Evaluation of Loss-of-Coolant Accident Simulation Tests with the Fuel Rod Analysis Code FRAPTRAN-1.4

Abstract

An integrated computer model for reactor fuel cladding rupture under loss-of-coolant accident (LOCA) conditions has been implemented in the transient fuel rod performance code FRAPTRAN. The model treats the zirconium alloy solid-to-solid phase transformation kinetics, cladding oxidation, cladding deformation, and eventually cladding rupture concurrently. It is for use together with the recently developed finite element based solution module in FRAPTRAN-1.4. The model has been employed to calculate ex-reactor single-rod transient burst tests in which the rod internal pressure and the heating rate were kept constant during the tests. The calculations are compared with experimental data on cladding rupture strain, temperature and pressure. Furthermore this model in FRAPTRAN-1.4 has been used to evaluate a LOCA simulation test within the IFA-650 series performed in the Halden boiling heavy- water reactor. The Halden test was made on a pressurized water reactor fuel rod with Zircaloy-4 cladding. It simulated the blowdown and refill (heatup) phase of a LOCA. The results of the FRAPTRAN-1.4 calculations are compared with experimental data on (i) maximum hoop strain at rupture, (ii) cladding diameter increase versus axial position, (iii) internal rod pressure versus time, (iv) peak cladding temperature at rupture, and (v) post-test cladding outer surface layer thickness. The results are discussed in terms of various uncertainties in the calculations. Nevertheless, the new implementation of the cladding rupture models indicates an overall improvement of the code for fuel rod LOCA analysis.