Effect of reactor vessel cooling insulation and reflector heat pipes on the temperatures of a pebble-bed reactor using a system CFD approach

Abstract

This study investigates firstly, the effects of varying insulation thickness and flow path width between the core barrel and RCCS-concrete pressure vessel of an upward-flow helium-cooled pebble bed HTGR. The effects on component temperatures, parasitic heat loss as well as accident condition temperatures are determined. Secondly, the effect of reflector heat pipes for passive heat removal during transient events is also investigated. The thermo-fluid performance of the reactor with these modifications is investigated using the system code Flownex. A 2D-axisymmetric model of the reactor is used that accounts for the core, bottom, side and top reflectors, core barrel, reactor cavity cooling system (RCCS) and the concrete vessel.

Changing the thickness of the insulation has a significant effect on the heat removal by the RCCS, as well as the temperatures of the core barrel and reflectors, but a negligible effect on the temperatures of the core. In the selected examples the heat pipes have a marked influence on the temperatures of the core, reflectors and core barrel. While the maximum temperatures reached by the core during a loss-of-cooling event are not affected by the heat pipes, the time-temperature exposure of fuel is reduced. This is significant because it provides an additional method to potentially reduce fission product release under accident conditions