Analysis of the Processes in Spent Fuel Pools in Case of Loss of Heat Removal due to Water Leakage

Table 4

Sequence of events, in the case of water leakage from spent fuel pools of Ignalina NPP.

Time ATHLET-CD	Time ASTEC	Time RELAP/SCDAPSIM	Events	Comments
0	0	0	Start of water leakage from the spent fuel pools	(1)
60,000 s (16.7 h)	60,000 s	60,000 s	The uncovering of fuel starts. Start of fuel heat up process	(2)
210,000 s (58.3 h)	240,000 s	220,000 s	The peak temperature of fuel reaches 650–700°C. Steam-zirconium reaction starts	(3)
215,000 s (59.7 h)	215,000 s	215,000 s	All SFAs are uncovered
235,000 s (65.3 h)	235,000 s	215,000 s	The water leakage is terminated	(4)
300,000 s (83.3 h)	300,000 s	300,000 s	Water injection starts	(5)
315000 s (87.5 h)	315000 s	305000 s	Water level increase up to the bottom part of SFA. Start of fast steam generation and fast exothermic oxidation	(6)
325,000 s (90.2 h)	317500 s	318000 s	The peak temperature of fuel in central part reaches 1500°C. Melting of stainless steel (distancing grids). Damages of fuel rods	(7), (8), (9)
333,000 s (92.5 h)	319000 s	325000 s	The peak temperature of fuel in central part reaches maximum	(10)
441000 s (122.5 h)	440000 s	417,000 s	All SFA are flooded by water	(11)

Comments:
(1) It is that assumed constant water leakage flow rate is 21.1 kg/s.
(2) The uncovering of fuel leads to increase in gamma radiation.
(3) Due to overheating of uncovered fuel assemblies, the oxidation of zirconium and hydrogen generation begins.
(4) Modelling assumption: small part of water at the bottom of the pool still remains.
(5) Water injection flow rate 27.8 kg/s is assumed taking into account the capacity of water makeup system in Ignalina NPP.
(6) Fast steam generation starts due to contact of water with hot bottom part of SFAs. Start of fast steam-zirconium reaction.
(7) Distancing grids at the ends of the SFA produced from the zirconium and niobium alloy; therefore melting of stainless steel distancing grids in central part does not lead to fuel rods dropping from SFA. Release of radioactive fission product starts.
(8) Zirconium and stainless steel grids in the SFAs are not modelled.
(9) Due to overheating of claddings in the long term the embrittlement and failure of claddings should appear earlier, comparing to ATHLET-CD calculations.
(10) Relocation of melted claddings starts, after reaching 1900°C temperature.
(11) Temperature of melted material and all SFA decreases below 100°C.