An analysis on the steam generator tube rupture events with core damage

Abstract

This paper investigates risk significance of Steam Generator Tube Rupture (SGTR) events accompanying a core damage and a direct release of radionuclides into the atmosphere bypassing the containment. Analyses on a spontaneous steam generator tube rupture (S-SGTR) event leading to a core damage and a consequential steam generator tube rupture (C-SGTR) event caused by a station black out (SBO) event are performed for APR1400 using MELCOR 2.2. Both events led to a direct radiological release to the atmosphere. The amount of radiological release was more than that of the Chernobyl and the Fukushima accident. Operator actions to mitigate the radiological releases are investigated. For the S-SGTR scenario, recovery of either auxiliary feed water (AFW) or safety injection alone was not able to prevent significant radiological releases. Combined action of opening of atmospheric dump valve (ADV) and supply of AFW to the steam generators (SGs) resulted in an aggressive depressurization of the primary system and an actuation of safety injection tank. Recovery of liquid inventory and minimizing the U-tube leakage prevented core damage. For C-SGTR scenario, delayed operator action until the time of U-tube creep rupture was not able to prevent significant radiological releases. C-SGTR had to be prevented by a supply of AFW to SGs or rapidly depressurizing the reactor coolant system before an occurrence of U-tube creep rupture.