H.S. Kamath

Densification behaviour of UO2 in six different atmospheres

Abstract The shrinkage behaviour of UO 2 has been studied using a dilatometer in various atmospheres of Ar, Ar–8%H 2 , vacuum, CO 2 , commercial N 2 and N 2 +1000 ppm of O 2 . The onset of shrinkage occurs at around 300–400 °C lower in oxidizing atmospheres such as CO 2 , commercial N 2 and N 2 +1000 ppm O 2 compared to that in reducing or inert atmospheres. Shrinkage behaviour of UO 2 is almost identical in Ar, Ar–8%H 2 and vacuum. The shrinkage in N 2 +1000 ppm O 2 begins at a lower temperature than that in the commercial N 2 . The mechanism of sintering in the reducing, inert and vacuum atmospheres is explained by diffusion of uranium vacancies and that in the oxidizing atmospheres by cluster formation.

Nondestructive Determination of Relative Plutonium Content in MOX Fuel Pins for Pressurized Heavy Water Reactors Using Passive Gamma Scanning

Abstract Passive gamma scanning (PGS) for detecting the variations in relative PuO2 content (composition) in (U,Pu) mixed oxide (MOX) fuel pins with low PuO2 content (0.4%) for pressurized heavy water reactors is demonstrated. Experiments are carried out with MOX pins of varying compositions with a NaI(Tl) detector. An annular detector was used to improve the counting statistics, and it was demonstrated that a change of PuO2 content of ±0.02% could be detected at a very low level of PuO2 content of 0.4%. It was also observed that a larger-sized PuO2 agglomerate of 1 mm located near the surface of the pellets could be detected during PGS of the welded fuel pins.

Densification behaviour of ThO2–PuO2 pellets with varying PuO2 content using dilatometry

Abstract The shrinkage behaviour of ThO 2 , ThO 2 –30%PuO 2 , ThO 2 –50%PuO 2 and ThO 2 –75%PuO 2 pellets has been studied using a dilatometer in inert (Ar) and reducing atmospheres (Ar–8%H 2 ). The effects of dopants of CaO and Nb 2 O 5 on shrinkage of the oxides of the above Pu/(Pu+Th) ratios were also studied. Out of the two dopants studied, CaO was found to give larger shrinkage for all the Pu/(Pu+Th) ratios covered in this study. It was also found that the shrinkage was marginally larger in Ar–8%H 2 than in Ar atmosphere. Addition of PuO 2 to ThO 2 enhanced sintering. This was found to be true for both the dopants. During the sintering of ThO 2 , a prominent peak was observed in the shrinkage curve at around 100–300 °C. This peak was attributed to the pressure increase of the trapped gases which subsequently release at high temperatures.

Nondestructive characterisation of MOX fuel rods using gamma autoradiography (GAR)

During the fabrication of mixed uranium-plutonium oxide (MOX) fuel rods, two important characteristics to be checked in as-fabricated fuel pins are plutonium enrichment and plutonium dioxide agglomerates. The mixed oxide fuel pellets are made via mechanical mixing of uranium dioxide and plutonium dioxide powders by cold compaction and sintering. The chance of loading a wrong Pu enrichment pellet and having pellets with plutonium dioxide agglomerates in a fuel pin cannot be ruled out. A simple nondestructive evaluation technique is felt necessary to ensure at the last stage (in the welded pins) to check these two characteristics.During the fabrication of MOX fuel rods for Boiling Water Reactors at Advanced Fuel Fabrication Facility of BARC at Tarapur, Gamma-auto-radiography was successfully used to evaluate composition of MOX pellets and to detect presence of PuO2 agglomerates in the peripheral region. The fuel pins are allowed to be in contact with industrial X-ray films loaded in cassettes for a long time and the processed films are carefully evaluated. Experiments were made to standardise the conditions for distinguishing fuel pellets of different composition by gamma-auto-radio-graphy of fuel pins loaded with pellets of different composition. Gamma-auto-radiography of fuel pins containing agglomerates of different sizes was also carried out. This paper describes the experimental details of the technique, results obtained and compare with other nondestructive evaluation techniques available.

Fabrication of (U-Pu) Mixed-Oxide Fuel for Pressurized Heavy Water Reactors

Indian pressurized heavy water reactors (PHWRs) have been operated over the years using natural UO2 bundles. An attempt has been made recently to use (U-Pu) mixed-oxide (MOX) fuel bundles containing 0.4% PuO2 to increase the burnup and reduce fuel inventory. This paper outlines the design changes made in the fuel elements and presents the important steps in fabrication of MOX fuel, the novel quality and process control procedures developed, and our experience during the fabrication of MOX fuel assemblies irradiated in one of the commercial PHWRs.