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Alternation of the melting temperature of irradiated mixed oxide (MOX) fuel in fast reactors with progress of burnup was determined in relation to the actinide fractions contained and of the oxygen-to-metal (O/M) ratio. Based on ideal solution models of UO 2 -PuO 2 and UO 2 -PuO 2 -Am 2 O 3 systems and on correlations obtained of the measured melting with O/M ratio and with burnup, an equation was derived expressing the estimated melting temperature T rev (K) as function of the factors mentioned above that affect the temperature:, where X 1 is the plutonium fraction (Pu/(Pu+U)), X 2 the americium fraction (Am/(Pu+U+Am)), X 3 the burnup (GWd/t), and X 4 the O/M ratio (−) (not exceeding 2.00). Toward advanced stage of burnup, the tendency of irradiated fuel melting temperature to lower with burnup progress presented a tendency to level off its rate of descent.

The melting (solidus) temperatures of irradiated mixed oxide fuels were measured and the compositions of the fuels on the temperature measurement date were calculated. The fuels contained about 29wt% Pu initially and were irradiated up to 124GWd/t in the experimental fast reactor JOYO. A melting temperature correlation was obtained by an experimental regression analysis using 21 measurements:where Tm is the expected melting temperature (K), XI the plutonium fraction (Pu/(Pu+U)) and X z the americium fraction (Am/(Pu+U+Am)) at the measurement, and XB the burnup (GWd/t). The equation shows that the melting temperature decreases by 5, 4, and 3 K per 10 GWd/t at 50, 100 and 150 GWd/t, respectively. The effects of actinides, such as Pu and Am, on melting temperature could be explained by an ideal solution model. However, the decreases caused by soluble fission products could not be well explained in terms of the ideal solution model. KEYWORDS: melting t e m p e r a t u r e , m i x e d o x i d e f u e l , fast reactor, irradiated fast reactor oxide fuel, solidus temperature, t h e r m a l arrest, experimental regression analysis, ideal solut i o n , burnup dependency, s a t u r a t i o n of temperature decrease, a m e r i c i u m f r a c t i o n , actinides

Alternation of the melting temperature of irradiated mixed oxide (MOX) fuel in fast reactors with progress of burnup was determined in relation to the actinide fractions contained and of the oxygen-to-metal (O/M) ratio. Based on ideal solution models of UO 2-PuO 2 and UO 2-PuO 2-Am 2 O 3 systems and on correlations obtained of the measured melting with O/M ratio and with burnup, an equation was derived expressing the estimated melting temperature T rev (K) as function of the factors mentioned above that affect the temperature: T rev = 3, 138 − 497X 1 + 60X 2 1 − (1, 000 − 2, 850X 1)(2.00 − X 4) − (1, 206 − 782X 1)X 2 − (1.06 − 1.43X 1)X 3 + 0.0008((1.06 − 1.43X 1)/0.66) 1.5 X 2 3 , where X 1 is the plutonium fraction (Pu/(Pu+U)), X 2 the americium fraction (Am/(Pu+U+Am)), X 3 the burnup (GWd/t), and X 4 the O/M ratio (−) (not exceeding 2.00). Toward advanced stage of burnup, the tendency of irradiated fuel melting temperature to lower with burnup progress presented a tendency to level off its rate of descent.

The MOX (mixed oxide) fuels, 29.97wt% (analytical value is 28.08wt%) plutonium in 8.3 or 12.1wt% enriched uranium were irradiated up to 120GWd/t in the MARK-I1 core of JOYO. The irradiated MOX fuels were dissolved in 8 M nitric acid solution. Anion exchange chromatographic procedures were developed for the isolation of neptunium contained in these MOX solutions. Neptunium-237 was separated from the other actinides by using the developed chromatographic technique and the content of 237Np was determined by a spectrometry.It was found that the content of 23iNp slightly increased with increasing burnup. The content did not exceed 0.08 wt% in the 30 wt% MOX fuels irradiated up to about 120 GWd/t. Model calculation by ORIGEN-2 reproduces well the observed content of 23iNp.KEYWORDS: m i n o r actinide, neptunium 297, mixed oxide fuels, fast reactors, i o n exchange, isotope analysis, alpha spectrometry