The operation of light water reactors requires enriched uranium. Just a few countries in the world master the technically complex and expensive process of enriching uranium. In order to use natural uranium with a 0.7 percent fraction of fissile Uranium-235 for generating electricity, heavy water reactors have been developed.
Heavy and light water reactors
Those reactors use heavy water as moderator. It consists of deuterium (D). In contrast to the “usual” hydrogen (H), deuterium has one neutron in its atomic nucleus. Deuterium only occurs as a 0.015 percentage share based on naturally available hydrogen. Light water (H2O) is however used as coolant.
Heavy water has a lower capacity for neutron absorption than light water, as well as a lower moderating effect. Thus, for the operation of a plant based on heavy water, bigger amounts of the moderator are necessary. The result is a 10-fold increase of the volume of the reactor pressure vessel (compared to a light water reactor with the same rated power). The advantage of heavy water reactors is the independence from supplies of enriched uranium.
In such a reactor design the heavy water is confined in an unpressurized container. The pressurized tubes which contain the fuel rods and the coolant are directed through this container.
Light water flows around the fuel rods, cools the rods and thereby takes up heat. This heat is transferred to a steam power process via a heat exchanger.
The rated power of these heavy water reactors is considerably smaller compared to light water reactors due to high construction costs. Light water reactors generally have an electrical power output of around 1 GW, while the one of heavy water reactors reaches only about a third of this value (ca. 300 MW).
Light water graphit-moderated reactors
Nuclear reactors of this type have been developed and construction in the Sowjet Union in order to generate electricity and to produce weapon-grade plutonium.
This reactor consists of a graphite block (pure carbon) permeated by pressurized tubes containing water. These tubes contain the fuel rods with enriched uranium. In addition, control rods for sustaining the fission reaction are necessary.
The neutrons released during nuclear fission are moderated by the graphite, heat the water in the tubes and initiate further fission.
If cooling water is lost, the intensity of the nuclear fission increases, as the moderator is still in place. The lack of heat dissipation by the coolant causes the reactor to heat up. At temperatures above 600°C, the graphite is prone to catch fire. The only way to prevent this, is the introduction of the control rods in time.
The reactor in Chernobyl has been of this kind. Due to the high temperatures, the control rods were bent out of shape and therefore were not able to stop a run-away nuclear chain reaction.
