Accident mitigation for spent fuel storage in the upper pool of a Mark III containment

• The upper pool is a possible choice to store spent fuels.
• The Kuosheng plant plans to store fuel discharged 15 years ago in the upper pool.
• The loss-of-coolant accident for the Kuosheng upper pool is analyzed using GOTHIC.
• The leakage, spray effect, and air ventilation are included in the model.
• The spray activation ensures the safety of the spent fuel in the upper pool.

The upper pool (UP) is a specific design of the Mark III containments. Basically, no fuel can be stored in the UP when the reactor criticality is achieved. The Kuosheng plant in Taiwan has two BWR/6 units with Mark III containments. The spent fuel storage capacity of this plant will become insufficient in coming years. Unfortunately, the license application for the dry cask storage has not been approved. Taiwan Power Company has a temporary solution to store spent fuel with cooling time of 15 years in the UP. Heat up mitigation for the UP by spray is also planned. In this study, the loss-of-coolant accident of the UP is analyzed using GOTHIC. The calculated results show that the pool spray can make up the UP inventory against small leakage. The spent fuel is sufficiently cooled if the spray mitigation maintains the pool level above the fuel. On the contrary, large leakage resulting in drainage of the entire pool allows airflow entering the fuel region to enhance the cooling effect. The case with the highest fuel cladding temperature occurs with a medium sized leakage because the partially uncovered fuels cannot be adequately cooled by either water or air. With 200-gpm spray, the calculated highest peak cladding temperature is 472.9 °C which is well below the threshold causing significant fission product release. The capability of the spray mitigation to maintain the pool level determines whether the spent fuel will be heated up. Based on the results, the activation of spray can ensure the thermal safety of the spent fuel in the UP during a loss-of-coolant accident.