Bruce NGS A Unit 3 and Bruce NGS B Unit 6 Licence Condition 15.3 Limit Exceedance

Introduction:

The purpose of this memo is to inform the Commission of a discovery issue at Bruce NGS A Unit 3 and Bruce NGS B Unit 6 where a licence condition limit has been exceeded.

Background Information:

During a hearing held in 2018 to consider Bruce Power's request for a renewal of its Power Reactor Operating Licence (PROL), CNSC staff reported that Bruce Power's current pressure tube fracture toughness models did not support the safety case to operate any pressure tube with a hydrogen equivalent concentration, or [Heq], in excess of 120 ppm. CNSC staff added that, for Bruce Power to be authorized to operate with the pressure tubes with an [Heq] in excess of 120 ppm, Bruce Power would have to satisfactorily demonstrate that the condition of the pressure tubes could support safe operation beyond 120 ppm [Heq].

In its decision, the Commission issued a Power Reactor Operating Licence, or PROL, 18.01/2028 [3], with Licence Condition 15.3, which states: "Before hydrogen equivalent concentrations exceed 120 ppm, the licensee shall demonstrate that pressure tube fracture toughness will be sufficient for safe operation beyond 120 ppm".

The Compliance Verification Criteria in Section 15.3 of Licence Conditions Handbook, LCH-PR-18.01/2028-R002 [4], establish the compliance requirements for Bruce Power. Bruce Power is required to submit to the CNSC a fracture toughness model for review and acceptance and no unit is authorized to operate above the 120 ppm [Heq] limit. The LCH also specifies that Commission approval is required for continued operation of units containing pressure tubes with a [Heq] exceeding 120 ppm.

Discovery Issue:

As required by the CNSC, Bruce Power has reported to the CNSC by way of two separate REGDOC-3.1.1 reports [1, 2] that Units 3 and 6 have exceeded Licence Condition 15.3 limit on hydrogen equivalent concentration [Heq] of 120 parts per million (ppm). This new information was obtained from the analysis of pressure tube samples from Units 3 and 6, which are both in a safe shutdown state. The sampling was conducted according to the inspections plans.

CNSC staff are of the opinion that the sample results from pressure tubes in Units 3 and 6 put into question the accuracy of the predictive model. The information submitted to CNSC staff in support of the current model predicted that all Bruce A and B pressure tubes should have [Heq] values below 120 ppm.

Discussion:

At the time of writing this memo, CNSC staff are evaluating:

• if the measured elevated hydrogen concentration was caused by a new phenomenon specific to these units or their pressure tubes;
• if the pressure tube inspections previously investigated locations identical or similar to those sampled in Units 3 and 6;
• the adequacy of the calculation/model/inputs to the model; and
• regulatory actions.

CNSC staff conclude that Bruce Power did not knowingly operate these units with [Heq] in pressure tubes above 120 ppm, and that this was a discovery after the shutdown of Units 3 and 6

Safety Impact:

Both Units 3 and 6 are shutdown (Unit 3 for a scheduled outage and Unit 6 for Major Component Replacement or MCR). As such, there is no safety concern arising from the measured hydrogen concentration in their pressure tubes. Although the units will be shutdown for a number of months, no restart is allowed without prior approval from the CNSC.

While Units 3 and 6 are shutdown, Bruce Power continues to operate Units 1, 2, 4, 5, 7 and 8. Bruce Power has confirmed to CNSC staff that this is safe given the fact that the [Heq] level has no impact on fracture toughness at high temperature during full power operation; CNSC staff agree with this assertion. The high [Heq] affects the pressure tube resistance to fracture at temperatures lower than the normal operating temperatures. Of specific regulatory concern would be an unlikely transient (called a cold overpressure transient) should it occur during heat up of the pressure tubes on restart of a reactor.

CNSC staff note that the reactors are designed with consideration of an event of a pressure tube rupture and adequate defence-in-depth design provisions are incorporated to ensure safe shutdown for the protection of the public and the environment. The defence-in-depth approach to design assures that there are multiple provisions and barriers to prevent an accident and mitigate their consequences, should they occur.

Actions taken by staff:

After being informed of this discovery, CNSC staff have directed Bruce Power to promptly conduct further assessments of the fitness for service of the operating Units (1, 2, 4, 5, 7 and 8), and to determine whether they remain within the licensing basis. Bruce Power’s response will be provided to staff by July 15, 2021.

Next Steps:

Bruce Power will conduct a more detailed assessment of the safe continued operability of Bruce Units 1, 2, 4, 5, 7 and 8. Bruce Power will also assess whether operation of these Units remain within the licensing basis, given the discrepancy between the model predictions and inspections results in Units 3 and 6, and submit results of the assessment to CNSC.

Other licensees are being directed to assess impact of this discovery on Canadian NPP fleet through 12(2) requests.

Staff will continue evaluating information as it becomes available and will update the Commission of any future developments.

References:

1. Bruce Power Email, J. Thompson to L. Sigouin, “REGDOC-3.1.1 B-2021-98077 DR”, July 5, 2021.
2. Bruce Power Email, J. Thompson to L. Sigouin, “REGDOC-3.1.1 B-2021-93819”, July 8, 2021.
3. CNSC Power Reactor Operating Licence PROL18.01/2028 “NUCLEAR POWER REACTOR OPERATING LICENCE BRUCE NUCLEAR GENERATING STATIONS A AND B”, October 1, 2018.
4. CNSC Licence Conditions Handbook LCH-PR-18.01/2028-R002 Bruce Nuclear Generating Stations A and B Nuclear Power Reactor Operating Licence PROL18.01/2028, May 25, 2020.