Home / IBDP Physics- E.4 Fission- IB Style Questions For HL Paper 1A

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

IBDP Physics HL Paper 1 -FA 2025- All Chapters

Question

Which of the below is A suitable material for use as a moderator in a nuclear reactor.
(A) cadmium
(B) concrete
(C) uranium-238
(D) water
▶️ Answer/Explanation
Detailed solution

A moderator slows down fast neutrons to thermal energies to sustain fission.
Good moderators have light nuclei that can slow neutrons through elastic collisions.
• Cadmium: neutron absorber (control rods)
• Concrete: shielding material
• Uranium-238: fissionable material
• Water: contains hydrogen, excellent moderator
✅ Answer: (D) water

Question

In a nuclear reactor model, 4 neutrons are emitted per fission. Control rods absorb \( N_c \) neutrons, \( N_1 \) neutrons are lost, and the rest cause further fissions. What values of \( N_c \) and \( N_1 \) maintain a steady reaction?
(A) \( N_c = 1 \), \( N_1 = 1 \)
(B) \( N_c = 1 \), \( N_1 = 3 \)
(C) \( N_c = 2 \), \( N_1 = 1 \)
(D) \( N_c = 2 \), \( N_1 = 2 \)
▶️ Answer/Explanation
Detailed solution

For a steady chain reaction, exactly 1 neutron per fission should cause another fission.
Total neutrons per fission = 4
Neutrons causing fission = 4 – \( N_c \) – \( N_1 \) = 1
So \( N_c + N_1 = 3 \)
Checking options:
(A) 1 + 1 = 2 ≠ 3
(B) 1 + 3 = 4 ≠ 3
(C) 2 + 1 = 3 ✓
(D) 2 + 2 = 4 ≠ 3
✅ Answer: (C)

More DP Physics HL Paper 1 Questions..
Scroll to Top