NuclearPowerPlants.doc 1 K. E. Holbert

NUCLEAR POWER PLANTS Similar to fossil power plants, nuclear power plants produce steam to drive a turbine-generator set to make electricity. The heat source for today’s nuclear power plants is the fissioning of uranium in contrast to combustion for the fossil units. Nuclear Fuel The fuel for nuclear power plants is enriched uranium. Natural uranium is composed, for all practical purposes, of 99.3% of the isotope U-238 and only 0.7% of U-235. U-235 readily fissions with thermal (low energy) neutrons whereas U-238 does not. Therefore, an artificial enrichment process is employed to raise the fraction of U-235 to a few percent (e.g., 3% to 4%).

The reactor fuel is uranium dioxide (UO2), which is formed into a ceramic fuel pellet that is compatible with the water coolant. The fuel pellets are stacked into a Zircaloy clad fuel rod. Many fuel rods are placed in a square lattice to construct a fuel assembly (see Figure 1). A couple hundred fuel assemblies are generally needed to fuel the entire reactor core. The reactor core is housed in a reactor pressure vessel that is composed of steel 8 to 10 inches thick.

Figure 1. Nuclear fuel assembly with 15x15 array [source: DOE Office of Civilian Radioactive Waste Management, http://www.ocrwm.doe.gov/].

NuclearPowerPlants.doc 2 K. E. Holbert

Fission In these reactors, nuclear fission is utilized to release binding energy of the atom. Fission occurs when a neutron absorbed into the nucleus causes atomic instability such that the nucleus fractures. The fission of the uranium expels ν neutrons and releases heat energy (Q).

( ) Qnn +++→→+ 10

AZ

AZ

*23692

10

23592 XXUU 2

2

1

1ν

The emitted neutrons (ν ≈ 2-3 neutrons per fission) sustain the chain reaction. The atomic fission produces 2-3 fission fragments (X). For each fission, approximately 200 MeV of energy is released as the nucleus is split apart. Using this value allows us to calculate the number of atoms of U-235 that must be fissioned per second to produce a thermal power of one watt.

sec

fissions1012.3J101.6022

MeV1sW

JnMeV/fissio200

W1 1013- ×=

×

⋅

(1)

Since each fission event corresponds to the splitting of exactly one atom, to release one joule of energy requires that 3.12×1010 U-235 atoms be fissioned. The equivalent mass of material can be found using Avogadro’s number (NAv)

( ) 235Uofg1022.1moleatoms/g106.022

moleg/g235atoms235U1012.3 1123

10 −×=

−×

−−× − (2)

Example: Determine the mass of U-235 necessary to produce one megawatt-day (MWd) of thermal energy. First, determine how many joules of energy are in a MWd:

( ) J1064.8day

hr24hr

s3600sMW

J10dMW1 106

×=

⋅

⋅

Then from using Equation (2), we find the mass of U-235 needed to produce 1 MWd of energy

235/MWdUofg05.1dMW

J1064.8J

g1022.1 1011

−=

⋅

×

× −

Roughly, one gram of U-235 can produce a thermal energy of 1 MW for 24 hrs. The overall energy release from the fuel is termed the fuel burnup, which is expressed in units of megawatt-days per metric ton of uranium (MWd/MTU) where uranium refers to both U-235 and U-238.

These reactors use normal (light) water as both the reactor coolant and the neutron moderator, and as such, they are called light water reactors (LWRs). The moderator slows down the high-energy neutrons born from the fission process to thermal energy levels at which the neutrons can cause further fissions, that is, achieve a self-sustaining chain reaction. Electric Generating Stations There are several reactor (power plant) types in use today around the world. In the Unites States, nearly all electric power plants are either of the pressurized water reactor (PWR) or the boiling water reactor (BWR) types. Pressurized water reactors are the dominant power reactor type and are also the basis of naval reactors. There are about 100 power reactors in the U.S. today. Nuclear power plants are base loaded because of their high capital and low fuel costs.

NuclearPowerPlants.doc 3 K. E. Holbert

The nuclear power plant is typically divided into two parts: (1) the nuclear steam supply system (NSSS) and (2) the balance of plant (BOP) which includes the turbine-generator set and feedwater train. Like coal-fired power plants, the nuclear units operate on a Rankine cycle. The PWR has a thermal efficiency of about 33%, but the BWR thermal efficiency is about 30%. For safety reasons, the steam temperatures and pressures are lower in the nuclear plant as compared to a fossil generating station. Reactor safety basically comes down to making sure that heat is adequately removed from the core in order to avoid release of radioactivity from the plant. This can be ensured by maintaining the fuel temperature below its melting temperature of about 5000°F, and by keeping the cladding temperature below the point at which the zirc-water reaction occurs (≈2200°F). Neutron absorbing material is employed in control rods to regulate the fission reaction in the reactor core.

PWRs utilize two coolant loops with an intermediate heat exchanger (steam generator), see Figure 2. The reactor core heats the primary reactor coolant (water) to about 600°F at which point it flows to the steam generator. After transferring its heat to the secondary water in the steam generator, the temperature of the primary coolant is reduced to about 540°F. The reactor coolant is then circulated by a pump back to the reactor core. The reactor coolant is kept at a pressure of about 2200 psia which prevents the primary coolant from boiling. The secondary water, which receives the transferred heat, is changed to steam that is piped to the turbine. The remainder of the plant is very similar to a large coal-fired unit in that the exiting steam from the turbine is condensed and sent back to the steam generator via feedwater heaters and pumps.

Figure 2. Pressurized water reactor (PWR) [source: TVA]

BWRs utilize a single coolant loop and therefore boil water in the reactor core. The

produced steam is sent directly to the turbine (see Figure 3). The steam pressure is about 1100 psia, which corresponds to a steam temperature of 556°F. Jet pumps at the reactor pressure vessel provide forced coolant circulation through the reactor core.

NuclearPowerPlants.doc 4 K. E. Holbert

Figure 3. Boiling water reactor (BWR) [source: TVA]

Emissions Nuclear power plants primarily produce two byproducts: (1) thermal emissions, and (2) radioactive materials.

Nuclear power plants are often associated with the tall hyperbolic-shaped natural-draft cooling towers. This association is the result of three factors. First, the lower thermal efficiency of nuclear plants compared to their fossil counterparts dictates that nuclear plants must inherently reject a larger percentage of their produced heat to the environment. Further, fossil units release heat through the flue gas stack, but nuclear plants must dissipate their heat through condenser cooling. Second, nuclear units were built with large electric outputs to take advantage of economy of size. Third, the construction of these large thermal power plants coincided with the recognition of environmental effects (i.e., thermal pollution) from cooling water discharge. Thermal pollution is the introduction of waste heat into bodies of water supporting aquatic life. The heat addition reduces the water's ability to hold dissolved gases, including oxygen which aquatic life requires; although aquatic life growth is usually enhanced by warm water. The radioactive materials produced in the reactor core come from two sources: (1) fission products, and (2) activation products (transuranics). When considering the fission process and the fact that the rest mass energy of a neutron is about 940 MeV, this means that for every one gram of U-235 fissioned, there is about 1 gram of fission products produced. The transuranics are heavy atomic number isotopes that are formed when the neutrons liberated in the fission process are captured but do not cause a fission. Nuclear waste is classified as either high-level or low-level waste. The high-level waste includes fission products and transuranic isotopes, and is highly radioactive and must be stored for long periods. Low-level waste includes clothing, rags, and tools, which are sealed in a drum for ultimate placement in a dedicated landfill.