Introduction of Transient Heat Transfer Experiment on Tube Flow for RIA Applications

Reactivity insertion events in power reactors can be divided broadly into control system failures, control rod ejections, events caused by coolant/moderator temperature and void effects, and events caused by dilution or removal of coolant/ moderator poison. Except the control rod ejections, most reactivity insertion events lead to relatively slow reactivity additions and they are considered as transients rather than accidents. However, the control rod ejection event is categorized to design basis accidents since it causes very rapid reactivity insertion. In pressurized water reactors (PWRs), the control rod ejection events can occur by mechanical failure of the control element assembly nozzle or control rod dive mechanism housing. In that cases, the coolant pressure ejects a control rod assembly completely out of the core. Interim acceptance criteria and guidance against the RIA are issued by NRC in 2007 and 2015. Unfortunately, those newly raised RIA safety criteria encroach previous safety margin due to the adoption of conservative evaluation method and high uncertainty in the analysis. Therefore a best estimate method is strongly required to secure the decreased safety margin by reducing the uncertainty in the RIA evaluations. However, there is a huge lack of knowledge related to the RIA up to now due to the difficulty and high costs for the RIA experiments. Especially, experimental studies on the thermal hydraulic phenomena are insufficient yet since it is difficult to realize the power excursion which simulates the RIA condition. In this paper, we introduce current status of the ongoing thermal hydraulic RIA experiments by KAERI. In these experiments, the transient heat transfer characteristics including boiling curves will be measured by adopting simple internal tube flow under simulated RIA conditions. Current experimental designs and major specifications of the test loop under construction are presented and the experimental plan is exhibited.