Reactor, breeder temperature effects

Irradiation at high temperatures can lead to brittle fracture of stainless steels used as cladding in fast liquid metal breeder reactors. The effects of irradiation on metals is discussed in more detail in a later chapter of this module. 

One effect of an increase in temperature which is important enough to merit discussion as a separate topic is the reactivity change due to formation of voids in the coolant, either as part of the normal operating condition, as in the boiling water reactor, or under accident conditions in reactors such as the pressurized water reactor or the fast breeder reactor. The effects of void formation are similar to those of reduction in coolant density already considered, e.g., reduced moderation, giving increased leakage and resonance capture, and reduced absorption, leading to an increase in the thermal utilization, but the effects can be more dramatic on account of the more rapid possible variation of coolant density. 

Studies have been carried out at BNL on control requirements for an LMFR experiment. The control requirements depend not only on the choice of operating temperatures, the possible xenon and fission-product poisoning, etc., but also on conceivable emergency situations. such as errors in fuel concentration control. In a reactor with a full breeding blanket, the control requirements may have to include the effect of complete loss of the breeder fluid. 

The limitations of motallurgical knowledge at present lead to the conclusion that tantalum will be one of the be.st container materials for those plutonium alloys. The high-temperature strength properties and the heat-transfer properties of tantalum are excellent moreover, it is weldable. The parasitic capture cross section of tantalum would be intolerable in an epithermal or thermal power breeder reactor and, although relatively large in a fast spectrum, its effect on neutron economy in a fast reactor can be made small, if not minor, by careful design. 

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