How can nuclear reactors be dangerous?

Answer 1

The purpose of a nuclear reactor is to create and sustain a fission chain reaction in order to produce heat to make steam to drive turbines and produce electrical power (extremely simplified explanation).

A fission chain reaction is the interaction of neutrons with fissile materials (elements that can be fissioned). Some enriched fuel (such as uranium-238) is introduced into the reactor core. It produces neutrons as radiation. If more fissile material is present ("fuel" such as uranium-235), that interaction repeats to make more neutrons, and so on. A nuclear reactor is designed to sustain a fission chain reaction and control the rate at which that reaction occurs.

The nuclear core of a reactor, where the nuclear fuel is, needs to be shielded so that the radiation and any radioactive components inside do not escape into the general environment.

The primary radiation type inside the core is neutron radiation. One of the best shields for neutron radiation is a hydrogen-dense substance, thus pure water is often used. Water, if circulated in a heat sink system, also serves as a heat-removal method (cooling system).

The enclosure for the core and the water pool is a containment vessel made out of something that is airtight and which shields against other kinds of direct radiation. Steel is a common material, perhaps supplemented by concrete and other reinforcement to guard against being damaged by an earthquake, explosion, or an airplane impact (for example).

There is usually an outer containment building that encloses the reactor containment vessel itself. This is a sort of "second line of defense" in case the reactor is breached. Containment buildings are designed to withstand extremely high internal pressures (such as superheated steam) and forces of almost any direction and realistic magnitude. The specific shape of buildings at a nuclear plant are part of their function. The concave towers that are so symbolic of nuclear power are a common design for cooling towers (and not the reactor core itself). Domes are a common shape for reactor containment buildings for a variety of reasons that have mostly to do with encouraging steam condensation.