Understanding CANDU Fuel Bowing in Dryout: An Industry Approach

Abstract

Fuel element bow induced by dryout could potentially perturb the coolant flow distribution and heat transfer from the fuel element to the coolant. Some accident scenarios leading to dryout of the fuel element are: loss of power regulation, pump trip, pump seizure, small and large break loss of coolant accidents. In these accidents, it is desirable to show with confidence that the fuel remains sufficiently cooled to maintain its geometry, even if it is in dryout. This can be demonstrated if fuel elements are separated from each other and from the pressure tube, with a sufficient (and stable) gap. Therefore, the prediction of the amount of bow, and its effect on heat transfer conditions is required for the assessments.

The utilities have joined force in launching an experimental investigation at Stern Laboratories to characterize the bowing phenomena.

This program will investigate the amount of deflection, transient and permanent, that results from accident conditions which cause a dry patch on one side of the sheath. This is expected to bound the consequences of fuel bowing due to dryout. Since the accident transients begin at full power and high coolant pressure (about 10 MPa) they generate sharp thermal gradients (dry patch) and it is necessary to develop a simulation with representative dry fuel sheath conditions initiated from a normal full power and coolant state.

The amount of bow is driven by thermal gradients in both the fuel pellets and the sheath, therefore, the thermal gradients should be representative. This program is structured in a series of tests progressing from simple representation to complex simulation. It is divided into 3 experimental phases:

Phase 1 - Thermalhydraulic simulation of fuel element bow by a heated tube.

Phase 2 - Thermal and mechanical bow with a simulator which accounts for pellet / fuel sheath interaction with internal pellet temperature distributions,

Phase 3 - Fuel element bow with a simulator using Zircaloy-4 fuel sheath and internal heater with UO₂ fuel pellets.

This paper describes the strategy of the program and presents some of the test results from Phase 1.