Analysis of mechanical and thermal consequences of core disruptive accident: Approach for current and future SFRs

For the 500 MWe mixed oxide fuelled Prototype Fast Breeder Reactor (PFBR), which is under construction in India, Unprotected Transient Over Power Accident (UTOPA) is found to be not energetic, i.e. it does not have potential to release the mechanical energy. However, during the pre-disassembly phase, the main vessel temperature rises to 767 K and the hot pool temperature is 960 K. Unprotected Loss of Flow Accident (LOFA) is however, energetic. During pre-disassembly phase, the temperature of main vessel remains at 683 K and the hot pool temperature rises to 853 K. By providing high safety margin, a Core Disruptive Accident (CDA) was defined as beyond design basis event which has a work potential of 100 MJ. The mechanical and thermal consequences of pre-disassembly and CDA phases are analysed in detail. The analysis includes determination of mechanical straining of main vessel and its internals, impact of sodium slug on the bottom of the top shield, sodium release to reactor containment building (RCB), design loads and analysis for core catcher. Mechanical analysis is carried out using the finite element code called 'FUSTIN' which uses Arbitrary Lagrangian Eulerian Co-ordinate system for describing fluid motions and convected co-ordinate system for modeling the geometrical non-linearity of structures. Further to overcome the limitations of the algorithm to handle the complicated core boundary movements with the presence of reactor internals, particularly, the control plug is a 'two phase fluid element' is developed and implemented in the code. With this element, the core bubble boundary nodes need not be purely Lagrangian. For the thermal consequences, commercial codes are employed. For ensuring the structural integrity of decay heat removal exchangers and intermediate heat exchangers, mockup tests were conducted with 1/13th scaled down models. Based on the numerical and experimental investigations, safety criteria specified for PFBR were respected with comfortable margin. Based on the PFBR analysis and further through introduction of innovative shutdown systems and decay heat removal system, it is proposed to consider CDA as non-energetic for the future SFRs. However, thermal consequences would be investigated comprehensively