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April 21, 2005

Comments

Air Rift

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Brian Mays

I would like to add some additional information to Chris's very practical explanation.

As Chris explained, a significant portion of the large margins that are built into nuclear plant designs is due to uncertainty. When these plants were designed and originally licensed over three decades ago, experience with nuclear power was limited to the first-generation reactors -- test reactors, military reactors, and reactors that were significantly different than the later designs. In addition to the uncertainties in the measurement of operating parameters that Chris mentions, the engineers designing the current generation of nuclear power plants simply did not know with a large amount of precision how some of the materials and components would behave over their licensed operating lifetime or how the plant would respond in some rare, but plausible, situations. Thus, they always used very conservative estimates in their designs.

With several decades of experience operating these plants, however, nuclear operators and engineers now have much better knowledge of how they work. Some of the concerns that the engineers originally had have simply proven not to be a problem, others have turned out to be much less of a problem than was originally estimated.

Since the level of uncertainty is now lower, the conservatisms applied to account for this uncertainty are less severe. Therefore, although some of the original margins have been reduced, this can be done without compromising safety.

Chris Wells

Dear JLK,

I wanted to reply to a concern of yours from the Nuclear Plant Uprate article. From the article it seemed you were concerned that plant equipment was not always replaced during an uprating and that this reduced safety margins. This is not quite accurate.

Let me explain by way of example: Let us say that the ultimate design limit of xyz flow is 150 units (I am using generic values). This is the absolute maximum capacity that the system is designed for, and we never want to come close to it. So, because we don’t want to challenge this design limit, we set a safety limit of 100.

Now, ideally we would want to run right at our safety limit of 100 for maximum efficiency. The problem is that in the real world we have to take into account questions like “how well can we accurately measure the value of flow” ? (or temperature, or piping stress, or neutron flux, it doesn’t matter, the principle is the same .) During initial operations, with limited operating experience and older instrumentation, we may have said we only know the value of flow accurately within 10 units. Therefore we would back off of our safety limit and set our operating limit at 90 units.

A few years down the road advances in technology and an increased operating experience base allow us to say that we accurately know the value of flow within 5 units. Armed with this new and improved knowledge we set the new operating value of flow at 95, run the plant more efficiently, and reap the benefits of increasing our electrical capacity without having to actually build a new power plant.

It is important to note that neither our safety limit of 100 nor our design limit of 150 have changed. We have exactly the same amount of designed “safety margin” as before, 50 units.

Now, not all plant uprates are a result of the above principle. Sometimes, we really do run out of room between operating and safety limits. These are the cases in which plant equipment is upgraded or replaced.

I hope my explanation helps,

Sincerely,

Chris Wells
Nuclear Engineer

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About The Editor

  • Searching For The Truth
    JLK is an intellectual property attorney living in the U.S. Northeast.

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