Enhancement of Cesium Release from Irradiated Fuel at Temperature above 2,800 K

Abstract

2. Results and Discussion The total fractional releases during the two tests were estimated from changes of the off-line measured γ -ray spectra just before and after the tests because the energy resolution of on-line γ -ray measurement was insufficient due to influence of electromagnetic waves from high frequency induction coil. The fractional releases of 134 Cs, 137 Cs and 106 Ru at the end of tests evaluated by using several γ -ray peaks of isotopes are summarized in Table 1. In the evaluation, no release of Eu was assumed because the on-line measurement showed almost no change of γ -ray intensities of Eu during the tests. Moreover, one of radionuclides, that is, Eu had to be chosen as standard for comparison of the γ -ray spectra between before and after the test because the off-line γ -ray measurements were performed using different systems. The fractional releases of Cs were evaluated by averaging those obtained at the peaks of gamma energy to be 99.8% and 85% in VEGA-3 and -1, respectively. The main reason for larger fractional release of VEGA-3 than that of VEGA-1 is considered that the fuel of VEGA-3 experienced higher maximum temperature for relatively longer duration than those of VEGA-1. Figure 2 shows the fractional release histories of Cs estimated from the on-line γ -ray measurement and temperatures measured in the VEGA-1 and -3 tests. The curves were plotted so that the first and second plateaus of VEGA3 might be superposed on the second and third plateaus of VEGA-1. In order to quantify the differences of release behavior in VEGA-3 and -1 tests due to maximum temperature and its duration, the release rate coefficients 11) used commonly as a parameter of radionuclide release models were evaluated and compared with each other. The release rate coefficient is essentially equal to a fractional release rate per minute to the current inventory. The Cs release rate coefficients every minute calculated from the two tests results are shown in Fig. 3. The fitting curves of the release rate coefficients in VEGA-1 and -3 mostly follow an Arrhenius form excepting at the three temperature plateaus and in particular high temperature portion above 2,800 K. The regression equations of release rate coefficient, k (min −1 ), calculated by the least