Design Modification Implementations for Mitigating the Reactor Inner Zone Inlet Header Temperature in CANDU Reactor Units

The Reactor Inner Zone Inlet Header (RIZIH) temperatures have raised more rapidly in the CANDU units in general, compared to the original aging predictions. This adverse trend is caused by a degradation mechanism that affecting heat exchange efficiency in certain areas of the process systems. The main contributor to the RIZIH temperature increase is the fouling in the preheaters and steam generator/boilers due to magnetite deposits on tube internal diameters. The RIZIH temperature rise had caused units de-rates to ensure the reactor safety and comply with the regulatory requirements. As reported in a previous PVP paper (PVP2017-65096), multiple design alternatives were considered and evaluated to address the adverse condition, the best design option with a piping modification by adding external feedwater bypass of high pressure heater was selected to improve RIZIH temperature control. Following the conceptual engineering, preliminary design and detail design, the engineering change was implemented in two CANDU reactor units between 2018 and 2019. This paper reports out the field physical implementations and discusses the effectiveness of the design change on mitigating the RIZIH temperature rise, it also presents the operational and financial benefits actualized through observations of the two implementing units.