The Reactor Inner Zone Inlet Header (RIZIH) temperatures have raised more rapidly in the CANDU units in general, compared to the original aging predictions. The RIZIH high temperature alarms are required to be monitored to prevent operation outside the safe operating envelope as supported by the safety analysis. To prevent RIZIH temperature alarms, increasing RIZIH temperatures are being mitigated through changes in unit operating conditions which have caused power derates (as low as 87% Full Power) and a loss of revenue. In order to address the above concerns, a CANDU utility has initiated a study to evaluate multiple design alternatives and develop the best design option to improve RIZIH temperatures control. Seven design alternatives were assessed to a sufficient level based on their risk, suitability, economics, and effectiveness in managing RIZIH temperature with minimum system level impacts to interfacing systems. The study concluded that the external feedwater bypass of High Pressure Heater is the preferred option.

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