Fuel and Graphite Temperatures in a Micro Nuclear Reactor During a DLOFC

The Advanced Micro Reactor (AMR), a high-temperature gas-cooled prismatic block reactor, is being designed to produce 10 MW of thermal power. The active core consists of an inner graphite reflector, fuel graphite block assemblies arranged in three rings, and an outer graphite reflector (OR). The system code Flownex SE has been used to set up an axisymmetric network model of the reactor to study the thermal-hydraulic behavior of the reactor under steady-state conditions and during a depressurized loss-of–forced cooling (DLOFC) event.

The helium coolant enters the reactor at 320°C, flows up in the risers in the OR to the upper plenum, down through the core, and exists the lower plenum at 750°C. In the risers, the coolant is preheated to 324.8°C. Under steady-state conditions, the top of the core is on average 403°C cooler than the bottom of the core, and the maximum fuel and graphite temperatures are 1032.4°C and 809.0°C, respectively.

During the DLOFC, the reactor endeavors to heat up the upper part of the core, cool down the lower part of the core, and set up the required temperature gradient in the radial direction to remove the decay heat and the excess heat accumulated in the solids. When the DLOFC starts and the reactor is scrammed, the fuel temperatures drop steeply, along with the drop in power, until they are in equilibrium with the corresponding graphite temperatures, and then follow the graphite temperatures as the decay heat decreases and the graphite heats up or cools down.

This paper focuses among other things on the accumulation and release of heat by the fuel and solids during the DLOFC. The thermal behavior of the inner ring of the fuel block assemblies receives special attention.