Pressure Tube / Calandria Tube Post-Contact Necking Behaviour in CANDU Reactors

Abstract of the technical presentation presented at:
7th International Workshop on CANDU Safety Association for Sustainability (CANSAS–2018)
October 15‒18, 2018

Prepared by:
M. El-Hawary and N. Mesmous
Canadian Nuclear Safety Commission

Abstract:

In CANDU reactors, fuel is located inside pressure tubes (PT) surrounded by low-pressure calandria tubes (CT) submerged in relatively cold moderator fluid. The moderator acts as a heat sink to prevent the fuel channel (PT&CT) from failing under accident conditions when an overheated PT balloons into contact with a CT.

Contact-boiling experiments were conducted to determine the limiting reactor conditions leading to fuel channel failure. Experiments were conducted with full-size PT&CT and with a graphite heater representing reactor fuel. For test conditions leading to channel failure, rupture(s) occurred at random locations on the tube surface, with necking clearly visible at these locations. This occurred even though the test conditions were uniform in the axial direction.

The mathematical model developed to simulate the post-contact phenomenon includes several heat transfer and creep models, and correlations. Here are two observations about these correlations:

• The CT creep correlation includes a term representing the necking behaviour. The model is therefore capable of predicting the accelerated creep rate prior to the failure known as necking.
• The model correlations are mostly non-linear, leading to a highly non-linear model. The non-linear behaviour manifests itself as cliff edges predicted under certain reactor conditions. Under these conditions, large increases in CT creep can be predicted for small variations in the reactor independent parameters of heat flux and moderator subcooling.

It is the objective of this paper to provide high-level information on the necking phenomenon from the perspective of material science, and to show how necking can be distinguished from the cliff edge predicted by the model. Both transients are similar in producing sudden increases in the CT creep under certain conditions.

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