This paper evaluates the so-called weak heat conduction mechanism, i.e., the heat transfer from heated-up fuel channels in the radial direction to cooled channels through the adjacent graphite columns in the RBMK-1500 reactor. The influence of this mechanism on the calculation results for a long-term loss-of-coolant accident is investigated.

Two possibilities for modeling the heat transfer in the radial direction, between adjacent graphite columns, using the system code RELAP5 are presented: (a) employing the interstructure heat conduction model built into the RELAP5-3D code and (b) employing the model of the reactor gas circuit, which supplies a mixture of gases into the reactor cavity.

Both means allow one to predict the localized heatup in the RBMK core. However, the modeling of the reactor gas circuit in parallel with the reactor cooling circuit decreases the calculation time-step size quite significantly. The analysis results demonstrate the capability of the RELAP5-3D code to model heat conduction in the radial direction between different heat structures much more easily and a hundred times faster.