In a research reactor, the large pool water is at the heart of safety because a thermal design flow during a normal operation or a flow path for a natural circulation during an abnormal reactor trip shall be supplied for core cooling. Most research reactors include a reactor in the enormous pool, and are designed using an open type reactor and pool. Thus, the primary coolant containing many kinds of radio-nuclides can be mixed to the reactor pool. Because operators can operate near the pool top during a normal operation, the bulk flow toward the top of the open type pool should be designed to be minimized.