Why cant a nuclear power plant explode like a nuclear bomb?

Answer 1

A fission bomb explodes because of a sudden assembly (driven by conventional explosives) of a very supercritical mass of fissile material, followed by a precisely timed neutron pulse at the optimal point of supercriticality.

A fission reactor cannot do this, for the following reasons:

• most fission reactors do not contain fissile material, only fissionable material
• a fission reactor has no mechanism for sudden assemblyof a very supercritical mass
• fission reactors operate exactly at critical, so the reaction neither dies out nor rises
• fission reactors use a continuous neutron source at startup that is removed once the reactor is running, not a precisely timed pulsed neutron source like used in fission bombs
• if a fission reactor using fissile material did go supercritical enough (without the automatic SCRAM system shutting it down first) to get a small nuclear yield, it would be a fizzle yield only capable of melting the core and thereby instantly dissembling the supercritical mass

Definitions:

• Fissionable material - material capable of sustaining fission, but only in presence of a moderator to slow neutrons to prevent their capture by Uranium-238 or other isotopes with large capture crosssections for fast neutrons
• Fissile material - weapons grade material requiring no moderator and capable of fast fission. required for both bombs and fast breeder reactors.

Answer 2

A nuclear power plant operates using controlled nuclear reactions to generate electricity, whereas a nuclear bomb relies on uncontrolled reactions to cause a massive explosion. The design and safeguards in place in a nuclear power plant prevent the conditions needed for a nuclear explosion to occur. The fuel used in a power plant is different from that used in a nuclear bomb and is not enriched to the same level.