Measurement of transient fission gas release from high-burnup MOX fuel under a simulated reactivity-initiated accident condition using fission gas dynamics testing technique

Taniguchi, Yoshinori ; Urano, Kenta; Mihara, Takeshi  ; Udagawa, Yutaka  ; Kakiuchi, Kazuo ; Katsuyama, Jinya  

To investigate the fission gas release behavior of MOX fuel under reactivity-initiated accident (RIA) conditions, a RIA-simulated test on a high-burnup MOX fuel irradiated up to about 64.5 GWd/t (Test FGD-3) was conducted at the Nuclear Safety Research Reactor (NSRR) in JAEA by using recently developed Fission Gas Dynamics (FGD) testing technique. The concept of the FGD tests is to evaluate fission gas release during RIA-simulated test by measuring the pressure transient inside a rigid chamber containing the test fuel rod. We utilize Linear Variable Differential Transformer (LVDT)-type pressure sensor which less affected by gamma and/or neutron field in the NSRR core than conventional strain gauge-type pressure sensor. The maximum fuel enthalpy during Test FGD-3 was evaluated as 276 J/g, which is almost the same value as that of a previous FGD test on a high-burnup UO fuel (about 61 GWd/t) (Test FGD-2). The measured pressure increased from 0.1 MPa to eventually stabilized at about 0.75 MPa: this increase of pressure roughly corresponds to a transient FGR of about 28%, which is higher than that obtained in Test FGD-2 (about 18%). Sensitivity analyses of effective gas permeability for axial gas communication inside the FGD-3 test fuel rod using fuel performance code RANNS showed that apparent gas permeability of the FGD-3 fuel was much higher than that of the FGD-2 fuel. These results suggest that transient fission gas release from high-burnup MOX fuel exceeds that from UO fuel with similar burnup levels, and a significant portion released shortly after energy injection.