Subchannel modeling of single rod bowing in a bundle geometry

During operation of a nuclear power plant, nuclear fuel assemblies in a reactor core are in strong temperature and radiation fields. With fuel burnup and the presence of thermo-mechanical stresses, gaps between fuel assemblies start to increase, leading to permanent fuel assembly bowing. The importance of fuel assembly bowing has been emphasised in the past for various reasons, one of which is an additional correction factor in models predicting the departure from the nucleate boiling ratio (DNBR). Before modelling bowing of entire fuel assembly rod bundle, it is important to fully understand the case of single rod bow first. In this study, an assessment of subchannel code CTF v3.5 in addressing the effect of rod bowing on DNBR is performed. Two well-known approaches for the critical heat flux (CHF) prediction are used: the W3 correlation and Groeneveld look-up table. Four different rod bowing geometries in a 4 × 4 rod bundle from the CHF data bank (Reddy and Fighetti, Reddy and Fighetti) are modelled: rod bowing with a gap closure of 50% and 85% of the nominal gap, and two configurations of rod bowing to contact. Comparison of calculated critical power resulted in an agreement of ±10% with measured values for the straight rod bundle and partial rod bowing configurations. Results for the rod bowing to contact tests are less in agreement with the experimental values. An explanation of CHF reduction due to rod contact is suggested. It is related to the change in affected subchannel coolant flow distribution and heat transfer from the heated rods to the coolant. This conclusion is also supplemented by a detailed CFD simulation of a two-rod model. Illustrative calculation of internal rod power, obtained with neutron transport code SERPENT, has shown that rod bowing leads to internal rod power redistribution as such that it reduces high local wall temperature at the contact point.