Question

The critical mass of fissionable material is
A. 75 kg
B. 1 kg
C. 20 kg
D. 10 kg

Answer 1

Hint: A subcritical masses a mass of fissile material that does not have the ability to sustain a fission chain reaction.

Complete step by step answer:
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fission cross-section), density, shape, enrichment, purity, temperature, and surroundings.
When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature, or neutron population. A mass may be exactly critical at a particular temperature. Fission and absorption cross-sections increase as the relative neutron velocity decreases. As fuel temperature increases, neutrons of a given energy appear faster and thus fission/absorption is less likely.
In the case of plutonium –239
The critical mass is about 10 kg
If the mass of fissionable material exceeds a critical value, chain reaction, no self propagation fission reaction or self propagating fission reaction takes place.

So, the correct answer is “Option D”.

Note: Critical mass, in nuclear physics, the minimum amount of a given fissionable material necessary to achieve a self. Sustaining fission chain reaction under starting condition. The critical size is the minimum size of a nuclear reactor core or nuclear weapon that can be made for a specific geometrical arrangement and material composition. The critical size must at least include enough fissionable material to reach critical mass. If the size of the reactor core is less than a certain minimum, too many fission neutrons escape through its surface and the chain reaction is not sustained.