J.P. Panakkal

Fracture toughness and fracture surface energy of sintered uranium dioxide fuel pellets

The fracture properties of sintered uranium dioxide fuel pellets for thermal reactors play an important role in the formation of stress-induced cracks and fission-gas swelling during the reactor operation. The fracture toughness or critical stress intensity factor, K~c, and the fracture surface energy, 7~, (energy required to create unit area of fractured surface) are two parameters which can be used for predicting the mechanical behaviour of the fuel pellets. The reported values of fracture surface energy, 7s, for uranium dioxide pellets show a large variation (0.4 to 8 J m-Z) [1, 2]. An attempt was made to study the variation of Kit and 7s in sintered uranium dioxide pellets in the density (4) range of 9.86 to 10.41 g cm 3 using Vickers indentation technique, now commonly used for determination of fracture parameters of brittle and ceramic materials [3-6]. Half-penny shaped cracks are developed in ceramics and the crack length is related to the fracture parameters (K~c and 7s). A new parameter called fracture modulus Ef = (Klc):/7s was also calculated from the experimental data. Uranium dioxide pellets of different densities (90 to 95% theoretical density) were made from a single batch of green powder (ammonium diuranate route) by cold compaction and sintering at an average temperature of 1680°C for 5h. The geometric density of the pellets was measured as specified by ASTM standard C-766-79 for standard uranium dioxide pellets. The height of the pellets varied from 14 to 16mm and the average diameter of the pellets was 12.35 mm. Vickers hardness indentation was made on metallographically polished specimens using a universal hardness tester (Eseway, Type: DVRB-M) and the hardness was determined. A load of 294N was applied and the radial crack lengths in the four directions were measured using an optical microscope at x 400 immediately after the indentation. The ratio of the crack length (C) to half diagonal of indentation (a) made on the samples was greater than 2. Fig. la presents a photomicrograph of a specimen showing the cracks when viewed in an optical microscope. Fig. l b shows the crack tip ending when viewed through a scanning electron microscope. A minimum of four indentations were made on each of the samples and the average crack length (C) of each indentation and the average of hardness values were used for the calculation. Indentations resulting in chipping were discarded and fresh indentations were made. It was confirmed that there was no change in crack length as a function of time after the indentation was made. The overall average crack-length (C) and the volume fraction porosity (p) in the pellets were plotted (Fig. 2). The data were fitted to a straight line C(#m) = 1073 to 3990p, withacoefficientofregression r = 0.96. The fracture toughness, KIo, was calculated from the average crack length and the hardness using the equation Kjc = f(E/H)~/2(p/c3/2) (1)

Use of ultrasonic velocity for measurement of density of sintered fuel pellets

The possibility of using ultrasonic wave velocity measurements for monitoring sintered density of nuclear fuel pellets has been explored. The variation of longitudinal ultrasonic velocity in sintered uranium dioxide pellets in the range of 89-96% of theoretical density has been studied using an ultrasonic thickness/velocity meter. The longitudinal wave velocity has been correlated with the density of sintered uranium dioxide pellets. The possible applications of this nondestructive evaluation technique in the quality control of nuclear fuels is discussed.

Use of autoradiography for checking plutonium enrichment and agglomerates in mixed-oxide fuel pellets inside welded fuel pins

Abstract Autoradiography of nuclear fuel pins containing uranium-plutonium mixed-oxide pellets has been tried as a simple and inexpensive non-destructive technique for assessing plutonium enrichment and detecting the presence of plutonium agglomerates, if any, in the pellets. The optical density of autoradiographs obtained by placing industrial X-ray films in contact with a fuel pin containing pellets of varied PuO2 content has been correlated with the plutonium enrichment of the pellets. Autoradiographs of plutonium agglomerates varying in size from 125 to 2000 μm incorporated in pellets with a nominal composition of UO2 - 4 %wt PuO2 inside welded fuel pins have been analysed. This paper describes the techniques used and the results obtained.

Use of ultrasonic velocity for nondestructive evaluation of ferrite content in duplex stainless steels

Abstract The possibility of using ultrasonic velocity measurements for the nondestructive estimation of the ferrite content in duplex stainless steels has been explored. The variation of the longitudinal ultrasonic velocity in specially prepared duplex stainless steel coupons having a ferrite content in the range 40–55 vol% has been studied using an ultrasonic thickness/velocity meter. The longitudinal wave velocity has been correlated to the ferrite content of the samples estimated by X-ray diffraction and quantitative image analysis.

Analysis of optical density data generated from neutron radiographs of uranium-plutonium mixed oxide fuel pellets inside sealed nuclear fuel pins

Abstract A quantitative analysis of neutron radiographs of welded nuclear fuel pins containing uranium-plutonium mixed oxide fuel pellets has been carried out to obtain a simple model for the transmission of neutrons through fuel pins during neutron radiography. The optical density data obtained by detailed microdensitometer scanning across the image of pellets of varying plutonium enrichment has been correlated to the product of the macroscopic neutron cross section and the distance traversed by the neutrons. Based on the experimental data, a simple model which can be applied to fuel pins of different dimensions and plutonium enrichment has been derived.

Monitoring plutonium enrichment in mixed-oxide fuel pellets inside sealed nuclear fuel pins by neutron radiography

Abstract The possibility of using neutron radiography for monitoring plutonium enrichment in mixed-oxide fuel pellets inside welded fuel pins has been explored. Neutron radiographs of pellets containing 0–6% weight PuO 2 have been analysed by microdensitometry. The optical density of the radiographs has been correlated to the compositional variations in the fuel pellets studied. Analysis of the experimental results is presented.

RADIOGRAPHIC EVALUATION 0F FBTR FUEL PIN END PLUG WELDS: USE OP DEFECT STANDARD AND MICRODENSITOMETRY

Abstract The inspection of Fast Breeder Test Reactor ( PBTR) fuel pin end plug welds by X-ray radiography presents certain problems in detecting extremely small defects in the weld zone because of the special geometry of the components. The use of a defect standard in addition to the conventional image quality indicators has been explored. Characterisation of the radiographs of standard defects and evaluation of typical defects from weld radiographs have been carried out using a scanning microdensitometer.

Nondestructive Characterization of Mixed Oxide Pellets in Welded Nuclear Fuel Pins by Neutron Radiography and Gamma-autoradiography

Nondestructive evaluation of nuclear fuel pellets after the welding of fuel pins plays a vital role in assuring a safe and reliable operation of reactors. Some of the important characteristics to be monitored in low plutonium enriched mixed oxide fuel pellets are plutonium enrichment, size of plutonium dioxide agglomerates, incorrect loading and geometric shape. Experiments were carried out at Bhabha Atomic Research Centre, Bombay on experimental fuel pins containing mixed oxide pellets of different geometry (solid and annular), of different plutonium enrichment (0–6 w% of plutonium dioxide) and containing PuO2 agglomerates of size 125–2000 microns to evaluate these characteristics nondestructively. Neutron radiography of these fuel pins was carried out using a swimming pool type reactor “APSARA”. Results of quantitative evaluation of the neutron radiographs and a simple model correlating neutron interaction probability and the optical density are presented. Gamma autoradiography of these fuel pins showed that these parameters could be evaluated with a few limitations. This paper presents the experimental details, quantitative analysis of the radiographs by microdensitometry and merits and demerits of neutron radiography and gamma autoradiography for nondestructive characterisation of nuclear fuel pellets.

Monitoring internal pressure of sealed nuclear fuel pins: a simple technique

Abstract There is a need for a non-destructive evaluation technique for monitoring internal pressure of sealed nuclear fuel pins pre-pressurized with helium, to ensure a more complete quality assurance. A simple technique for assessing the internal pressure by monitoring the radiographic image of a small pressure-sensitive insert introduced into the fuel pin has been demonstrated. The results of the experiment are discussed in this paper.