b'Figure 2. RELAP5-3D predictions of the fuel and cladding temperature, TREAT core power, and capsule pressure during TWIST LOCA transient. Blowdown at t = 0 secondscapsule that can house up to a 50-pelletin the fuel. After the desired conditions fuel rodlet connected to a lowerare reached, the second segment is pressure expansion tank via a remoteinitiated by opening the valve to blow-triggered valve. Opening this valvedown the water in the specimen-con-triggers a rapid depressurization andtaining capsule into the expansion tank. draining of the water surrounding theAt the same time the valve is opened, fuel simulating the blowdown portionTREAT power is reduced to simulate of a LOCA. A typical TWIST LOCAdecay heat in the fuel rod. Together, experiment consists of two segments.these two segments allow for TWIST In the first, the fuel rod is brought upto simulate a prototypic LWR LOCA in to quasi-steady-state conditions meantwhich cladding temperature rapidly to mimic the operating conditions of aincreases up to ~100C/s due to the rod in a commercial light water reactorredistribution of stored energy in the (LWR). This is done by ramping upfuel, followed by further clad heat up TREAT power over a period of seconds,from the simulated decay heat. The then holding the power constant forconditions have been shown to match ~30 seconds resulting in a radialwell with predictions for limiting rods temperature profile throughout the fuelin LWRs. and linear heat rate that is consistentAccomplishmentswith commercial LWRs. Achieving aLeveraging modeling and simulation prototypic radial temperature profile istools, the design of the TWIST experi-important as it allows for returning testment vehicle is nearing completion. fuel to its operating thermomechanicalRELAP5-3D modeling of TWIST has state, though the ~seconds timescaleshown that it is capable of meeting will not allow for full reconditioningthe thermal-hydraulic objectives of of the fuel microstructure. The radialsimulating a LOCA from steady-state temperature profile also establishesoperating conditions. The ability to representative stored energy conditions 2021|AFC ACCOMPLISHMENTS 145'