The SPEAR-BETA's fission gas release model was modefied by use of effective thermal conductivity and axial fission gas mixing between fuel-clad gap and plenum. With use of this modified code fission gas release was analyzed under a reactor operating condition with rapid power ramping.
Effective fuel thermal conductivity for which the effect of fuel cracking is taken into account was devised in calculation of fuel temperature distribution and inner gas pressure under rapid power ramping. Mixing and dilution effect due to fission gas flow and diffusion was also taken into account in computing the gap condition. The axial fission gas flow model was solved by the Crank-Nicholson method, and the finite difference method was used to realize fast running time.
The present modified fission gas release code was validated by comparing its predicted results with experimental results from the Studsvik ramping tests and calculated results with use of unmodified SPEAR-BETA code and with use of FEMAXI-IV code. Predicted results with use of the modified code showed better agreement with experimental results from the Studsvik tests than latters' calculated results. It was found that the fuel centerline temperature resulting from use of the modified heat conduction model was higher by 200°C than that resulting from use of the unmodified heat conduction model. The fraction of fission gas release predicted by using the modified code that took into account the axial gas flow was larger by nearly 6 % than that calculated by use of the unmodified code.
Effects on fission gas release of gap width, fuel grain size, terminal power level and power ramp rate also were evaluated with use of the modified code.