Pre- and Post-Test CATHENA Simulations for RD-14M Critical Break Experiments

Abstract

Historically, peak fuel element simulator (FES) sheath temperatures in RD-14M Loss-of-Coolant Accident (LOCA) experiments have not exceeded 550 degrees C. However, in licensing analysis scenarios, peak sheath temperatures during the early blowdown phase of a LOCA have been predicted to reach or exceed 1000 degrees C. Experimental data at these conditions can aid in the validation of codes used for licensing analysis purposes. A series of critical break LOCA experiments was performed in RD-14M to provide experimental FES sheath temperatures up to 1000 degrees C. This paper summarises the CATHENA simulations used to help design the test series. Post test simulations of selected tests are also discussed. For this test series, RD-14M was modified to use a single channel per pass; all other channels were isolated at the headers. No emergency core cooling was used. Experiments were conducted either with the power supplies ramped to decay levels 2 s after initiating the break or with the power supplies left at initial conditions until the test was terminated by a process protection trip. The FES trip temperature was increased to 1000 degrees C for the final test. A CATHENA scoping analysis predicted an inlet header break between 15 mm and 20 mm at a loop flow of 3.7 Us would produce a critical break with this geometry. Experimental results confirmed these predictions. For experiments conducted with an 18 mm inlet header break with no power ramp down, a peak sheath temperature of 968 degrees C was reached. CATHENA accurately predicted the flow split point in the channel. The code overestimated the top, centre FES temperature by 141 degrees C. This is considered to be a conservative estimation of the peak sheath temperatures.