Nuclear Energy

Nuclear Energy

• Conversion of energy from nuclear bonds of U-235 isotope into thermal energy.– Nuclear fission – the splitting apart of atomic

nuclei releases nuclear energy– Radioactive decay – when parent isotope (U-235)

emits alpha, beta particle or gamma radiation• 1 g of U-235 contains 2 – 3 million x the energy as 1g of

coal

Nuclear Energy

• Uranium is mined from a ore of UO2 (rock)– 99% of mined UO2 is U-238, not easily fissionable

– 1% of UO2 is U-235 but must be enriched to at least 3% to sustain chain reaction

• U-235 is the most easily fissionable isotope

• 1 ton of ore = 6.6 pounds of uranium1 ton of ore = 6.6 pounds of uranium– 2001 - 78.9 million pounds2001 - 78.9 million pounds– Largest U deposits are in AustraliaLargest U deposits are in Australia

Mining Uranium

Chain reaction:•Neutron strikes atom•Splits atom into 2 or more parts•Releases more neutrons, heat•Additional neutron in turn, promote more fission reactions = chain reaction•Large amounts of energy are released

Nuclear Energy• Daughter products of U-235 fission are barium & krypton.• Many other radioactive daughter products are released.• Nuclear reactor will harness the kinetic energy from the 3 neutrons

in motion which produces a self-sustaining chain reaction.

How Reactor Works

• Nuclear fuel stored in containment structure• Fuel rods – contain pellets of (U-235)

– Hundreds of bundles of fuel rods in core• Heat from fission heats water till steam is

produced– The rest of the process is exactly the same as coal

power plants (steam turns turbines = generate electricity)

• Light water reactors are most common & the only kind used in US

Controlling Nuclear Fission

• For fission to begin, reactions in the must be slowed down by a moderator– Moderator is usually water or graphite

• Excess neutrons produced must be absorbed by control rods to maintain fission reaction at desired rate.

Radioactive Waste• Not a combustion reaction

– All CO2 comes from mining, transporting, processing

• Many radioactive isotopes are made during the fission process (waste)– Ex. Ba-142 (4 months) , I-129 (15.7 million

years)

• Human health risk – 10 half lives• Storage (@ plants):

– Pools, above ground facilities & metal containers

Fig. 16-16, p. 372

Small amounts of radioactive gases

Uranium fuel input (reactor core)

Control rodsContainment shell

Heat exchanger

Steam Turbine Generator

Waste heat

Electric power

Hot coolant

Useful energy 25%–30%Hot

water outputPumpPump

Coolant Pump Pump

Moderator

Cool water input

Waste heat

Shielding Pressure vessel

Coolant passage

Water CondenserPeriodic removal and storage of radioactive wastes and spent fuel assemblies

Periodic removal and storage of radioactive liquid wastes

Water source (river, lake, ocean)

Radioactive Waste

• Half-life for U-235 is 704 million years!• Radiation is measured in a variety of units:

– Becquerel (Bq) – rate at which a sample of radioactive material decays.

• 1 Bq = decay of 1 atom/second

– Curie = 37 billion decays/second

Radioactive Waste• Spent rods are stored in a water pool to cool

– Running out of pool storage

Radioactive Waste

• After spent fuel rods are cooled considerably, they are sometimes moved to dry-storage containers made of steel or concrete.

Radioactive Waste• Long term storage??

– Bury it deep underground.

– Shoot it into space.– Bury it in the Antarctic

ice sheet.– Bury it in the deep-

ocean floor that is geologically stable.

– Change it into harmless or less harmful isotopes.

Proposed site in Nevada (Yucca Mountain) – near faults & groundwater; 2010 site was no longer funded. Alternative site not agreed upon.

Nuclear Accidents

• Chernobyl, Ukraine – (1986) explosion in reactor

• 3 Mile Island, PA (1979) – partial meltdown of core

• Fukushima, Japan (2011) – involving 6 units; EQ & tsunami cut off all emergency electricity required to cool core & waste storage.

Case Study: The Chernobyl Nuclear Power Plant Accident

• The world’s worst nuclear power plant accident occurred in 1986 in Ukraine.

• The disaster was caused by poor reactor design and human error.

• By 2005, 56 people had died from radiation released.– 4,000 more are expected from thyroid cancer and

leukemia.

What Happened to Nuclear Power?

• Multi billion-dollar construction costs.

• Higher operation costs and more malfunctions than expected.

• Poor management.

• Public concerns about safety and stricter government safety regulations.

• Increased price of uranium

• Increased concerns of terrorist attacks

NUCLEAR ENERGY

• A 1,000 megawatt nuclear plant is refueled once a year

vs

• a coal plant requires 80 rail cars a day.

Figure 16-20Figure 16-20

NUCLEAR ENERGY

• Currently there are 104 plants in US– 70% of energy in France

• At least 228 large commercial reactors worldwide (20 in the U.S.) are scheduled for retirement by 2012.– Many reactors are applying to extent their 40-

year license to 60 years.– Modern reactors are much more safe– Aging reactors are subject to degredation and

corrosion.

New and Safer Reactors

• Pebble bed modular reactor (PBMR) are smaller reactors that minimize the chances of runaway chain reactions.

Figure 16-21Figure 16-21

New and Safer Reactors

• Some oppose the pebble reactor due to:– A crack in the reactor could release

radioactivity.– The design has been rejected by UK and

Germany for safety reasons.– Lack of containment shell would make it

easier for terrorists to blow it up or steal radioactive material.

– Creates higher amount of nuclear waste and increases waste storage expenses.

Fig. 16-18, p. 373

Decommissioning of reactorFuel assemblies

ReactorEnrichment of UF6 Fuel fabricationFuel fabrication

(conversion of enriched UF(conversion of enriched UF66

to UOto UO22 and fabrication of and fabrication of

fuel assemblies)fuel assemblies) Temporary storage of Temporary storage of spent fuel assemblies spent fuel assemblies underwater or in dry underwater or in dry caskscasks

Conversion of U3O8 to UF6

Uranium-235 as UFUranium-235 as UF66 Plutonium-239 as PuOPlutonium-239 as PuO22

Spent fuel Spent fuel reprocessingreprocessing

Low-level radiation Low-level radiation with long half-lifewith long half-life

Geologic disposal of moderate &

high-level radioactive

wastesOpen fuel cycle today

“Closed” end fuel cycle