2.1 INTRODUCTION

In common with pressurised water reactors (PWRs), boiling water reactors (BWRs) and advanced gas-cooled reactors (AGRs) most fast power reactors use oxide fuel. There is a certain amount of experience with metal fuel in the United States, and there is interest in the use of metal fuel and other ceramic fuels such as nitride or carbide for future fast reactors. This chapter deals mainly with the oxide and metal fuels of which there is most experience, and it covers other fuel materials in less detail.

Since oxide fuels are widely used much of this chapter applies as well to thermal reactors as to fast reactors. The main difference is that new fast reactor fuel usually consists of a mixture of plutonium and uranium dioxides whereas at present most thermal reactors use enriched uranium dioxide with a small amount of plutonium present after irradiation. The use of mixed uranium and plutonium diox­ide (MOX) fuel in thermal reactors is increasing. Most water-cooled thermal reactors have fuel clad in zirconium alloy, but AGRs use stainless steel, and because of the similarity of the coolant temperat­ures there is much in common between the behaviour of AGR fuel and that of a sodium-cooled fast reactor.

There is an important difference, however. In all systems it is eco­nomically desirable to irradiate the fuel as long as possible before it

is removed from the reactor. In a thermal reactor the limit is set by the loss of reactivity as fission products that absorb thermal neutrons accumulate. In a fast reactor the loss of reactivity is much smaller, especially if the internal breeding is high, so the irradiation limit is different. It is set by the need to be sure that the cladding will remain intact and not allow radioactive material to escape into the coolant. As a result the maximum burnup is very high in a fast reactor. The neutron irradiation accompanying this burnup damages the cladding and the irradiation limit is set by the need to remove the fuel before the cladding loses its integrity.

A very thorough account of reactor fuel in general, and in partic­ular of oxide fuel for fast reactors, is given by Olander (1976). It is particularly valuable for its treatment of the theory of many aspects of fuel behaviour.