off strong gamma radiation that requires shielding for safety. This adds a weight penalty in mobile applications.The major criterions considered in the selection of fuels are: Avoidance of gamma in the decay chain Half life Particle range Watch out for (alpha, n)reactionsRadium-226 is a naturally occurring isotope which is formed very slowly by the decay of uranium-238. Radium-226 in equilibrium is present at about 1 gram per 3 million grams of uranium in the earths crust. Uranium mill wastes are readily available source of radium-226 in very abundant quantities. Uranium mill wastes contain far more energy in the radium-226 than is represented by the fission energy derived from the produced uranium. Strontium-90 gives off no gamma radiation so it does not necessitate the use of thick lead shielding for safety.strrrontium-90 does not exist in nature, but it is one of the several radioactive waste products resulting from nuclear fission. The utilizable energy from strontium-90 substantially exceeds the energy derived from the nuclear fission which gave rise to this isotope. Once the present stores of nuclear wastes have been mined, the future supplies of strontium-90 will depend on the amount of nuclear electricity generated hence strontium-90 decay may ultimately become a premium fuel for such special uses as for perpetually powered wheel chairs and portable computers. Plutonium-238 dioxide is used for space application. Half life of tantalum-180m is about 1015 years. In its ground state, tantalum-180 (180Ta) is very unstable and decays to other nuclei in about 8 hours but its isomeric state, 180m Ta, is found in natural samples. Tantalum 180m hence can be used for switchable nuclear batteries.

Fig. 3.1 relation between energy and power density

A current flowing through the device generates heat (I2R losses). As long as the temperature increase does not cause a phase change, nothing happens. However, if the current increases enough so that corresponding temperature rise causes a phase change, the polymers crystalline structure disappears, the volume expands, and the conducting carbon chains are broken. The result is a dramatic increase in resistance. Whereas before in the phase change a polymer-carbon combination may have a resistance measured milliohms or ohms, after the phase change the same structures resistance may be measured in mega ohms. Current flow is reduced accordingly, but the small residual current and associated I2R loss is enough to latch the polymer in this state, and the fuse will stay open until power is removed.

3.2 ENERGY FROM RADIOACTIVITYLet us do a calculation to estimate the energy in 1000 kg of U-235. Its half life is 4.5 109years. The energy released in each alpha particle is 4.27 MeV. The decay constant is k = ln(2)/(half-life) ~ 4.9 10-18sec-1. The number of nuclei is N = 1000 kg / (235 mass of proton) ~ 2.6 1027. The activity is thus k N ~ 1.3 1010sec-1. Now, power is activity energy per particle, which is about 0.0085 W. If we convert all this energy to electricity, we might barely have enough to power an LED! If we replace U-235 with Cs-137 with a half life of about 30 years, it would yield 1 MW of power, which is sizable, yet not even close enough to run a power plant. The conversion could be lossy too. This hints at the domains of operation: long lasting power supply, low power, high energy. Electrical technology being mature, we consider harnessing and/or storage in terms of electricity.3.3 ATOMIC BATTERIES-ENERGY AND POWERThe family of devices that convert these radiations to electrical power is known as Atomic Batteries. Conversion is classified into two main types: Thermal and non-thermal. Thermal conversion uses the energy in the radiation to heat a target, which is then converted to electricity, for instance using a thermocouple. Non thermal converters do not depend on the thermal energy of radiation; for instance radiation can be used to induce charge which is then converted to electricity. The efficiency of these can be about 20% at best. To compare their performances, we consider the power and energy density of storage devices. Such a plot is known as a Ragone plot. [2,3] Fig.3.2 presents a Ragone plot of atomic batteries made of some commonly used radioactive materials and compares them to commonly used batteries. The sloped lines are constant-time lines. The numbers for atomic batteries were calculated as shown in the previous section, assuming an efficiency of 0.1. This shows that atomic batteries are high energy density and relatively low power density devices.

Fig.3.2 Ragone plot for various batteries including Atomic batteries.

The costs that matter are the cost per unit energy ($/kJ) and the cost per unit power ($/kW). The costs for various commonly used batteries were obtained from a market survey - by obtaining the cost per kilogram of the packaged product and using Fig. 1 to get to $/kJ of the storage system. The costs of atomic batteries were approximated to the cost of the radioactive material required to build it, since the $/kg of these materials are many orders of magnitude higher than the cost of processing or packaging. The cost of most of these substances were found to be about 105$/kg. Fig.3.3 shows the data obtained from this analysis. We observe that some of the atomic batteries, especially those of Sr-90 and Cs-137 are comparable in power density to chemical batteries and lower in cost.

Fig.3.3 Cost per unit energy ($/kJ) plotted against power density (W/kg) of various batteries.

3.4 APPLICATIONThe applications follow from the position of these in the Ragone plot. These batteries have been employed to enable compact and high energy capacity power generators for applications ranging from implantable cardiac pacemakers to space stations. [4, 5] Currently, radioisotope power generators are being developed for realizing safe, compact, high energy capacity, and long lifetime batteries for remote wireless sensor microsystems in applications ranging from environmental health monitoring to structural health monitoring. [6, 7] These are also employed in a variety of industrial applications including electron capture devices for gas chromatography.

3.5 ENERGY CONVERSIONThe thermal atomic battery converts the atomic energy into heat first and then electricity. While thermal-to-electric conversion techniques has been studied extensively, there are many of them are available to be generalized to heat source powered by radioisotopes. To date, thermal converters include following forms, thermionic converter, radioisotope thermoelectric generator, thermo photovoltaic cell, alkali-metal thermal to electric converter and Sterling radioisotope generator. The conversion of atomic energy to heat is quite simple. In thermal conversion atomic battery, the radioisotope, called fuel, is placed in a container. Alpha particles generated by alpha decay or beta particles generated by beta decay can easily interact with atoms of shielding materials and lose energy. This part of energy is dissipated in the form of heat. Therefore, the container and the radioisotope itself are used as heat source in thermal conversion atomic battery. In thermionic converter, the heat generated from radioactive decay is used to heat a hot electrode to emit electron through thermionic emission at temperature 1500-2000 K. [1] The emitted electron is collected by a cold electrode. [1] Plasma, usually consisting of Cs vapor, is maintained between the two electrodes to reduce the work needed for electron emission, magnify the currency, which is favorable to increase efficiency and modify the electron conducting property between electrodes. [2-4]. the efficiency can also be increased by lowering the potential difference between the top of the potential barrier in the interelectrode space and the Fermi level of the anode. [5] However, the efficiency of the thermionic converter cannot exceed 90% of Carnot efficiency. [3] In practice, the efficiency of thermionic converter can be close to 20%. Radioisotope thermoelectric generator makes use of Seebeck effect to directly transfer the temperature difference between heat source and heat sink to electricity. Its structure is quite simple. The hot end of a thermopile is attached to the heat source, and its cold end is attached to the heat sink, usually the ambient temperature. Thermopile for power generation is usually made of pairs of connected P-type and N-type semiconductors. In presence of temperature difference, the velocity of charge carrier in both semiconductors is different, which forms current. Due to its simplicity, radioisotope thermoelectric generator is very reliable and can be made very small, and thus widely used in spacecraft. However, the efficiency of this converter is very low, about 7% in practical use. One possible way to improve efficiency is to hybridize the system with other converter. The working temperature of a typical radioisotope thermoelectric generator is much lower than that of thermionic converter. The hot end temperature is 811 K while the cold end temperature is 394 K. Thermo photovoltaic cell is similar to other photovoltaic cell in that it converts the energy of photons generated by thermal emission of the heat source into electricity. Wien's Law gives out the relation between wavelength with peak light flux and the temperature asmax= b TWhere b is Wien's displacement constant and T is temperature. Considering that a reasonable temperature of radioisotope heater unit is around 1200K-1500 K, the peak wavelength of the thermal emission is within the infrared light spectrum and the band gap of the photovoltaic cell should be smaller than solar cell that mainly uses visible light for energy conversion. Several thermo photovoltaic techniques have been proposed with reported efficiency up to 23%. These pilot projects are encouraging to result in higher conversion efficiency than current state-of-the-art level.The alkali-metal thermal to electric converter is a relatively young technique. This converter is basically a concentration cell with sodium vapor at 600-1000 K and solid sodium at 100-500 K as electrode as well as solid beta-alumina as electrolyte. The reaction in the cell absorbs heat. Thus additional heat is required to maintain the cell at operating temperature. Some models have been reported with efficiency of 14-18% while the theoretical efficiency can reach 20%-40%. The Stirling radioisotope generator converts the heat from radioisotope to dynamic motion by a Stirling engine and then a generator converts the motion to electricity. A Stirling engine is a heat engine operating by cyclic compression and expansion of working fluid at different temperature levels. It is classified as an external combustion engine since the heat transfer take place through the engine wall and there is no mass exchange of working fluid with outside of engine, which is perfect for external heat source as radioisotope heater unit. NASA, DoE and Lockheed Martin have jointly launched project to develop this type of converter for the use of space craft.In this project, helium is used as working fluid and efficiency as high as 23% has been achieved in experiments. The greatest challenge to date of Stirling generator is its reliability. Compared to other form of converter, it has moving parts and its vibration may harm other facilities within the spacecraft.New technology is needed to overcome these disadvantages.

3.6 SELETION OF RADIOISOTOPESThere are several criteria in selecting the radioisotope fuel. Firstly, the radioisotope should produce high energy decay. Alpha particle has a typical dynamic energy of 5 MeV while the figure of beta particle is typically 1 MeV. Secondly, the radiation must be easily absorbed and converted into heat. In general, alpha radiation has most significant heat effect; beta radiation can give off considerable amounts of gamma radiation through secondary radiation; gamma neutron radiation can easily penetrate common shielding material. Therefore, radioisotope of the type of alpha decay is favorable. Thirdly, the half-life of radioisotope must be long enough to release energy at a relatively constant rate for a reasonably long time. The life span of a thermal atomic battery is usually expected to be decades. Therefore, the half-life of radioisotope must be at least several decades. In the case of spacecraft, the energy density of radioisotope must be high to reduce the load of spacecraft. No more than 30 radioisotopes satisfy the first two criteria. Currently, Pu-238, Cm-244 and Sr-90 are most widely used radioisotopes. Other isotopes such as Po-210, Pm-147, Cs-137, Ce-144, Ru-106, Co-60, Cm-242 have also been studied for this use.Within these candidates, Pu-238 has the lowest shielding requirement and longest half-life. It needs less than 2.5mm of lead shielding to screen radiation. As a fuel for thermal atomic battery, it usually takes the form of PuO2with a half-life of 87.7 years. Sr-90 has much lower energy density and produces gamma radiation. However, it is still used in some cases since it is relatively cheap. Am-241 has a half-life of 432 years but only 1/4 of energy density of Pu-238 and requires at least 18 mm lead shielding to screen the penetrating radiation it produced.3.7 NUCLEAR FISSIONRTGs andnuclear powerreactors use very different nuclear reactions. Nuclear power reactors use controllednuclear fission. When an atom of U-235 or Pu-239 fuel fissions, neutrons are released that trigger additional fissions in achain reactionat a rate that can be controlled with neutron absorbers. This is an advantage in that power can be varied with demand or shut off entirely for maintenance. It is also a disadvantage in that care is needed to avoid uncontrolled operation at dangerously high power levels. Chain reactions do not occur in RTGs, so heat is produced at an unchangeable, though steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. An accidental power excursion is impossible. However, if a launch or re-entry accident occurs and the fuel is dispersed, the combined power output of the nowradionuclidesset free does not drop. In an RTG, heat generation cannot be varied with demand or shut off when not needed. Therefore, auxiliary power supplies (such as rechargeable batteries) may be needed to meet peak demand, and adequate cooling must be provided at all times including the pre-launch and early flight phases of a space mission.3.7.1 RTG FOR INTERSTELLER PROBESRTG have been proposed for use on realistic interstellar precursor missions andinterstellar probes.An example of this is theInnovative Interstellar Explorer(2003current) proposal from NASA.A RTG using241Am was proposed for this type of mission in 2002.This could support mission extensions up to 1000 years on the interstellar probe, because the power output would be more stable in the long-term than plutonium.Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products.An interstellar probe proposal from 1999 suggested using threeadvanced radioisotope power source(ARPS).The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.One mission proposed using the electricity to powerion engines, calling this methodradioisotope electric propulsion(REP).3.8 NUCLEAR POWER SYSTEM IN SPACEKnown spacecraft nuclear power systems and their fate. Systems face a variety of fates, for example, Apollo's SNAP-27 were left on the Moon.Some other spacecraft also have small radioisotope heaters, for example each of the Mars Exploration Rovers have a 1 watt radioisotope heater. Spacecraft use different amounts of material, for example MS Curiosityhas 4.8kg ofplutonium-238 dioxide,while theCassinispacecraft has 32.7kg.Name and/or modelLaunchedFate/location

MSL/Curiosity roverMMRTG2011Mars surface

Apollo 12SNAP-27ALSEP1969Lunar surface (Ocean of Storms)

Apollo 13SNAP-27 ALSEP1970Earth re-entry (over Pacific near Fiji)

Apollo 14SNAP-27 ALSEP1971Lunar surface (Fra Mauro)

Apollo 15SNAP-27 ALSEP1971Lunar surface (HadleyApennine)

Apollo 16SNAP-27 ALSEP1972Lunar surface (Descartes Highlands)

Apollo 17SNAP-27 ALSEP1972Lunar surface (TaurusLittrow)

Transit-4A SNAP-3B1961Earth orbit

Transit 5A3 SNAP-31963Earth orbit

Transit 5BN-1 SNAP-31963Earth orbit

Transit 5BN-2 SNAP-9A1963Earth orbit

Transit 91964Earth orbit

Transit 5B41964Earth orbit

Transit 5B61965Earth orbit

Transit 5B71965Earth orbit

Transit 5BN-3SNAP-9A (1)1964Failed to reach orbit[41]

Nimbus-BSNAP-19(2)1968Recovered after crash

Nimbus-3SNAP-19 (2)1969Earth re-entry 1972

Pioneer 10SNAP-19 (4)1972Ejected from Solar System

Pioneer 11SNAP-19 (4)1973Ejected from Solar System

Viking 1lander modified SNAP-191976Mars surface (Chryse Planitia)

Viking 2lander modified SNAP-191976Mars surface

CassiniGPHS-RTG(3)1997OrbitingSaturn

New HorizonsGPHS-RTG (1)2006Leaving the Solar System

GalileoGPHS-RTG (2),1989Jupiter atmospheric entry

UlyssesGPHS-RTG (1)1990Heliocentric orbit

LES-8MHW-RTG1976Neargeostationary orbit

LES-9MHW-RTG1976Neargeostationary orbit

Voyager 1MHW-RTG(3)1977Ejected from Solar System

Voyager 2MHW-RTG(3)1977Ejected from Solar System

Fig.3.4 spacecraft nuclear power systems

3.9 INDUCED GAMMA EMISSIONInphysics,induced gamma emission(IGE) refers to the process of fluorescent emission ofgamma raysfrom excited nuclei, usually involving a specificnuclear isomer. It is analogous to conventionalfluorescence, which is defined as the emission of aphoton(unit of light) by an excited electron in an atom or molecule. In the case of IGE, nuclear isomers can store significant amounts of excitation energy for times long enough for them to serve as nuclear fluorescent materials. There are over 800 known nuclear isomers but almost all are too intrinsically radioactive to be considered for applications. As of 2006there were five proposed nuclear isomers that appeared to be physically capable of IGE fluorescence in safe arrangements:tantalum-180m,osmium-187m,platinum-186m],hafnium-178m2 andzinc-66m.

Fig. 3.5 Energetics of IGE from 115In. Arrows are photons, (up) absorption, (down) emission. Horizontal lines represent excited states of in involved in IGE

Induced gamma emission is an example of interdisciplinary research bordering on both nuclear physics and quantum electronics. Viewed as anuclear reactionit would belong to a class in which only photons were involved in creating and destroying states of nuclear excitation. It is a class usually overlooked in traditional discussions. In 1939Pontecorvoand Lazardreported the first example of this type of reaction.Indiumwas the target and in modern terminology describingnuclear reactionsit would be written115In (,')115mIn. The product nuclide carries an "m" to denote that it has a long enough half life (4.5 hr in this case) to qualify as being anuclear isomer. That is what made the experiment possible in 1939 because the researchers had hours to remove the products from the irradiating environment and then to study them in a more appropriate location.With projectile photons, momentum and energy can be conserved only if the incident photon, X-ray or gamma, has precisely the energy corresponding to the difference in energy between the initial state of the target nucleus and some excited state that is not too different in terms of quantum properties such as spin. There is no threshold behavior and the incident projectile disappears and its energy is transferred into internal excitation of the target nucleus. It is aresonantprocess that is uncommon innuclear reactionsbut normal in the excitation of fluorescence at the atomic level. Only as recently as 1988 was the resonant nature of this type of reaction finally proven.Such resonant reactions are more readily described by the formalities of atomic fluorescence and further development was facilitated by an interdisciplinary approach of IGE.There is little conceptual difference in an IGE experiment when the target is anuclear isomer. Such a reaction asmX(,')X wheremX is one of the five candidates listed above, is only different because there are lower energy states for the product nuclide to enter after the reaction than there were at the start. Practical difficulties arise from the need to ensure safety from the spontaneous radioactive decay of nuclear isomers in quantities sufficient for experimentation. Lifetimes must be long enough that doses from the spontaneous decay from the targets always remain within safe limits. In 1988 Collins and coworkersreported the first excitation of IGE from a nuclear isomer. They excited fluorescence from thenuclear isomertantalum-180m with x-rays produced by anexternal beam radiotherapylinac. Results were surprising and considered to be controversial until the resonant states excited in the target were identified.[5]Fully independent confirmation was reportedby the Stuttgart Nuclear Group in 1999.

3.9.1 DISTINCTIVE FEATURESIf an incident photon is absorbed by an initial state of a target nucleus, that nucleus will be raised to a more energetic state of excitation. If that state can radiate its energy only during a transition back to the initial state, the result is ascattering processas seen in the schematic figure. That is not an example of IGE. If an incident photon is absorbed by an initial state of a target nucleus, that nucleus will be raised to a more energetic state of excitation. If there is a nonzero probability that sometimes that state will start a cascade of transitions as shown in the schematic, that state has been called a "gateway state" or "trigger level" or "intermediate state". One or more fluorescent photons are emitted, often with different delays after the initial absorption and the process is an example of IGE. If the initial state of the target nucleus is its ground (lowest energy) state, then the fluorescent photons will have less energy than that of the incident photon (as seen in the schematic figure). Since the scattering channel is usually the strongest, it can "blind" the instruments being used to detect the fluorescence and early experiments preferred to study IGE by pulsing the source of incident photons while detectors were gated off and then concentrating upon any delayed photons of fluorescence when the instruments could be safely turned back on. If the initial state of the target nucleus is a nuclear isomer (starting with more energy than the ground) it can also support IGE. However in that case the schematic diagram is not simply the example seen for115In but read from right to left with the arrows turned the other way. Such a "reversal" would require simultaneous (to within