IB Physics SL E.4 Fission questions

Answer: D. Control rods absorb neutrons, so inserting them further reduces the number available to sustain fission and lowers the fission rate. Fission products in removed fuel are radioactive, so the fuel requires long-term storage.
A: confuses shielding, which limits radiation escaping the reactor, with control of the neutron population driving fission.
B: correctly controls the rate, but shielding does not remove the need for long-term storage of radioactive fuel.
C: withdrawing control rods absorbs fewer neutrons, tending to increase rather than decrease the fission rate.
Remember: control rods regulate fission; shielding limits radiation exposure, while radioactive waste needs long-term management.

Answer: B. A long half-life means the activity of a radioactive sample decreases slowly, so some fission products can remain radioactive over long periods. Secure long-term containment reduces exposure to radiation and prevents radioactive material from reaching the environment.

A: confuses shielding, which reduces radiation reaching people, with changing unstable nuclei into stable nuclei. C: assumes that stopping fission also stops radioactive decay; fission products continue to decay after removal. D: treats a half-life as a fixed expiry time for every nucleus; after one half-life, about half the radioactive nuclei are expected to have decayed.

Remember: shielding reduces exposure; it does not stop radioactive decay.