Taking advantage of the laws of nature is the fundamental method of achieving nuclear reactor safety. The prime objective of nuclear safety is to keep the large quantity of fission products in their proper place in the fuel. A number of separate inherent safety features are employed toward this general objective.

Use of Uranium Dioxide Fuel. The ceramic form of uranium dioxide in the form of sintered pellets is the nuclear fuel in today’s nuclear power reactors. It has an inherent safety advantage in that this material has the property of retaining most fission products, even when overheated. This property has become particularly evident during routine power reactor operation. For example, large water power reactors have operated with a number of experimental fuel assemblies. Failure of the metallic fuel clad­ding was expected and in a number of cases did occur. However, opera­tion continued with coolant contamination essentially at normal levels for

a number of months before shutdown for a scheduled refueling operation. The ability of the uranium dioxide to retain fission products, even in the presence of completely severed cladding tubes, is even more effective than originally had been expected and thus provides substantial natural safety for reactors using this fuel material.

Use of Low Enrichment Fuel. The uranium dioxide used for fuel in today’s power reactors is enriched in the easily fissionable isotope 235U, a factor of only three or four times its natural level. Thus, it is possible to use a natural safety factor, owing to the uranium composition, technically called the Doppler effect. This effect operates to reduce immediately the rate at which fissions occur in the fuel whenever the fuel temperature rises significantly. This inherent safety feature is the basic reason why a power reactor in no way resembles or could even act as a bomb which would con­tinue to release energy with no inherent “shutoff mechanism.” Since dur­ing normal reactor operation the average temperature of uranium dioxide is thousands of degrees less than its melting temperature of about 5,000° F, considerable elbow room is available for this effect to operate before significant melting of the fuel would occur in any accident.

Safety by Water Moderator. The use of ordinary water as both cool­ant and neutron moderator in today’s water-cooled power reactors pro­vides additional natural safety. If the reactor core should increase in power for any reason, the temperature of the moderator water would also tend to rise. In fact, in a boiling water reactor the amount of steam generated within the reactor would tend to rise also. In turn, the density of the water would decrease, and the efficiency of the water to moderate the neutrons would be decreased. Therefore, an inherent reactor shutdown mechanism would come into play. This is also an inherent reason why our reactors are so easily controlled and why they always operate stably. This major safety precaution results from the selection of water as the coolant and modera­tor of large power reactors.

The above three examples identify strong inherent safety features of today’s large water-cooled power reactors. They were major considera­tions during the period that the suppliers were evolving their product line in the nuclear power reactor business. The more inherent safety that the product itself possesses, the safer the overall product must be.