CANDU-Specific Severe Core Damage Accident Experiments in Support of Level 2 PSA

This paper describes progress made in CANDU severe core damage accident experiments conducted with a single channel of one-fifth scale containing twelve simulated fuel bundles in an inert atmosphere. The geometry of the reference fuel channel considered in this study is a pressure tube-calandria tube composite, with the pressure tube ballooned into circumferential contact with the calandria tube. The tests are conducted to develop a better understanding of the behaviour of a CANDU fuel channel array when the CANDU reactor core undergoes a postulated, low probability severe core damage accident. The data are required to ensure that the core damage models used in MAAP CANDU code are adequate. MAAP CANDU code is used to calculate the progression of a severe core damage accident in a CANDU reactor to support Level 2 Probabilistic Safety Assessment (PSA) activities. Experimental results from this study showed the development of time-dependent sag when the reference channel temperature exceeds 800C, thus suggesting that creep is the mode of channel deformation. Significant localized wall thinning and high longitudinal strain localization were observed at the bundle junctions along the bottom-side of the channel, at a distance one to two bundles away from the channel mid-point. Thus the debris formed by this mechanism is expected to be two to three bundles long. A creep-assisted deformation mechanism is proposed to support the findings. The experiments showed that the end-load applied by the channel during heat-up and during cool-down is small. Scaling up the data showed that the forces are not sufficient to pull a full-size CANDU channel out from the end-fitting. Therefore, it may be possible to pump coolant into the channel for accident management, without inducing failure of the channel during the early stages of the sag phenomena prior to the development of significant wall-thinning at the bundle junctions.