Plutonium rock-like fuel burning in LWR

Plutonium rock-like oxide(ROX) fuel burning in LWR has been studied. To improve reactivity insertion accident(RIA) behavior of zirconia(ZrO₂) type ROX(Zr-ROX) fuel PWR, small negative Doppler reactivity coefficient of the fuel is increased with the additives such as 24mol% ThO₂ or 15mol% UO₂ in the fuel. There is also an approach of a heterogeneous core with 1/3 ROX and 2/3 UO₂ fuels. From the loss of coolant accident(LOCA) analysis of Zr-ROX fuel PWR, the importance to decrease the large power peaking is shown. The ThO₂ additive can make it easier to flatten the power distribution in the core, and improve not only the reactivity accident behavior but also the LOCA behavior. The power flattening can also be achieved by reducing the content of Gd₂O₃ mixed in ZrO₂ and adding Er₂O₃ in place.

In the case of weapons-grade plutonium burning, the plutonium transmutation rate in Zr-ROX fuel LWR is about 0.9tonne/GWe/300 days, and far larger than that of full MOX LWR. The additives of ThO₂ or UO₂ decrease the plutonium transmutation rate, yet it is still larger than that in full MOX LWR by more than 2 times. Even in 1/3 Zr-ROX fuel core, the transmutation rate is comparable with the full MOX case. Total amount of discharged plutonium becomes less than 1/4 to 1/6 in these cores.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen: II. Reaction rates

Kinetics of the carbothermic synthesis of UN from UO₂ in an NH₃ stream and a mixed 75% H₂ + 25% N₂ stream were studied in the temperature range of 1400–1600°C by X-ray analysis and weight change measurement of the sample. The weight change was divided into two parts; i.e. weight loss due to carbothermic reduction of UO₂ and weight loss due to removal of carbon by hydrogen. The former followed the first-order rate equation −1n(1 − ₀) = k₀t, and the latter the rate equation of phase boundary reaction 1 − (1 − _c)^1/3 = k_ct. The apparent activation energy of the former was in the range of 320–380 kJ/mol. The value of the latter in an NH₃ stream was 175–185 kJ/mol, which was smaller than that in a mixed 75% H₂ + 25% N₂stream (285 kJ/mol). In this method, the rate of the removal of carbon by hydrogen determines that of the formation of high purity UN.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen— I synthesis of high purity UN

Formation of uranium mononitride by the reaction between UO₂ and C in an ammonia stream, a mixed 75%H₂-25%N₂ stream and a mixed 8%H₂-92%N₂ stream has been studied at 1400–1600°C. For the formation of high purity UN, there exists a minimum mixing ratio ($\text{C}\text{UO}{}_2$). It largely depends on the reaction atmosphere and temperature. The mixing ratio in the three atmospheres decreases in the following order: ammonia >75%H₂ + 25%N₂ > 8%H₂ + 92%N₂, and it decreases with increasing temperature. The total amount of impurities (carbon + oxygen) in the UN obtained is in the range of 500–1000 ppm. The time necessary for completion of the reaction in ammonia is shorter than in a mixed hydrogennitrogen.