Chapter 13

Chapter 13

Energy from Nuclear Power

Introduction

• Read the intro to CH. 13 (on page 331) and be able to answer the following ?s:– What was the cause and effect of the accident in

Japan?– How much of Japan’s electricity is supplied by

nuclear power?

NUCLEAR ENERGY IN PERSPECTIVE13.1

The Nuclear Age

• In the 1960s and early 70s, utility companies moved ahead with plans for numerous nuclear power plants

• Research to use nuclear power to generate electricity to prevent the pollution formed by fossil fuels and to solve the problem of resource limitations

• Perception of nuclear power plans was one of optimism

Curtailed

• 1975 utilities stopped ordering nuclear power plants and existing orders were canceled

• The perception of nuclear power has been one of pessimism– Public opinion has the greatest impact on future

developments of nuclear energy

Since the early 1970s, when orders for plants reached a peak, few utilities called for new plants and many canceled earlier orders. Nevertheless, the number of plants in service increased steadily as plants under construction were completed. The number of operating plants peaked at 112 and is holding steady at 103

The Shoreham Nuclear Plant on Long Island, NYWith little electricity produced, this plant was closed because of concerns about whether surrounding areas could be evacuated in case of an accident. The plant is now being dismantled

Global Picture

• Nuclear power plants generates about 16% of the world’s electricity

• France and Japan remain fully committed to pushing forward with nuclear programs– France now produces 78% of its energy with

nuclear power with plans to push it to 80%• As of 2005, the U.S. had 103 nuclear plants

operating, producing 20% of U.S. electricity

Nuclear share of electrical power generationIn 2005, those countries lacking fossil fuel reserves tended to bet he most eager to use nuclear power (Source: Date from International Atomic Energy Agency)

HOW NUCLEAR POWER WORKS13.2

Objective

• Control nuclear reactions so that energy is released gradually as heat

• Heat is used to boil water and produce steam, which then drives conventional turbogenerators

From Mass to Energy

• Differs from generating electricity using fossil fuels– Fossil fuels – chemical reactions remain

unchanged at the atomic level– Nuclear energy involves changes at the atomic

level through fission or fusion

Nuclear Energy

Fission• A large atoms of one

element is split to produce two smaller atoms of different elements

Fusion• Two small atoms combine

to form a larger atoms of a different element

• In both fission and fusion, the mass of the product(s) is less than the mass of the starting material, and the lost mass is converted to energy in accordance with the law of mass energy equivalence (E = mc2)

Nuclear Energy

• The amount of energy released by the mass-to-energy conversion is tremendous– The sudden fission or fusion of a mere 1 kg of

material releases the explosive energy of a nuclear bomb

– Controlled fission releases the energy gradually as heat

Nuclear FissionSplitting of certain large atoms into smaller atoms

Nuclear Fusion Fusing together of small atoms to form a larger atoms

The Fuel for Nuclear Power Plants

• All current nuclear power plants employ the fission of uranium 235 (U-235)

• The element Uranium occurs naturally in various minerals in Earth’s crusts

Uranium

• Exists as two isotopes– Isotope – any given element that contains

different numbers of neutrons, but the same number of protons and electrons• U-238• U-235

– The numbers (238, and 235) is called the mass number of the element. It is the sum of the number of neutrons and the number of protons in the nucleus of the atoms

– U-235 will readily undergo fission, but U-238 will not

Fission

• It takes a neutron hitting the nucleus at jus the right speed to cause U-235 to undergo fission

• The fission reaction gives off several more neutrons and releases a great deal of energy– Chain reaction occurs when the neutrons cause

other fissions, which release more neutrons, which causes other fissions• Fission products:

– Radioactive by-products, heat, neutrons

Chain Reaction

Nuclear Fuel

• To make nuclear “fuel” uranium ore is mined, purified into uranium dioxide (UO2) and enriched

• 99.3% of uranium found in nature is U-238– Enrichment involves separating U-235 from U-238

to produce a material containing higher concentrations of U-235

Nuclear Bomb

• When U-235 is highly enriched, the spontaneous fission of an atom can trigger a chain reaction

• A nuclear bomb is the result of an uncontrolled fission of a high grade U-235

Nuclear Reactor

• Designed to sustain a continuous chain reaction but not allow it to amplify into a nuclear explosion

• Uranium is enriched to 4% U-235– This prevents nuclear explosion

• Consists primarily of an array of fuel and control rods

• Generates an enormous amount of heat

Moderator

• Chain reaction can be sustained in a reactor only if a sufficient mass of enriched uranium is arranged into a geometric pattern and is surrounded with a material called a moderator– Moderator slows down neutrons that produce

fission so they are traveling at the right speed to trigger another fission.

Fuel Rods

• Enriched UO2 is made into pellets that are loaded into long metal tubes– Loaded tubes are called fuel elements or fuel rods– Over time daughter products that also absorb

neutrons accumulate in the fuel rods and slow down the rate of fission and heat production

– The highly reactive spent fuel elements are removed and replaced with new ones

Control Rods

• Chain reaction in the reactor core is controlled by rods of neutron-absorbing material (typically cadmium) called control rods

• Chain reaction is started and controlled by withdrawing and inserting the control rods as necessary

Nuclear ReactorIn the core of a nuclear reactor, a large mass of uranium is created by placing uranium in adjacent tubes, called fuel elements. The rate of the chain reaction is moderated by inserting or removing rods of neutron-absorbing material between the fuel elements

Nuclear ReactorThe fuel and rods are surrounded by the moderator fluid, near-pure water

Nuclear Power Plant

• Heat from the reactor is used to boil water and provide steam for driving conventional turbogenerators

LOCA

• If the reactor vessel should break, the sudden loss of water from around the reactor, called a “loss-of-coolant)accident” (LOCA) could result in the core’s overheating, resulting in a meltdown

Warm-Up

• What is the difference between fusion and fission?

• What isotope of uranium is used in fission reactions?

• What are the products of fission?

Comparing Nuclear Power with Coal Power

Fuel Needed - Nuclear• Requires 1.5 tons of raw

material– Mining causes much less harm

to humans and environment• Fission of about 1 pound of

uranium fuel releases the energy equivalent to burning 50 tons of coal– About 60 tons of uranium is

sufficient to run for as long as two years

Fuel Needed - Coal• Coal plant consumes 2 – 3

million tons of coal– Obtained through strip

mining• Acid mine drainage, erosion

– Obtained through deep mining• Human costs in the form of

accidental deaths and impaired health

Comparing Nuclear Power with Coal

CO2 Emissions – Nuclear• Does not emit ANY CO2 into

the atmosphere while producing energy– However, fossil fuels are used

in the mining and enriching of uranium, construction of plants, the decommissioning of the plant after it is shut down and the transportation and storage of waste

CO2 Emissions – Coal • Emits more than 10 million

tons of CO2 into th4e atmosphere– Since coal pants also need to

be constructed, coal and waste ash, the extra fossil fuel consumption can apply to coal

Comparing Nuclear Power to Coal

SO2 and other Emissions – Nuclear

• Produces no acid-forming pollutants or particulates

SO2 and other Emissions – Coal

• Emits more than 300,000 tons of SO2, particulates, and other pollutants– Leads to acid rain and health-

threatening air pollution

Comparing Nuclear Energy with Coal

Radioactivity - Nuclear• Releases low levels of

radioactive waste gases

Radioactivity - Coal• Releases 100 x more

radioactivity than a nuclear power plant because of the natural presence of radioactive compounds in coal– Uranium, thorium


Solid Wastes - Nuclear• Produces about 250 tons of

highly radioactive wastes – Requiring safe storage and

ultimate safe disposal• Safe disposal is an

unresolved problem

Solid Wastes - Coal• Produces about 600,000

tons of ash requiring land disposal.


Accidents – Nuclear• Range from minor

emissions of radioactivity to catastrophic releases that can lead to widespread radiation sickness, death, cancer, widespread and long lasting environmental contamination

Accidents – Coal • Worst case scenario

– Fatalities to workers and a destructive fire

THE HAZARDS AND COSTS OF NUCLEAR POWER FACILITIES

12.3

Radioactive Emissions

• When an element undergoes fission, the split “halves” are atoms of lighter elements– These are the DIRECT products of fission• Typically unstable isotopes of their respective elements

• Unstable isotopes are called radioisotopes– Radioisotopes become stable by spontaneously

ejecting subatomic particles (alpha/beta particles and neutrons), high energy radiation (gamma rays and X rays) or both

Radioactive Emissions

• Radioactivity is measured in curies• The particles and radiation emitted from

radioisotopes are called radioactive emissions– Many materials in and around the reactor may be

converted to unstable isotopes and become radioactive by absorbing neutrons from the fission process• These indirect products (unstable isotopes) of fission,

along with the direct products (spent fuel) are the radioactive wastes of nuclear power

Biological Effects of Radioactive Emissions

• Radioactive emissions can penetrate biological tissue and damage it– Radiation displaces electrons from molecules

leaving behind charged particles, or ions• Therefore emissions are called ionizing radiation

– Ionizing radiation can break chemical bonds or change the structure of the molecule and inhibit its function

Biological Effects of Radioactive Emissions

• High Doses – Prevent cell division• In medical applications – chemotherapy• However, if whole body is exposed, a general blockage

of cell division occurs that prevents the normal replacement or repair of blood, skin, and other tissues– “radiation sickness”

» May lead to death a few days, or months after exposure» Very high levels can cause immediate death

Biological Effects of Radiation Exposure

• Low Dose– Damages DNA – Cells with damaged DNA may then begin growing out

of control• Forms malignant tumors or leukemia• If damaged DNA is a sex cell, it can result in birth defects• Other effects include weakening of the immune system,

mental retardation, and the development of cataracts

• Low level effects may go unseen until many years after the even

Exposure to Radiation

• Health effects are directly related to exposure– Evidence for this hypothesis comes from studies of

patients with various illnesses who were exposed to high levels of X rays in the 1930s• People in these groups developed higher-than-normal

rates of cancer and leukemia

• There is no agreement among health care agencies as to what a safe level of exposure is

Sources of Radiation

• Background exposure– Uranium and radon gas that occurs naturally in the

Earth’s crust– Medical and dental X rays– Cosmic radiation from outer space

Radiation from Nuclear Power Plants

• When operating normally, they are generate less radiation than normal background radiation– Because the direct fission products remain within

the fuel elements, and the indirect products are maintained within the containment building that houses the reactor

– Public exposure from normal operations of a power plant is less than 1% of the natural background

Then Why the Concern?

• Problems arise from the storage and disposal of radioactive wastes and the potential for accidents

Radioactive Wastes

• Radioactive Decay– As unstable isotopes eject particles and radiation,

they become stable and cease to be radioactive.• As long as the radioactive materials are kept isolated

from humans and other organisms, the decay proceeds harmlessly

Half – Life

• The time for half of the amount of a radioactive isotope to decay

• Half-life of an isotope is always the same, regardless of starting amount

• Each particular radioactive isotope has its own characteristic half life– Range from fraction of a second to many

thousands of years

Half – Life

• Uranium fission results in a heterogeneous mixture of radioisotopes

• Some of this material that has been created by the neutron bombardment of U-238 can be recovered and recycled in an operation called preprocesses– U-235, PL-239 – U.S. until recently prohibited the practice because

of concerns over plutonium and nuclear weapons

Requires Long-Term Containment

Disposal of Radioactive Wastes

• Development of nuclear power went on with out fully addressing the issue of what to do with radioactive wastes

• Originally thought to burry solid waste deep in stable rock formations, sealed in containers– This has not yet happened

Short Term Contamination

• Allows the radioactive decay of short-lived isotopes

• In 10 years, fission wastes lose more than 97% of their radioactivity

• Wastes can be handled much more easily and safely after this loss occurs

Long-Term Containment

• EPA has recommended a 10,000 year minimum to provide protection from the long-lived isotopes

• Government standards require isolation for 20 half-lives– Plutonium has a half-life of 24,000 years!• Therefore would require 240 half lives in order to be

safe

Tanks and Casks

• For short-term containment, spent fuel is first stored in deep swimming pool-like tanks on the sites of nuclear power plants

• The water in these tanks dissipates waste heat and acts as a shield against the escape of radiation

• The storage pools can typically hold 10-20 years of spent fuel– Pools reached 50% capacity in 2004 and will be at 100%

by 2015

Tanks and Casks

• After a few years of decay, the spent fuel may be placed in air-cooled dry casks for interim storage until long-term storage becomes available – Casks are engineered to resist floods, tornadoes,

and extremes of temperatures

Accumulated Waste

• World commercial reactors accumulate 9,000 tons of waste a year, reaching 270,000 tons at the end of 2006– All stored on site at the plants– 53,000 tons are generated by the U.S.

Military Radioactive Wastes

• Manufacture of nuclear weapons• In U.S. – liquid high-level wastes stored have leaked into the

environment and contaminated wildlife, sediments, groundwater and soil• Recent documents revealing past accidents have been

made available to the public– Hanford, Washington; Fernald, Ohio; Oak Ridge, Tennessee;

Savanna River, South Carolina– Deliberate releases of uranium dust, xenon-133, iodine-131,

and tritium gas have been documented

Military Radioactive Wastes

• Manufacture of nuclear weapons• Former U.S.S.R– Chelyabinsk-65 located in Ural Mountains– For 20 years discharged nuclear wastes into the

Techa River and then into Lake Karachay• 1,000 cases of leukemia have been traced• Lake dried up in 1967 and winds blew radioactive dust

with 5 million curies across the countryside– Russian authorities have filled the lake with concrete, rocks,

and soil.

Megatons to Megawatts

• End of the Cold War – U.S. and nations of the former U.S.S.R agreed to

dismantle nuclear weapons, and close remaining plutonium weapons productions facilities

• Megatons to Megawatts program– Partnership where private U.S. companies oversee

the dilution of weapons-grade uranium to power plant grade• Sells it to U.S. power plants

Megatons to MegawattsCylinders containing nuclear-warhead-derived fuel from Russia are unloaded in the U.S. This program as eliminated 10,700 nuclear warheads and currently supplies a large proportion of fuel for the U.S. nuclear power plants

High Level Nuclear Waste Disposal

• Geologic burial is the solution for most countries– No nation has developed plans to the point of

actually carrying out the burial• Can’t find any site that is suitable• Ones that were proposed have questions raised about

safety– Cannot guarantee that a rock formation will remain stable and

dry for tens of thousands of years

U.S. Storage

• Efforts have been hampered by a severe “not in my backyard (NIMBY) syndrome

• Nuclear Waste Policy Act of 1982 committed the federal government to begin receiving nuclear waste from power plants in 1998

• 1987 Congress stopped looking for a place and selected Yucca Mountain, in NW Nevada

Yucca Mountain

• Nevadans fought the selection and passed a state law in 1989 that prohibits anyone from storing waste in the site

• Federal government can override state laws • Nevada site has undergone intensive study

over the past 20 years, costing $4 billion

Nuclear Power Accidents

• Three Mile Island– 3/28/1979 suffered a partial meltdown due to

human/equipment failures and a flawed design – Steam generator shut down automatically because

of a lack of power in its feed water pumps• Valve on top of the generator opened in response to

the gradual buildup of pressure• Valve remained in open position and drained coolant

water from the reactor vessel

Three Mile Island

• Operators responded poorly to the emergency and shut off the emergency cooling system

• Gauges told operators the reactor was full of water, when it needed it (lack of coolant)

• Core was uncovered for a time and suffered partial meltdown

• 10,000,000 curies of radioactive gas were released into the atmosphere

Three Mile Island

• Situation was eventually brought under control• Radioactive contamination occurred inside the

containment building– Cleanup is almost as expensive as building a new

plant– No plants to restart reactor– GPU Nuclear have since paid $30 million to settle

claims from accident, even though it has never admitted to any radiation caused illness

Chernobyl

• 4/26/1986• Engineers disabled the power plant’s safety

systems, withdrew control rods, shut off the flow of steam to generators, and decreased the flow of coolant water in the reactor

• Did not allow for the radioactive heat energy that would still be generated by the reactor core after it had been shut off

Chernobyl

• Lacking coolant, the reactor began to heat up• Extra steam generated could not escape and

rapidly boosted energy production of the reaction

• Steam explosions blew off the 2,000 ton top off the reactor

• Reactor melted down• Fire was ignited in graphite, burning for days

Results

• 50 tons of dust an debris carrying 100-200 million curies of radioactivity were released that rained radioactive particles over 1000s of square miles– 400x the radiation fallout from the bombs

dropped on Hiroshima and Nagasaki in 1945

Consequences

• 135,000 people were evacuated • Reactor was eventually sealed in a

sarcophagus of concrete and steel• Soil remains contaminated with radioactive

compounds• 2 of the engineers died from the explosion• 28 of the personnel brought in to contain the

aftermath died later of radiation sickness

Consequences

• Main health impact has been outbreak of thyroid cancer to children who drank contaminated milk containing radioactive iodine

• Constructed shelter over the managed reactor is in a state of decay– $800 million “New Safe Confinement”

construction began in 2006

Could it happen here?

• NO– Chernobyl reactor used graphite as a moderator

rather than water – The water is incapable of developing a power

surge more than 2x their normal power– LWR (light water reactors) have backup systems to

prevent the core from overheating • Reactors are housed in a thick, concrete-walled

containment building designed to withstand explosions

Safety and Nuclear Power

• U.S. and Nuclear Regulatory Commission upgraded safety standards in the technical design of the plants but in maintenance procedures and training of operators

Passive Safety

• Involves engineering devices and structures that make it virtually impossible for the reactor to go beyond acceptable levels of power, temperature and radioactive emissions

• Operation depends only on standard physical phenomena, such as gravity and resistance to high temperatures

Active Safety

• Relies on operator-controlled actions, external power, electrical signals, etc

New Generation of Reactors

• Generation I– Earliest, developed in 1950s and 1960s– Few are still operating

• Generation II– Majority of today’s reactors

• Generation III – Newer designs with passive safety features and

smaller, simpler power plants

Generation III Reactors

Core is surrounded by 3 concentric structures; a reactor pressure vessel, in which heat from the reactor boils water directly into steam; a concrete chamber, and water pool, which together contain and quench steam vented from the reactor in an emergency; and a concrete building, which acts as a secondary containment vessel and shield. Any excessive pressure in the reactor will automatically open valves that release steam into a quench pool, reducing the pressure. Water from the quench pool can, if necessary flow downward to cool the core

Terrorism and Nuclear Power

• Potential threats– Destroy control building and bring on a LOCA– Attack plant and overcome guards– Obtaining spent-fuel rods to make a “dirty bomb”• Rain radioactivity over a vast area

Economic Problems

• Energy demand was less than expected– Postponed orders for all types of power plants

• Increasing safety standards drove up prices• Withdrawal of government subsidies• Public protests delayed construction and

opening of plants• Safety systems do not always prevent

accidents

Operating Life Spans

• Originally thought a nuclear power plants life span was 40 years

• World-wide, more than 107 nuclear plants have been shut down after an average operating lifetime of 17 years

Operating Life Times

• Reasons for disparity of projected vs. actual average– Embrittlement– Corrosion

Embrittlement

• As neutrons from fission bombard reactor vessel and other hardware, it causes metals to become brittle enough that they may crack under thermals tress

Corrosion

• Result of steam generation• Water inside pipes contain corrosive

chemicals, that over time, causes cracks to develop in some of the pipes

MORE ADVANCED REACTORS13.4

Breeder (Fast-neutron) Reactors

• Recall when U-235 atom fissions, two or three neutrons are ejected– Only one of these is used for the chain reaction– Rest are absorbed by something else

• Breeder reactors work so that non-fissionable U-238 absorbs the extra neutrons, which are allowed to maintain their high speed

Breeder Reactors

• When U-238 absorbs excess neutrons it is converted to plutonium (Pu-239) which can be purified and used as a nuclear fuel– Fast-neutron reaction may produce more fuel than

it consumes– These reactors are operated for military purposes

Risks of Breeder Fission

• Consequence more serious if meltdown were to occur– Due to large amounts of Pu-239– Pu has long half life (24,000 years)– Plutonium can be purified and fabricated into

nuclear weapons easier than U-235

Fusion Reactors

• d-t reaction– Hydrogen fusion is promoted as the ultimate

solution– Most current designs do not use regular hydrogen

but isotopes of hydrogen• Deuterium (2H) and tritium (3H)

d-t Reaction

• Deuterium is a naturally occurring non-reaction isotope that can be extracted from hydrogen in seawater

• Tritium is an unstable gaseous radioactive isotope that must be produced artificially – Since Tritium is dangerous, fusion rectors could

easily become a source o radioactive tritium leaking into the environment

Present State of Fusion Reactors

• Still an energy consumer rather than a producer

• It takes an extremely high temperature and pressure to get hydrogen atoms to fuse

• Very expensive, research is still taking place

FUTURE OF NUCLEAR POWER13.5

Opposition

• 1. General distrust of technology they do not understand

• 2. Critical of the way nuclear technology is being managed

• 3. Problems involving lax safety, operator failures, and cover-ups by nuclear plants and their regulatory agencies

Opposition

• 4. Problems of high costs of construction and unexpectedly short operational lifetimes

• 5. Nuclear industry has repeated presented energy as safe, however, accidents do occur

• 6. Nuclear power plants are sources for terrorist attacks

• 7. Disposal of nuclear waste

Mismatch

• Nuclear energy mostly competes with coal fired power plants to produce electricity

• Our nation is still dependent on crude oil for transportation

Chapter 13

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