Documentation of 37-Element, Nominal Pressure Tube, Coolant Mixing Model

Abstract

In this report a new subchannel mixing model that is based on an initial model from references [1,2] is documented. The model is intended for use in the fuel channel behaviour code, FACTAR. The new model predicts the steady state, single-phase (vapour) coolant mixing in 37-element bundles for nominal geometry pressure tubes only. The operating conditions on which the model is based are consistent with those found in the early and late blowdown stages following a large break loss of coolant accident (LBLOCA). It is assumed that coolant mixing is a process consisting of three mixing mechanisms; turbulent diffusion mixing, mixing due to diversion flow around appendages, and buoyancy induced mixing. The model makes use of subchannel mass flow fraction correlations developed from the output of the subchannel mixing code ASSERT PV V3.0. The model predicts the coolant mixing from the 1st bundle to the 12th bundle of a channel. The model implicitly predicts the quantity of heat that is lost to the moderator. The model makes use of mean cross-flow mass transfer correlations developed from the output of ASSERT PV V3.0 in order to improve prediction of coolant mixing under buoyancy dominated flows. The new model gives reasonably accurate predictions of subchannel mixing for both convection and buoyancy dominated flows, and also gives reasonably accurate predictions of heat lost to the moderator. The new model discussed in this report relies heavily on the use of the ASSERT PV V3.0 subchannel mixing code. There is some speculation as to the ability of ASSERT to properly predict coolant mixing under low flow conditions. For this reason, included in this report is a discussion of the applicability of ASSERT PV V3.0 to the prediction of subchannel mixing under low flow conditions. This discussion revolves around some informative input from the developers of ASSERT. Based on this input it has tentatively been concluded that within the boundary operating conditions and geometry specifications examined in this report, ASSERT results are reasonably trustworthy.