Ken-ichi Shiozawa

Fabrication of (U, Pu)N fuel pellets

Abstract Uranium-plutonium mixed nitride, (U, Pu)N, was fabricated by the carbothermic reduction in a N 2 -8% H 2 mixed gas stream from UO 2 and PuO 2 . The influence of the sintering conditions, such as milling time, compacting pressure, sintering temperature, sintering atmosphere and pore former content, on the characteristics of (U, Pn)N pellets was systematically investigated. High density (~ 95% TD) pellets were obtained by sintering at 2023 K in an Ar-8% H 2 or Ar stream without loss of plutonium using the highly activated powder. On the other hand, low density (~ 85% TD) pellets with excellent thermal stability were fabricated by sintering with pore former particles. The difference in the microstructure or composition was discussed in connection with the sintering conditions. Certain increase in the lattice parameter of (U, Pu)N was observed during sintering stage.

Electrolysis of plutonium nitride in LiCl-KCl eutectic melts

Abstract The electrolysis of plutonium nitride, PuN, was investigated in the LiCl–KCl eutectic salt with 0.54 wt% PuCl3 at 773 K in order to understand the dissolution of PuN at the anode and the deposition of metal at the cathode from the viewpoint of the application of a pyrochemical process to nitride fuel cycle. It was found from cyclic voltammetry that the electrochemical dissolution of PuN began nearly at the theoretically evaluated potential and this reaction was irreversible. Several grams of plutonium metal were successfully recovered at the molybdenum electrode as a deposit with a current efficiency of about 90%, although some fractions of the deposited plutonium often fell from the molybdenum electrode.

Chemical forms of solid fission products in the irradiated uranium—plutonium mixed nitride fuel

Chemical forms of solid fission products in the irradiated (U, Pu)N fuel were estimated by both thermodynamic equilibrium calculation and electron microprobe analysis on burnup simulated samples prepared by carbothermic reduction. Besides the MX type matrix phase dissolving zirconium, niobium, yttrium and rare earth elements, the existence of two kinds of inclusion was recognized. One is URu3 type intermetallic compound constituted by uranium and platinum group elements. The other is an alloy containing molybdenum as a principal constituent. Furthermore, the swelling rate due to solid fission products precipitation was evaluated to be about 0.5% per %FIMA.

Determination of nitrogen in UN, PuN and (U,Pu)N by oxidation in circulating oxygen and gas chromatographic measurement of the combustion gases

Abstract A simple and accurate method for the determination of nitrogen in uranium- and plutonium-bearing materials was developed. The loss of nitrogen by oxidation of the sample before analysis was prevented by pulverizing, weighing and packing the sample into a tin capsule in a glove-box with a high-purity argon atmosphere. Nitrogen was determined by oxidizing the nitrides in the tin capsule in circulating oxygen at 850 °C and analysing the combustion gases by gas chromatography with thermal conductivity detection. The relative standard deviation was about 0.7% and the time required to analyse one sample was about 10 min for successive analyses. Skilled techniques for glove-box work are not necessary. The method is applicable not only to the analysis of research samples but also to the quality control of nitride fuel production lines.

Electron microscope observation of epitaxial UO2 films

Abstract Epitaxial fcc UO 2 films were obtained on (100) vacuum cleavage faces of NaCl, KCl, KBr, NaF and LiF, and a transmission electron microscope observation was made. Films of (100) orientations having extra spots due to micro-twins were most frequently observed for substrate temperatures of 350 to 550 °C for NaCl, 350 to 450 °C for KCl, 300 to 400 °C for KBr and 500 to 550 °C for LiF. Films prepared on NaF in the temperature range from 350 to 550 °C were observed to have a perfect (100) orientation without extra spots, while those prepared on LiF at 500 °C were sometimes observed to have overlapping (100) and (211) orientations. Registration of fission fragment tracks as straight lines about 100–200 A wide, of bright contrast were observed in films irradiated with thermal neutrons.

He+ ion energy loss parameter in thin films of UO2

Abstract The stopping cross sections of He + ions in vacuum evaporated films of UO 2 have been measured at incident energies from 0.3 to 2.0 MeV. A procedure has been described for obtaining true stopping cross sections by using thin films having various thicknesses. The energy loss parameter of He + ions was determined using slopes at the origin in relative energy loss vs. thickness curves.