Robert H. Fisher, age 14, of Fredericton, New Brunswick, Canada, for his question:
Why is expensive heavy water used in atomic reactors?
Most heavy water is produced by an expensive process of electrolysis. It looks and feels like ordinary water, but seeds refuse to germinate in it and tadpoles cannot survive in it. However, in the atomic reactor it does what ordinary water cannot do.
It slows the neutrons that speed from the nuclear fission and makes it possible to transfer heat to turn electric power generators. Heavy water has many other uses now and in the future. But at present, its most useful role is in the nuclear reactor.
In the core of the reactor, nuclear fission shoots off neutrons at fairly high speeds. A moderating agent is needed to slow their speeds and a cooling agent is needed to carry the heat to where it is needed. Ordinary water could be a fine moderating agent, except for one thing. The hydrogen atoms in its molecules tend to grab and hold onto flying neutrons. The water gets too hot and boils away.
However, heavy water absorbs very few stray neutrons. Instead it tends to slow down those speeding from the fission in the core of the reactor. Heat, as we know, is the motion of particles of matter and speeding neutrons in the reactor to create very high temperatures. Heavy water moderates their speeds and cools things down. It also makes it possible to carry the heat from the core. This heat supplies the energy to boil water and create steam the steam used to generate electricity.
The difference between ordinary water and heavy water is heavy hydrogen. The differences between ordinary hydrogen and heavy hydrogen is one neutron. The nucleus of ordinary hydrogen has but one proton. The nucleus of heavy hydrogen has one proton plus a neutron. Its atomic mass is about twice as great and the extra neutron gives it different properties. This hydrogen isotope is called deuterium. When deuterium replaces the ordinary hydrogen in water molecules we get deuterium oxide, alias heavy water.
Deuterium oxide, naturally, is heavier than ordinary water. Its extra neutrons also give it different properties. It freezes at 3.83 degrees centigrade and boils at 101.42 degrees centigrade. The freezing and boiling points of ordinary water, as we know, are 0 degrees centigrade and 100 degrees centigrade. It also tends to discharge more slowly in the process of electrolysis; hence, particles of heavy water gather on the electrolyte cells.
Heavy water also is a more useful cooling agent because ordinary water boils away at a slightly lower temperature. But as a moderating agent ordinary water is useless because its ordinary hydrogen atoms merely capture and absorb the speeding neutrons. Heavy water already has a quota of extra neutrons in its hydrogen isotopes. This may explain why it does not absorb any more and merely slows down those emitted by nuclear fission in the reactor.
Concentrating quantities of deuterium and deuterium oxide is so expensive that their uses are limited. In the future, scientists hope to find less costly ways to produce them. If and when this happens, they may be able to separate and use the heavy water present in sea water. It is estimated that the oceans contain enough of it to yield billions of times more energy than all our reserves of coal and oil.