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Letter to Ontario Power Generation regarding Regulatory Hold Point 4 – Darlington Unit 3

Regulatory Operations Branch

July 8, 2023

Mr. Richard Geofroy
Senior Vice President, Nuclear
Ontario Power Generation Inc.
1 Holt Road South, P.O. Box 4000
Bowmanville, ON L1C 3Z8

Ms. Subo Sinnathamby
Senior Vice President, Refurbishment
Ontario Power Generation Inc.
1855 Energy Drive, 3C14
Courtice, ON L1E 0E7

SUBJECT: Darlington NGS - Unit 3 Return to Service: Request for CNSC Consent to Remove Regulatory Hold Point-4, Prior To Exceeding 35 % Full Power

Dear Mr. Geofroy and Ms. Sinnathamby:

The purpose of this letter is to respond to Ontario Power Generation’s (OPG) submission [1], which requests removal of Regulatory Hold Point-4 (RHP-4), allowing operation above 35 % full power for the refurbished Darlington Unit 3 reactor, and provides the associated Completion Assurance Document.

Canadian Nuclear Safety Commission (CNSC) staff’s assessment [2] of OPG’s submission [1], as well as all supporting documentation, concluded that OPG has met all the pre-requisites established by the Commission to remove RHP-4. Therefore, as the person authorized by the Commission and pursuant to Licence Condition 15.4 of the Darlington Nuclear Generating Station Power Reactor Operating Licence 13.03/2025, I consent to remove RHP-4 and OPG may now proceed with operation above 35 % full power for the Unit 3 reactor.

This authorization is with a condition pertaining to the two Unit 3 fuel channels (FC), X10 and F21, with incorrect pressure break-down orifices (PBO) installed in their inlet feeder. OPG has provided sufficient information and mitigating actions to demonstrate that Unit 3 can be operated safely to 100% Full Power (FP) operation, with different flow conditions and mitigating measures implemented. OPG assessments indicate this to be a short-term solution, with initial mitigating measures for covering the first 80 full power days (FPD) until a second fueling strategy and additional studies are completed for a longer term solution [3].

My consent is based on a risk informed decision, used in special circumstances, taking into account CNSC staff’s assessment [2], in which CNSC staff conclude that this item will be resolved post RHP-4 removal and is being tracked through Action Item 2023-13-28040 [3]. Failure to address the requests stipulated in Action Item 2023-13-28040 by the prescribed due date to CNSC staff satisfaction, will require OPG to seek Commission approval for continued operation.

Yours sincerely,

Ramzi Jammal
Executive Vice-President and
Chief Regulatory Operations Officer
Regulatory Operations Branch

c.c: A. Viktorov, A. Mathai, M. Rickard, C. Loreti, N. Abonasara, A. Oussoren, C. Badal, C. Brown, C. Chan, Darlington Site Office


  1. OPG Letter, R. Geofroy and S. Sinnathamby to R. Jammal, “Darlington NGS Refurbishment: Unit 3 Return to Service: Request for CNSC Consent Prior to Exceeding 35% Full Power (RHP4)”, dated July 6, 2023.
  2. CNSC staff document entitled: “Request by Ontario Power Generation Inc. to Remove Regulatory Hold Point for Prior Consent Prior to Exceeding 35% Full Power (RHP4)”, dated July 8, 2023.
  3. CNSC letter, A. Mathai to R. Geofroy and S. Sinnathamby, “Darlington NGS - Refurbishment - Flow Blockage Checks and Flow Distribution Checks Follow-Up - New Action Item 2023-13-28040”, dated July 7, 2023.

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