I need to apologize. This is one of the few genuine nuclear engineering questions that we've gotten on this board - and I've been so busy that I didn't take the time to answer it when you first posted. Anyway, the stipulation that keff has to be less than .95 is a requirement from the US Code of Federal Regulations (10 CFR 50.68). If you go to the federal register 63 FR 63130, 11/12/98) you will find the published regulation - and a short history of why it was there and references to previous federal register notices that led up to it. Long and short, is that it deals with the use of high density fuel racks and unborated water and provides a "margin of safety" when accounting for instrument uncertainties. (Note: the federal register is available on the web) Patricia Lougheed
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Thank you very much for your help. I accessed the FR and found the information. For the last part of my search, I am trying to find out the origin of the value selected. I have checked the ADAMS document database and found documents all the way back to 1962 which had an acceptance criteria of 0.90, but by 1968, the value 0.95 was being proposed in licensing submittals. If you are wondering why so much interest, I am helping an engineer at EPRI prepare a technical paper.
Bill - you have gone far beyond my knowledge base. I thought it had to do with the degraded boroflex issue that arose in the 1990's (see generic letter 96-04) but if you've researched it all the way back to 1968, then it's obviously not that. It may have been due to a switch in acceptable probability: going to the 95th percentile; however this would have been in the very early days of the AEC - before the first reg guides were issued. I can ask around and see if anybody remembers; also if you're with EPRI, you might talk to your official Research counterpart. Patricia Lougheed
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Hi. I have been doing nuclear criticality safety for over 30 years and occasionally check this forum. I am not, at this point, upset or appalled, but we need to clear some things up before this thread goes any further. Specifically, it is inappropriate, i.e., it can be misleading or confusing to the point of causing an accidental nuclear criticality, to define any specific numeric value of delta keffective as the "criticality safety margin" at a given nuclear site or for a given transport package, and then rely upon that delta to provide adequate safety. As a specific example, it is not prudent to state that the safety margin for spent nuclear fuel is 5% because keffective may not exceed 0.95 and leave the matter at that.
I scarcely know where to begin to explain this conundrum further, because I don't know the extent of the audience or what they have in mind. However to be very brief, I will begin by agreeing in spirit with other posters on this topic: requiring that there be a delta keffective is a valid part of an entire scheme of criticality safety, just as a house needs a foundation, but a foundation alone is uninhabitable. I also fully acknowledge that regulatory bodies require that the license or other Authorization Basis documents specify either a specific minimum value of delta keffective or specify the methodology by which it can be derived. In other words, these documents promise that no calculated value of any credible assembly (e.g., interacting array) of fissile material will exceed the value X, which is 1.0 minus this delta, insofar as validated neutronics calculations can determine. Again, this is equivalent to stating that if the foundation of a house is level, square, and sound, a liveable house can be built upon it.
To reiterate, an assembly of fissile material may be unsafe at a keffective of 0.90 or completely safe at a keffective exceeding 0.98. There is no magic number, no single safety margin, and no simple answer, although I agree that regulatory bodies have to start somewhere, and in the case of spent nuclear fuel, they are currently using 0.95. I plan to alert my colleagues nationwide to the existence of this discussion thread so that they may monitor it and add to or subtract from my remarks.
Thanks for your post. I quite agree with your statement and I don't think that either Bill's original question or my replies were trying to, in any way, imply that criticality is something that can be assumed to be "safe" just because of the keff value. The first thing I learned in nuclear engineering school is that you have to expect criticality at any time!
However, if you read what Bill is trying to do, he is looking at the regulatory aspect and where and why the regulations changed to specify the 0.95 value. If you, or any of your collegues, especially given the history that you have, can shed any light on this, I'm sure he'd be appreciative. I'd also be interested, as from what he's learned so far, it was before I entered the field (I'm a relative "newcomer" with only 24 years in the industry.)
The nuclear industry in general needs to ensure that the wisdom is passed down. If you look at thread466-46737, you'll see some lamentation going on about the "death" of the nuclear industry. One of the side effects I've seen, still being in it, is the frequent reinvention of the wheel because new people coming in have no ideas what has happened before. Therefore, I hope that you and your collegues to join in the forum discussions frequently - starting new topics as well as posting on other ones. I'd really like to see more activity here, and not just a one-sided regulatory view (I'm afraid that when I post, I scare others off!)
Patricia Lougheed
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Since we all agree with Ivon in principle, let me not retread that path. Let me simply acknowledge that the overarching regulations in both the DOE and the NRC world vis-a-vis nuclear criticality safety are the ANSI/ANS-8 National Consensus Standards. These standards do not anywhere mandate a secific delta-k value as a subcritical margin - for very good reasons. The overriding SHALL statement is: "Before starting a new operation with fissile materials or before an existing operation is changed, it shall be determined that the entire process will be subcritical under both normal and credible abnormal conditions."
Obviously to "quantify" this statement becomes very judgmental - but that's life in the safety arena and can not be avoided. The concern is that some will impose a specified margin of subcriticality, whether in a regulatory guide, CFR regulation, or where ever, and then others will blindly apply/approve it when it may not be sufficient.
Thus the "easy way out" is to mandate a delta-k in regulations, but it is truly not adequate to comply with the ANS-8 requirement. Tom McLaughlin, ANS-8 Chair, LANL
