15 new reactor applications to NRC

[mheslep]

Nuclear-Plant Analyses Ordered
http://online.wsj.com/article/SB120847674445424819-email.html

The NRC has received applications for 15 new reactors, with many more expected. The revival is driven by concerns about growing power needs and global-warming emissions from power sources such as coal.

Article also mentions that the interest in new plants is leading to heightened scrutiny of existing plants; a few licenses are at risk.

Nearly half the U.S. fleet of 104 reactors faces license expirations from 2009 through 2015. No utility requesting a license extension has been refused to date, although the NRC sometimes attaches conditions to renewals.

Why the additional scrutiny?

One challenge concerned metal used in equipment near the reactor pressure vessel -- the heart of the nuclear plant -- and is subject to forces that can accelerate aging. The commission has identified metal fatigue as fundamental risk in aging plants.

 

[vanesch]

Why the additional scrutiny?

This is not something that people "discover" now, it was the initial reason for a prediction of a limited life time of a power plant (the reactor vessel is such a big component, that it is economically not viable to replace it: better build a new power plant).

What actually happens, is that neutron irradiation of carbon steel hardens it, and shifts upwards the "ductile-brittle" temperature limit. That means that carbon steel becomes brittle at low temperatures. This is a known phenomenon, and at the plant construction, with big safety margins, people estimated when this boundary ductile-brittle could compromise security. Note that it is at *low* temperatures that the problem occurs. The vessel of a hot, working reactor is not going to become brittle. But the danger is that when cold water is injected, one doesn't want to come close to this boundary.

As such, people put samples of the carbon steel of the vessel closer to the reactor core, where they get a higher neutron flux than the actual vessel. The follow-up of the brittleness of these samples gives one a "look in the future" of the behaviour of the actual vessel, and in most cases, this is way better than anticipated 30 years ago (under worst-case estimations and with safety margins). This is what allows one to extend the life time of the vessel without danger: there are samples that have "already lived 10 years more" concerning neutron irradiation, and that have still acceptable ductile-brittle transition temperatures.

Russians even did some experiments heating the used carbon steel to about 450 degrees (C), and then it turns out that the accumulated radiation damage seems to be "wiped out" - if this turns out to be reliable, it means that by a heat treatment, one can "renew" the vessel - concerning radiation damage - indefinitely.

 

[mheslep]

Yes. I had the impression from the article that, while the radiation effects on steel were understood, inspections were showing the process more advanced than expected. One possible reason for this is that US plants are being run harder than originally planned. A surprising fact at the end of the article: decades ago the US's 100 nuclear plants provided 20% of US electrical. Now many years later after substantial demand growth and no new plants, these 100 still provide 20% of US electrical, so they are being improved and run quite hard.

-Edit: I don't consider any of this a 'bad' thing, just interesting. Appears the average nuke plant gets more / closer inspection than a NASA spacecraft .

 

[gmax137]

We call "running the plant harder" a "power uprate." The time-dependence of the embrittlement is mostly due to the integrated neutron fluence, so you would think with higher power level in the core, the fluence would go up and the embrittlement would occur faster. But - the fluence at the vessel wall also depends upon the design of the core (what enrichment and burnup each individual fuel assembly has). It turns out in many cases that "modern" core designs result in lower fluence than that assumed in the original analyses. So, with calculation of the actual embrittlment during the fuel cycles that have actually been run, it may be that the predicted vessel life is longer, even accounting for the power uprate. Finally, this concern was addressed in detail by the NRC before they licensed the plant to begin with and again before they amend the license to permit the uprate. Look on the nrc website (nrc dot gov) for a document called RS-001, this is the NRC guidance on evaluating power uprates.