15-5 What Are the Advantages and Disadvantages of Nuclear Energy?

• Concept 15-5 Nuclear power has a low environmental impact and a very low accident risk, but high costs, a low net energy yield, long-lived radioactive wastes, vulnerability to sabotage, and the potential for spreading nuclear weapons technology have limited its use.

How Does a Nuclear Fission Reactor Work? (1)

• Controlled nuclear fission reaction in a reactor– Light-water reactors

• Fueled by uranium ore and packed as pellets in fuel rods and fuel assemblies

• Control rods absorb neutrons

How Does a Nuclear Fission Reactor Work? (2)

• Water is the usual coolant

• Containment shell around the core for protection

• Water-filled pools or dry casks for storage of radioactive spent fuel rod assemblies

Light water reactors

• 85% of world’s nuclear generated electricity (100% in US)

• High inefficient in terms of energy conversion (up to 83% lost as waste heat)

Nuclear fuel

• Made from uranium ore• Enriched to 3% of radioactive isotope U-235• Made into pellets, size of pencil, energy

equivalent to 1 ton of coal• Pellets are packed into large pipes-fuel rods

• Rods are grouped together into fuel assemblies, these assemblies are placed into reactor core

Controlling the reaction

• Control rods placed between rods

• Control rods moved in and out of the assemblies, absorbing neutrons which trigger the chain reaction

• Water circulates through the assemblies, removing the heat, keeping the rods from melting

What happens if there is no water?

How nuclear reactors generate electricity

• Superheated water turns into steam

• Steam passed through turbine

• Physical motion of the turbine is converted into electrical energy

Steam from Nuclear power plants• Superheated water after used in the turbines goes into

a condenser

• Condenser requires cold water source which is why most plants are located next to water

• Pipes with the hot water are circulated through a container filled with cold water , heat is exchanged

• Hot water is either discharged into river, ocean… or vented into the atmosphere as steam

Light-Water-Moderated and -Cooled Nuclear Power Plant with Water

Reactor

Fig. 15-17, p. 387

Small amounts of radioactive gases

Uranium fuel input (reactor core)

Control rodsContainment shell

Waste heatHeat exchanger

Steam Turbine Generator

Hot coolant Useful electrical

energy 25%–30%

Hot water output

PumpPump

Coolant Pump Pump

ModeratorCool water input

Waste heatShielding

Pressure vessel

Coolant passageWater Condenser

Periodic removal and storage of radioactive wastes and spent fuel

assemblies

Periodic removal and storage of radioactive

liquid wastes

Water source (river, lake, ocean)

Reactor in Japan

After 3 or 4 Years in a Reactor, Spent Fuel Rods Are Removed and Stored in Water

Spent Fuel rods• After about 3-4 years of use, the Fuel rods become

spent-level of fission drops beneath a certain level

• Rods are taken out of reactor stored nearby in water filled pools or dry casks

• Stored until they cool down enough to be shipped for permanent storage or to be recycled

• These storage facilities are next to the reactor plants, vulnerable to terrorist attack or accidents

Spent fuel reprocessingThe spent fuel rods are sent to a facility which separates

plutonium from spent fuel for further use as a new generation of fuel or as material used to make atomic weapons.

1. First the fuel is chopped up, by remote control, behind heavy lead shielding.

2. These chopped-up pieces are then dissolved in boiling nitric acid, releasing radioactive gases in the process.

3. The plutonium is separated from the acid solution by chemical means, leaving large quantities of high-level radioactive liquid waste and sludge behind.

4. After it has cooled down for several years, this liquid waste will have to be solidified for ultimate disposal, while the separated plutonium is fabricated into nuclear fuel or nuclear weapons.

Japan and the Soviet Nuclear program connection

As a result of a successful program between the Soviet Union and USA, many nuclear weapons have been destroyed.

The fuel from a number of Soviet weapons was sold to Japan to be used for fuel in their reactors

So plutonium is used in at least one of the damaged reactors in Japan

What Is the Nuclear Fuel Cycle?• Mine the uranium

• Process the uranium to make the fuel

• Use it in the reactor

• Safely store the radioactive waste

• Decommission the reactor

Fig. 15-19, p. 389

Fuel assemblies Decommissioning of reactor

Enrichment of UF6

ReactorFuel fabrication

(conversion of enriched UF6 to UO to UO2 and fabrication of fuel assemblies)

Temporary storage of spent fuel assemblies underwater or

in dry casksConversion of U3O8 to UF6

Uranium-235 as UF6 Plutonium-239 as PuO2 Spent fuel

reprocessing

Low-level radiation with long half-life

Geologic disposal of moderate- and high-level radioactive wastes

Open fuel cycle today Recycling of nuclear fuel

What Happened to Nuclear Power?

• Slowest-growing energy source and expected to decline more

• Why?– Economics– Poor management– Low net yield of energy of the nuclear fuel cycle– Safety concerns– Need for greater government subsidies– Concerns of transporting uranium

Case Study: Worst Commercial Nuclear Power Plant Accident in the

U.S.• Three Mile Island– March 29, 1979– Near Harrisburg, PA, U.S.– Nuclear reactor lost its coolant– Led to a partial uncovering and melting of the

radioactive core – Unknown amounts of radioactivity escaped– People fled the area– Increased public concerns for safety

• Led to improved safety regulations in the U.S.

Case Study: Worst Nuclear Power Plant Accident in the World

• Chernobyl– April 26, 1986 – In Chernobyl, Ukraine– Series of explosions caused the roof of a reactor

building to blow off– Partial meltdown and fire for 10 days– Huge radioactive cloud spread over many countries

and eventually the world – 350,000 people left their homes– Effects on human health, water supply, and

agriculture

Oak Ridge Reactor

Remains of a Nuclear Reactor at the Chernobyl Nuclear Power Plant

Level of contamination

Human casualties of Chernobyl

56 people lost their lives as a direct result of radiation poisoning or fire

Thyroid cancerFrom drinking Milk killed 10-12 thousand

Nuclear Power Has Advantages and Disadvantages

• Advantages

• Disadvantages

Fig. 15-21, p. 391

TRADE-OFFSConventional Nuclear Fuel Cycle

Large fuel supply Cannot compete economically without huge government subsidies

Advantages Disadvantages

Low environmental impact (without accidents)

Low net energy yieldHigh environmental impact (with major accidents)Emits 1/6 as much CO2 as

coal

Environmental costs not included in market price

Risk of catastrophic accidents

Moderate land disruption and water pollution (without accidents)

No widely acceptable solution for long-term storage of radioactive wastes

Moderate land use

Low risk of accidents because of multiple safety systems (except for Chernobyl-type reactors) Spreads knowledge and

technology for building nuclear weapons

Subject to terrorist attacks

Fig. 15-22, p. 392

TRADE-OFFS

Coal vs. Nuclear

Coal NuclearAmple supply Ample supply of

uraniumHigh net energy yield

Low net energy yield

Very high air pollution

Low air pollution

High CO2 emissions Low CO2 emissions

Much lower land disruption from surface mining

High land disruption from surface mining

High land use Moderate land use

Low cost (with huge subsidies)

High cost (even with huge subsidies)

Nuclear Power Plants Are Vulnerable to Terrorists Acts

• Explosions or meltdowns possible at the power plants

• Storage pools and casks are more vulnerable to attack

• 60 countries have or have the ability to build nuclear weapons

Dealing with Radioactive Wastes Produced by Nuclear Power Is a Difficult Problem

• High-level radioactive wastes – Must be stored safely for 10,000–240,000 years

• Where to store it– Deep burial: safest and cheapest option– Transportation concerns– Would any method of burial last long enough?– There is still no facility: NIMBY scenario

• Can the harmful isotopes be changed into harmless isotopes? (working on it, $$$)

Case Study: Experts Disagree about What to Do with Radioactive Wastes in the U.S.

• 1985: plans in the U.S. to build a repository for high-level radioactive wastes in the Yucca Mountain desert region (Nevada)

• Problems– Cost: $58–100 billion– Large number of shipments to the site: protection

from attack?– Rock fractures– Earthquake zone– Decrease national security

What Do We Do with Worn-Out Nuclear Power Plants?

• Decommission or retire the power plant• At least ½ of US plants are scheduled to close by next

year• Some options– Dismantle the plant and safely store the radioactive

materials– Enclose the plant behind a physical barrier with full-

time security until a storage facility has been built– Enclose the plant in a tomb• Monitor this for thousands of years

Can Nuclear Power Lessen Dependence on Imported Oil, Reduce Global Warming?

• Nuclear power plants: no CO2 emission

• Complete Nuclear fuel cycle: emits CO2 (not like Fossil fuels)

• Opposing views on nuclear power and global warming– Nuclear power advocates– 2003 study by MIT researchers: low grade ore– 2007: Oxford Research Group: reactor a week for 70 years

still only produce 20%

Will Nuclear Fusion Save Us?

• “Nuclear fusion is the power of the future and always will be”

• Still in the laboratory phase after 50 years of research and $34 billion dollars: energy

• 2006: U.S., China, Russia, Japan, South Korea, and European Union– Will build a large-scale experimental nuclear fusion

reactor by 2040

Experts Disagree about the Future of Nuclear Power

• Proponents of nuclear power– Fund more research and development– Pilot-plant testing of potentially cheaper and safer

reactors– Test breeder fission and nuclear fusion

• Opponents of nuclear power– Fund rapid development of energy efficient and

renewable energy resources

Science Focus: Are New and Safer Nuclear Reactors the Answer? (1)

• Advanced light-water reactors (ALWR) – Built-in passive safety features

• High-temperature-gas-cooled reactors (HTGC)

Avoids problems with present water based systems, but still not 100% reliable

Alternatives

• Pebble bed modular reactor (PBMR)– Pros: no need to shut down for refueling– Cons: graphite protective coatings, vulnerable– China, South Africa

• Breeder nuclear fission reactors– Creates plutonium from uranium– France built one then shut it down 2 years later

Pebble Bed Reactors

Breeder reactors

Science Focus: Are New and Safer Nuclear Reactors the Answer? (2)• New Generation nuclear reactors must satisfy

these five criteria1. Safe-runaway chain reaction is impossible2. Fuel can not be used for nuclear weapons3. Easily disposed of fuel4. Nuclear fuel cycle must generate a higher net

energy yield than other alternative fuels, without huge government subsidies

5. Emit fewer greenhouse gases than other fuels

Exit questions for 15.5

1. Describe the nuclear fuel cycle2. How do we deal with the highly reactive

radioactive wastes produced by nuclear power plants? How should we?

3. What is the difference between nuclear fusion and fission? What is fusion’s potential as a energy source

4. Summarize arguments for and against nuclear power