Basic Radiological Emergency Response Training Information: Nuclear Reactors

The major difference between nuclear power plants and fossil fuel power plants is that nuclear plants use the process of fission to make steam instead of burning gas, coal or other fossil fuels. The fission process and how nuclear power plants operate will be discussed in this section.

The process of fission starts with the neutron and the target nucleus of an atom (usually made of uranium). The neutron is absorbed in the target nucleus. When the neutron is absorbed the energy level of the target nucleus increases. This increase in energy "splits the atom". The resulting products are more neutrons to fission other targets, fission products (which are different radioactive elements), and heat.

REACTOR SYSTEMS

There are two types of nuclear power reactors used in the commercial generation of electricity in Minnesota.

Boiling Water Reactors (BWRs) actually boil the water in the reactor core. The steam is transferred to the turbine which turns the generator to make electricity. After the steam loses energy to turn the turbine it is condensed back into water and returned to the reactor.

PRESSURIZED WATER REACTOR SYSTEM

Pressurized Water Reactors (PWRs) keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.

Most nuclear power plants have cooling towers or other systems to cool water used in the plant before it is released back into lakes or rivers. The water being cooled is from systems that are separated from radioactive water. The condensation coming from the cooling tower is not radioactive.

Since radioactive materials can be dangerous, nuclear power plants have many safety systems to protect workers, the public, and the environment. These safety systems include shutting the reactor down quickly to stop the fission process, cooling the reactor core down and carrying heat away from it, and containing the radioactivity to prevent it from escaping into the environment.

PROTECTIVE BARRIERS

The uranium of the nuclear fuel is fabricated into pellets then placed in welded metal rods. The rods are put together into an assembly called a fuel bundle. Several hundred fuel bundles are placed into a reactor pressure vessel before the fission process takes place. The reactor pressure vessel is located in a containment building made of thick concrete and steel walls.

In a reactor incident, the temperature in the pellets may raise to a point where the fuel melts. Temperature or other factors may cause the rod to crack. Cracks allow fission products to leak into the reactor vessel. If there is a leak in the reactor pressure vessel, the fission products may leak into the containment. If a leak in the containment occurs or if the nuclear plant operators vent the containment a plume of radioactivity may escape into the environment.

The first protective barrier is the reactor fuel cladding or the walls of the fuel rod. The second barrier are the walls of the reactor pressure vessel. The third barrier is the walls of the containment building also known as primary containment

SUMMARY

Nuclear power plants vary in design but always keep safety in mind. The safety systems and barriers to release of radioactivity minimize the risk of incidents and exposure to the public in case there is an incident.

Information courtesy of the CDC - educational material.