The Radiant Heat Transfer in the High Temperature Reactor Core After Failure of the Afterheat Removal Systems

In the case of hypothetical accidents, temperatures of ∼2000 to 3000°C are expected in the core of a pebble-bed high temperature reactor (HTR). At such high temperatures the transport of heat by radiation is the most important mechanism. For the calculations of temperature pattern in the reactor core, the effective thermal conductivity λ_eff of the pebble bed must be known. Two models predicting λ_eff are represented. They are the cell model of Zehner and Schluender and the modified radiation model of Vortmeyer, which has been extended to high temperatures. A transient measurement method was used to determine the effective thermal conductivity of pebble beds of graphite and of zirconium oxide at temperatures up to 1000 and 1500°C, respectively. The theoretical λ_eff values are compared with experimental results. The theoretical values of λ_eff predicted by the Zehner-Schluender formula are too low, while the λ_eff values of the modified radiation model are somewhat too high. Corrections to both formulas were made. Finally, it is demonstrated which values of λ_eff are predicted by high temperatures in the pebble bed of the HTR.