Development of Experimental Techniques / Facilities at JAEA R&D Centers

Nuclear Fuel Cycle Engineering Laboratories ■14-3 Study on Basic Properties for Establishment of a
Fast Reactor Cycle System–Melting Temperatures and Phase Diagram of MOX Fuel–(b)

Temperature (K)(a)

Fig.14-5 Microstructures of heated MOX with 40% Pu content(a) The sample was encapsulated in W and heated to 2963K.
Precipitates of W metal and Pu oxide were observed by electronprobe micro analyses in the grain boundary.
(b) The sample was heated to 2978K using a W capsule with aRe inner. Grain growth was observed, and there was no traceof melting.

It is essential that nuclear fuels are operated in an undamaged
condition during their entire lifetime. Therefore, the maximum
temperature of the fuel is limited by the melting temperatureof MOX.
JAEA has developed a fast reactor recycle technology. As apart of the development, the basic properties of uranium andplutonium mixed oxide (MOX) fuels that are used as fuel inthe fast reactor have been studied. The melting temperature of
MOX fuels was measured during the period of 1960-1970’s asfunctions of Pu content and O/M ratio, and their phase diagram
was evaluated. Unfortunately, analyses of the melted samples
were not performed in those studies. As a result, the measured
melting temperature of the actual compounds is unreliable.
In this study, the melting temperature was investigated by a
new measurement technique using a W capsule with a Re inner.
It was found that the previously obtained data was not correct,
because a reaction between the sample and the W capsulematerial occurred and the compound which formed affected
the melting. The melting temperature of MOX fuels wasdetermined to be a function of the Pu content and O/M ratio,
and the phase diagram of the UO2-PuO2 system was revised forthe first time in about 40 years.
Fig.14-5 (a) and (b) show the microstructures of the heated
MOX fuel with about 40% Pu content. The sample shown inExperimentThis work Solidus
Liquidus
Kato et al. Solidus
Liquidus
Aitken et al. Solidus
Liquidus
Lyon and Baily Solidus
LiquidusCalculation
This work
Epstein
AdamsonPuO2 (wt%)

Fig.14-6 Solidus and liquidus temperatures in theUO2-PuO2 system

Fig.14-5 (a) was encapsulated in W and heated to 2963K, which
was the method used in the conventional melting temperaturemeasurements. The sample was transformed into a block, andthe precipitates of metallic W and Pu oxide were observed in
the grain boundary. A reaction occurred when the MOX fuelscontained more than 30% Pu.
The sample shown in Fig.14-5 (b) was heated to 2973K
inside a W capsule with a Re inner. Grain growth was observed,
but no traces of melting, such as deformation was seen. Theseobservations show that the previously obtained data is notcorrect, and a reaction between the MOX and the W capsulematerial occurred and affected the melting measurements.
The melting temperature was determined by the thermalarrest method which was observed in the heating curve whena sample melted. Fig.14-6 shows the measurement results ofsolidus and liquidus temperatures which were measured byusing the W capsule with a Re inner. The measurement results
were evaluated by the ideal solution model, and the phase
diagram of the UO2-PuO2 system was revised. The calculated
results represented the experimental data with an accuracy of±20K. It was found that the solidus temperatures of fast reactor
MOX fuels were 50-100K higher than the existing data hadshown.

Reference
Kato, M. et al., Solidus and Liquidus Temperatures in the UO2-PuO2 System, Journal of Nuclear Materials, vol.373, issues 1-3, 2008, p.237-245.

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JAEA R&D Review 2008