R.B. Bhatt

A novel method of measuring the delay between pre-ionizing and main discharges in TE gas lasers

A simple and reliable method of measurement of the very short delay (tens of nanoseconds) between the pre-ionizing and main discharge currents in a TEA CO2 laser operated in the spiker sustainer configuration has been demonstrated. In this method light emitted from the pre-ionizing spark and the main discharge are first collected separately using optical fibres and then the delay between the onsets of their emission is measured with the help of simple opto-electronic circuits. This method is noise free, since it does not involve measurement of large pulse currents, and can be useful for the measurement of similar delays in the operation of other TE gas lasers.

Ultrasonic decontamination of prototype fast breeder reactor fuel pins.

Fuel pin decontamination is the process of removing particulates of radioactive material from its exterior surface. It is an important process step in nuclear fuel fabrication. It assumes more significance with plutonium bearing fuel known to be highly radio-toxic owing to its relatively longer biological half life and shorter radiological half life. Release of even minute quantity of plutonium oxide powder in the atmosphere during its handling can cause alarming air borne activity and may pose a severe health hazard to personnel working in the vicinity. Decontamination of fuel pins post pellet loading operation is thus mandatory before they are removed from the glove box for further processing and assembly. This paper describes the setting up of ultrasonic decontamination process, installed inside a custom built fume-hood in the production line, comprising of a cleaning tank with transducers, heaters, pin handling device and water filtration system and its application in cleaning of fuel pins for prototype fast breeder reactor. The cleaning process yielded a typical decontamination efficiency of more than 99%.

Laser-Assisted Decontamination of Fuel Pins for Prototype Fast Breeder Reactor

Decontamination of fuel pins is an important process step in nuclear fuel fabrication. Decontamination assumes greater significance with respect to fuels containing plutonium, owing to both plutonium’s high radiotoxicity arising from its long biological half-life and its relatively short radioactive half-life. The advantages of using a laser to decontaminate such radioactive surfaces over conventional cleaning techniques are well recognized. This paper describes detailed process optimization and field implementation of laser-assisted decontamination of fuel pins of the Prototype Fast Breeder Reactor (PFBR). A short-pulsed Nd-YAG laser has been effectively used to decontaminate the fuel pins by exposing their outer surface to laser radiation of an appropriate fluence. The laser parameters were controlled to achieve the required cleaning without causing any clad surface damage. Achievement of such was confirmed by evaluating the laser-cleaned surface using scanning electron microscopy, chemical composition studies by electron probe microanalysis, and Vicker’s microhardness test.

Microstructural characterisation of a dissimilar weld of alloy D9 and AISI 316M stainless steel produced using a 2.5 kW CW CO 2 laser

The dissimilar weld, a fuel clad tube (D9 alloy) and an end plug made of AISI 316M forms a part of the fuel pin in the 500 MW Indian Prototype Fast Breeder Reactor (PFBR). The present study aimed at the development of a crack resistant microstructure of the dissimilar weld by optimisation of 2.5 kW CO 2 laser welding parameters. The D9 alloy, because of its low Cr eq /Ni eq ratio (close to 1), solidified first as primary austenite which makes its welds particularly susceptible to solidification cracking. In this study, the primary mode of solidification in the fusion zone was modulated by controlling the relative degree of fusion of the fuel clad tube and the end plug. A defect free weld with predominantly a primary ferrite mode of solidification was obtained by optimising laser parameters and displacing the focused laser beam towards the end plug side to achieve a greater contribution of end plug material to the fusion zone. The formation of a crater and associated defects at the termination site of lase...