Investigating the effect of the miniature reactor cooling spiral on increasing the reactor operating time

The Miniature Neutron Source Reactor is a tank-in-pool research reactor that uses highly-enriched uranium as fuel, light water as coolant, and beryllium as reflector. During the operation of the reactor, the coolant temperature increases and leads to negative feedback in the reactor. Although the negative reactivity is amongst the advantages of this reactor and an inherent safety feature; it causes a decrease in both the additional available reactivity and the reactor operation time. Therefore, short operation time (about 2.5 hours) at nominal power is one of the main limitations of the MNSR reactor. The natural convection in the core, vessel and pool of the reactor was modeled through a CFD analysis and the effect of the cooling coil at the top of the reactor tank on increasing the reactor operation time was investigated. To reduce the computations the details were decreased by considering the reactor core as a porous medium and a heat source with a constant power of 30kw. The experimental values ​​and those ​​obtained from the numerical solution are in good agreement. Results show a steady upward slope in the temperature rise of the coolant in the absence of coil, and an about-2-hours rise of the temperature in its presence. After this 2-hours period, the increasing rate decreases and the temperature fluctuates in a certain range. Compared to the case without the cooling coil, the average temperature is reduced by 3 degrees and the reactor operation time is increased by 1.5 hours.