Nuclear Batteries Department of Electrical Engineering_AGI_Jaipur_2013-2014 Page 1 CONTENTS 1. Introduction 3 2. What are nuclear power batteries? 5 3. Types of nuclear batteries 6 a. Thermal converters 6 i. Thermionic Converters 6 ii. Radioisotope thermoelectric generator 7 iii. Thermo photovoltaic cells 9 iv. Alkali-metal thermal to electric converter 10 b. Non-thermal converter 13 i. Direct charging generators 13 ii. Betavotaic 13 iii. Alphavoltaics 15 iv. Optoelectric 15 v. Reciprocating electromechanical battery 16 4. Fuel consideration 17 5. Advantages 20 6. Drawbacks 21 7. Applications 22 8. Conclusion 26
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Nuclear Batteries
Department of Electrical Engineering_AGI_Jaipur_2013-2014 Page 1
CONTENTS
1. Introduction 3
2. What are nuclear power batteries? 5
3. Types of nuclear batteries 6
a. Thermal converters 6
i. Thermionic Converters 6
ii. Radioisotope thermoelectric generator 7
iii. Thermo photovoltaic cells 9
iv. Alkali-metal thermal to electric converter 10
b. Non-thermal converter 13
i. Direct charging generators 13
ii. Betavotaic 13
iii. Alphavoltaics 15
iv. Optoelectric 15
v. Reciprocating electromechanical battery 16
4. Fuel consideration 17
5. Advantages 20
6. Drawbacks 21
7. Applications 22
8. Conclusion 26
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LIST OF FIGURE
SR.
NO.
NAME PG. NO.
1. THERMIONIC CONVERTER
7
2. RADIOISOTOPE THERMOELECTRIC GENERATOR 8
3. THERMOPHOTOVOLTAIC CELLS 10
4. AMTEC Direct Energy Conversion 12
5. BETAVOLTAIC CELL 14
6. Radioisotope piezoelectric generator 16
7. Power Density 19
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CHAPTER 1
INTRODUCTION
A 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 today‟s 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 1950‟s.
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
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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.
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CHAPTER 2
WHAT ARE NUCLEAR POWER BATTERIES?
The terms atomic battery, nuclear battery, tritium battery and radioisotope generator are used to
describe a device which uses energy from the decay of a radioactive isotope to generate
electricity. Like nuclear reactors they generate electricity from atomic energy, but differ in that
they do not use a chain reaction. Compared to other batteries they are very costly, but have
extremely long life and high energy density, and so they are mainly used as power sources for
equipment that must operate unattended for long periods of time, such as spacecraft, pacemakers,
underwater systems and automated scientific stations in remote parts of the world.
Nuclear battery technology began in 1913, when Henry Moseley first demonstrated the beta cell.
The field received considerable in-depth research attention for applications requiring long-life
power sources for space needs during the 50s and 60s. In 1954 RCA researched a small atomic
battery for small radio receivers and hearing aids. After RCA development, over the years after
many types and methods has been developed. The scientific principles are well known, but
modern nano-scale technology and new wide band gap semiconductors have created new devices
and interesting material properties not previously available.
Batteries using the energy of radioisotope decay to provide long-lived power (10–20 years) are
being developed internationally. Conversion techniques can be grouped into two types: thermal
and non-thermal. The thermal converters (whose output power is a function of a temperature
differential) include thermoelectric and thermionic generators. The non-thermal converters
(whose output power is not a function of a temperature difference) extract a fraction of the
incident energy as it is being degraded into heat rather than using thermal energy to run electrons
in a cycle. Atomic batteries usually have an efficiency of 0.1–5%. High efficiency betavoltaics
have 6–8%.
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CHAPTER 3
TYPES OF NUCLEAR BATTERIES
Nuclear batteries are mainly classified into two main categories:
I. THERMAL CONVERTERS
The thermal converters are the devices which convert heat energy to electrical energy i.e. whose
output power is a function of a temperature differential. Thermal converters are also classified
into four types.
1. THERMIONIC CONVERTER
A thermionic converter consists of a hot electrode which thermionically emits electrons over a
potential energy barrier to a cooler electrode, producing a useful electric power output. Caesium
vapor is used to optimize the electrode work functions and provide an ion supply (by surface
contact ionization or electron impact ionization in a plasma) to neutralize the electron space
charge.
The scientific aspects of thermionic energy conversion primarily concern the fields of surface
physics and plasma physics. The electrode surface properties determine the magnitude of
electron emission current and electric potential at 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 the work function, which is the barrier that limits
electron emission current from the surface and essentially is the heat of vaporization of electrons
from the surface. The work function is determined primarily by a layer of caesium atoms
adsorbed on the electrode surfaces. The properties of the interelectrode plasma are determined by
the mode of operation of the thermionic converter. In the ignited (or “arc”) mode the plasma is
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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.
Fig. 1 THERMIONIC CONVERTER
2. RADIOISOTOPE THERMOELECTRIC GENERATOR
A radioisotope thermoelectric generator (RTG, RITEG) is an electrical generator that obtains its
power from radioactive decay. In such a device, the heat released by the decay of a suitable
radioactive material is converted into electricity by the See beck effect using an array of
thermocouples. RTGs have been used as power sources in satellites, space probes and unmanned
remote facilities, such as a series of lighthouses built by the former Soviet Union inside the
Arctic Circle. RTGs are usually the most desirable power source for robotic or unmaintained
situations needing a few hundred watts (or less) of power for durations too long for fuel cells,
batteries, or generators to provide economically, and in places where solar cells are not practical.
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Safe use of RTGs requires containment of the radioisotopes long after the productive life of the
unit. RTGs were developed in the US during the late 1950s by Mound Laboratories in
Miamisburg, Ohio under contract with the United States Atomic Energy Commission.
Critics argue that they could cause environmental and security problems as leakage or theft of
the radioactive material could pass unnoticed for years, particularly as the locations of some of
these lighthouses are no longer known due to poor record keeping. In one instance, the
radioactive compartments were opened by a thief. In another case, three woodsmen in Georgia
came across two ceramic RTG heat sources that had been stripped of their shielding. Two of the
three were later hospitalized with severe radiation burns after carrying the sources on their backs.
The units were eventually recovered and isolated. There are approximately 1,000 such RTGs in
Russia. All of them have long exhausted their 10-year engineered life spans. They are likely no
longer functional, and may be in need of dismantling. Some of them have become the prey of
metal hunters, who strip the RTGs' metal casings, regardless of the risk of radioactive
contamination.
Fig. 2 RADIOISOTOPE THERMOELECTRIC GENERATOR
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3. THERMOPHOTOVOLTAIC CELLS
Thermo photovoltaic (TPV) energy conversion is a direct conversion process from heat
differentials to electricity via photons. A basic thermophotovoltaic system consists of a thermal
emitter and a photovoltaic diode cell.
The temperature of the thermal emitter varies between different systems from about 900 °C to
about 1300 °C, although in principle TPV devices can extract energy from any emitter with
temperature elevated above that of the photovoltaic device (forming an optical heat engine). The
emitter can be a piece of solid material or a specially engineered structure. A conventional solar
cell is effectively a TPV device in which the Sun functions as the emitter. Thermal emission is
the spontaneous emission of photons due to thermal motion of charges in the material. For
normal TPV temperatures, this radiation is mostly at near infrared and infrared frequencies. The
photovoltaic diodes can absorb some of these radiated photons and convert them into free charge
carriers, that is electricity.
Thermophotovoltaic systems have few, if any, moving parts and are therefore very quiet and
require low maintenance. These properties make thermophotovoltaic systems suitable for
remote-site and portable electricity-generating applications. Their efficiency-cost properties,
however, are often rather poor compared to other electricity-generating technologies. Current
research in the area aims at increasing the system efficiencies while keeping the system cost low.
In the design of a TPV system, it is usually desired to match the optical properties of thermal
emission (wavelength, polarization, direction) with the most efficient conversion characteristics
of the photovoltaic cell, since unconverted thermal emission is a major source of inefficiency.
Most groups focus on gallium antimonide (GaSb) cells. Germanium (Ge) is also suitable.[1]
Much research and development in TPVs therefore concerns methods for controlling the
emitter's properties.
TPV cells have often been proposed as auxiliary power conversion devices for regeneration of
lost heat in other power generation systems, such as steam turbine systems or solar cells.A
prototype TPV hybrid car was even built.
TPV research is a very active area. Among others, the University of Houston TPV Radioisotope
Power Conversion Technology development effort is aiming at combining thermophotovoltaic
cell concurrently with thermocouples to provide a 3 to 4-fold improvement in system efficiency
over current radioisotope thermoelectric generators.
TPVs have significant promise for efficient and economically viable power systems for both
military and commercial applications. Compared to traditional nonrenewable energy sources,
burner TPVs have little NOx emissions and are virtually silent. Solar TPVs, on the other hand,
are a source of entirely renewable energy with no emissions. Compared to photovoltaics, TPVs
can be more efficient owing to recycling of unabsorbed photons. However, the structure of TPVs
is more complex, and losses at each energy conversion step can result in a lower efficiency than
that of photovoltaics. Further developments must be made to the absorber/emitter and PV cell to
realize its full potential as a renewable energy source. Unlike PVs, however, when TPVs are
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used with a burner source, they provide on-demand energy. As a result, no form of energy
storage is needed. In addition, owing to the PV‟s proximity to the radiative source, TPVs can
generate current densities 300 times that of conventional PVs.
Fig. 3 THERMOPHOTOVOLTAIC CELLS
4. ALKALI-METAL THERMAL TO ELECTRIC CONVERTER
The alkali metal thermal to electric converter (AMTEC), originally called the sodium heat engine
(SHE) was invented by Joseph T. Kummer and Neill Weber at Ford in 1966.
This device accepts a heat input in a range from about 900 K–1300 K and produces direct current
with predicted device efficiencies of 15-40%. In the AMTEC sodium is driven around a closed
thermodynamic cycle between a high temperature heat reservoir and a cooler reservoir at the heat
rejection temperature. The unique feature of the AMTEC cycle is that sodium ion conduction
between a high pressure or activity region and a low pressure or activity region on either side of
a highly ionically conducting refractory solid electrolyte, is thermodynamically nearly equivalent
to an isothermal expansion of sodium vapor between the same high and low pressure.
Electrochemical oxidation of neutral sodium at the anode leads to sodium ions which traverse the
solid electrolyte and electrons which travel from the anode through an external circuit where they
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perform electrical work, to the low pressure cathode, where they recombine with the ions to
produce low pressure sodium gas. The sodium gas generated at the cathode then travels to a
condenser at the heat rejection temperature of perhaps 400–700 K where liquid sodium reforms.
The AMTEC thus is an electrochemical concentration cell which converts the work generated by
expansion of sodium vapor directly into electric power.
The converter is based on the electrolyte used in the sodium-sulfur battery, sodium beta"-
alumina, a crystalline phase of somewhat variable composition containing aluminum oxide,
Al2O3, and sodium oxide, Na2O, in a nominal ratio of 5:1, and a small amount of the oxide of a
small cation metal, usually lithium or magnesium, which stabilizes the beta" crystal structure.
The sodium beta"-alumina solid electrolyte (BASE) [ceramic] is nearly insulating with respect to
transport of electrons, and is a thermodynamically stable phase in contact with both liquid
sodium and sodium at low pressure.
Single cell AMTECs with open voltages as high as > 1.55 V and maximum power density as
high as > 0.50 W/cm² at temperature of 1173K (900°C) have been obtained with long term stable
refractory metal electrodes.
Efficiency of AMTEC cells has reached 16% in the laboratory. High voltage multi-tube modules
are predicted to be 20% to 25% efficient, and power densities up to 0.2 kilowatts per liter appear
to be achievable in the near future. Calculations show that replacing sodium with a potassium
working fluid increases the peak efficiency from 28% to 31% at 1100 K with a 1 mm thick
BASE tube.
AMTEC requires energy input at modest elevated temperatures, and not at a specific wavelength,
it is easily adapted to any heat source, including radioisotope, concentrated solar, external
combustion, or nuclear reactor. A solar thermal power conversion system based on an AMTEC
has advantages over other technologies (including photovoltaic systems) in terms of the total
power that can be achieved with such a system and the simplicity of the system (which includes
the collector, energy storage (thermal storage with phase change material) and power conversion
in a compact unit). The overall system could achieve as high as 14 W/kg with present collector
technology and future AMTEC conversion efficiencies. The energy storage system outperforms
batteries, and the temperatures at which the system operates allows long life and reduced radiator
size (heat reject temperature of 600 K). Deep-space applications would use radioisotope
thermoelectric generators; hybrid systems are in design.
While space power systems are of intrinsic interest, terrestrial applications will offer large scale
applications for AMTEC systems. At the +25% efficiency projected for the device and projected
costs of $350/kW, AMTEC is expected to prove useful for a very wide variety of distributed
generation applications including self-powered fans for high efficiency furnaces and water
heaters and recreational vehicle power supplies cathodic protection of pipelines, remote
telemetry from oil well sites are other areas where this type of electrical generation might be
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used. The potential to scavenge waste heat may allow for integration of this technology into
general residential and commercial cogeneration schemes although costs per kilowatt-hour
would have to drop substantially from current projections.
Fig. 4
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II. NON-THERMAL CONVERTERS
Non-thermal converters extract a fraction of the 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 five classes.
1. DIRECT CHARGING GENERATORS
In the first type, the primary generators 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 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 H.G.J. Moseley constructed the first of these. Moseley‟s apparatus consisted of
a glass globe silvered 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.
2. BETAVOLTAICS
Betavoltaics 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 is used to generate electricity (thermoelectric and thermionic sources),
betavoltaics use a non-thermal conversion process; converting the electron-hole pairs produced
by the ionization trail of beta particles traversing a semiconductor.
Betavoltaic power sources 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.
The primary use for betavoltaics is for remote and long-term use, such as spacecraft requiring
electrical power for a decade or two. Recent progress has prompted some to suggest using
betavoltaics to trickle-charge conventional batteries in consumer devices, such as cell phones and
laptop computers. As early as 1973, betavoltaics were suggested for use in long-term medical
devices such as pacemakers.
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Although betavoltaics use a radioactive material as a power source, the beta particles used are
low energy and easily stopped by shielding, as compared to the gamma rays generated by more
dangerous radioactive materials. With proper device construction (that is, proper containment), a
betavoltaic device would not emit dangerous radiation. Leakage of the enclosed material would
engender health risks, just as leakage of the materials in other types of batteries leads to
significant health and environmental concerns.
As radioactive material emits, it slowly decreases in activity. Thus, over time a betavoltaic
device will provide less power. For practical devices, this decrease occurs over a period of many
years. For tritium devices, the half-life is 12.32 years. In device design, one must account for
what battery characteristics are required at end-of-life, and ensure that the beginning-of-life
properties take into account the desired usable lifetime.
Liability connected with environmental laws and human exposure to tritium and its beta decay
must also be taken into consideration during risk assessment and product development.
Naturally, this increases both time-to-market and the already high cost associated with tritium. A
2007 report by the UK government's Health Protection Agency Advisory Group on Ionizing
Radiation declared the health risks of tritium exposure to be double those previously set by the
International Commission on Radiological Protection located in Sweden.
Fig. 5 BETAVOLTAIC CELL
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3. ALPHAVOLTAICS
Alpha voltaic power sources are devices that use a semiconductor junction to produce electrical
particle from energetic alpha particles.
4. OPTOELECTRIC
An opto-electric nuclear battery is a device that converts nuclear energy into light, which it then
uses to generate electrical energy. A beta-emitter such as technetium-99 or strontium-90 is
suspended in a gas or liquid containing luminescent gas molecules of the excimer type,
constituting a "dust plasma." This permits a nearly lossless emission of beta electrons from the
emitting dust particles. The electrons then excite the gases whose excimer line is selected for the
conversion of the radioactivity into a surrounding photovoltaic layer such that a lightweight, low-
pressure, high-efficiency battery can be realised. These nuclides are relatively low-cost
radioactive waste from nuclear power reactors. The diameter of the dust particles is so small (a
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 (such as krypton,
argon, and xenon) 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
wide forbidden zones as e.g. diamond which convert the optical energy generated from the
radiation into electrical energy.
The battery would consist of an excimer 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 a photocell with a bandgap tuned for the excimer.
When the beta active nuclides (e.g., krypton-85 or argon-39) emit beta particles, they excite their
own electrons in the narrow excimer band at a minimum of thermal losses that this radiation is
converted in a high band gap photovoltaic layer (e.g. in p-n diamond) very efficiently into
electricity. 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 battery's net power.
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