Evaluation of spent fuel pool temperature and water level during SBO

The objective of the present study is to evaluate water level of a spent fuel pool during a station blackout (SBO) event which was really caused in the Fukushima-Daiichi (Fukushima-I) accident. The water level during the event can be calculated using a computer code based on the inventory of water in the spent fuel pool and decay heat of spent fuel assemblies. However, a calculation model should be prepared and a longer CPU time is required to obtain the result. If the water level change can be calculated by a hand calculation, it is fast and convenient to obtain the result. Therefore, the calculation results in terms of timings of saturated condition and boil-off are compared to the hand calculation results. It has been shown that the hand calculation results about the saturated and boil-off timings have good agreement with the calculated results using the RELAP5-3D code. The calculation model using the RELAP5-3D is verified using the water level data measured during the Fukushima-I accident. The spent fuel has a dryout phenomenon after the collapsed water level decreases below the top of the active fuel. The calculation model is verified using the measured data under an atmospheric pressure. The code can trace the collapsed water level to cause dryout at the top of the fuel assembly.