Reply #9: Thanks for the explanation [View All]

[caraher]

I'm not fully conversant with the details of these systems, so additional technical information is useful. I definitely understand better the issue with the steam bubbles.

Still, it's not clear to me what the safety margins really are - sure, BWRs do operate at lower pressures and the torus helps, but there does seem to be a consensus that the original Mark I designs weren't adequate in some accident scenarios (which is http://www.nytimes.com/2011/03/16/world/asia/16contain.html?_r=3&pagewanted=1&seid=auto&smid=tw-nytimesscience">why they did retrofits including addition of venting systems).

It's interesting to read http://www.gereports.com/setting-the-record-straight-on-mark-i-containment-history/">GE's rebuttals to criticisms of http://www.gereports.com/the-mark-i-containment-system-in-bwr-reactors/">Mark I containment. What's I find telling is that most of what they say is couched in terms of regulatory compliance. For instance (from the second link):

Claim: The Mark I was “cheaper and easier to build – in part because they used a comparatively smaller and less expensive containment structure.”

Fact: Because of the pressure suppression capability designed into the Mark I, we are able to have a smaller containment design. The pressure suppression technology enables the Mark I to reduce the pressure in the containment vessel by condensing steam in the suppression pool. Safety remained our top priority, and the Mark I design met all NRC design criteria.

I don't see how their "Fact" in any way contradicts the claim. They agree that the containment system is smaller and do not address the economics. Rather than give an assessment of safety directly, they simply say their design met NRC criteria. I don't think regulatory compliance is in dispute. A better question may be the adequacy of those criteria.

From the first link, we're told

The Mark I containment has a proven track record of safety and reliability for over 40 years and there are 32 BWR Mark I reactors operating as designed worldwide.

While the technology was commercialized 40 years ago, it has continued to evolve. Over the last four decades, the Mark I has been modified in the form of retrofits to address technology improvements and changing regulatory requirements.

All of the modifications were made in accordance with regulatory requirements. In the United States, for example, the NRC issued a generic industry requirement in 1980 for the Mark I containment that the industry used to make modifications.

We understand that all of the BWR Mark I containment units at Fukushima Daiichi also addressed these issues and implemented modifications in accordance with Japanese regulatory requirements.

Here they cast fixes as "technology improvements" and responses to "changing regulatory requirements" without any mention of reasons for changing regulations. The regulations changed for a simple reason - a better understanding of the hydrodynamics of certain accident scenarios revealed unacceptable risks (by NRC and industry standards) in the original design.

The fact that GE reports the Fukushima plants as being in compliance with all applicable regulations certainly calls into question any assurance of safety based merely on compliance with government standards...