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planned for a burn-up range of 1.5 to 6 at% and up to 10 at% as soon as difficulties in restarting the
Phénix reactor have been overcome.
2.2.1.5 Fabrication of nitride fuel including MAs
Nitride has many advantageous properties as advanced fuel such as high-thermal conductivity,
good FP retention, high heavy metal density and mutual solubility.
In 1970s and 1980s, laboratory studies on a fabrication process for mixed nitride fuels has
been performed at CEA. The fuel was seen as the best advanced fuel for FR application mainly due to
its higher breeding gain and its absence of fuel/sodium reaction in the case of clad failure. The process
developed was the carbothermic reduction of mixed oxide in an atmosphere of nitrogen, followed by
decomposition of higher nitrides to mononitride. This dry preparation process was the most capable for
industrial applications. Nitride pellet fuel pins have been fabricated for irradiation test (NIMPHE
programme) in the Phénix reactor. The ITU fabricated, at the same time, nitrides by sol-gel process for
irradiation in the HFR reactor (NILOC experiments). The fabrication technology for mixed nitride fuel
based on the experience of carbide fuel has been developed, in both the Paul Scherrer Institute in
Switzerland [62] and the Bhabha Atomic Research Center in India [63].
During the past 10 years, research on fabrication technology for mixed uranium-plutonium
nitride fuel has been performed at JAERI with a view to an advanced fuel cycle system. Nitride pellets
have been fabricated and supplied for measurement of their characteristics and for irradiation tests [64].
PNC has also fabricated some nitride fuel pellets and measured the fuel characteristics.
Pellet fuel
In 1960s and 1970s, actinide mononitride was mainly synthesised by a hydriding-nitriding
route from the actinide metal, but recently the conversion of oxide to the nitride by carbothermic
reduction has been improved [65,66]. Uranium-plutonium mixed nitride and neptunium-bearing nitride
have been synthesised by reduction of the dioxide with graphite, usually in a nitrogen-hydrogen mixture.
Pellet-type fuel is fabricated by milling, compacting and sintering in a similar way to MOX fuel. No
important problems appear to remain except in the enrichment and recycling of 15 N.
The use of 15 N would be preferable because of the massive formation of 14 C by the 14 N(n,p) 14 C
reaction. Both 15 N and 14 C would be difficult to retain during the nitride dissolution in the PUREX
reprocessing, while they could be easily recovered during the pyrochemical reprocessing with fused salt
electrorefining. In view of the cost for the enrichment of 15 N, the content of which is only 0.365% in
natural nitrogen, the recycling of 15 N would be one of major issues for feasibility of nitride fuel.
Particle fuel
Remote fabrication from particles has advantages with materials of high radioactivity, so
techniques for the fabrication of nitride particles by a sol-gel process have been developed [62-67].
Minor actinides separated from the high-level waste (HLW) as nitrates are converted into solids. The
feed solution is prepared by mixing actinide nitrate solution, carbon powder, hexamethylenetetramine
(HMTA) and urea. HMTA decomposes to form ammonia on heating to about 80-100°C, so gel
particles are formed as a mixture of the actinide oxide and carbon, which is converted to mononitride by
carbothermic reduction in N 2 -H 2 mixture. The particles of pure UN with low oxygen and carbon
contents have been successfully fabricated in Japan and Switzerland, and the method has also been
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