Narrowing transient testing pulse widths to enhance LWR RIA experiment design in the TREAT facility

Abstract

The Transient Reactor Test (TREAT) Facility provided thousands of transient irradiations and plays a crucial role in nuclear-heated safety research. The facility's flexible design and multi-mission nature saw historic experiments for numerous reactor fuels and transient types but was never specifically adapted to address very-brief pulse transients akin to postulated Light Water Reactor (LWR) Reactivity Initiated Accidents (RIA). Since the behaviors of fuel under these conditions depends strongly on energy input duration and resulting cladding time/temperature response under pellet cladding mechanical interactions, this three-year project was conceived to investigate new pulse-narrowing capabilities. Kinetic models showed incremental improvements for minor facility enhancements including increased reactivity step insertions (to initiate the power pulse) and slightly-increased transient rod drive speed for pulse termination ("clipping"). Replacing peak fuel assemblies, repositioning non-transient control rods to hold down the "hot side" of the core, and balancing against required excess reactivity needs, the limiting fuel assembly power can be reduced by ~20 %. This is a remarkable discovery of latent capability in TREAT, not only in enabling the subject capabilities for reduced pulse widths, but also significantly "uprating" the core's transient energy capacity. Incremental improvements can likely enhance TREAT's capability into BWR-relevant missions; briefer PWR pulse shapes can only be achieved with an advanced clipping system. Helium-3 ( 3 He) was found to offer the greatest benefits for clipping design and overall performance. A unique concept showed great promise for enabling a 3 He-based system with a reasonable cost; this design concept will likely become the focal point of future work.

High-burnup Experiments in Reactivity Initiated Accidents (HERA)

“…For TREAT's ~1 m reactor length, it takes ~280 ms for the control rods to fully insert themselves. However, pulses with FWHM of 89 ms have been demonstrated, and planned plant modifications involving a He-3 clipping system are predicted to be able to achieve pulse FWHM as low as 50 ms, 18 within reach of representative PWR HZP RIA pulse widths and overlapping nicely to the upper end of pulse widths that CABRI is capable of producing, which more naturally overlaps with NSRR capability.…”

The Transient Reactor Test Facility (TREAT)

“…Transient operation can be "clipped," based on the desired test conditions, by rapidly inserting the transient control rods to narrow the TREAT natural pulse width to <90 ms (full width at half max) and terminate the reactor power. Further upgrades are planned for expanding TREAT's clipping capability to include even narrower pulses when needed [8]. Under certain conditions, a state-of-the-art reactor trip system will initiate the rapid insertion of all rods; however, as with the air-cooling system, the trip system is not credited in the reactor safety basis.…”

Simulation of a research reactor reactivity transient with deterministic and GPU-assisted Monte Carlo reactor kinetics codes

“…Deeper understanding of local power distribution after inadvertent reactivity insertions may help in the determination of appropriate safety limits. Other applications can include advanced simulation of reactor pulses which can be used for several purposes, e.g., testing fuel behavior during reactivity initiated accidents in the TREAT facility [5].…”