Development and Benchmarking of Mechanistic Channel Deformation Models in RELAP/SCDAPSIM/MOD3.6 for CANDU Severe Accident Analysis

In different stages of postulated severe accidents in CANDU reactors, the fuel channels may experience a series of thermomechanical deformations, some of which may have significant impacts on accident progression; however, they have not been mechanistically modeled by integrated severe accident codes such as MAAP-CANDU and SCDAP/RELAP5. This paper focuses on the development and benchmarking of mechanistic models for pressure tube (PT) ballooning and sagging phenomena during the fuel channel heatup phase as well as for the sagging of fuel channel assemblies during the core disassembly phase. These models, which are based on existing phenomena in literature, are coupled with RELAP5 and/or integrated into RELAP/SCDAPSIM/MOD3.6 as new SCDAP subroutines to provide more robust treatment of the deformation phases of severe accidents.

The ballooning of a PT will lead to contact with its calandria tube (CT) and occurs during conditions where the coolant pressure is moderately high. At initial contact the high contact thermal conductance and the large temperature difference between the two tubes result in a large transient heat flux that challenges the channel integrity through potential film boiling on the outer calandria surface if moderator subcooling is low. A one-dimensional ballooning and contact model (BALLON) has been developed. BALLON calculates the ballooning-driven transverse strain of PT and CT and modifies the effective conductivity of the annulus before and after contact.

Pressure tube sagging is the dominant deformation mechanism at low pressures and occurs at relatively high PT temperatures. A model based on simple beam theory (SAGPT) has been developed. SAGPT calculates the longitudinal strain and the deflection of PT, and it also determines PT-to-CT sagging contact. The sagging and disassembly of the entire fuel channel assembly occur when the fuel channels are uncovered and the moderator heat sink is lost; thus, the entire PT-CT assembly sags together, possibly contacting channels at lower elevations. A model named SAGCH is created to track fuel channel assembly sagging after moderator boil off and also determines the extent of channel-to-channel contact, channel disassembly, suspended debris bed characteristics, and eventual core collapse.

This paper presents detailed descriptions of the models, the coupling schemes, and their benchmark against experiments, together with an extensive review of relevant studies in the literature.