Dynamics and Control of Molten-Salt Breeder Reactor - 2017

Nuclear Power Industry

Singh, and three co-authors, reported in 2017 that preliminary results of the dynamic analysis of a two-fluid molten-salt breeder reactor (MSBR) system were investigated. Based on an earlier work on the preliminary dynamic model of the concept, the model presented here is nonlinear and has been revised to accurately reflect the design exemplified in ORNL4528.

A brief overview of the model followed by results from simulations performed to validate the model is presented. Simulations illustrate stable behavior of the reactor dynamics and temperature feedback effects to reactivity excursions. Stable and smooth changes at various nodal temperatures are also observed. Control strategies for molten-salt reactor operation are discussed, followed by an illustration of the open-loop load-following capability of the molten-salt breeder reactor system. It is observed that the molten-salt breeder reactor system exhibits “self-regulating” behavior, minimizing the need for external controller action for load-following maneuvers.

The authors concluded that an enhanced nonlinear dynamic model for a two-fluid MSBR plant has been developed. The nodalized model accurately represents the reference design presented in the work of Robertson et al.. The model structure has been extended to the MSRE, and a companion paper comparing results of simulation with experimental data is also in preparation.

The simulation of dynamic response to reactivity perturbations illustrates the stability of the two-fluid model. Upon positive reactivity insertion, the model shows a prompt jump in power and the corresponding neutron density, as expected. The prompt jump is followed by an exponential fall to nominal power. Local peaks are seen on the otherwise smooth curve due to the introduction of delayed neutrons by the circulating fuel. Some higher-order peaks are also seen. A corresponding change in the feedback reactivity to compensate for the step insertion is also observed. Similar stability is observed in the case of periodic reactivity excursions.

A major observation is the open-loop load-following capability of the MSBR system to changes in demand power. The natural temperature feedback of the fuel salt and graphite (and to a smaller extent the fertile salt) to changes in power extraction at the ultimate heat sink seems to be sufficient to control the MSBR system. The resulting reactivity change compensates for the change in demand power and the reactor power is observed to change in a stable fashion. This self-regulation can be leveraged in well-designed MSBR systems to be used as on-demand and grid-stabilizing power generators in an economy that increasingly favors solar and wind, which are clean but intermittent power sources. Furthermore, temperatures in the salt-carrying sections of the plant are observed to be varying around an average.

This means that the temperature of the hottest node decreases while the temperature of the coldest node increases during a load-following maneuver. Thus, salts in the various parts of the MSBR system do not freeze during load-following maneuvers leading to safe and stable operation. While these results are only accurate to a certain degree, it is remarkable that despite having a lower delayed neutron fraction than pure U-235, and further losses in delayed
neutrons due to circulation, the MSBR system still displays a high degree of stability. Even more impressive is the reactor’s self-regulation in response to changes in demand power. This latter characteristic is an emergent behavior of the total feedback of this reactor system. It can be leveraged to engineer a truly “walk away” safe next-generation reactor.