Emerging Safety Problem in CANDU reactors

Norman Rubin
June 13, 2001

Recently discovered flaws in the CANDU reactor at Point Lepreau in New Brunswick (Canada) have raised concerns about safety, inspection and management issues associated with the Canadian CANDU reactor design, in Canada and internationally.

Specifically, CANDU reactors essentially identical to the flawed Point Lepreau reactor have already been built in India, Pakistan, South Korea, Argentina, and Romania, and two more are currently being built in China. The flaw consists of a potential for unanticipated sudden Loss of Coolant Accidents (“LOCAs”) arising from failures in so-called Feeder Pipes through two mechanisms – one of which has been almost totally ignored by Canada’s nuclear regulator, despite already having caused two Feeder Pipe failures at Point Lepreau – the first in 1997 and the second on March 8, 2001.

Energy Probe, an independent non-governmental nuclear watchdog organization in Canada, has reviewed expert evidence establishing that this long-ignored failure mechanism – known as Stress Corrosion Cracking (SCC) – has the potential to cause far more serious failures than the two that occurred at Point Lepreau, and to do so with little warning. Specifically, the two Feeder Pipe cracks at Point Lepreau were both in the axial or “lengthwise” direction, and therefore produced detectable leaks before the pipes broke. But experience with natural-gas pipelines subject to SCC shows that the same mechanism can also produce much more serious cracks in the circumferential or “crosswise” direction, which can produce “guillotine” pipe failures with no prior detectable leaks. Such an event in any two of a CANDU reactor’s 760 Feeder Pipes would produce a potentially catastrophic “Beyond Design Basis” loss of coolant accident, or LOCA.

The feeder pipes contain essential cooling water at enormous pressure – approximately 100 times the pressure of a kitchen pressure cooker – and that water would immediately “flash” into steam if the pipes broke, leaving the fuel uncooled. In the CANDU reactor, a well-known design problem means that a loss of coolant inherently causes an increase in the power level, and heat output, of the nuclear fuel, placing enormous pressure on the reactor’s emergency shutdown systems. However, even after a successful shutdown, the fuel in a CANDU reactor produces approximately 140,000,000 watts of heat – heat which must be removed by circulating water, or the highly radioactive fuel will overheat and begin to release radioactive gases, or even melt.

In the latest Significant Development Report, presented to their ruling Commissioners on May 29, 2001, staff of the Canadian Nuclear Safety Commission (CNSC) have finally conceded that:

  • Both Feeder Pipe failures at Point Lepreau were apparently caused by SCC – and not by the more predictable Erosion-Corrosion or “wall-thinning” mechanism;
  • These failures have implications for other CANDU reactors in Canada and internationally;
  • The CANDU Owners’ Group (COG) has notified its foreign members of this problem, and its significance; and
  • COG has formed a “feeder working party” to attempt to resolve this problem.

However, the CNSC and Canada’s nuclear industry have in fact been unwilling to face the hazard of SCC in feeder pipes since the 1997 failure at Point Lepreau made the problem obvious to more alert and more independent observers. For example, when metallurgist Dr. Michael Moles (one of Canada’s most prominent experts in SCC) testified about the problem before the CNSC on December 14, 2000, and predicted future failures through SCC, his testimony was met with opposition from both CANDU operators and safety regulators. Pierre Charlebois, Chief Nuclear Engineer of Ontario Power Generation or OPG – the largest owner of CANDU reactors in the world – suggested that the 1997 failure at Point Lepreau happened only because of physical conditions that were “a bit unique” and chemical conditions inside the pipe that were also “somewhat unusual.” At that same meeting, the CNSC’s Jim Blyth – the regulatory expert responsible for ensuring that reactor operations do not threaten public health and safety – responded as follows:

“I think it is important to recognize that we have one isolated event of this occurring. It did leak before it broke – well, it did leak; it never did break. In fact, under very, very unusual circumstances, highly unusual circumstances that have not been replicated elsewhere. I don’t believe it has been demonstrated that this mechanism is in fact active in these reactors. I believe the one incident can be explained.”

In this short statement, Blyth manages to make two serious and frightening errors: First, he minimizes the safety significance of Stress Corrosion Cracks in Feeder Pipes, by assuming that all such failures will leak before they break. Second, he totally dismisses the possibility of future Feeder Pipe failures through SCC – a reassurance that was proven false by the second such failure at Point Lepreau, less than three months after Blyth gave this false reassurance!

As if to prove beyond all doubt that they are all still asleep on the hazard of SCC failures, the staff of the CNSC released their brand-new Annual Review of the Canadian Nuclear Power Industry at the Commission Meeting of May 29-30, 2001 – almost four years after the first SCC failure, and almost three months after the second. That 36-page document spends only two paragraphs discussing feeder pipes, both of them dealing only with the more predictable Erosion-Corrosion or “wall-thinning” mechanism. The word “stress” does not appear once in the entire document.

Energy Probe reaches several findings and conclusions from this story, so far:

1. The CANDU reactor has a design flaw that makes it subject to potentially catastrophic LOCAs with little or no warning – at least under the present inspection procedures and practices.

2. In operating inherently hazardous technologies like CANDU nuclear reactors, we must anticipate safety-critical component failures rather than idly waiting for them to occur.

3. If the operators of nuclear reactors will not take this “active” approach, the safety regulator must insist on it, or withdraw permission for the reactors to operate, since they are not safe.

4. If this approach fails, and an unanticipated failure in a safety-critical component occurs, operators and regulators must never succumb to the foolish and dangerous human tendency to dismiss the incident as unique or never to be repeated, unless they have proof that it is true.

5. The Canadian CANDU nuclear establishment – designers, operators and regulators alike – have demonstrated in this instance that they do not understand or agree with the simple and prudent principles that are essential to safe operation of inherently hazardous technologies like CANDU. They have also demonstrated that they are unwilling to learn from their history – even when that history includes a serious safety-critical component failure caused by an unanticipated mechanism – and have therefore failed to earn the public’s trust.

Norman Rubin is Director of Nuclear Research with Energy Probe, an independent Canadian research and advocacy group focusing on energy and environmental issues.

For more on CANDU click here or here

Please also see: Is There Stress Corrosion Cracking in Pickering ‘A’?

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One Response to Emerging Safety Problem in CANDU reactors

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