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Risk of cracks in pressure tubes of Canada’s ageing nuclear reactors – how long can they keep operating safely?

The regulatory violations at the Bruce station are the latest indication that the industry’s approach to managing the aging of pressure tubes, and predicting deuterium ingress, may be breaking down.

At issue is the industry’s ability to accurately predict how long Canada’s aging nuclear reactors, many of which have already exceeded their 30-year design life, can continue to operate safely

Reactors at Bruce nuclear station violated terms of operating licence,   MATTHEW MCCLEARN  Globe and Mail, 19 Juy 21,Two reactors at the Bruce Nuclear Generating Station have violated the terms of its operating licence, its operator and the federal regulator have revealed.

Bruce Power, which operates the plant in Kincardine, Ont., announced in a July 13 statement that pressure tubes in Unit 3 and Unit 6 were found to have “higher-than-anticipated readings.” The following day, the Canadian Nuclear Safety Commission (CNSC) issued its own statement saying hydrogen equivalent concentration (Heq) levels in some of the station’s pressure tubes exceeded the allowable limit of 120 parts per million.

Pressure tubes are six-metre-long rods that contain bundles of uranium fuel. A CANDU reactor contains several hundred of them – and they are considered the principal life-limiting component of Canada’s reactor fleet. Pressure tubes with high Heq levels are at risk of developing blisters and cracks that could cause them to fracture.

Citing an ongoing “regulatory process” that “will continue to evolve,” Bruce Power did not answer questions from The Globe and Mail regarding how many tubes were affected or how much they exceeded the allowable limit……………..

At issue is the industry’s ability to accurately predict how long Canada’s aging nuclear reactors, many of which have already exceeded their 30-year design life, can continue to operate safely……….

Frank Greening, a retired OPG employee who worked for more than a decade with pressure tubes, said the Unit 6 tube reading is unprecedented and puts the regulator in a difficult position………….

Pressure tubes deteriorate as they age, picking up deuterium (an isotope of hydrogen) through a corrosion process known as deuterium ingress. In combination with other aging processes, deuterium ingress causes tubes to grow in length and diameter, known as creep, which allows more coolant to bypass the fuel bundles, lowering the margin of safety. Over time, tube walls become thinner and more brittle, which can cause them to crack and eventually fracture.

In January, 2019, the CNSC renewed Bruce Power’s licence to operate the Bruce station for 10 years, to 2028. However, the regulator insisted that before Heq levels exceeded 120 ppm, Bruce Power would have to prove that its pressure tubes could continue to operate safely above that level. If any pressure tube reached the limit, it declared, the operator would have to shut down the reactor.

At the time, Bruce Power promised to “extend the validity limits of the existing fracture toughness model to 140 ppm of [Heq] in pressure tubes by the end of 2018 and to 160 ppm of [Heq] by the end of 2019.”

But the CNSC said it received a new fracture toughness model for review this May. “No decisions regarding acceptance of the model have been made at this time,” it said.

The regulatory violations at the Bruce station are the latest indication that the industry’s approach to managing the aging of pressure tubes, and predicting deuterium ingress, may be breaking down.

It shows their predictions aren’t worth beans,” Dr. Greening said. “Their predictions are failing. And this is not the first time.”

In March, The Globe reported that, since 2017, CNSC staffers had expressed concerns about unreliable data from pressure tube inspections by OPG at its Pickering plant, east of Toronto. CNSC staffers warned that measuring and predicting deuterium ingress is “potentially one of the biggest issues currently faced by the Industry.”………. https://www.theglobeandmail.com/business/article-reactors-at-bruce-nuclear-station-violated-terms-of-operating-licence/

July 20, 2021 - Posted by | Canada, Reference, safety

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