Candu reactor

Candu reactors are nuclear reactor that differ from every other nuclear reactor, in that they do not require the expensive nuclear enrichment.

Uranium the metal at the core of all existing nuclear reactors, is radioactive. The two most common isotopes of Uranium, Uranium-235 and Uranium-238 differ significantly in their half-life. U-235 decays more quickly, consequently, more than 99 percent of Uranium is U-238.

Atoms of both isotopes occasionally decay, and emit a Neutron. The nuclear chain reaction at the centre of an exploding atomic bomb, and in a nuclear reactor, relies on Neutrons striking other Uranium atoms, triggering them to split into two smaller atoms, releasing a large amount of energy.

Like atomic bombs nuclear reactors require a relatively large mass of Uranium, to increase the chance that a recently emitted neutron will strike another Uranium atom. Splitting the nucleus is much more likely when it is struck by a relatively slow moving Neutron.

Conventional reactors rely on control rods, made from something that slows Neutrons, to control the rate of the nuclear reactions. Cadmium is a common choice for the control rods, and is a vulnerability. Cadmium is a soft metal, and, like all metals, it expands, when heated. Technicians can lose control over a reactor when the vulnerable control rods expand and get jammed in their tubes.

Candu reactors use heavy water as their coolant, surrounding, and circulating between tubes full of Uranium pellets. Heavy water itself slows Neutrons.

The strong advantage Candu reactors have over light-water reactors is that they do not suffer catastrophic failure if a blockage results in the coolant boiling away. If the heavy-water coolant boils away it is no longer slowing Neutrons, so the chain reaction ends.