Design and fabrication of cladding tube burst test apparatus with high pressurization rates

A test apparatus which can rapidly increase the internal pressure of a LWR fuel cladding was designed and fabricated to obtain the basic information on failure behavior of the high burnup fuel rod under a reactivity initiated accident (RIA) condition. The apparatus aims to simulate rapid pellet-cladding mechanical interaction (PCMI) which occurs in the high burnup fuel rod during a pulse irradiation. Oil pressure was selected to expand the fuel cladding uniformly in the present apparatus, taking account of the results from the NSRR pulse-irradiation experiments suggesting strong influence of fission gas in the UO2 pellet on the PCMI. In order to pressurize and burst the fuel cladding in a very short period corresponding to the pulse width in the NSRR experiments, the authors adopted a pressurization method that a high-pressure line pre-pressurized up to 170 MPa was instantaneously connected with the fuel cladding by opening a solenoid valve. The fabricated apparatus achieved the maximum pressurization rate of 3.4 MPa/ms at room temperature and the unirradiated PWR-type Zircaloy-4 cladding failed in 70 ms. The pressurization rate could be successfully controlled in a wide range of 0.002 to 3.4 MPa/ms by equipping a bypass line and a flow control valve in the pressurization system. Since the hydrided fuel cladding tested by the present apparatus exhibited the same failure morphology as the high burnup fuel failed in the NSRR pulse irradiation, it is considered that the present apparatus can satisfactory simulate the PCMI during the pulse irradiation. Preliminary tests were conducted changing parameters such as pressurization rate, test temperature, and axial constraint condition. The results confirmed that the apparatus worked as designed. (author)