Suppose you are in charge of a fission nuclear reactor, and you notice that the amount of xenon-135 in the reactor core has built up to the extent that it is compromising the operation of the reactor. What factor(s) could this build-up be due to? Select one or more: a. The water level was allowed to drop too low b. The control rods were removed too far c. Running the reactor at too low a power level d. The fuel rods have reached their end of life
The correct answer and explanation is:
The correct answer is c. Running the reactor at too low a power level.
Explanation:
Xenon-135 (Xe-135) is a byproduct of nuclear fission and acts as a strong neutron absorber. It is produced when iodine-135 (I-135) undergoes neutron capture and decays to xenon-135. Normally, xenon-135 is transitory because it decays relatively quickly (with a half-life of about 9.2 hours). However, its buildup can be problematic under certain conditions.
When the reactor operates at low power levels, the rate of production of iodine-135 is reduced. However, the rate of decay of xenon-135 remains constant. As a result, xenon-135 accumulates faster than it can be consumed in the reaction, leading to the phenomenon known as “xenon poisoning.” This buildup of xenon-135 can absorb neutrons that are critical for sustaining the fission chain reaction, thus impeding the reactor’s ability to operate efficiently.
The other options do not directly cause xenon-135 buildup:
- a. The water level was allowed to drop too low: While water serves as a coolant and neutron moderator, a drop in water level affects heat removal and safety, but it does not directly influence the buildup of xenon-135.
- b. The control rods were removed too far: Control rods are used to absorb neutrons and regulate the fission chain reaction. However, the removal of control rods increases reactivity rather than causing an increase in xenon-135 buildup.
- d. The fuel rods have reached their end of life: When fuel rods reach the end of their life, they typically contain a higher concentration of fission products, including xenon-135. However, this is not the main reason for the build-up of xenon-135. Running the reactor at low power is the more critical factor in causing the excess accumulation.
In summary, running the reactor at too low a power level allows xenon-135 to accumulate, causing operational difficulties in nuclear reactors.