T.R.G. Kutty

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.

Studies on hot hardness of Zr and its alloys for nuclear reactors

Abstract The hardness behaviour of pure Zr, Zircaloy 2, Zr–2.5Nb, Zr–2.5Nb–0.5Cu and Zr–1Nb–1Sn–0.1Fe alloys (composition in wt%) were evaluated from ambient temperature to 1173 K using a hot hardness tester in order to compare the hardness and to extract information on the strength behaviour from the hot hardness data using standard empirical relations. This paper summarizes the experimental hot hardness data of several Zr alloys and discusses the results with the help of their microstructure, alloy content and atomic radii of the constituents.

Development of creep curves from hot indentation hardness data

Indentation creep is defined as time dependent motion of a hard indenter into a solid under constant load and temperature. This technique is particularly advantageous for health hazardous radioactive nuclear materials since small samples are required for this test and a large number of data could be generated on the same sample. The only requirement for evaluation of indentation hardness and creep is a metallographically polished specimen surface. Several investigators have correlated hot hardness with creep and have estimated activation energy for creep and stress exponent for steady state creep rate. The aim of the present work is to evaluate hot hardness and in turn creep behavior of U-Zr alloy fuels containing 5, 10 and 15 wt.% of Zr and fuel cladding materials namely stainless steel (type 316) and zircaloy-2.

Densification behaviour and sintering kinetics of PuO2 pellets

Abstract The sintering behaviour of PuO 2 pellets has been studied using a dilatometer in inert, reducing and oxidising atmospheres. The shrinkage rate was found to be maximum for pellets sintered in Ar–8% H 2 atmosphere. The mechanism for the initial stage of sintering was determined using rate controlled sintering (RCS) technique and was found to be volume diffusion for both inert and reducing atmospheres. The activation energy for the initial stages of sintering was found to be 210 and 159 kJ/mol for Ar and Ar–8% H 2 atmospheres, respectively. The mechanism for the sintering in oxidising atmospheres could not be evaluated using RCS technique.

Hot hardness and indentation creep studies on Zr1Nb1Sn0.1Fe alloy

Abstract The hot hardness behaviour the Zrue5f81Nbue5f81Snue5f80.1Fe alloy was evaluated from room temperature to 1173 K at 100 K intervals. The hardness versus temperature data for this alloy can be represented by the relationship H = κ exp( −BT). The indentation creep measurements of the above mentioned alloy were carried out using a load of 300 g at 573, 673, 773, 873 and 973 K. The stress exponent obtained from hardness-time plots was found to be temperature dependent and decreases from a value of nearly 42 at 573 K to ∼ 5 at 973 K. The activation energy for creep was also found to be temperature dependant for this alloy. At least four different domains of creep were observed in the temperature range of 573–973 K. The mechanism in the temperature regime of 573–673 K was found to be athermal.

Densification behaviour of UO2–50%PuO2 pellets by dilatometry

Abstract The sintering behaviour of UO 2 –50%PuO 2 pellets has been studied using a dilatometer in inert, reducing and oxidising atmospheres. The shrinkage begins at a much lower temperature in oxidising atmosphere such as CO 2 and commercial N 2 . The shrinkage rate was found to be maximum for pellets sintered in N 2 atmosphere. The mechanism for the initial stage of sintering was found to be volume diffusion for both oxidising and reducing atmospheres. The activation energy for the initial stages of sintering was found to be 365 and 133 kJ/mol for Ar–8%H 2 and CO 2 atmospheres, respectively. The activation energy obtained using the Dorn method matches well with that obtained using the rate controlled sintering process. The lower activation energy obtained in the oxidising atmosphere is explained with the help of models available in the literature.

Sintering studies on UO2–PuO2 pellets with varying PuO2 content using dilatometry

Abstract The sintering behaviour of UO 2 , PuO 2 , UO 2 –20%PuO 2 , UO 2 –50%PuO 2 and UO 2 –76%PuO 2 pellets have been studied using a dilatometer in inert, reducing and oxidizing atmospheres. The onset of shrinkage occurs by about 400°C lower in oxidizing atmosphere such as CO 2 and commercial N 2 than that occurs in reducing atmosphere. PuO 2 and UO 2 –76%PuO 2 showed an expansion on heating in Ar at ∼1000°C. UO 2 –20%PuO 2 pellet sinters slightly better in Ar than in Ar–8%H 2 . From the shrinkage rate curves, it was found that the maximum shrinkage rate occurs in commercial nitrogen atmosphere for UO 2 –50%PuO 2 . For the other compositions, the maximum shrinkage rate was observed for Ar atmosphere. The sintering behaviour of above mentioned pellets was discussed with the help of point defect model and the possible mechanisms were suggested.

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.

Thermal expansion of AlU and AlUZr alloys

Abstract The thermal expansion of Alue5f810.3U, Alue5f816.2U, Alue5f822U, Alue5f88.1Uue5f81Zr, Alue5f816.2Uue5f81Zr, Alue5f822Uue5f82Zr, Alue5f830ue5f8U2Zr and Alue5f846Uue5f83Zr alloys (all compositions are in wt%) were measured from room temperature to 550°C using a high temperature dilatometer. The coefficient of thermal expansion (CTE) was found to increase with temperature for Alue5f88.1Uue5f81Zr and Alue5f810.3U alloys while for higher uranium containing ternary alloys namely Alue5f830Uue5f82Zr and Alue5f846Uue5f83Zr the CTE decreased with temperature. The CTE of Alue5f8U and Alue5f8Uue5f8Zr alloys generally decreased with increase in uranium content. The differences in the thermal expansion behaviour of these binary and ternary alloys have been attributed to their microstructures and the amount of UAl 3 , UAl 4 and Al in these alloys.

Identation creep of Zircaloy-2 welds of nuclear fuel pins

The indentation method was found to be a suitable method for determination of the creep properties of small weld joints of nuclear fuel pins. The activation energy for the creep for TIG welded sample was found to be ~40% more than the base metal. Both base metal and resistance-welded specimens have the same activation energy for creep and match with self-diffusion values of Zricaloy. Since the value of n estimated in the study is in the range of 5.5-7.6 the most probable mechanism is dislocation creep in the power-law regime.