Simulation of the Lower Head Boiling Water Reactor Vessel in a Severe Accident

Table 1

Main events of the accident sequence during the simulation of a large LOCA with simultaneous loss off-site power and failure of the ECCS.

Time (s)	Event
0	Reactor is operating at steady state condition at 100% of power
20	Guillotine rupture in the recirculation line and loss of AC Power
	Scram of the reactor
	Trip of all feedwater pumps
	No injection of the ECCS is assumed
21	Vessel water level reaches level 4 (low level)
30	Feedwater flow decreases to zero
22	Vessel water level reaches level 3 (low level)
24	Vessel water level reaches low-low water level (level 2). Main steam isolation valves (MSIV) close
27	Vessel water level reaches low-low-low water level (level 1). Second signal to start LPCI and LPCS due to low water level, but all the ECCSs are considered out of order
510	The meltdown begins when the region of the core exceeds the temperature for eutectic melting of stainless steel clad control rods with B4C (boron carbide) absorber material. This temperature is about 1,500 K
682	Some drops of corium fall in the lower region of the core and are quenched
1570	An important amount of molten corium starts to form the molten pool
2300	The reactor core reaches the highest temperature (3000 K, fuel bundle 323, axial node 1). The radius of the molten pool is 1.22 m. Volume of molten pool is 3.81 m^3
2320	The most important amount of melt material slump in the lower head of the vessel when the crust fails, this is observed in the highness of the bed debris that increases 0.40 m in about 100 s. The radius of the molten pool is 0.82 m, and its volume is one third lower.
2590	Volume of molten pool is 1.16 m^3 with a temperature of 2873 K (fuel bundle 322, axial node 10). The total mass of UO2 in the pool is 8770 kg. The code crashed before RPV failure