Assessment of creep-fatigue damage at main vessel triple point of Prototype Fast Breeder Reactor after core disruptive accident

Abstract

The main vessel (MV) carries all the major reactor components including core and liquid sodium filled to 12.4 m height. The reactor core along with the inner vessel is placed on the grid plate, which is supported on the core support structure (CSS). The CSS is supported on the MV through a support shell, welded to the dished end at the point where crown and knuckle joins. The CSS support shell to MV joint is called as triple point and damage at this point is of concern. The core catcher (CC) is placed below the CSS and is also supported on the CSS support shell. The triple point is subjected to a dead load of 920 t. During the unlikely event of core disruptive accident (CDA) a high dynamic pressure will act on the core catcher (CC), which in turn exerts a high load on the triple point. Also the hot molten core debris will come to the CC, which is close to the MV dished end. This will result in increase of temperature in the cold pool, which will cause significant creep damage at triple point during post accident heat removal phase. As the triple point is the only junction, which is supporting the reactor core and CC through CSS support shell, its failure can cause serious consequences to the integrity of MV. Hence, triple point is analysed to ensure its integrity during the transient pressure loading and subsequent post accident heat removal (PAHR) phase of CDA. The stress intensities during transient pressure are found to be within the limits. The subsequent creep damage during post accident heat removal (PAHR) phase is also assessed. Based on the apportioned creep damage of 0.4 for category 4 events, the triple point can be kept at a temperature of 923 K (650°C) or 973 K (700°C) effectively for a period of 1.2 × 104 h (500 days) or 662 h (27.6 days) respectively which are acceptable with comfortable margin. More details are presented in the paper.