AP1000 containment insufficient for DBA, engineer claims

29 April 2010

A US nuclear consultant engineer has criticised the Westinghouse AP1000 containment design, arguing that it could release radiation in case of a design-basis accident. Westinghouse has denied the claims.

Hypothetical DBA radiation leak in AP1000
A leaking pressure vessel could release radioactive gas directly into the atmosphere during a design-basis accident, according to Fairewinds' Arne Gunderson.


"The steel containment in the AP1000 design has no backup secondary concrete containment behind it [as most existing US PWRs do] to capture post-accident radiation that leaks out," said Arnie Gunderson, chief engineer, Fairewinds Associates. "Again, the problem with the AP1000 is that there is no backup system. Nuclear plants have been licensed under redundant safety features in order to protect public health and safety, and the containment redundancy is missing from the AP1000."

He also said: "Research shows that there have been more than 80 documented problems with containment systems in the U.S. during the past 45 years. Four of these 80 cases have been through-wall rust holes that completely penetrated the steel containment liner. In 2009 alone, there were three significant containment problems on existing reactors": a through-containment hole at Beaver Valley, a 60-foot crack in Crystal River's concrete containment and a snapped post-tensioned containment tendon at the unstarted plant Bellefonte.

Instead of a containment building, the AP1000 design has a shield building that is vented to the external environment. In an emergency, the shield building draws in air through vents near the top of the structure, and sprays water on to the containment vessel from tanks stored in the roof. The water and air cool the containment vessel passively. Gases are vented out of a chimney in the roof of the structure.

AP1000 PWR emergency passive cooling
Credit: Image courtesy www.climateandfuel.com
A combination of air drawn through a chimney and water held in tanks on the roof of the AP1000 protects it passively against a design-basis accident

Gunderson made two major criticisms with this design. First, he said that the hole in the roof is not filtered, and any leaking radiation is released directly into the environment.

Second, he says that the design of steel containment would be difficult to inspect for corrosion. He says, "This gap between the concrete shield building and the AP1000 steel containment allows for numerous locations where rust can develop on the steel containment. Moisture and corrosive agents can flourish in this gap outside the containment. Inspection of these inaccessible locations in the AP1000 is extraordinarily difficult to detect until the rust creates a hole completely through the steel. Due to the unique AP1000 design features, the likelihood of a hole caused by rust in the AP1000 containment is much greater than the rust holes that have already occurred in existing steel containment liners."

Gunderson said that he does not believe that Westinghouse has studied previous industry experience with holes in containment, and consequently erroneously assumes that radiation breaching the containment is extremely rare. He said that as a consequence of the design, a design-basis accident could release 10 times as much radiation as the US NRC allows.

He concludes that Westinghouse should be required to fit filters in the roof to capture any leaking radioactivity.

Gunderson said that the criticisms were part of a containment analysis prepared for the NRC – his third. His remarks were part of a press conference with other environmental organisations, including the Blue Ridge Environmental Defense League, which is antinuclear.

Westinghouse spokesman Vaughn Gilbert responded vigorously to the claims:

"We disagree completely and unequivocally with every conclusion that was put forward. We are certainly never surprised when an antinuclear group with an antinuclear agenda puts forth antinuclear comments. The reality is that the steel in question is 1.75 inches thick, it is corrosion-resistant, and it is highly unlikely corrosion would ever be an issue. Contrary to what they reported, if corrosion were to begin, it would be quickly discovered in a manner that is prompt and appropriate, and it would be remedied before it would come close to being a problem. The announcements were plain and simple wrong."

The company also responded with a more detailed rebuttal. It argued first that the containment vessel design is different, and more accessible and has corrosion mitigation. It also argued that the water corrosion rates that Gunderson assumed 'are not credible,' and corrosion would anyway be picked up by periodic operational testing and inspection. It also argued that the DBA emergency passive water cooling system would tend to capture fission products, not release them into the environment.


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