How does a heavy water nuclear reactor work?

Answer 1

Uranium and Plutonium atoms require nuetrons moving at a certain speed, with a certain amount of kenetic force, to fission properly and often, and to achieve this speed, a neutron moderator is placed between the neutron source and the fuel, which slows the neutrons down by causing them to hit its molecules. Water is often used, since the energy transfer is much more efficient, as hydrogen atoms are almost identical in size to neutrons, possesing only one proton (like two billiard balls striking each other), but hydrogen atoms sometimes absorb neutrons, meaning less get through to cause fissions, and once the concentration of fissionable material drops blow a certain percentage (usualy somewhere around 5%) fission is no longer maintainable. Heavy water posses hydrogen atoms with one extra neutron, so althought the energy transfer is slightly less efficient than with hydrogen atoms, there is much less chance of the atoms abosorbing neutrons, and so many more neutrons get through, allowing the reactor to run on fuel with much lower concetrations of fisionable material (even as low as 0.7%, the natural level of U-235 in Uranium ore). Thus somereacotrs using heavy warer as a neutron moderator (such as the CANDU) can even run on the waste from other, "light water moderated" reactors (light water is just another name for normal water, as opposed to heavy water).

Answer 2

Heavy water acts as a neutron moderator. The neutrons formed by fission come off at high velocity, but U-235 absorbs neutrons much more strongly when they are slowed down, so all so-called thermal reactors use a moderator to slow them down until they achieve the average velocity appropriate to the atoms of the moderator which is at approx 300 deg C in a water reactor. Other moderators can be light water or graphite. Heavy water has the advantage over light water that it absorbs fewer neutrons so natural uranium can be used, whilst light water requires enriched uranium fuel. Graphite in a very pure form is also a good moderator but requires a gas coolant, carbon dioxide has been used in magnox and AGR reactors, but there is a time limit on the life of the reactor due to graphite corrosion, and these are now obsolete as far as new builds are concerned. The Soviet RBMK reactors used graphite but with a water coolant inside pressure tubes, but after Chernobyl there will certainly be no more of these built.