Nuclear reactors are based on the "fission" process for generating heat energy to produce electricity. Fission is a chain reaction process that needs to be controlled for smooth and safe operation of the reactor. The materials used for controlling the speed of neutrons produced during fission are known as moderators. There are several common moderators used in nuclear reactors,such as graphite, beryllium and heavy water. These materials are selected as moderators on the basis of some very specific properties and requirements.
Properties of Graphite Moderators
Graphite is a commonly used neutron moderator in nuclear reactors. It has satisfactory purity (99 per cent pure, with ash content less than 300 ppm and boron less than about 2 ppm) available at reasonable price. It is thermally stable, but at elevated temperatures it can react with oxygen and carbon dioxide in the reactor decreasing the effectiveness. It can also form carbides (a compound composed of carbon with another element) after reacting with some metals and metal oxides. Despite being a non-metal, graphite has good heat conducting property, which is an important property of neutron moderators. The basic drawbacks of graphite in nuclear moderation are the chances of oxidation in presence of air, low strength and the low density. Its dimensions may change under the influence of radiations in the reactor.
Properties of Ordinary Water & Heavy Water Moderators
Ordinary water is used in nuclear reactors as a moderator for a number of reasons. The low cost of water, easy availability and excellent slowing-down property makes it a fine neutron moderator. Ordinary water can be used as a moderator only if enriched uranium fuel is used. It can be employed as both, a moderator and a coolant in the nuclear reactor. The main drawbacks of ordinary water is its relatively low boiling point. Heavy water on the other hand overcomes the major drawback of ordinary water. Unlike ordinary water, it can perform satisfactorily with natural uranium fuel also; it yields highly enhanced neutron economy (unlike ordinary water, it absorbs the neutrons), allowing the reactor to operate without fuel enrichment facilities and generally enhancing the ability of the reactor. Unlike graphite moderator, it does not oxidise. Heavy water moderated nuclear reactors are smaller and require considerably less nuclear fuel. The drawback associated with heavy water is the considerably higher cost than ordinary water.
Properties of Zirconium Hydride Moderator
Zirconium hydride is not a usual material used as a moderator in nuclear reactors. When powdered, the material has poor thermal conductivity and hence requires special cooling provisions in high power reactors operating at high to moderate temperature. Zirconium hydride is employed in the "triga" reactor with enriched uranium as the fuel element.
Properties of Beryllium and Beryllium Oxide Moderator
Beryllium is superior to graphite as a neutron moderator in nuclear reactors. Beryllium has been used in some reactors and beryllium oxide is proposed as a neutron moderator for high-temperature gas-cooled nuclear fission reactors. Beryllium is high in cost compared to graphite and heavy water, which often replace it.