A General Approach to Nuclear Fission Sustainability and the Need for Specific Solutions. A Case Study on a New Coolant

Extensive exploitation of nuclear raw materials requires the use of “nuclear breeding”, which is a phenomenon that can be attained in fast reactors. However, those reactors have had a complex history with some drawbacks and some important nuclear-policy attacks, as the INFCE initiative launched inside IAEA in 1978. Two points were very relevant in that context: the extensive use of plutonium recycling and an inherent property of fast reactors that could induce positive feedback between reactivity and thermal-hydraulics. In fact, a partial or total loss of coolant could convey a tremendous injection of reactivity, which could produce a catastrophe. An alternative to breeding in critical fast reactors is presented by hybrids, which are subcritical reactors which need an external neutron source for keeping their neutron population alive. Besides that, design and natural responses of the reactor systems against accidental initiating events have to be considered for arriving to the concept of Residual Safety beyond Design Limits. Such a final safety level will depend quite a lot on the type of coolant and the way the fuel is conformed into elements of a given geometry.