How does nuclear reaction take place in a nuclear plant?

Answer 1

In a nutshell, a typical controlled nuclear reaction involves an equilibrium between the rate of splitting Uranium-235 atoms and the production of neutrons from those splits. The neutron is what causes the atom to split. The splitting generates between 1 and 2.5 (on average) neutrons, with some sources saying up to 3.5 neutrons. What you want, on the large scale, is a one-to-one ratio - one split yields one neutron which yields one split. This is what we call KEffective = 1.

Each of those splits produces about 200 MeV of energy due to the loss of binding energy (Strong Atomic Force), which is translated from the calculated loss of mass of about one fifth of a proton. This does not seem like much but, on the large scale, it adds up quickly. That energy is manifest in various forms, most usually, heat, which is carried away to generate steam to turn turbines that turn generators.

We manage that "magic" KEffective with moderators and control rods. The control rods provide gross control, primarily startup and shutdown, and tuning at lower powers, while the moderator, usually water, provides fine control, at higher powers. What happens is that the water has a negative reactivity coefficient, which means that, as it gets hotter, reactivity (KEffective) decreases, and vice versa. This makes the reaction self modulating.

In this scheme, what we call a light water moderated fission reaction, loss of that water, say from a depressurization incident that flashes all of the water to steam, results in instant subcriticality, and the reactor shuts down. Of course, that water must be replaced after the control rods are inserted, otherwise decay heat can cause fuel damage, as we unfortunately are experiencing with the Fukushima Daiichi facility in Japan.