ML021070151

10 Page Response to Questions

ML021070151
Person / Time
Site:	Diablo Canyon
Issue date:	04/11/2002
From:	Page W Pacific Gas & Electric Co
To:	Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation
References
+sispmjr200505, -RFPFR
Download: ML021070151 (18)

Text

NRC/PG&E Open Meeting, San Francisco, CA Diablo Canyon Independent Spent Fuel Storage Installation Response to NRC question William D. Page Senior Engineering Geologist PG&E Geosciences Department April 11, 2002

Question:

Explain Degree of Confidence in Results n Input parameters used for modeling potential large-scale rock mass movements are realistic and conservative n Confidence in predicted foundation conditions at CTF, ISFSI Pads and ISFSI cutslopes

Input Parameters for Modeling n Geometry of clay beds well understood n Groundwater conditions known, clay beds assumed saturated

Dip Direction Change in dip direction

Temporary perched water on clay beds after storms Main water table

Large-scale Mass Movements n Geologic interpretations of extent of clay beds is conservative, but not extreme n Potential slide planes are chosen to follow the full extent of more extensive clay beds and step between clay beds, this assumes minimum rupture of rock n Rock to rock contact along potential slide plane along clay beds not factored into model, this would increase the clay strength from that used

Clay Beds Not Continuous Clay Bed Extent Based on Thickness Thin areas where rock contact occurs across clay bed Thick areas of clay bed Potential slide plane smoothes Shears offset clay bed undulations of clay bed by breaking through rock Potential Slide Plane Breaks through Rock along Clay Bed

Evidence of No Landslides at ISFSI n No evidence on pre-1971 air photos n No evidence in studies for and excavation of borrow site n No evidence of any fissures or fissure fills in trenches for ISFSI

Assumed Displacement of Large Scale Slide Mass n Fractures in the slope larger than 3 to 4 inches would have left a record on the slope K No vegetation lineaments (similar to the zones of intense growth in filled trenches)

K No open fractures or soil-filled fractures in trenches on slope n Hillslope is 430,000 years old n Subjected to many large earthquakes K Assumed 4 inches would occur in one slide event

Q3 Q4 Q2 Marine Marine Terraces Terrace deposits Q1

Slope 430,000 years ago 1971 slope Q5 Q4 Marine wave-cut platform older than 430,000 Marine wave-cut platform years (430,000 years old)

Hill Slope is 430,000 Years Old, but degraded a Few Tens of Feet

Clay beds at base of modeled large-scale movements extrapolated to pre-1971 slope

Results of Sensitivity Study Clay Bed Strengths 200 Median Deep 800 psf / 15 deg Model stress range Model Shear stress (psi) 3000 psf / 22 deg 150 Shallow stress 800 psf / 36 deg Model stress range 0 psf / 40 deg range 2500 psf / 23 deg 100 800 psf / 26 deg 0 psf / 37 deg 50 Strength used in study 0

0 50 100 150 200 250 Normal Stress (psi)

Confidence in Predicted Foundation Conditions at CTF, ISFSI Pads and Cutslopes n High confidence in rock types predicted K Sandstone K Dolomite K Friable Sandstone K Friable Dolomite K Clay beds

Interpretations with Less Certainty n Locations and percentage of rock types not known with certainty n Friable diabase may be encountered and is expected to have the same properties as friable sandstone n Attitude of clay beds uncertain, more clay beds may be exposed n Precise location of faults uncertain, other shear zones are expected

Conclusion n High degree of confidence that there will be no significant surprises n Features will be mapped during construction n Planned mitigation measures will be applied as appropriate