27th Annual Report, Volume I, Exhibit 9.0, Pacific Gas and Electric’s Response to the Diablo Canyon Independent Safety Committee Twenty-seventh Annual Report on the Safety of Diablo Canyon Nuclear Power Plant Operations – July 1, 2016 to June 30, 2017

Recommendations:
PG&E should perform additional study of submarine landslide-induced tsunami hazards at DCPP and its environs.
Basis for Recommendation:
The DCISC believes that a probabilistic analysis would provide the annual frequency of various tsunami “sizes” at the DCPP site, including estimates of the various uncertainties. Here the word “size” might have one of several meanings, including tsunami maximum height, tsunami run-up, tsunami volume (related to its force on structures), or other possible endpoints. The DCISC endorses developing an estimate (or a useful upper bound) on the annual frequency of a tsunami-caused core-damage accident at DCPP. Such a Core-damage Frequency (CDF) estimate could be used by decision-makers and the public to understand whether the overall CDF risk from tsunamis is (or is not) an important contributor to the total CDF from all accidents at DCPP. Developing a probabilistic “understanding” does not, in the DCISC’s view, necessarily mean performing a full-blown quantitative probabilistic analysis of the tsunami hazard. Instead, it might involve something less, such as a demonstrably conservative bounding analysis of the annual probabilities of various tsunami “sizes,” or an analysis that aims for a realistic probabilistic description but might have very large uncertainties, if that is the best that can be accomplished. Perhaps the desired upper-bound CDF estimate would be easier to develop in a defensible way than a quantified realistic CDF.

James H. Welsch D. Halpin,
Diablo Canyon Power Plant
P.O. Box 56, Avila Beach, CA 93942
805.545.3242 JMW1@pge.com

December 15, 2017

PG&E Letter ISC-17-001

Dr. Peter Lam
c/o The Diablo Canyon Independent Safety Committee
857 Cass Street, Suite D
Monterey, CA 93940

Response to the Diablo Canyon Independent Safety Committee Twenty-Seventh Annual Report on the Safety of Diablo Canyon Nuclear Power Plant Operations – July 1, 2016 to June 30, 2017

Dear Dr. Lam:

On November 13, 2017, Pacific Gas and Electric Company (PG&E) received the Diablo Canyon Independent Safety Committee’s (DCISC) Twenty-Seventh Annual Report on the Safety of Diablo Canyon Operations for the period of July 1, 2016, to June 30, 2017. The DCISC made One Recommendation during this report period, to perform additional study of submarine landslide-induced tsunami hazards at Diablo Canyon Power Plant (DCPP) and its environs.

Consistent with and as part of the peer reviewed seismic probabilistic risk assessment requirements (that will be submitted to the NRC), PG&E has conservatively assessed a bounding risk assessment of potential seismically induced tsunamis creating waves larger than 14m and 26m. To assess the significance of the generation of a tsunami wave coincident with an earthquake that impacts DCPP, a sensitivity calculation was performed.  This sensitivity shows that inclusion of a conditional tsunami has an insignificant impact on the risk to the seismic core damage frequency or seismic large early release frequency.

We are pleased that the DCISC has once again concluded that PG&E operated DCPP safely during the report period. As you are aware, operating the plant conservatively to protect public health and safety is our highest priority, and we will continue to ensure that we fulfill this commitment.

We welcome the DCISC’s independent review and oversight, which contributes to the continued safe operation of DCPP.

Sincerely,

James M. Welsch
Vice President, Nuclear Generation and Chief Nuclear Officer

cc/enc:

Dr. Robert J. Budnitz
Dr. Peter Lam
Richard McWhorter
Dr. Per F. Peterson
Robert W. Rathie
Ferman Wardell
Robert R. Wellington

For more information contact:

Diablo Canyon Independent Safety Committee
Office of the Legal Counsel
857 Cass Street, Suite D, Monterey, California 93940
Telephone: in California call 800-439-4688; outside of California call 831-647-1044
Send E-mail to: dcsafety@dcisc.org.