Electricity from Nuclear

Advantages of Nuclear Power

• Nuclear electricity is reliable and relatively cheap (with an average generating cost of 2.9 cents per kW/h) once the reactor is in place and operating.

• Large reserves of Uranium in United States - Fuel for nuclear power plants will not run out for tens of thousands of years

• Nuclear power plants contribute no greenhouse gasses and few atmospheric pollutants

Disadvantages of Nuclear Power

• Uranium is ultimately a nonrenewable resource.• Nuclear power plants are extremely costly to

build.• The slight possibility that nuclear power plants

can have catastrophic failures.• Large environmental impact during the mining

and processing stages of uranium are numerous.• Nuclear waste (Spent nuclear fuel) is extremely

hazardous and must be stored safely for thousands of years.

Comparing Uranium to Coal*1 kg of uranium-235 will generate as much energy

as 3,000 tons of coal without CO2 emissions

Comparing Uranium to Coal

Coal Power Plant Environmental Concerns

Nuclear Power PlantEnvironmental Concerns

Nuclear Power in the U.S.

• The U.S. has 104 nuclear power plants (more than any other country)

• Combined, these reactors produce about 100 billion watts of electricity (20% of the total)

Nuclear Power in the U.S.

• No new reactors have been completed in the U.S. since 1979.

• Why???– High Operating Costs– Liability Problems– Wastes– Health Concerns– Terrorism– Public Misconceptions

Illinois Nuclear Power

• Nuclear power typically accounts for nearly one-half of Illinois electricity generation and more than one-tenth of all the nuclear power generated in the United States.

• With 11 operating reactors at six nuclear power plants, Illinois ranks first among the States in nuclear generation.

• The growth of the Illinois nuclear industry is due largely to State government initiatives, which began encouraging nuclear power development in the 1950s

Illinois Nuclear Power

• The origin of all of the commercial and military nuclear industries in the world can be traced back to December 2, 1942 at the University of Chicago.

• On that day, a team of scientists under Dr. Enrico Fermi initiated the first controlled nuclear chain reaction.

Illinois Nuclear Power

• There are 6 operating nuclear power plants in Illinois: Braidwood, Byron, Clinton, Dresden, LaSalle, and Quad Cities.

• With the sole exception of the single-unit Clinton plant, each of these facilities has two reactors.

• The two reactors at Braidwood and both reactors at Byron are pressurized light water reactors (PWR).

• The reactors at the other facilities (Clinton, Dresden, LaSalle, and Quad Cities are boiling water reactors (BWR).

Fission and Fusion

• Fission: A heavy nucleus is split into smaller nuclei, releasing energy

• Fusion: Two light nuclei fuse into a heavy nucleus, releasing energy– Fusion generates much more energy than

fission– Fusion of hydrogen into helium is what

provides power to the sun (and thus to the Earth)

Energy from Fission

• When a neutron strikes a uranium-235 nucleus, it splits into two nuclei, releasing energy

• Example: n + 235U -> 93Kr + 141Ba + 2n• One neutron goes in, two neutrons come out

Chain Reactions

• If at least one neutron from each fission strikes another nucleus, a chain reaction will result

• An assembly of uranium-235 that sustains a chain reaction is critical

Power Plants and Bombs

• Natural uranium contains 99.3% uranium-238 and 0.7% uranium-235

• Only uranium-235 can create fission chain reactions– Nuclear power plants use uranium that has

been enriched to 3% uranium-235– Nuclear bombs use uranium that has been

enriched to 95% uranium-235• 3% enriched uranium can never explode!

Nuclear Bombs

• Since neutrons initiate fission, and each fission creates more neutrons, there is potential for a chain reaction

Nuclear Bombs

• Have to have enough fissile material around to intercept liberated neutrons

• Critical mass for 235U is about 15 kg, for 239Pu it’s about 5 kg

• Bomb is dirt-simple: separate two sub-critical masses and just put them next to each other when you want them to explode!– difficulty is in enriching natural uranium to

mostly 235U

Nuclear Fuel Cycle

Mining and Milling

• Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found.

• From these, the mined uranium ore is sent to a mill which is usually located close to the mine.

Mining and Milling

• At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock.

• It is then recovered from solution and precipitated as uranium oxide (U308) concentrate.

– Sometimes this is known as "yellowcake“

Conversion

• Because uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant.

Enriching

• Need to enrich uranium to at least 3% U-235 for a power plant

• Gaseous Diffusion Method– UF6 (hexafluoride) gas heated– U-238 is heavier than U-235 – Hexafluoride Gas can be separated into two streams

• Low velocity U-238• High Velocity U-235

• Centrifuge Method– Gas spun in centrifuge– Lighter U-235 will separate from heavier U-238

Fuel Conversion

• Enriched Uranium transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets.

• These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods.

• The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor.

Fuel Packaging in the Core

• Rods contain uranium enriched to ~3% 235U

• Need roughly 100 tons per year for a 1 GW plant

• Uranium stays in three years, 1/3 of the fuel rods are cycled annually

The Reactor Core

• The reactor core consists of fuel rods and control rods– Fuel rods contain enriched

uranium– Control rods are inserted

between the fuel rods to absorb neutrons and slow the chain reaction

• Control rods are made of cadmium or boron, which absorb neutrons effectively

Moderators• Neutrons produced during fission in the core are

moving too fast to cause a chain reaction– Note: This is not an issue with a bomb, where fissile

uranium is so tightly packed that fast moving neutrons can still do the job.

• A moderator is required to slow down the neutrons

• In Nuclear Power Plants water (light or heavy) or graphite acts as the moderator– Graphite increases efficiency but can be unstable

(Chernobyl)

Light vs. Heavy Water

• 99.99% of water molecules contain normal hydrogen (i.e. with a single proton in the nucleus)

• Water can be specially prepared so that the molecules contain deuterium (i.e. hydrogen with a proton and a neutron in the nucleus)

• Normal water is called light water while water containing deuterium is called heavy water

• Heavy water is a much better moderator but is very expensive to make

Boiling Water Reactor (BWR)

• Heat generated in the core is used to generated steam through a heat exchanger

• The steam runs a turbine just like a normal power plant

Pressurized

Water Reactor (PWR)

Pressurized Water Reactor (PWR)

• Water in the core heated top 315°C but is not turned into steam due to high pressure in the primary loop.

• Heat exchanger used to transfer heat into secondary loop where water is turned to steam to power turbine.

• Steam used to power turbine never comes directly in contact with radioactive materials.

PWR vs. BWR

Spent Nuclear Storage

• Fuel rods must be replaced every 3 years.

• Spent Fuel Rods are temporarily stored in special ponds which are usually located at the reactor site.

• The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.

Spent Fuel Rods Radiation

• Spent fuel assemblies taken from the reactor core are highly radioactive

• Contain plutonium-239, which is radioactive.

• Forms from the reaction of U-238 and neutrons in the core

Fate of Spent Fuel

There are two alternatives for spent fuel: 1. Reprocessing to recover the usable portion of it 2. Long-term storage and final disposal without

reprocessing.

Uranium Reprocessing

• Spent fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%.

• About 3% of spent fuel comprises waste products and the remaining 1% is plutonium produced while the fuel was in the reactor

• Reprocessing extracts useable fissile U-238 and Plutonium from the spent fuel rods

Uranium Reprocessing

• Most of the spent fuel can be reprocessed but isn’t in the U.S.

• Federal law prohibits commercial reprocessing because it will produce plutonium (which can be used both as a fuel and in constructing bombs)

Nuclear Waste Disposal

• In the U.S., no high-level nuclear waste is ever disposed of--it sits in specially designed pools resembling large swimming pools (water cools the fuel and acts as a radiation shield) or in specially designed dry storage containers. 

• Spent nuclear fuel must be isolated for thousands of years

• After 10,000 years of radioactive decay, according to EPA standards, the spent nuclear fuel will no longer pose a threat to public health and

Yucca Mountain

• The United States Department of Energy's long range plan is for this spent fuel to be stored deep in the earth in a geologic repository, at Yucca Mountain, Nevada. 

Concerns about Yucca Mountain

• Possible health risks to those living near Yucca Mountain and transportation routes

• Eventual corrosion of the metal barrels containing the waste

• Located in an earthquake region and contains many interconnected faults and fractures– These could move groundwater and any

escaping radioactive material through the repository to the aquifer below and then to the outside environment

Reactor Safety

• These are the only major accidents (Three Mile Island and Chernobyl) to have occurred in more than 12,700 cumulative reactor-years of commercial operation in 32 countries. 

• The risks from nuclear power plants, are minimal compared with other commonly accepted risks

Breeder Reactor• In a fission reactor, both uranium-235 and

plutonium-239 can be used as nuclear fuel• Most of the uranium in a typical reactor is uranium-

238, which cannot be used as nuclear fuel• When a high-energy neutron collides with the

nucleus of uranium-238, plutonium-239 is sometimes produced

• Could a nuclear reactor create more fuel while it generates energy? Yes, this is a breeder reactor

Breeder Reactor• To convert uranium-238

to plutonium-239, you need fast neutrons

• Therefore, water (which acts as a moderator) cannot be used as the coolant

• Liquid sodium or high pressure gasses are used instead

Uranium-238 only captures fast (high-energy) neutrons

The Core of a Breeder Reactor

• A breeder reactor core consists of a normal uranium-235 reactor (moderated by graphite rods) surrounded by a jacket of uranium-238

• The uranium-238 will be converted to plutonium-239 by neutrons escaping from the core

Orange: uranium-238

Green: enriched uranium-235

Blue: graphite rods

A Typical Breeder Reactor

• Because liquid sodium becomes radioactive, it is separated from the rest of the power plant using a heat exchanger

• High pressure gas-cooled reactors are also being researched

Comments on Breeder Reactors

• Breeder reactors must be tightly controlled, because plutonium-239 can be used to build atomic weapons

• Liquid sodium is extremely reactive (and dangerous) thus high-pressure gasses are preferred as the coolant

• Only France and Japan are actively pursuing breeder reactor technology

• Breeder reactors are Illegal in the united States due to safety and terrorist concerns.