Transient non-boiling heat transfer in a fuel rod bundle during accidental power excursions

One important aspect in analysis of the unprotected transient overpower accident is the prediction of time and location of fuel pin failure. The accuracy of such predictions decisively depends of the adequate modeling of the heat transfer processes involved. The present investigation focusses on the fundamental heat transfer aspects in the first phase of the power-excursion transient before boiling of the coolant starts, by solving the four coupled three-dimensional time-dependent energy equations for the fuel rod (fuel, gap, clad) and the coolant. The analysis surpasses similar previous studies, e.g. by Gopalakrishnan, in several important aspects. Most notable is that the geometry of the fuel rod bundle is correctly described in its three dimensions and the turbulent energy transport within the coolant is modeled more realistic and in a consistent manner over the entire Prandtl number range of interest using results from a specially tailored phenomenological turbulence model. The physical problem studied is the transient non-boiling heat transfer of a cylindrical fuel rod consisting of fuel, gap, and cladding to a steady, fully developed turbulent flow. A digital computer program has been developed, on the basis of finite-difference techniques, to solve the governing partial differential equations with their associated subsidiary conditions. (Auth.)