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IBDP Physics- E.4 Fission- IB Style Questions For SL Paper 1A -FA 2025

IBDP Physics SL Paper 1 -FA 2025- All Chapters

Question 

Which two components of a nuclear power station are primarily responsible for ensuring that a controlled chain reaction is maintained?

(A) Control rods and heat exchangers
(B) Moderators and shielding
(C) Shielding and heat exchangers
(D) Moderators and control rods
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{D}} \)

A nuclear reactor uses a moderator to slow down fast neutrons. Slower neutrons are more effective at producing further fission in many reactor fuels, helping the chain reaction to continue.

Control rods absorb neutrons and are inserted or withdrawn to control the rate of the chain reaction.

Therefore, moderators and control rods are the two components primarily responsible for maintaining a controlled chain reaction.

Shielding provides protection from radiation, while heat exchangers transfer thermal energy. These are important reactor components but do not primarily control the chain reaction.

Hence, the correct answer is \( \boxed{\mathrm{D}} \).

Question 

Radioactive oxygen-15 \(\left(^{15}_{8}\mathrm{O}\right)\) decays according to the following equation:

\(^{15}_{8}\mathrm{O}\rightarrow X+\beta^{+}+\nu_{\mathrm{e}}\)

What are the proton number and the nucleon number of nuclide \(X\)?

 Proton number of \(X\)Nucleon number of \(X\)
(A)\(7\)\(14\)
(B)\(9\)\(14\)
(C)\(7\)\(15\)
(D)\(9\)\(15\)
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{C}} \)

In beta-plus (\(\beta^+\)) decay, a proton in the nucleus changes into a neutron, producing a positron and an electron neutrino:

\(p\rightarrow n+\beta^{+}+\nu_{\mathrm{e}}\)

Therefore, the proton number decreases by \(1\), while the nucleon number remains unchanged.

For \(^{15}_{8}\mathrm{O}\):

\(\text{proton number of }X=8-1=7\)

\(\text{nucleon number of }X=15\)

Thus, \(X\) has proton number \(7\) and nucleon number \(15\).

Hence, the correct answer is \( \boxed{\mathrm{C}} \).

Question

What are the principal roles of a moderator and of a control rod in a thermal nuclear reactor?
 Role of moderatorRole of control rod
A.increases kinetic energy of neutronsmaintains a constant rate of reaction
B.increases kinetic energy of neutronsabsorbs energy transferred in the reactor
C.reduces kinetic energy of neutronsmaintains a constant rate of reaction
D.reduces kinetic energy of neutronsabsorbs energy transferred in the reactor
▶️ Answer/Explanation
Detailed solution

In a thermal reactor, fast neutrons produced by fission are less likely to cause further fission of \(\,^{235}\text{U}\) than slow (thermal) neutrons.
A moderator slows neutrons down by repeated collisions, so it reduces the kinetic energy of neutrons.

Control rods (made of neutron absorbers such as boron, cadmium, or hafnium) absorb neutrons.
By inserting or withdrawing the rods, the reactor can be kept critical so the chain reaction proceeds at a constant rate.

✅ Answer: (C)

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