raskasvesimoderointiinsa

Raskasvesimoderointiinsa, often translated as heavy water moderation, is a method used in some types of nuclear reactors to control the nuclear fission chain reaction. This technique relies on the properties of heavy water, which is water where the protium atoms (hydrogen-1) have been replaced by deuterium atoms (hydrogen-2). Deuterium has a much lower neutron absorption cross-section compared to protium, meaning it is less likely to absorb neutrons.

In a nuclear reactor, neutrons are emitted during the fission of nuclear fuel, such as uranium. These neutrons can then go on to cause further fissions, sustaining a chain reaction. However, many of these neutrons are too fast to efficiently cause fission in common nuclear fuels like natural uranium. This is where a moderator comes in. The moderator's role is to slow down these fast neutrons to thermal energies, where they are much more likely to be absorbed by the fuel and induce further fission.

Heavy water is an exceptionally effective moderator because it slows neutrons down efficiently while absorbing very few of them. This low absorption rate allows reactors using heavy water as a moderator to sustain a chain reaction using natural uranium, which has a lower concentration of fissile uranium-235 compared to enriched uranium. Reactors that utilize heavy water moderation, such as the CANDU (CANada Deuterium Uranium) reactor design, can therefore operate without the need for uranium enrichment. The moderator is typically held within the reactor core, surrounding the fuel assemblies. The efficient moderation provided by heavy water is a key factor in the design and operation of these reactors, contributing to their ability to use natural uranium fuel.