Question

The core of a nuclear reactor generates a large amount of thermal energy from the decay of fission products, even when the power-producing fission chain reaction is turned off. Would this residual heat be greatest after the reactor has run for a long time or short time?

a) After a long time
b) After a short time
c) Residual heat is the same regardless of time
d) Residual heat is zero

Answer 1

Residual heat peaks shortly after a reactor is shut down due to the rapid decay of highly radioactive fission products. Thus, the correct option is b) After a short time.

Step-by-step explanation:

The residual heat generated in a nuclear reactor is greatest after a short time. This might seem counterintuitive at first, but it is due to the nature of the decay processes involved in nuclear fission. When a fission reaction occurs, various radioactive isotopes, called fission products, are produced. These isotopes undergo radioactive decay, emitting particles and converting into stable isotopes over time. The decay process releases heat as a byproduct. The rate of decay is initially high, and therefore, the heat produced is greatest shortly after the reactor is shut down.

In the first moments after shutdown, the reactor's fission chain reaction is turned off, but the fission products are still highly radioactive and decay rapidly. The heat generated during this short time is the result of the cumulative decay of these isotopes. As time progresses, the rate of decay decreases, leading to a reduction in the amount of residual heat produced. Therefore, the residual heat is at its peak shortly after the reactor has been in operation and is subsequently reduced over time.

Understanding the dynamics of radioactive decay is crucial for reactor safety and design, ensuring that cooling systems can effectively manage the residual heat during and after the reactor's operational phase.

Thus, the correct option is b) After a short time.