ASSESSMENT OF AGING OF ZR-2.5NB PRESSURE TUBES IN CANDU(TM) REACTORS

In modern CANDU (TM) nuclear generating stations, pressure tubes of co ld-worked Zr-2.5Nb material are used in the reactor core to contain th e fuel bundles and the heavy water (D2O) coolant. The pressure tubes o perate at an internal pressure of about 10 MPa and temperatures rangin g from about 250 degrees C at the inlet to about 310 degrees C at the outlet. Over the expected 30 year lifetime of these tubes they will be subjected to a total fluence of approximately 3 x 10(26) n m(-2). In addition, these tubes gradually pick up deuterium as a result of a slo w corrosion process. When the hydrogen plus deuterium concentration in the tubes exceeds the hydrogen-deuterium solvus, the tubes are suscep tible to a crack initiation and propagation process called delayed hyd ride cracking (DHC). If undetected, such a cracking mechanism could le ad to unstable rupture of the pressure tube. A fitness-for-service met hodology has been developed which assures that this will not happen. A key element in this methodology is the acquisition of data and unders tanding-from surveillance and accelerated aging testing-to assess and predict changes in the DHC initiation threshold, the DHC velocity and the fracture toughness (critical crack length) as a function of servic e time. The most recent results of the DHC and fracture toughness prop erties of CANDU pressure tubes as a function of time in service are pr esented and used to suggest procedures for mitigation and life extensi on of the pressure tubes. Crown copyright. (C) 1997 Published by Elsev ier Science B.V.