Void Reactivity Effect in the MTIR-SKD Multipurpose Test Research Reactor

The problem of the positive void reactivity effect is considered for reactors operating in the fast neutron spectrum, in particular, for the VVER-SKD reactor. The relevance of the study is explained by the need to ensure the safety of these reactors under the conditions of out-of-design accidents. The study is focused on the MTIR-SKD reactor application as an experimental platform for analysis of the void reactivity effect. It is shown that void reactivity effects with the opposite signs can be obtained by changing the composition of the fuel and reflectors. It is expected that a negative void reactivity effect will be implemented to ensure safety at the first stage of work with the reactor, and it may be changed later to the positive one. In the first part of the work, the neutron balances have been calculated, which reveal the mechanisms of the void reactivity effect formation. It is shown that a steel reflector and the plutonium-240 isotope significantly affect the positive void reactivity effect. Options to achieve negative and zero void reactivity effects are discussed, whereas the positive effect can be achieved using high-background plutonium, which eliminates the risk of uncontrolled reactor runaway. The results can be used as a basis for justifying safe operation of the VVER-SKD reactor with a positive void reactivity effect. This work has both scientific and practical significance for the development and design of safe new-generation nuclear reactors that provide reliable reactivity control under various operating conditions.