b'tent with a decay energy heatupsurrounding the fuel. At the same (DEH) have been performed on HButime the power of TREAT is reduced fuel. In DEH conditions, the fuelto simulate decay heat within the fuel. is isothermally heated at a rate ofAn illustration of the two-SP design is ~5C/s. The ability to simulate SEHshown in Figure 1.conditions, representative of LBLOCAFigure 2 shows the fuel centerline and scenarios, within the TWIST designouter cladding temperature histories will provide extremely importantfor the PWR LBLOCA and TWIST HBu insights as to how the thermomechan- LOCA. Figure 3 compares the radial ical response of the fuel and claddingtemperature distribution within the impact FFRD behavior. fuel at various times throughout the Accomplishments transient. In the case shown here, As the primary goal of the TWISTthe fuel rod simulated had a burnup HBu LOCA experiments is to investi- of approximately 70 GWd/t and was gate FFRD under representative LOCAoperating at a steady-state power of conditions, these conditions needed23 kW/m prior to the LOCA; however, to be determined. To do this, a 4-loopmanipulation of TREAT reactor power PWR LBLOCA was simulated usingand TWIST operating conditions, the TRACE thermal-hydraulic systemsenables the ability to simulate a wide code. The model included a core-widevariety of temperature histories.power and burnup distributionThe design of the TWIST LOCA consistent with that of a PWR coreexperiments will allow for first-of-at the end of a 24-month cycle. Thea-kind testing on HBu fuel under TRACE model was tightly coupledrepresentative LBLOCA conditions. to the BISON fuel performance codeThis capability will provide insights using the BlueCRAB MOOSE-wrappedinto to the complex interactions application. This tight couplinggoverning FFRD behavior such as fuel enables detailed simulation of thestresses, transient fission gas release, thermomechanical response of a highand cladding ballooning and burst. burnup fuel rod during an LBLOCA. The work performed here enabled the These representative conditionscompletion of the preliminary design were then used as a target for theof the TWIST experiment device for TWIST HBu LOCA experiments.HBu fuel testing, a level 2 milestone. Through an iterative effort betweenThe progress made on the TWIST the mechanical design modificationsHBu experiment vehicle will enable and neutronic, thermal-hydraulic, andirradiation of HBu fuel samples from fuel performance analysis, excellentthe Byron Nuclear Generation Station agreement between the PWR LBLOCAfuel shipment which is scheduled to and TWIST HBu LOCA simulationscommence in the first half of fiscal was achieved. To accomplish this, ayear 2025.two-state point (SP) approach is used. In State Point 1 (SP-1), the test fuel rod is brought up to temperatures consistent with PWR operating condi-tions. In State Point 2 (SP-2), a valve is opened, rapidly removing the water 2023|AFC ACCOMPLISHMENTS 111'