IB Physics SL E.4 Fission questions

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Question 1

Paper 2Easy9 marks

One possible neutron-induced fission reaction of uranium-235 is

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n

The following rest masses are available:

Particle or atomRest mass / u
²³⁵U235.043930
¹n1.008665
¹⁴¹Ba140.914411
⁹²Kr91.926156
(a)

Distinguish between spontaneous fission and neutron-induced fission.

[2]
(b)

For the reaction and data above, use 1 u c² = 931.5 MeV and 1 MeV = 1.60 × 10⁻¹³ J.

(i)

Determine the energy released in the reaction, in MeV.

[3]
(ii)

Using your answer to (b)(i), express the energy released in joules.

[2]
(c)

Explain how neutrons released in fission can sustain a chain reaction in uranium-235.

[2]

Question 2

Multiple choiceMedium1 mark

In a fission power plant, the fission rate has risen above its intended value. Fuel removed after use contains fission products. Which paired action correctly addresses the reactor rate and the removed fuel?

Show answer and explanation

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.

Question 3

Multiple choiceMedium1 mark

A reactor operator is planning long-term storage for fission products, some of which have long half-lives. Which statement correctly links their nuclear behaviour to the need for this storage?

Show answer and explanation

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.

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