Recoupling and decoupling of parallel loops in simulated PIUS-type reactor shutdown and restart transients

The shutdown and restart transients in the PIUS-type reactor are simulated experimentally and analytically. A half-height, 1/2,400 volumetrically-scaled low-pressure facility was used for experiments and a one-dimensional model was employed for analysis. The passive shutdown of the PIUS-type reactor is achieved by terminating the pumped circulation of reactor coolant through the primary loops and thereby allowing a natural circulation between the primary-loop riser section and a borated cold water pool. It is shown that the natural circulation grows quickly owing to a dynamic force caused by the riser-section fluid deceleration following the pump trip. The restart of the reactor, on the other hand, requires the primary loops to be decoupled from the borated pool. The reactor restart can be achieved by simply restarting the coolant pump and then controlling the pump speed. The natural circulation between the primary-loop riser section and the reactor pool is terminated quickly owing to a dynamic force caused by the riser-section fluid acceleration following the pump restart. (author)