An Overview of Welding Aspects and Challenges during Manufacture of Intermediate Heat Exchangers for 500mwe Prototype Fast Breeder Reactor

Abstract

Prototype Fast Breeder Reactor (PFBR) is first of its kind 500MWe pool type, sodium cooled nuclear reactor which is presently in advanced stage of construction at Kalpakkam, India. Intermediate Heat Exchanger (IHX) is a shell and tube type, counter current, sodium to sodium heat exchanger. IHX is a very important, massive, over dimensional (∼42 tones in weight, ∼2 meters in diameter and ∼18 meters in length) and critical component of reactor, as it transfers heat from the radioactive primary circuit sodium to non-radioactive secondary circuit sodium forming the boundary between these two circuits. The principal material of construction of IHX is austenitic stainless steel grade 316LN. Each IHX has 3600 nos. of straight seamless tubes which are rolled and then welded to the either ends of top and bottom tubesheets by autogenous pulsed Gas Tungsten Arc Welding (GTAW) process. The shell welding around the tube bundle is extremely difficult and challenging task due to small gap between the tube bundle and shell inside diameter. Special arrangements were made to avoid arc strike or fusion on the tube during shell welding around the tube bundle.A mechanical hardfaced seal arrangement at the interface of the IHX outer shell and the inner vessel stand pipe is the chosen design concept to ensure leak tightness in the IHX penetrations. Based on radiation dose rate & shielding considerations during maintenance, handling and decommissioning, nickel based ENiCr-B hard facing alloy (Colmonoy-5) was chosen to replace the traditionally used cobalt based stellite alloys to improve the resistance to high temperature wear, especially galling of mating surfaces in sodium environment. The more versatile Plasma Transferred Arc Welding (PTAW) and Gas Tungsten Arc Welding (GTAW) were used for deposition of Colmonoy-5. The hardfacing on the seal ring is really challenging task, as the diameter is too large and thickness & width is too small and which has hardfacing on all mating surfaces. Extensive varieties of trials were conducted on the mockup to optimize the hardfacing process along with heat treatment cycle to obtain minimum distortion deploying special tools and fixtures.Intermediate Heat Exchanger involves 10mm and 28mm thick borated stainless steel components around the tube bundle and bottom portion confirming to ASTM 887, Type 304 B4, Grade B classification to reduce the radiation level in secondary sodium system while flowing through the IHX. Various types of welding trials were conducted on the test coupons for welding procedure qualification of borated stainless steel components using versatile grades of welding consumables. More than hundred nos. of trial coupons welded using 10 varieties of welding consumables have shown fissures and cracks in weld, as these welding consumables found not having good compatibility with SS 304 B4, Grade B borated steel. Special development work was taken-up for the first time in the country by developing a special grade ∼1% borated welding consumable (GRINOX-308BRN electrode) specifically for welding of borated steel components of IHX. This paper highlights on welding aspects and challenges during fabrication of Intermediate Heat Exchangers for 500MWe Prototype Fast Breeder Reactor.