To increase the proliferation resistance of plutonium, a coconversion method of adding plutonium nitrate and uranyl nitrate to a mixed oxide powder was devel­oped inJapan. In the MH method, about 71 of a mixed solution of uranyl nitrate and plutonium nitrate with a concentration of about 250gl~1 of heavy metal, is fed into a denitration vessel. The diameter and height of this silicon nitride vessel are about 50 and 6 cm, respectively. After microwave irradiation (2450 MHz, 16 kW), PuO2 + UO3 is formed, and then this product is calcined to PuO2 + U4O9 + U3Ogx in air for 2h at 750 °C. Subsequently, this mixture is reduced to PuO2 + UO2 (MH-MOX) powder under an atmo­sphere of N2-5% H2 mixed gas, at the same tempera­ture used for calcination.52 The obtained MH-MOX powder has sufficiently good powder characteristics to allow fabrication of MOX pellets of more than 95% TD.52,53 Full details of the MH method have been given elsewhere.53-56 With the MH method, the generation of radioactive liquid waste containing plutonium is reduced compared with other conver­sion processes.

Figure 12 shows microstructures which were observed by scanning electron microscopy (SEM) at 10 000-fold magnification, in the PuO2 powder (A) prepared by the oxalate precipitation method and MH-MOX powder (B). The microstructures of MH-MOX powder and UO2 powder (prepared by the ADU process) calcined at various temperatures have been reported in Asakura et a/.52

Examples of the characteristics of PuO2 and MH-MOX powders are shown in Table 4.

The values vary depending on the conversion condi­tions described above.