Pressure-tube and calandria-tube deformation following a single-channel blockage event in ACR-700

One postulated accident scenario for the Advanced CANDU Reactor (ACR-700™) is the complete coolant flow blockage of a single pressure-tube (PT). The flow is not monitored within each individual PT, thus during the early stages of this accident the reactor remains at full power and full pressure, resulting in rapid coolant boil-off and fuel overheating. Melting of the Zircaloy (Zry) components of the fuel bundle can occur, with relocation of some molten material to the bottom of the PT, which may cause failure of the PT and/or the calandria-tube (CT). We analyzed several key phenomena occurring after the blockage, including coolant boil-off, cladding heat-up and melting, dripping of molten Zircaloy (Zry) from the fuel pin, thermal interaction between the molten Zry and the PT, localized bulging of the PT, and interaction of the bulged PT with the CT. The main findings of the study are as follows: (1) Most coolant boils off within 3 s of accident initiation. (2) The Zry cladding starts to melt between 7 and 10 s after accident initiation. (3) The very high heat-up rate typical of the flow blockage accident sequence ensures that the molten Zry would drip to the bottom of the PT. (4) After contacting the molten Zry, the PT and CT bulge out radially under the effect of the reactor pressure. (5) PT/CT failure occurs only if the postulated mass of molten Zry in contact with the PT is sufficiently large, i.e., >100 g. The characteristic time scales for this 100-g case are as follows: - PT bulging starts within 3 s of Zry/PT contact; - PT makes contact with the CT in another 3 s; - CT bulging starts in approximately 1 s; - CT failure occurs within another 6 s. Thus, the duration of the PT/CT deformation transient is 13 s, which gives a total duration of the accident (from PT blockage to PT/CT failure) of 20–23 s. The relatively simple models developed in this study and the estimates generated with these models provide a solid physical framework for the key phenomena in the single-channel flow blockage event in ACR-700. As such, they can also assist in the interpretation and verification of future analyses of this event conducted with more sophisticated codes and tools.