Consider the following statements : 1. The chain reaction process is used in nuclear bombs to release a vast amount of energy, but in nuclear reactors, there is no chain reaction. 2. In a nuclear reactor, the reaction is controlled, while in nuclear bombs, the reaction is uncontrolled. 3. In a nuclear reactor, all operating reactors are ‘critical’, while there is no question of ‘criticality’ in case of a nuclear bomb. 4. Nuclear reactors do not use moderators, while nuclear bombs use them. Which of the above statements about operational principles of a nuclear reactor and a nuclear bomb is/are correct?

Nuclear reactors and nuclear bombs both rely on nuclear chain reactions, making statement 1 incorrect [2]. In a nuclear reactor, the chain reaction is 'controlled' using control rods to maintain a steady energy output, whereas a nuclear bomb involves an 'uncontrolled' chain reaction that grows exponentially to cause an explosion, making statement 2 correct [1]. Regarding statement 3, 'criticality' refers to a state where the neutron population is steady (multiplication factor k=1), which is the standard operating state for reactors [2]. While bombs must reach a 'supercritical' state (k>1) to explode, the concept of criticality is fundamental to the physics of both devices. Statement 4 is incorrect because reactors use moderators (like water or graphite) to slow neutrons and sustain the reaction, whereas most nuclear bombs do not use moderators as they rely on fast neutrons for rapid energy release [3].

Sources

1. [1] https://www.osti.gov/opennet/manhattan-project-history/Science/NuclearPhysics/chain-reaction.html
2. [2] https://en.wikipedia.org/wiki/Nuclear_reactor_physics
3. [3] https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work