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Introduction to
Nuclear Power

Introduction to Nuclear Power

To provide the power for a turbo- generator,
nuclear power plants rely on the heat energy generated from nuclear
fission. In this process, the nucleus of a heavy element, such as uranium
or plutonium, splits when bombarded by a neutron in a nuclear reactor. The
fission process for uranium atoms typically yields two smaller atoms
called fission fragments; two or three neutrons, plus about 200 Million
electron volts of nuclear energy in the form of radiation and heat.
Because more neutrons are released from a uranium fission event than are
required to initiate the event, the reaction can become self sustaining--a
chain reaction--under controlled conditions, thus producing a tremendous
amount of energy.

In the vast majority of the world's nuclear
power plants, the heat energy generated by uranium fuel is transferred to
ordinary water and is carried away from the reactor's core either as steam
in boiling
water reactors (BWRs) or as superheated water in pressurized-water
reactors (PWRs). In a PWR the superheated water in the primary cooling
loop flows through a special heat exchanger called a “steam generator”
that is used to boil water and create steam in a secondary loop that feeds
the turbo-generator. The two loop design of a PWR keeps the radioactivity
isolated and only clean steam is circulated through the turbine. This
helps to minimize maintenance costs and radiation exposures to the plant
personnel.

In the BWR reactor design, the water is
boiled inside the reactor core and then sent directly to the
turbo-generator to produce power. The BWR is a simpler design, but has the
disadvantage that the turbo-generator and more of the plant become
slightly radioactive over time. In either a boiling-water or
pressurized-water installation, steam under high pressure turns a steam
turbine whose shaft is connected to an electric generator.

Although nuclear generating stations sound
complex, the turbo-generator and many auxiliary systems are similar to
conventional fossil fuel plants. The main difference is the nuclear heat
source and the reactor safety systems. Boiling-water and pressurized-water
reactors are sometimes called light-water reactors (LWRs), because they
utilize ordinary water (H2O) to transfer the heat energy from reactor to
turbine in the electricity generation process. In other reactor designs,
the heat energy is transferred by heavy water (D2O), helium gas, or a
liquid metal such as lead or sodium.

Because the cooling water absorbs some of the
neutrons needed for the nuclear chain reaction and the natural uranium
absorbs others, the concentration of the U-235 isotope in uranium must be
increased (enriched) above the level of natural uranium to assist in
sustaining the nuclear chain reaction in the reactor core. The process of
enrichment is explained below.

The fuel core for a light-water nuclear power
reactor can have up to 800 fuel assemblies. An assembly consists of a
group of sealed fuel rods, each filled with UO2 pellets, held in place by
end plates and supported by metal spacer-grids to brace the rods and
maintain the proper distances between them. The fuel core can be thought
of as a reservoir from which heat energy can be extracted through the
nuclear chain reaction process. During the operation of the reactor, the
concentration of U-235 in the fuel is decreased as those atoms undergo
nuclear fission to create heat energy. Some U-235 atoms are converted to
atoms of fissile Pu-239, some of which will, in turn, undergo fission and
produce energy. The products created by the nuclear fission reactions are
retained within the fuel pellets and these become neutron-absorbing
products (called "poisons") that act to slow the rate of nuclear fission
and heat production. As the reactor operation is continued, a point is
reached at which the declining concentration of fissile nuclei in the fuel
and the increasing concentration of poisons result in lower than optimal
heat energy generation, and the reactor must be shut down temporarily and
refueled. The fraction of the reactor's fuel core replaced during
refueling is typically one-fourth for a boiling-water reactor and
one-third for a pressurized-water reactor.

The amount of energy available in nuclear
fuel can be expressed in "full-power days.” The number of full power days
between refueling outages is related to the amount of fissile U-235
contained in the fuel assemblies at the beginning of the cycle but it is
also limited by safety and engineering considerations.

The amount of energy extracted from nuclear
fuel is called “burn up” which is expressed in terms of the heat energy
produced per initial unit of fuel weight. Burn up is commonly expressed as
megawatt days thermal per metric ton of initial heavy metal.

Less than 1 percent of the uranium is
typically burned in a reactor before it is discarded as “spent fuel”. This
is because of the buildup of poisons in the fuel from the fission process
and because the metal cladding on the fuel weakens over time as it is
exposed to radiation. A higher percentage of U-235 in the core at the
beginning of a cycle will permit the reactor to be run for a greater
number of full power days. It is also possible to recycle the spent fuel
in LWRs to extract more energy from the remaining unused uranium. However,
in the United States, the “once-through fuel cycle” is currently preferred
over recycling.

The nuclear fuel cycle for typical
light-water reactors is illustrated in Figure 1. The cycle consists of
"front end" steps that lead to the preparation of uranium for use as fuel
and "back end" steps that are necessary to safely manage, and dispose of
the highly radioactive spent nuclear fuel. The front end of the nuclear
fuel cycle commonly is separated into the following steps: exploration,
mining, milling, conversion, enrichment, and fabrication

Figure 1. The Nuclear Fuel Cycle

Exploration. The nuclear
fuel cycle begins with the exploration for uranium deposits. Many types of
sophisticated techniques are used to find uranium such as geophysical and
geochemical analyses, satellite and airborne radiometric surveys, water
sampling, and drilling. Because uranium deposits usually occur in discrete
little pockets, rather than long continuous seams like coal and oil, the
exploration process can be difficult and expensive. A drill hole, for
example, can just miss a huge deposit, thereby giving a false indication
that there is no uranium present. Likewise, the drill hole can also
produce misleading results when it hits a tiny pocket of uranium.
Therefore, many drill holes are usually needed to characterize the extent
of the ore deposit. EIA collects and reports data on uranium drill holes
and expenditures as an indicator of exploration activity.

Once a uranium deposit has been discovered,
geologists go to work to evaluate the extent and grade (concentration) of
the ore deposit. Mining engineers then evaluate how best to extract the
ore and what it will cost. This data is combined to generate a “uranium
reserves estimate” that is basically a table of how much uranium can be
extracted at a particular cost. The reserves estimate can then be compared
with the market price of uranium to tell whether it makes sense to open a
mine.

Mining. If the mining
operation appears profitable, the next step in the fuel cycle is to begin
mining uranium ore. Uranium ore is the raw rock or gravel material that
comes from the ground. It can be extracted through conventional mining in
open pit and underground methods similar to those used for mining other
metals.

Another popular mining technique is called
solution mining or In situ leach mining. In this technology, a slightly
caustic liquid such as carbonated water is injected underground to
dissolve the uranium in place. The water containing dissolved uranium is
then pumped back to the surface where the uranium is extracted. The water
is then recirculated back underground and the process is repeated until
all of the uranium has been recovered. Some people liken this
environmentally friendly mining technique to operating a simple water
purification plant.

Uranium ores in the United States typically
range anywhere from about 0.05 to 0.3 percent uranium oxide (U3O8).
Uranium is also present in some phosphate rocks. Although the uranium
concentrations are quite low (50 to 200 parts per million) in the
phosphate rock, the mining operations can be profitable because of the
phosphates and phosphoric acid that is co-produced. At one time,
phosphates were a significant source of uranium production in the United
States.

Milling. The next step in
the nuclear fuel cycle, called milling, involves the purification and
concentration of uranium. Mined uranium ores normally are processed by
grinding the ore materials to a uniform particle size and then treating
the ore with acid to extract the uranium by chemical leaching. The milling
process commonly yields a dry powdered-uranium oxide (U3O8) material
called "yellowcake" because of its bright yellow color evident in Figure
2.

Figure 2. Yellowcake.

Source: U.S. Nuclear Regulatory Commission
File Photo

Uranium Conversion. Milled
uranium oxide, U3O8, must be converted to uranium hexafluoride, UF6, in
order to separate the individual U-235 atoms from the U-238 atoms. A solid
at room temperature, UF6 can be changed to a gaseous form at moderately
higher temperatures. The UF6 conversion product contains only natural, not
enriched, uranium. Currently, there is only one conversion plant in the
United States.

Enrichment. The
concentration of the fissionable isotope, U-235 (0.71 percent in natural
uranium) is less than that required to sustain a nuclear chain reaction in
light water reactor cores. Natural UF6 thus must be "enriched" in the
fissionable isotope for it to be used as nuclear fuel. Light-water reactor
fuel normally is enriched up to about 4 percent U-235. However, different
levels of enrichment for a particular nuclear fuel application may be
specified. The UF6 gas that remains has less than normal concentrations of
U-235 in it and is therefore called “the enrichment tails”, “depleted
uranium”, or simply “DU”.

Gaseous diffusion and gas centrifuge are the
commonly used uranium enrichment technologies. The gaseous diffusion
process consists of passing the natural UF6 gas feed under high pressure
through a series of diffusion barriers (semi porous membranes) that permit
passage of the lighter 235UF6 atoms at a faster rate than the heavier
238UF6 atoms. This differential treatment, applied across a large number
of diffusion "stages," progressively raises the product stream
concentration of U-235 relative to U-238. In the gaseous diffusion
technology, the separation achieved per diffusion stage is relatively low,
and a large number of stages is required to achieve the desired level of
isotope enrichment. Because this technology requires a large capital
outlay for facilities and it consumes large amounts of electrical energy,
it is relatively expensive.

In the gas centrifuge process, the natural
UF6 gas is spun at high speed in a series of cylinders. This acts to
separate the 235UF6 and 238UF6 molecules based on their slightly different
atomic masses. Gas centrifuge technology involves relatively high capital
costs for the specialized equipment required, but its power costs are
significantly less than those for the gaseous diffusion technology. New
enrichment technologies currently being developed are the atomic vapor
laser isotope separation (AVLIS) and the molecular laser isotope
separation (MLIS). Each laser-based enrichment process can achieve higher
initial enrichment (isotope separation) factors than the diffusion or
centrifuge processes can achieve.

Fabrication. For use as
nuclear fuel, enriched UF6 is next converted into uranium dioxide (UO2)
powder which is then processed into pellet form, shown in Figure 3.

Figure 3. Uranium Fuel Pellet.

Source: US NRC File
Photo

The pellets are then fired in a high
temperature sintering furnace to create hard, ceramic pellets of enriched
uranium. The cylindrical pellets then undergo a grinding process to
achieve a uniform pellet size. The pellets are stacked, according to each
nuclear core's design specifications, into tubes of corrosion-resistant
metal alloy. The tubes are sealed to contain the fuel pellets: these tubes
are called fuel rods. The finished fuel rods are bundled together in
special fuel assemblies that are then used to make up the nuclear fuel
core of a power reactor. An example of such an assembly is shown in Figure
4.

Figure 4. Reactor Fuel Assembly.

Source: NRC file photo

The back end of the cycle is divided into the
following steps: interim storage, reprocessing, and waste disposal.
Interim Storage. After its operating cycle, the reactor is shut down for
refueling. The fuel discharged at that time (spent fuel) is initially
stored under water at the reactor site in special leaktight pools with
cooling systems. The cooling systems are needed because the spent fuel
continues to emit radiation and give off heat even after it is removed
from the reactor. This is called “decay heat’ and the levels can be as
much as 5 percent of the original heat level. The spent fuel cools off and
after about 5 years it can be taken out of the water and placed in special
dry storage canisters made of concrete for long term storage. Reactor
owners have been able to store the fuel onsite safely now for decades.
However, some pools are beginning to fill up to capacity. At these sites,
special dry storage pads are being installed to store the fuel in dry
canisters until a final underground repository is constructed. These
storage locations are known as Independent Spent Fuel Storage
Installations (ISFSI) at the reactor site or at a facility away from the
site.

Reprocessing. Spent fuel
discharged from light-water reactors contains appreciable quantities of
fissile (U-235, Pu-239), fertile (U-238), and other radioactive materials.
These fissile and fertile materials can be chemically separated and
recovered from the spent fuel. The recovered uranium and plutonium can, if
economic and institutional conditions permit, be recycled for use as
nuclear fuel. Currently, plants in Europe are reprocessing spent fuel from
utilities in Europe and Japan. In the United States, DOE is conducting
research on advanced nuclear fuel cycles that may someday recycle the
spent fuel and extract more energy from the uranium. Advanced fuel cycle
research is also being conducted to reduce the amount of nuclear waste
that must be buried in the repository.

Waste Disposal. Although,
the safety record for spent fuel storage systems has been good, ultimately
the waste stream must be isolated from the biosphere until the
radioactivity contained in them has diminished to a safe level. Under the
Nuclear Waste Policy Act of 1982, as amended, the Department of Energy has
responsibility for the development of the waste disposal system for spent
nuclear fuel and high-level radioactive waste. Current plans call for the
ultimate disposal of the wastes in solid form in licensed deep, stable
geologic structures. A national repository at Yucca Mountain has been
chosen to store the waste, but progress on the facility has been delayed.
In the interim, spent fuel is being stored at about 70 sites across the
United States.