Experimental investigations on out-of-pile single rod test using fuel simulator and assessment of FRAPTRAN 2.0 ballooning model

The extent of clad tube ballooning is important when analyzing the upper limits of coolant-channel blockage and subsequent planning of emergency core cooling system (ECCS) design strategy. As per revised ECCS acceptance criteria, the safety-analysis code system should be able to predict precisely fuel rod behavior to simulate a realistic safety analysis under off-design conditions. Considering these aspects, the present investigation was carried out to access the capability of FRAPTRAN 2.0 code to predict the ballooning behavior of cladding at various heating rates and internal pressures. Three tests at (4.5, 5.5, and 6.5) MPa and heating rates of 1.7-4 K/s were performed on a facility named 'FISRBIT' (Facility to Investigate Single-Rod Behavior In Transient) under inert gas atmosphere. Transient temperature, pressure, and deformation were recorded during the experiment at three axial positions over the internally heated Zircaloy-4 clad tube. Ballooning started at the location with the highest temperature; then propagated in the axial directions. Under fast transient heating, the balloon was confined near the highest temperature site, but at slower rates, an axially elongated balloon was observed. The test time between initiation of ballooning and rupture varied from 25 to 100 s depending on heating rate and internal pressure. The maximum hoop-strain prediction based on the hoop-stress calculation by the Rosinger model was better than the FRACAS-I and BALON-2 models. Using the Rosinger model, the hoop stress increased gradually until burst; hence, it was judged to simulate the physics of ballooning adequately. The rupture timing prediction by the FRAPTRAN 2.0 code was sooner than in the experimental results. One reason for the early rupture prediction was the time-independent behavior of the plastic model adopted by the code for modeling the ballooning phenomenon.