Fuel composition stabilization in a subcritical liquid-salt transplutonium burner reactor

Abstract

Possible replenishment schemes ensuring the attainment and maintenance of an equilibrium fuel composition are studied for a subcritical liquid-salt transplutonium burner reactor now under development. The computational model describes fuel burnup in a two-component approximation singling out the transplutonium actinides and the remaining actinide fractions consisting mainly of plutonium. The analysis is limited to instantaneous replenishment control, directly proportional to the criticality and/or fuel composition unbalance. The case of small fuel composition unbalance, allowing linearization of the burnup equations, and the general case where Lyapunov functions are used for stability analysis are studied. It is shown that criticality unbalance control using the transplutonium actinide fraction does not secure fuel-composition stability. Control using the plutonium fraction does secure stability.The possibility of burning transplutonium elements in specialized reactors is now being carefully studied for the purpose of utilizing these elements. One such reactor is a liquid-salt subcritical reactor with an external source of neutrons. This reactor is an elaboration of the reactor in [1]. It has the classical scheme, for subcritical systems, with a target surrounded by the core. A lead-bismuth eutectic in which protons from an accelerator generate the primary neutrons serves as the target. The core is a 174 cm high cylinder with radius 94 cm, filled with salt in which 10.7 tons fuel is dissolved. In the working state with accelerator current 10 mA, the thermal power at subcriticality 0.01 reaches 1650 MW(t), which makes it possible to burn ~520 kg transplutonium elements per year.A considerable advantage of the reactor from the neutron-physical standpoint is the fast-intermediate energy spectrum of the neutrons, which is due to the use of the salt LiF + NaF + KF with a high content of dissolved actinides. Compared with a thermal spectrum, such a spectrum decreases the parasitic absorption on the elements of the solvent salt and the fission products and promotes direct fission of 241 Am, the predominant transplutonium element. As a result, transplutonium elements can be burned autonomously without using any additional seed material in the form of plutonium or high-enrichment uranium. For clarity, the burning of transplutonium elements in a reactor can be described as follows: 241 Am fed into the reactor partially decays and as a result of the radiative capture of a neutron and several subsequent decays partially transmutes into 238 Pu whose fission completes the transmutation. In the process, isotopes and other actinides ranging from uranium to curium, which are also present in the reactor fuel, are formed.In the present work, stable replenishment of the reactor ensuring that the fuel reaches and is stabilized in an equilibrium state is studied on a qualitative level.Two possible variants for determining fuel deviations from an equilibrium concentration were studied -indirect determination based on reactor criticalit...