Studies of the deteriorated turbulent heat transfer regime for the gas-cooled fast reactor decay heat removal system

Increased reliance on passive emergency cooling using natural circulation of gas at elevated pressure is one of the major goals for the gas-cooled fast reactor (GFR). Since GFR cores have high power density and low thermal inertia, the decay heat removal (DHR) in depressurization accidents is a key challenge. Furthermore, due to its high surface heat flux and low velocities under natural circulation in any post-LOCA scenario, three effects impair the capability of turbulent gas flow to remove heat from the GFR core, namely: (1) acceleration effect, (2) buoyancy effect and (3) property variation.This paper reviews previous work on heat transfer mechanisms and flow characteristics of the deteriorated turbulent heat transfer (DTHT) regime and decomposes governing non-dimensional groups to provide an insight for the GFR DHR system design. It is shown that by applying the developed methodology, the GFR's DHR system has a potential for operating in the DTHT regime and the gas DTHT regime is different from the liquid or super-critical fluid's DTHT regime. A description of an experimental facility designed and built to investigate the DTHT regime is provided together with the initial test results. The initial runs were performed in the forced convection regime to verify facility operation against well-established forced convection correlations. The results of the three runs at Reynolds numbers of 6700, 8000 and 12,800 show good agreement with the Gnielinski correlation for heat transfer, which is considered the best available correlation for forced convection over a wide range of Reynolds and Prandtl numbers. However, even in the forced convection regime, the effect of variation of the fluid properties was found to be significant.