Consequences of CANDU severe accidents initiated by steam generator tube ruptures

Source term release characteristics are analyzed for a SGTR (Steam Generator Tube Rupture) sequence for a CANDU type reactor in a multi-unit station of negative-pressure containment design. The postulated initiating event of this sequence involves multiple steam generator tube failures. The reactor is tripped manually but failure of emergency coolant injection and moderator cooling is postulated, leading to severe tore damage. The Safety Relief Valves (SRVs) are activated and at least one is postulated to fail to close and remains open throughout the accident because the main interest in this sequence is its potential pathway for direct discharge of radioactivity to the environment. The analysis was performed using MAAP-CANDU version 4.0.4A+. In the case where the SRV is stuck open the maximum release of CsI is approximately 0.72% of core inventory. This analysis indicates that the potential releases to the environment from a severe accident initiated by a steam generator tube rupture are relatively small for the multiple unit CANDU design. A comparison is made against published results for a Korean CANDU 6 unit and some of the key differences are discussed. (author)