Assessment of Fission Product Release Following Stagnation Feeder Break in CANDU 6

The objective of the fuel analysis is to estimate the quantity and timing of a fission product release from the fuels when a postulated single channel accident occurs in CANDU 6 reactors. Fission product release calculations consist of three parts, (i) the fission product inventory in the channel is estimated using the fuel performance evaluation code, (ii) based on the fuel temperatures following the accident, the fractional release of the different chemical species is estimated, and (iii) the release of different isotopes in the channel is determined by multiplying the fractional releases by the inventories. In this study, a calculation of fission product releases following a stagnation feeder break accident was carried out and the results were compared to the case of Wolsong 2/3/4. Feeder break is a single channel accident when the other channels remain intact in the CANDU core. For some ranges of a feeder break size, a flow in the channel can be stagnated due to a force balance between the upstream side and the downstream end. In the extreme, this can lead to a rapid fuel heat up and fuel damage, and the failure of a fuel channel. In this case, radionuclide can be released directly to the containment through the feeder break and to the moderator through the failed channel. This break scenario is called a stagnation feeder break. A stagnation feeder break can be postulated to occur in any of 380 channels in the reactor at any time during the reactor's operating life. Because of this, a stagnation feeder break is assumed to occur in the high-powered 'limiting' channel for the conservative safety assessment. This limiting channel has a channel power of 7.3 MW and the two central bundles at 935 kW. Here, 7.3 MW and 935 kW are the LCO (Limiting Condition for Operation) power values for a fuel channel and a fuel bundle, respectively