Unintended Consequences – Redesign for Aircraft Impact Rule Calls AP1000 Shield Design Into Question

One of the lessons that should have been learned during the first nuclear age was that rule changes aimed at solving a new “issue” can have unintended consequences in another area of plant design. The example I remember was the issue of emergency core cooling systems where regulators determined that the rate of flow from the engineered cooling systems was insufficient. They required plant designers to greatly increase the capacity of cooling pumps, only to realize that putting too much cold water into a hot plant all at once could cause a risk of thermal shock that might break other parts of the plant. (I may be getting some of the details wrong, but I am operating off of a dim memory of something I read long ago. Perhaps some of you have more direct experience that you want to share.)

Based on some recent news, it appears that the lesson of the importance of regulatory stability and system engineering consequences may not have been learned as well as some of us would have hoped. As a result of massive outside pressure to solve the “issue” of potential plant damage due to aircraft impact, the NRC issued a new rule. According to the Chattanooga Free Press, Westinghouse’s initial effort to redesign the containment structure for the AP1000, which had been issued a partial design certification well before the aircraft impact rule was issued, has resulted in an NRC concern about the structural integrity of the reactor shield. In order to satisfy the concerns, Westinghouse will apparently be required to do some additional testing that has the potential to delay its full design certification.

J.M. Bernhard Jr., CEO, chairman and president Shaw Group, a 20% owner of Westinghouse, has indicated that he does not expect the required testing to delay any of the current projects. Unfortunately for any sellers of Shaw Group stock, the market did not agree with that assessment; Shaw’s common stock price dropped by almost 10% yesterday. Toshiba, which owns 70% of Westinghouse, fell just 0.35%. Though I am not an investment advisor and do not offer stock advice, I personally view such occasions of panic selling of a good company like Shaw Group to be a buying opportunity.

Back in my submarining days, we had a saying related to the black art of passive sonar contract tracking that I like to use as part of my own investment strategy – “contacts do not zig in position”. It is a bit difficult to translate that saying, but it means that if you know where something is, it is not logical to assume that it will be in a completely different place the next time you sense it. It might change direction during the gap, but its ability to move to a new place is governed by speed and time since the last observation.

Update: (Posted at 0547 on 18 October 2009) The Charlotte Business Journal’s Power City Blog has a bit more detail about the issue in a story titled NRC Rejects Westinghouse Reactor’s Shield Building Design. According to John Downey, the problem is that Westinghouse has proposed an “innovative” shield building design that NRC reviewers think “will not work”. The NRC is leaving the solution to the problem up to Westinghouse, but the options appear to be more testing to prove the design choices or another attempt at a redesign to something more conventional that the NRC is more willing to approve.

As I understand it, part of the issue is the requirement for the shield building to support the six million gallon emergency cooling tank – which is at the top of the building – through all weather conditions and aircraft impacts. It was put there to provide the passive cooling flow driven by gravity that has been a big part of the “AP” design since the AP-600, which also received an NRC design certification before the implementation of the aircraft impact rule. I am only guessing here, but I presume that the redesign to prove protection against the already remote possibility of core damage due to an aircraft impact has added a significant amount of weight high up in the building, calling into question some parts of the structural support.

See how tangled things can get when you change engineering requirements based on political pressure?

Update: Dan Yurman has produced a more detailed article for the Energy Collective titled Westinghouse gets a scare from NRC. Worth a read.