ABSTRACT
A small amount of Plutonium-239 is subsequently produced from the Uranium-238 in the fuel rods. However, over time, this additional plutonium can generate nearly one-third of the total power in a light-water reactor at the end of a refueling cycle. Depending on the level of the neutron flux, the reduction in the concentration of U-235 and U-238 can occur at different rates. Various isotopes of plutonium are produced in thermal water reactors from the burnup of Uranium-238. The U-238 in the fuel is generally bonded together with two oxygen atoms to form a ceramic fuel pellet consisting of about 97% U-238 and 3% U-235. Changes in the temperature of the core can occur at different rates in the fuel and in the moderator, and certain parts of the core can run hotter or colder than others. Thus, core-wide temperature changes can affect the power level in many different ways. This chapter discusses how these reactivity changes can be predicted quantitatively.
