Integrating Chemistry With Maintenance During CANDU Reactor Lay-up for Retubing Outages
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Abstract
CANDU reactors were designed with a planned replacement of the horizontal zirconium alloy fuel channels midway through the operating life of the plant, i.e., following 25 to 30 years of service. The fuel channel includes the calandria tube, which isolates the primary coolant from the moderator, and the pressure tube, which contains the fuel and reactor coolant. At present, two of the units at Bruce A (in service 1977) and the CANDU-6 at Point Lepreau (in service 1982) are being retubed, and Wolsong Unit 1 (CANDU-6 in service 1983) and Gentilly-2 (CANDU-6 in service 1983) are preparing for retubing outages in 2009 and 2011, respectively. A refurbishment outage is essentially an extended maintenance outage. The considerations for lay-up of some systems (e.g., the end shield cooling and liquid zone control systems) are the same as for regular maintenance outages. Other systems (e.g., the primary heat transport system) require more attention because the reactor will be defuelled and retubed in the refurbishment outage, and therefore it be impossible to lay-up these systems as during a regular maintenance outage. It is essential to plan and maintain chemistry control during all phases of the ~18 month retubing outage, to manage the storage of the heavy water for the dismantled in-core systems and to minimize degradation of system components that are not being replaced, while allowing the maintenance activities to proceed. AECL is working closely with CANDU utilities to develop such lay-up chemistry guidelines. The coordination of chemistry control with planned maintenance is essential to minimize system degradation.
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