Use of Eutectic Na–Tl Coolant in a Modular Fast Reactor.

One of the main problems of fast reactors with sodium coolant is its high chemical activity in interaction with water and air. This requires complex safety systems, fire extinguishing, and diagnostics and complicates the design of steam generators and pipelines. The use of Na–Tl eutectic as a coolant for the first and intermediate circuits of a modular fast neutron reactor can alleviate this problem. Sodium–thallium eutectic (92.9%Na–7.1%Tl) has a much lower chemical activity than pure sodium and prevents or extinguishes its ignition by forming an inert surface layer. Also, this coolant has a higher boiling point and a lower melting point in comparison with sodium. The introduction of thallium into the coolant can change the neutron spectrum in the core and the neutronic characteristics of the reactor. In this paper, the effect of replacement of Na coolant with the Na–Tl eutectic on the neutronic characteristics of a modular fast reactor with metal fuel is studied. In addition, the influence of possible changes in the isotopic composition of thallium is studied. The activation of thallium isotopes in the coolant is simulated and their contribution to the gamma-radiation source is investigated. [ABSTRACT FROM AUTHOR]