Thermal-Hydraulic and Engineering Evaluations of New LOCA Testing Methods in TREAT

Design evaluations and thermal-hydraulic predictions were undertaken to compare three candidate options for loss-of-coolant accident (LOCA)–testing capabilities at the Transient Reactor Test facility (TREAT). These options included a small water capsule, a large natural circulation capsule, and a forced-convection water loop. All options were found to create reasonable approximations of radial temperature gradients in fuel specimens prior to blowdown to a secondary tank. Given its minimalist form factor, the small water capsule was predicted to perform remarkably well in mimicking the thermal response of a 20-pellet fuel rodlet during a postulated LOCA event, while sensitivity studies showed versatility in adjusting conditions for specific test phenomena priorities. The large natural circulation capsule predictions showed representative capsule pressure evolution but indicated it would likely struggle to capture the desired fuel temperature response in the blowdown transition, primarily due to nonrepresentative cooling as its tall water column was discharged. These results were compared to previous water loop evaluations that had shown a capability for representing ideal test conditions, especially for longer rodlet test specimens in small bundles, while being achievable in a self-contained form factor to facilitate test logistics. Considering the broader cost and capability spectrum needed to support fuel safety research, these efforts resulted in a recommendation to pursue the small water capsule for near-term data needs specific to high-burnup fuel fragmentation, while continuing detailed development of the water loop to address ultimate data needs.