Assessment of Aging of Zr-2.5Nb Pressure Tubes for Use in Heavy Water Reactor

Abstract

The pressure tubes of pressurized heavy water reactor (PHWR) operate under high temperature high pressure aqueous environment and are subjected to fast neutron irradiation. In order to assure the structural integrity of pressure tube during service, it is periodically examined by non-destructive examination (NDE) techniques. A typical Pressurized Heavy Water Reactor (PHWR) consists of few hundred horizontally placed coolant channels. The coolant channel comprises of a Zr-2.5% Nb pressure tube encircled by a Zircaloy-4 calandria tube and four garter spring spacers (Zr-2.5Nb-0.5Cu), which prevents these two tubes to come in contact during their service life. The pressure tube carries the nuclear fuel, high temperature high pressure heavy water coolant and is subjected to fast neutron irradiation. The integrity of pressure tube is central to the safety of PHWRs. To ensure this, they are periodically subjected to in-service inspection by nondestructive examination (NDE) techniques.  In this paper detailed study has been done on the operating conditions which lead to degradations in the pressure tube with respect to dimensional changes as a result of irradiation creep and growth, deterioration in mechanical properties due to irradiation embitterment, initiation and growth of new flaws like fretting damage due to debris and fuel element bearing pads. The absorbed hydrogen can also limit the life of a pressure tube due to the degradation mechanisms such as delayed hydride cracking (DHC), hydride blister formation and cracking and hydride embitterment. It is a also one of the regulatory requirement to periodically subject pressure tubes to in-service inspection by employing non-destructive examination techniques. All these aspects have been duly addressed in this paper.

Keywords: pressure tube, zircalloy-4, irradiation damage, delayed hydride cracking