Which isotope is the result of activation of impurities in the coolant?

• A. H-3
• B. Na-24
• C. N-16
• D. Ar-41

Answer: (D)

Neutron activation of trace impurities in the reactor coolant creates radioactive isotopes specific to those impurities. Argon is a common impurity in the coolant (often dissolved gas). When Argon-40 in that impurity captures a neutron, it becomes Argon-41, which is a short‑lived gamma emitter. That’s why this isotope appears as a result of coolant impurity activation.

The other isotopes come from different sources: tritium mainly from hydrogen/deuterium in the water; sodium-24 would require sodium in the coolant (not the typical impurity here); nitrogen-16 comes from interactions with the oxygen in water, not from a trace impurity. Thus Argon-41 best fits the idea of activation of impurities in the coolant.