The seismic CDFs do not assume that the earthquake has happened; rather, the frequency of earthquakes is
combined with the probability of core damage to estimate the seismic CDF. In fact, change in the frequency of
earthquake occurrence is the issue in GI-199.
Here’s some information on CDFs that I pulled from Appendix D of the G1-199 Safety/Risk Assessment report
that should help you to put the different CDFs into perspective:
Even the IP3 IPEEE indicated that seismic risk was much larger than internal events risk.
During our January 2011 visit, the licensee told us that they does not maintain their seismic PRAs of IP2 and
IP3. There’s little regulatory motivation for them to do so.
First, NUMARC 93-01 (which we endorsed in RG
1.160) excludes contributions from external events when determining risk-significant plant configurations
Second, RG 1.174 allows the use of qualitative arguments in certain situations where
less than a full-scope PRA is available.
Third, there is no regulation that requires Part 50 licensees to have a
So, it’s somewhat understandable that the licensee would focus on the internal event CDF (which is
readily available from the risk monitoring software used to support Maintenance Rule (a)(4) evaluations), and
tend to ignore the contributions from external events such as earthquakes and fires.
I expect that the staff will
re-examine some or all of these issues in the coming months.
From: Boska, John
Sent: Wednesday, March 23, 2011 8:34 AM
To: Coe, Doug; Beasley, Benjamin; Stutzke, Martin
Subject: question on core damage frequency
In the meeting the other day, I asked the question about CDF, and I still don’t understand it. If I go to a site and
ask the VP what the CDF is for his reactor, he will say about 1 E-5. But for Indian Point 3, we are saying the
seismic risk of CDF is 1 E-4.
This means the seismic risk of core damage is 10 times greater than all internal
If this is true, we should concentrate a lot more on seismic upgrades.
I suspect that
perhaps the seismic CDF starts by assuming the earthquake occurred, and then calculates CDF. If that is true,
it is not a true CDF as we use it for operating reactors.
Please let me know which is correct. Thanks.
[toggle_simple title=”Related articles” width=”600″]
- March 15th, 2011 – Plant Seismic information to Support NRC Hearing on Wednesday (enformable.com)
- March 14th, 2011 – What a licensee needs to determine to ensure not exceeding licensed limits (enformable.com)
- March 15th, 2011 – I am walking to the 6th floor to get permission to work on this. (enformable.com)
- March 16th, 2011 – NRC internally frustrated by lack of better response to ‘biased (MSNBC) report’ (enformable.com)
- March 14th, 2011 – GI-199 Questions – Due to uncertainties in the data NRC sending letter to US Plants (enformable.com)
- Strong ground motions from the 2011 Japan earthquake obtained from a dense nationwide seismic network (enformable.com)
- March 14th, 2011 – Seismic PRAs do not consider damage from earthquake aftershocks – Station Blackout Write-Up (enformable.com)
- March 14th, 2011 – Seismic PRAs do not consider damage from earthquake aftershocks – Need to predict the fragility of the plant SSCs following the main shock (enformable.com)
- March 14th, 2011 – Modelling Seismic Event Sequences like the one that happened to Fukushima (enformable.com)