ML19274E698
| ML19274E698 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos File:GEH Hitachi icon.png |
| Issue date: | 04/11/1979 |
| From: | GENERAL ELECTRIC CO. |
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| ML19274E695 | List: |
| References | |
| EDAC-117-217.13, NUDOCS 7904170188 | |
| Download: ML19274E698 (44) | |
Text
EDAC-117-217,13 PROBABILITY ANALYSIS OF SURFACE RUPTURE OFFSET CENEATH REACTOR BUILDING GENERAL ELECTRIC TEST REACTOR prepared for General Electric Company San Jose, California EDHC ENGINEERING DECIS!ON ANALYSIS COMPANY. INC.
480 CALIFORNIA AVE., SUITE 301 2400 MICHELSON DR:VE BURNiTZSTRASSE 34 PALO ALTO CALIF. 94306 IRVINE. CALIF. 92715 6 FRANKFURT 70. W. GERMANY
.i a
EDAC-117-217.13 PROBABILITY ANALYSIS OF SURFACE RUPTURE OFFSET BENEATH REACTOR BUILDING GENERAL ELECTRIC TEST REACTOR prepared for General Electric Company San Jose, California
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ENGINEERING DECISION ANALYSIS COMPANY, INC.
480 CALIFORNIA AVE SUITE 301 2400 MISHELSON DRIVE BURNITZGTR'7SE 34 PAL O ALTO. CAL IF. 94306 IRV!NE. CAllf. 927iS 6 IRANKF URT 70. W. GF RMANY
9 e
TABLE OF CONTENTS 9
Page EXECUTIVE
SUMMARY
ii e
1.
INTRODUCTION...........................
1-1 2.
ACCEPTANCE CRITERION.......................
2-1
- 3. MODEL...............................
3-1 4.
GE0 LOGICAL DATA..........................
4-1 46 5.
PROBABILISTIC ANALYSIS......................
5-1 Model Probabilistic Analysis...................
5-2 Alternate Probability Analysis..................
5-9 6.
EVALUATION OF CONSERVATISM....................
6-1 9
7.
CONCLUSIONS............................
7-1 REFERENCES e
APPENDIX A -- Geologic Bases for Probabilistic Analysis APPENDIX B -- Age of Sediments, General Electric Test Reactor Site, Vallecitos, California O
e F.DHC
EXECUTIVE
SUMMARY
A probabilistic analysis was conducted to quantify the likelihood of a surface rupture offset occurring beneath the General Electric Test Reactor (GETR) reactor building, which is located at the Vallecitos Nuclear Center. The objective of the analysis was to evaluate whether a conservative estimate of the probability of occurrence of a future surf ace offset is sufficiently low so that ground offset beneath the reactor building need not be considered as a design basis event.
A probabilistic model was developed which incorporated the observed data obtained from recent trench excavations. The likelihood of an offset occurring beneath the reactor building was compared to the U. S. Nuclear Regulatory Conrnission's (USNRC) criterion. This criterion is defined to be approximately 10-6 annual probability of occurrence for accident consequences if the basis for the prchability estimates can 'be shown to be conservative. The GETR is a test reactor and not a nuclear power plant, and thus the USNRC criterion for nuclear power plants represents a conservative basis for perfonning a probabilistic evaluation.
The calculations made to obtain the probability of occurrence were sufficiently conservative to comply with the intent of the criterion. The most important conservative assumption made is concerned with the possibility for potential exposure given that an cffset occurs.
Since the GETR is currently being modified to resist the effects of a 3-foot offset, it is realistically estimated that the probability of exposure consequences is conservatively at least one order of magnitude less than the calculated probabilities for offset beneath the reactor building which are given in this report.
HDHC 3,
The weighted average value for the annual probability of surface rupture offset occurring beneath the reactor building is less than 5x10-7 In addition, there is nearly a 90 percent confidence level of a 10-6 annual probability based on the most conservative assumptions.
In conclusion, the calculated probabilities are less than the criterion value of approximately 10-6, and reasonable qualitative arguments have been made to ensure that a realistic estimate of the probabilities of potential exposures would be lower.
Based on these findings, it is concluded that surface rupture offset of any size at the GETR need not be considered as a design basis event.
9 iii
9 l-1 9
1.
INTRODUCTION 9
A probabilistic analysis was conducted to quantify the likelihood of a surface rupture offset occurring beneath the General Electric Test Reactor (GETR) reactor building, which is located at the Vallecitos Nuclear Center. The objective of the analysis was to evaluate whether a conservative estimate of the probability of occurrence of a future offset is sufficiently low so that ground offset beneath the reactor building need not be considered as a design basis event.
A methodology based on probabilistic techniques (Ref. 1), which incorporated the observed data obtained from the recent trench excavations (Ref. 2), was developed. The likelihood of an offset occurring beneath the reactor building was compared to the USNRC criterion to determine whether surface rupture offset should be considered to be a design basis event.
g This report consists of seven chapters.
In Chapter 2, the USNRC criterion is discussed. Chapter 3 presents the conceptual model used to perform the probabilistic analysis, and Chapter 4 gives the basis for data pertinent to the model. The fonnulation of the probabilistic approaches are given in e
Chapter 5 and the results of the analyses using the geologic data are presented and discussed. Chapter 6 lists the conservative features of the analysis so that the results can be compared in a meaningful manner to the acceptance criterion. Finally, the conclusions of the analysis are e
presented in Chapter 7.
O EDAC
e 2-1 8
2.
ACCEPTANCE CRITERION O
This chapter discusses the basis for the criterion used to determine whether the calculated probability of occurrence of a future offset is sufficiently low to exclude surface rupture offset as a design basis event. The USNRC Standard Review Plan (SRP) (Ref. 3) gives acceptance criteria for excluding events from the design of nuclear power plant safety-related features. The GETR is a test reactor and not a nuclear power plant, and therefore the USNRC criteria represent a conservative e
basis for judging the results of the probabilistic analysis.
Probabilistic acceptance criteria are given in the SRP.
Section 2.2.3 gives an acceptance criterion of 10-6 annual probability of occurrence for accident consequences if the basis for the probability estimates can be shown to be conservative. The USNRC Standard Review Plan, Section
2.2.3 states
e "Accordingly, a conservative calculation showing that the probability of occurrence of potential exposures in excess of the 10CFR Part 100 guidelines is approximately 10-6 per year is acceptable if, when combined with reasonable qualitative arguments, the e
realistic probability can be shown to be lower."
The calculations made to obtain the probability of occurrence of any size offset beneath the reactor building are sufficiently conservative to O
c mply with the intent of this statement. On this basis, a probability of occurrence of approximately 10-6 was selected as the criterion for excluding surface rupture offset as a design basis event.
O EDRC
O 3-1 e
3.
MODEL 0
Figure 3-1 presents a cross-section of the GETR site, which shows Trenches B-1 and B-2 that were recently excavated and the two shears a
which were found (Ref. 2).
It is conservatively assumed that if an offset should occur, it would be located either on or between the two existing shears.
It is more likely that the offset will occur on the existing shears since this has been the state of nature for the past 4
128,000 to 195,000 years or longer.
If an offset occurs away from the existing shears, it is conservative to assume that it will occur between the two shears where it may or may not occur beneath the reactor building.
It is possible that it could occur outside the area between 9
the two shears.
In this event, there is no way that the reactor building could be affected by surface rupture offset.
Indications of shears outside this area (e.g., in Trench H, see Ref. 2) further support the belief that in a relative sense, future offsets would just as likely 9
occur outside the area of the model.
However, it is conservatively assumed that any future offset will occur either on or between the observed shears on which displacements have been occurring for at least 128,000 to 195,000 years and probably much longer.
9 The probability of occurrence of a future offset beneath the reactor building is separated into two probabilities, P1 and P, where:
2 P1=
annual probability that an offset will occur between shears B-2 and B-1/B-3 (i.e. not on the shears)
P2=
probability that the offset will occur beneath the reactor building, given that an offset occurs between shears B-2 and B-1/B-3 EDAC
3-2 The annual probability, P, that an offset will occur beneath the reactor building is the following equation.
P=P xP (3-1) y 2
Probability Pt reflects the probability of occurrence of offsets and their location either on or off the existing shears, while probability P2 reflects tne spatial distribution of offsets between the two shears and the relative size and location of the reactor building.
It 1, assumed that the area in the directions perpendicular to the model will behave similarly to the model. This assumption is supported by the geological evidence which is discussed in Chapter 4.
Thus, it is appropriate to use a one-dimensional model as shown in Figure 3-1.
9 O
O O
EDHC
e e
e e
e e
o e
e e
4
- =
=
t-700 TRENCH B 1 5
g
- 70(,
^
t Z_ 600 TRENCH B-2 f
-600 Z c
Sheer B-1/8-3 o
y500~ #
REACTOR BUILDING /
~500 b
b 400<
=400 d SCALE: HORIZONTAL-VERTICAL 0
200 Ittt
-W hC I'
FIGURE 3-1 CROSS-SECTI0tl 0F GETR SITE
O 4-1 9
4.
GE0 LOGICAL DATA O
The geological data bases used in the probabilistic analysis are presented in Appendicies A and B.
A sumary of the data used to develop the model is given in the following text.
Table 4-1 gives the offsets in feet during the past 128,000 to 195,000 years at shears B-2 and B-1/B-3 which were found during the recent trench excavations (Ref. 2).
Data from Trenches B-1 and B-2 (see Figure 3-1) establish that there are no shears between shears B-2 and B-1/B-3 for at least the last 128,000 to 195,000 years.
It is conservatively assumed in the model that shears parallel to shears B-2 and B-1/B-3 may be present in both directions away from the existing shears. Therefore the segment between tho shears B-2 and B-1/B-3 is typical and it is appropriate to define this as the area for analysis. Thus the model include: shears B-2 and 8-1/8-3 and the 1,320 feet space inbetween (see Figure 3-1).
9 The data obtained from the trench excavations indicate that it is probable t1.at future offsets which may occur on shears B-2 and 8-1/B-3 are equally likely to occur on either shear.
Similarly, if future g
offsets occur between the existing shears, it is geologically reasonable to assume that they will be symetrically distributed between the shears.
It was detennined that the width of a future offset at the ground surface a
will vary between 2 and 4 feet.
It it conservatively assumed in the analysis that the surface width will be 4 feet.
The data used in the model represents the data which is pertinent to the e
GETR. Data obtained from the e' r excavations do not influence the model or the probabilistic analysis.
EDAC e
O 4-2 9
TABLE 4-1 OBSERVED OFFSET DATA O
Offset During Time Period (ft)**
g Time Period (Before Present in Years)
Shear B-2*
Shear B-1/B-3*
0-8,000 to 15,000 0
0 8,000 to 15,000 -
17,000 to 20,000 3
3 e
17,000 to 20,000 - 70,000 to 125,000 8
12 70,000 to 125,000 - 128,000 to 195,000 or greater 80+
40+
0 See Figure 3-1 for location of shears 4D
- Each offset shown is maximum measured value and may have resulted from several events O
O 9
I!Ilill:
O 5-1 5.
PROBABILISTIC ANALYSIS O
A probabilistic methodology was developed and used to calculate the annual probability of an offset occurring beneath the GETR reactor building. Any size offset, whether it is small or large, is implicitly 8
included in the methodology. The objective of the analysis was to determine whether the probability of occurrence of any offset beneath the reactor building is sufficiently low so that surface rupture offset can be excluded as a design basis event.
Two approaches were used to obtain estimates of probabilities.
It was assumed in the first approach that the occurrence of offsets is a Poisson process. This is a standard assumption which is made in earthquake hazard and risk analyses (Ref. 4, 5, and 6).
It is assumed, in using a Poisson process, that the process is stationary, events are independent, and the probability of two or more events occurring in a short interval of time is negligible compared to the probability of a single event.
As part of the first approach a weighted estimate of the probability of an offset beneath the reactor building was obtained by integrating the g
model distribution (which is based on the observed data) over all values of the unknown parameters. This is a standard accepted technique, which is described in Reference 1.
In addition, the probability distribution for the unknown parameters was used to obtain probability level estimates g
of the offset occurrence.
As a benchmark for comparing the results from the model approach, Classical upper one-sided confidence limits were calculated directly and 9
HDHC
5-2 were shown to be similar to the results obtained from the first approach. This finding is reasonable, since prior distributions for the unknown parameters were assumed to be uniformly diffuse.
9 The methodology and analysis results for the two approaches are given below.
MODEL PROBABILISTIC ANALYSIS The annual probability, P, of an event of any size occurring beneath the reactor building is defined as follows (see Chapter 3):
P=PyxP2 (5-1) where:
P1=
annual probability that an offset will occur between 9
shears B-2 and B-1/B-3 (i.e. not on the shears)
P2=
probability that the offset will occur beneath the reactor building, given that an offset occurs between shears B-2 and B-1/B-3 9
Derivation of Probability P y Probability P is assumed to be a weighted Poisson distribution as 1
given by the following equatior which is for a time period of one year:
1 = 4Ae-A (5-2)
P where:
O A=
mean time rate of occurrence of offsets 4=
the probability that an offset will occur between the two shears, given that an offset occurs 9
EDRC
O 5-3 9
If values for the parameters A and 4 were knowa, it would be a simple procedure to compute P.
Since the values for the two parameters are t
not known, it is necessary to use the observed data to obtain estimates g
of their values.
The data are used to determine the joint probability density function p(A,4) which is given below.
p(A,$)=$L(A,4l data)p'(A,$)
(5-3)
S where:
p(A,4) posterior joint probability density function
=
of A and 4 0
p'(A,$)
prior joint probability density function
=
of A and &
L(A,4l data)
Likelihood of A and $, given data
=
normalizing constant so that the area under the
=
joint probability density function is unity Since there is not sufficient information to develop a prior probability density function, it is assumed that all values are equally likely to However, in order to study the sensitivity of this assumption, a g
occur.
prior distribution of the following form was used in developing the probabilistic model.
p,g'4), 8"A"~1 -6A
~ 6-1(1-$)"~1 '
e 4
(5-4)
_ r(a)
_r(c)r(v)/r(c+v)_
The first term in brackets is a ganma probability density function for parameter A, with scaling and shape f actors B and a, respectively.
The second bracketed term is a beta probability density function for parameter 4 with control variables ( and v.
Several prior distributions were considered, including diffuse distributions, by appropriately g
selecting values of the parameters.
EDRC e
e 5-4 8
The data at the GETR are given in the form of total offsets during periods of time dating back at least 128,000 to 195,000 years or longer.
For each period of time in which an offset occurred, one or more unknown a
number of events took place.
Rather than treating the number of events as a randan variable which caused the total displacement, it was assumed in developing the probabilistic model that this number is known.
It was found in studying the results from applying the methodology that the 9
assumed number of events does not significantly affect the results of the analysis. Thus, formulating the methodology in terms of an assumed number of offsets in each time period avoids the necessity of determining the distribution of offsets.
Subsequently it is shown that the results 4
are conservative and that this assumption is appropriate.
It should be noted in the following derivation that the probability function p(A,4) is conditional on knowing the number of Ns events.
9 For time period t, it is assumed that the total offset, A, (which j
j was measured) occurred due to nj assumed events.
The probability, p(n ), that nj events will occur 9 time period t is assumed to be j
j distributed as a Poisson process.
O (At ) j -At n
j e
j p(n ) =
(5-5) g nij 9
The probability of observing nj events in time period tj on either of the two shears is:
O (At ) s -At n
j e
j p(n ) =
(1-4)
(5-6) j nj i 4
EDHC e
O 5-5 O
Rather than using the total offset values given for each time period in Table 4-1, the assumed number of events, n, which produced the total j
offset values were used to obtain the likelihood function. This function is the joint probability of similar events for the four time periods for which shear displacements were recorded, (At ) I -At n
I 4
e n
8 j
L(A,4l data) = II (1-4) j (5-7) n ',
i=1 i
Retaining only terms involving A and 4, the posterior density G
function is obtained from equation 5-3 using equations 5-4 and 5-7.
-A(B+t)y +a-1 (-1(y_4)n+v-1 (5-8) n p( M ) = 4e 4
where:
4 t=
tj 4
""fy"i Solving for $ by requiring that the area under the density function is unity produces the following equation for p(A,4) p(A,4) = (B+t)"+"A"+ ~1 -A(84t)
,g-1(1-O"+"~I e
f(n +a) f(C)P(n+v)/r(C+n+v)
(5-9) 9 where:
f(+) is the ganna function.
A weighted estimate for P, which is designated as P, is obtained by 1
1 O
weighting the equation for P1 (see equation 5-2) by the probability density function for A and 4 (see equation 5-9) and integrating over all possible values of A ano 4 The weighted estimate F1 is given by the following equation.
EDAC 9
5-6
"+"+
(e n+a
(
(g+ Q+t Y*
(8+t)[(+n+v)
(5-10) 1 tg The parameters B, a, C, and v were investigated to detennine their effects on F.
By assigning the value of one to perameters ( and v, a uniformly 1
diffuse prior marginal probability density function P'(4) was used to compute F which produced conservative results when compared to using P'(4) 1 which was skewed toward the value of 4 equal to zero. Observations of surface faulting from past earthquakes suggest that the mean value for 4 is considerably less than 0.5; thus it reasonable to assume a uniform prior distribution for 4 Assuming that the marginal distribution p'(A) is uniformly diffuse is also conservative since the mean value of a O
unifonn unbounded density function is infinite, and it is reasonable and conservative to expect that the mean value of A is small.
To make A and 4 unitannly diffuse, density function values of B=0, a=1, O
(=1, and v=1 were assumed for the prior joint density function for A and 4 For this case, equation 5-10 can be simplified as follo'is:
n+2 F = (t 1)
(5-11) 1 n+2 It can be seen from equation 5-11 that 7 is very insensitive to large 7
values of n, which is assumed to be known.
In other words, the value of n, the assumed total number of events in time t, does not significantly affect F.
Since n may be small, a conservative bound on P is given 1
i by the following equation which is independent of n.
O F <f (5-12) 1 In the case of the GETR site, t is at least 128,000 to 195,000 years.
EDRC
5-7 An alternate approach to obtain values for P is based on the y
concept of probability limits (which for decision purposes can be considered as confidence limits). Using the probability density function p(A,4) the regio, of values for A and 4 where 4Ae A exceeds an assumed value for Py can be determined.
The integral of the density function in this region is the probability of exceeding P.
In analytical tenns y
the probability level, C, is given by the following equation for assumed unifonnly diffuse prior distributions for 4 and A.
n1 A
t n -At (1-P
-A)"+1 dA (5-13)
C=1-
]
e e
A where A' = lower limit where Ae-A =P y A" = upper limit where Ae-A =P y
Frm this equation, values of Py can be obtained so that there is a large probability that larger values will not occur. Corresponding values of P then can be obtained using equation 5-1.
Both approaches g
were used in the analysis and the results are given later.
Derivation of Probability P 2
Figure 5-1 shows the geometrical aspects of the probability density function for occurrence of an offset between the two shears (given that one occurs, but not on shears B-2 and 8-1/B-3).
Various shapes of probsbility density functions were investigated to determine the sensitivity of P to various assumptions. All shapes investigated had 2
e monotonically decreasing ordinates from the shears to the midpoint between the shears and considered to be symetric about the midpoint.
Geological evidence indicates that this is a reasonable assumption, since it is more likely that future offsets will be closer to the existing 4
shears.
NNSE
5-8 Because the reactor buildino is located one-quarter of the distance between the two shears, th! different density functions which were investigated produced values of P which are always less than or equal 2
to the values produced by a uniform distribution. Because of this 9
finding, it was conservatively assumed that P2 would be based on a uniform density function.
Since the fault is assumed to have a width greater than zero, the probability that an offset will intersect the foundation of the reactor building is given by the following equation:
P 2=
(5-14) where:
1: width of the reactor building L:
distance between the two shears O
b:
width of the offset at the ground surface For an assumed value of b equal to 4 feet and values of 72 feet and 1,320 feet for 1 and L, respectively, P was calculated to be 0.058.
2 Results of the Model Analysis Equations 5-12 and 5-14 were was used with equation 5-1 to calculate conservative weighted estimates of the probability P.
It was found that e
for values of time equal to 128,000 and 195,000 years, the annual prob-abilities of an offset occurring beneath the reactor building are less than 4.5 x 10-7 and 3.0 x 10-7, respectively. Values corresponding to the 95 percent and 50 percent probability levels were obtained using equations 5-1, 5-13, and 5-14 for values of n equal to 2 and 15.
In this part of the analysis, uniformly diffuse distributions were assumed for the prior density functions for A and 4 O
HDAC e
5-9 The probability values are given in Table 5-1.
Note that the probability level values are relatively insensitive to n.
This verifies the assumption made in the development of the methodology that the results would not depend on the number of offsets which occurred on shears B-2 and B-1/B-3 during the period up to 128,000 to 195,000 years ago.
Table 5-2 gives the probability levels corresponding to the criterion probability value of P equal to 10-6 ALTERNATE PROBABILITY ANALYSIS Confidence level probabilities can be determined using the one-sided g
Chi-squared distribution with two degrees of freedom. For this approach, each year is a trial and it was found that zero occurrence of offsets have occurred between the two shears for at least 128,000 to 195,000 years. The probability, P, of an offset beneath the reactor building g
consists of the product of P times P where P is computed for a y
2 y
given confidence level and P is identical to the value used in the 2
model probabilistic analysis. An equivalent approach to the Chi-squared distribution for large values of time is given by the following equations:
O C = (1 - e-P t)
(5-15) l P=
PyxP2 O
where C = Confidence level probability t = Total time that no offsets have occurred between shears B-2 and B-1/B-3 4
Probabilities P and associated confidence level values for the alternate approach are given in Table 5-1, and confidence levels corresponding to the criterion probability value of P equal to 10-6 are given in Table 5-2.
O EDRC e
e 5-10 9
TABLE 5-1 PROBABILITIES OF 0FFSET BENEATH REACTOR BUILDING G
Probability of Offcet Occurring Beneath Reactor Building Model A1proach Alternate Approach t=128,000 yrs t=195,000 yrs t=128,000 t=195,000 Analysis Basis n=2 n=15 n=2 n=15 years years Weighted Estimate (4.5x10-7)*
(3.0x10-7)*
95% Probability Level 1.1x10-6 1.3x10-6 7.0x10-7 8.4x10-7 1.4x10-6 8.9x10-7 50% Probability Level 2.2x10-7 2.9x10-7 1.5x10-7 1.9x10-7 3.1x10-7 2.1x10-7 9
3 = f, note that this estimate corresponds to the 63% confidence
- Based on 7 level based on the alternate approach O
4 O
EDRC e
e 5-11 9
TABLE 5-2 PROBABILITY LEVELS FOR OFFSET BENEATH REACTOR BUILDING FOR CRITERION PROBABILITY VALUE G
Probability of Model Approach Alternate Approach Offset Beneath
- t=128,000 yrs t=195,000 yrs t=128,000 t=195,000 Reactor Building n=2 n=15 n=2 n=15 years years 1x10-6 o,94 0.91 0.98 0.97 0.89 0.96 O
e O
4 9
EDHC e
e b
e PROBABILITY o
sq TRI AL DENSITY FUNCTION N~%
f s'
UNIFORM DENSITY FUNCTION,,,,,-
~ x __
1 DISTANCE y100'
-100 y a.
E600-
-soo z o
Soo-
[
Shear B-IIB-3 o
h REACTOR BUILDING >
~SCO dooj Shear B-2 SCALE: HORIZONTAL-VERTICAL 0
200
...i mC T
D m
am FIGURE 5-1 GE0 METRICAL PROBABILITY DENSITY FUNCTIONS FOR GETR SITE
4 6-1 O
6.
EVALUATION OF CONSERVATISM O
In development of the probabilistic model and methodology, conservative assumptions were made which, if relaxed, would produce lower probabili-ties than calculated. The following is a list of the conservative assumptions which were made in the analysis.
The annual probability was calculated for surface rupture offset g
beneath the reactor building and compared to a criterion value of 10-6 This implies an extremely conservative assumption of a unit probability for the occurrence of potential exposure given that the offset occurs. The GETR is currently being modified to g
resist the effects of a 3-foot offset anywhere on the site.
Thus, the probability of the occurrence of the exposure consequences can be conservatively estimated to be at least one order of magnitude less than the probability of occurrence of the e
offset alone.
Note that the potential consequences of an accident at the GETR are on the order of one-sixtieth of a nuclear power plant.
This assumption for potential exposure implies that all the probabilities given in Table 5-1 are less O
than 10-6 and many, including the wr'mhted estimate, are less than 10-7,
Age of unfaulted soil materials between shears B-2 and B-1/B-3 4
were assumed to be less than 195,000 years.
However, since the material beneath the reactor building is deeper and older than materials exposed in Trench B-1, this is a conservative estimate.
9 EDAC e
4 6-2 4
A very conservative geometric model for detennining probability P2 was developed.
It is also possible that a future offset will occur outside of the area between the two shears.
Because of limitations on the available data, the analysis was couched in terms of offsets of any size. The probability of exceeding the design criteria of three feet is much smaller than the probabilities presented in Table 5-1, which are for offsets of any size.
It is likely that the probability of exceeding three feet is at least one order of magnitude smaller than the probability of occurrence of an offset of any size.
The prior distributions for A and 4 were conservatively assumed.
The conclusions of the analysis are based on the total time period of 128,000 years.
Even lower probabilities are obtained 9
ff the upper bound value of 195,000 years is used.
The width of a future offset at the ground surface was assumed to be the maximum value of four feet, rather than a range between 4
two and four feet.
9 O
O EDRC e
4 7-1 4
7.
CONCLUSIONS O
The weighted estimate values for the annual probability of a surface rupture offset occurring beneath the reactor building are less than 10-6 These values correspond to the alternate approach confidence limit value of 63 percent. The probability limit values are all approximately 10-6 with the higher values being associated with the 95 percent level, which is a very conservative limit.
It can be conservatively stated based on the analysis that the probability of an offset of any size beneath the reactor building is 10-6 with a confidence limit of almost 90 percent based on the alternate analysis approach which is the most conservative.
9 The various elements of conservatism which were used in the analysis are listed in Chapter 6.
The most important assumption is concerned with the possibility for potential exposure, given that an offset occurs. The GETR is being modified to resist the effects of a 3-foot offset.
It is conservatively estimated that the probability of exposure consequer.ces is at least one order of magnitude less than the calculated probabilities given in Chaoter 5.
9 In conclusion, the calculated probabilities are less than the criteria value of approximately 10-6 and reasonable qualitative arguments have been made to insure that a realistic estimate of the probabilities of a
potential exposures would be lower.
Based on these findings, it is concluded that surface rupture offset at the GETR should be eliminated as a design basis event.
HDAC
REFERENCES e
o e
e 4
e EDHC
ep R-1 O
REFERENCES O
1.
Benjamin, J. R., and Cornell, C. A., Probability, Statistics, and Decision for Civil Engineers, McGraw-Hill Book Company,1970.
2.
iD Earth Sciences Associates, " Geologic Investigation, Phase 2, General Electric Test Reactor Site, Vallecitos, California," Report to General Electric Co., Vallecitos, California, February 1979.
3.
USNRC, " Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," LWR Edition, September 1975.
II 4
Cornell, C. A., " Engineering Seismic Risk Analysis," Bulletin of the Seismological Society of America, Vol. 58, No. 5, pp. 1,583-1,606, October 1968.
5.
Merz, H. A., and Cornell, C. A., " Seismic Risk Analysis Based on the g,
Quadratic Magnitude-Frequency Law," Bulletin of the Seismological Society of America, Vol 63, No. 6, pp. 1,999-2,006, December 1973.
6.
Algermissen, S. T., and Perkins, D. M., "A Probabilistic Estimate of Maximum Acceleratioa in Rock in the Contiguous United States," U. S.
Geological Survey, Open File Report 76-416, 1976.
9 9
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O O
O Appendix A GEOLOGIC BASES FOR PROBABILISTIC ANALYSIS O
O by Earth Sciences Associates 701 Welch Road Palo Alto, California 94304 O
O for General Electric Company Vallecitos, California O
O O
April 1979 Earth Sciences Associates
O Appendix A O
GEOLOGIC BASES FOR PROBABILISTIC ANALYSIS Introduction O
The purpose of this appendix is to provide the geologic bases for the model described in Chapter 3 of the probability analysis. Data have been accumulated through extensive investigations of geologic and tectonic conditions at, and in 8
the vicinity of, the Vallecitos Nuclear Center. The data collected, references cited, and interpretations made during those investigations are presented in a series of reports (ESA,1978a,b,c,d; 1979). Those data which form the geologic 0
bases for Table 4-1 used in the probability analysis are presented and discussed below under headings which relate to the various elements described in Chapter 4 of the main report.
O Location of Shears A series of trenches were excavated from the Vallecitos hill-front southwest O
to the southern boundary of the VNC to investigate all photolinears and geomor-phic features which were postulated as indications of young faulting at the GETR site. Three subparallel west-northwest trending shears were found in these trenches O
(see Figure A-1, following, and Figures 1 and 2 in ESA,1979). " Shear B-2" is ex-posed in Trench B-2 and ten of the B-2 series side trenches; " shear B-1/B-3" is exposed in Trenches B-1, B-3, T-1, and possibly T-2; and the "II-shear" was en-O countered in Trenches II,11-1, and possibly II-2. No other shears were found in any of these trenches.
O O
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9 O
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9
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\\\\ s e
\\
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\\\\N '5,
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4
'sN \\
\\
s1 a-3 f
- v,%
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osin a-2 a m m
' T.1 S
l N
N H-tronche O
O SCALE O
2000 e
feet Figure A-1: Location of shears in relation to GETR O
G Character of Shears g
The three shears at the GETR site show a common style or character of deformation. Each of the shear systems exhibits low angle, essentially reverse dip-slip offset along a main (first order) shear and associated second and third g
order deformation in the vicinity of the main shear. The characteristics of each order or scale of deformation, as observed in trench exposures, are outlined below.
First Order Deformation (main shear) g well developed shear fabric (shear foliation, claycy gouge, slicken-sides, striations, grooves / mullion structure) in well defined zone up to several inches thick continuity of shear to base of trench 4
truncates stratigraphic units juxtaposes datable soil-stratigraphic units of differing age shows evidence of repeated movement 9
commonly shows noticeable local change in dip of units across shear generally forms sole of zone of second and third order deformation Second Order Deformation (splays of main shears, sheared bedding contacts) show some degree of shear fabric development commonly conformable to, or coincident with, bedding contacts continuity of shear to base of trench generally subparallel to main shear offsets of stratigraphic units small where measurable do not truncate or offset soil-stratigraphic markers, where markers are present Third Order Deformation (joints, discontinuous shears, small-scale folds) show very minor (some slickensides, thin clay coatings, etc.) to no shear fabric along planar surfaces 9
A-2 Earth Sciences Associates
4 discontinuous (finite length) show very minor local to no offset commonly occur along bedding contacts, within claycy units and associated with folds O
Expected Width of Surface Rupture Zone On the basis of the observed characteristics listed above, it is evident that most of the inelastic strain associated with any single surface rupture event occurs 9
as offset along a main (first order) shear. Near surface splaying of main shears, as observed in trench exposures, may result in surface rupture within a zone up to two to four feet wide around the main shear.
O Both second and third order deformation probably reflect strain relief from some combi. nation of arabient, long-term stress and localized, short-lived stress conditions associated with surface rupture events. Where datable soil stratigraphic e
markers are present in Trenches B-1, B-2 and B-2 series side trenches, B-3, II, and T-1, it is apparent that second and third order deformation has not resulted in measurable surface offset at least in the last 70,000-125,000 years.
Thus, in terms of predicted future behavior, one would expect surface offset to occur within a narrow (2-4 foot wide) zone associated with a first order shear.
Associated second and third order deformation would occur as minor strain in 6
the vicinity of the first order shear, without surface offset.
Amount and Time of Measured Offsets on Individual Shears The ages of soil-stratigraphic horizons pertinent to assessing the time of offset on major shears at the GETR site are provided in Appendices A and B of the Phase II geologic report (ESA,1979).
O A-3 Earth Sciences Associates
O Direct measurements were made in the field of the amount of net reverse g
separation of the various marker units along shears B-2 and B-1/B-3. There was no displacement in any of the trenches of the organic horizons (Cambic) of the modern solum, whose minimum age is at least 8000 years. The measurements g
of stoneline and stage 5 palcosol displacements are tabulated in Table A-1, follow-ing, and are discussed in detail in Appendix B of the Phase II geologic report (ESA, 1979). Additional offset data were interpreted from the geologic logs of explora-g tory trenches 11 and T-1 by ESA in consultation with Dr. Roy Shlemon. These values are also shown in Table A-1. In addition to offsets of younger soil-strati-graphie units, the trench exposures reveal a cumulative offset of pre-70,000 to G
125,000 year old units of at least 40-80 feet along the major shears (see Trench II, B-1, B-2, and R-3 logs, ES A,1979, Appendix E).
G Ages of Geologic Units Between Shears B-2 and B-1/B-3 One element of the input to the probability model described in the main report is the age of unfaulted units in the reactor foundation area. In the Phase O
Il geologic report (ESA,1979), the units at the elevation of the GETR base slab were inferred to be Livermore Gravels on the order of one million years old on the basis of the following reasoning. Units exposed near the bottom of the deep northeast end of Trench B-1 beneath the main shear were interpreted to be Liver-more Gravels on the basis of lithology, continuity of stratigraphic units, and degree of consolidation. The subsurface projection of these units would intersect the O
GETR base slab, which is buried approximately 20 feet below parking-lot grade and about 25-30 feet below the pre-excavation natural grade. Limits on the age of the Livermore Gravels are not well known. Ilowever, as noted by Sarna-Wojcicki G
(1976), part of the section may be as much as 4.5 million years old. At the GETR S
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9 Table A-1 MAXIMUM MEASUllED DISPLACEMENTS (ft)(
8000 to 15,000 -
17,000 to 20,000 yrs.
17,000 to 20,000 -
70,000 to 125,000 -
0-8000 to 15,000 yrs.
(Pre-Cambic IIorizons 70,000 to 125,000 yrs.
128,000 to 195,000 yrs.
(Post-Cambic llorizons of Modern Solum (Pre-Stoneline and or greater Trench No.
of Modern Solum) and Post-Stoneline)
Post-Stage 5 Palcosol)
(Pre-Stage 5 Paleosol) 11-2 Shear 11-2 0
3.0 6.0 Il-2 0
5.5-11.0 2
0 2.5 3.9 and 3.6 3
0 0.5
- i.8 4
0 1.5 5
0 1.4 6
0 2.7 6
0 2.7 7
0 0.7 and 1.3 5.0-6.0 8
0 2.2 10 0
1.0 -
13 0
2.6 8.5-9.5 14 0
1.5 +
15 0
1.2 1.0-1.5 Maximum Value 0
3.0 3.0-8.0 806 m
Il-1/B-3 Shear Il-1 0
1.9 8.0-10.0
,g 11-3 0
10.0-11.0 T-1(2) 0 2.0 14.0 m
9-Maximum Value 0
2.0 12.0 40+
(D Dn 11 Shear g) rD II,11-1 0
1.5 4.0 m
Maximum Value 0
1.5 4.0 20+
w O
O.
(1) See Appendices A and 11 (ESA,1979) for discussion and additional details of data.
m, (2) Displacements inferred from trench logs in ESA,1979.
c0 m
G site, the presence of three very well 'eveloped palcosols above Livermore Gravels in Trenches 11 and B-3 (see Appendix B, ESA,1979) indicates that the Gravels are at least older than 350,000 years. Given that the Livermore Gravels at the site occur deep within the very thick section on the breached southern flank of g
the Livermore syneline (ESA,1978), it is reasonable to estimate their age as being on the order of at least a million years. If one assumes that the GETR is located on the up-thrown side of two splays of a major thrust fault at the site, one would g
have to infer that the Livermore Gravels beneath it would be even older than in-ferred for an unfaulted section.
In order to arrive at a conservative input for the probabilistie model, how-e ever, and in response to comments expressed by the NRC, this interpretation has been reevaluated. Dr. Roy Shlemon has reexamined exposures in Trench B-1, particularly in the area toward which a shear beneath the GETR structure woald 9
project, and has interpreted the geologic log of Trench B-1 in soil stratigraphic terms. As discussed in Appendix B by Dr. Shlemon, the conclusion reached is that units projecting to the base slab of the GETR are at least greater than 128,000 9
years old and may be significantly older, and that units of comparable age are exposed for the fulllength of Trench D-1 and above the main shear in Trench B-2.
O Expected Distribution of Future Surface Rupture Within the area of the model, offsets have occurred only on the B-2 and B-1/B-3 shears during at least the last 128,000 years and probably for a much I
longer period. Based on this historical record and on the fact that these two shears provide preferential planes of weakness on which failure can take place, it is more likely that future offsets will occur on these existing shears. On the basis of the e
age and amount of observed offsets (Table A-1), any future offset is as likely to occur on one of these shears as on the other.
O Earth Sciences Associates A-5
9 REFERENCES O
Earth Sciences Associates,1979. Geologic investigation, Phase II, General Electric Test Reactor Site, Vallecitos, California. February,1979: repart to General Electric Co., Vallecitos, California.
,1978a. Geologic investigation, General Electric Test Reactor Site, g
Vallecitos, California, February,1978: report to General Electric Co.,
Vallecitos, California.
,1978b. Geologic investigation, General Electric Test Reactor Site, Vallecitos, California, Addendum I, April 1978: report to General Electric Co., Vallecitos, California.
O
,1978c. Landslide stability, General Electric Test Reactor Site, Vallecitos, California, J. !v,1978: report to General Electric Co., Vallecitos, California.
,1978d. Geologic evaluations of GETR structural design criteria, report 2:
Ground motion and displacement on a hypothetical Verona fault: report to G
General Electric Co., Vallecitos, California.
Sarna-Wojcicki, A.M.,1976. Correlation of late Cenozoic tuffs in the central Coast Ranges of California by means of trace and minor elemer.t chemistry:
U.S. Geological Survey Prof. Paper 972,30 p.
6 9
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e A-6 Earth Sciences Associates
e Appendix B e
Age of Sediments General Electric Test Reactor Site Vallecitos Nuclear Center Alameda County, California O
e by Roy J. Shlemon e
for Earth Sciences Associates g
Palo Alto, California on behalf of General Electric Company Vallecitos, California 4
9 Roy J. Shlemon & Assoc., Inc.
P.O. Box 3066 Newport Beach, California 92663 O
April 1979 e
O Appendix B g
Age of Sediments General Electric Test Reactor Site Vallecitos Nuclear Center Alameda County, California O
INTRODUCTION The relative age of soil-stratigraphic units and intervening sediments at the General Electric Test Reactor site (GETR) has been spelled out in previous investigations (Earth Sciences Associates (ESA),1979, Appendices A and B). Par-g ticularly documented is the age of the section at and adjacent to shear planes exposed in Trenches B-1 and B-2 (see trench logs, ESA,1979, Appendix E). At question, however, is the age of sediments and soils between the two shears, es-g pecially those directly opposite the GETR. This study points out the most probable age of this section based on field examination and interpretation of applicable trench logs, especially the area between stations 3+54 and 4+26 (ESA,1979, Fig.
g E-4). The rationale for assignment of ages is indicated in sections following.
SOIL-STR ATIGRAPIIIC DATING G
Trenches in the GETR area expose perhaps the best known Quaternary se-quence of sediments and soils in the Central Coe'. Ranges of California. Observed, for example, are at least three discrete, strongly-developed buried paleosols (Pale-4 xeralfs) in Trenches B-3 and }{ (ESA,1979, Appendix B); each forming on older alluvial / colluvial deposits. Also, though not everywhere present, the base of the modern colluvium is marked by a stoneline, useful for correlation in the GETR 9
area.
B-1 4
O Except for mean-residence-time radiocarbon dates obtained from organic g
horizons of the modern solum, the absolute age of GETR soil stratigraphy is un-known. Approximate ages, however, can be determined by observation of degree of soil profile development and by association of soils and sediments with Quater-g nary epochs of regional climatic change. The basic principles of relative dating are spelled out in ESA,1979 (Appendices A and B), and are reviewed briefly here as applied to assessing age of sediments between shears B-1/B-3 and B-2.
O Major changes of climate and vegetation during the Quaternary resulted in alternating epochs of regionallandscape stability and instability. Times of glacial and/or pluvial conditions in latitudes comparable to that of the GETR site 9
were generally characterized by relative landscape instability. As a result, collu-viation and sediment production increased, represented in the GETR area as pied-mont fan and debris deposits. Epochs of landscape instability correspond approxi-9 mately with low stands of sea level (glacio-eustatie) deemed in the marine oxygen-isotope chronology as stage 2 (youngest),4,6,8, etc. (see ESA,1979, Appendix A, Figs. A-4a and A-4b). Conversely, during interglacial/interpluvial epochs, relative landscape stability prevailed, often giving rise to regional soil formation.
The equivalent marine isotope stages are designated, correspondingly,1 (present),
3, 5, 7...ete.
O GETR SITE STR ATIGRAPHY g
The dated soil-stratigraphic section at the GETR site, based on association with the marine isotope chronology, is portrayed diagrammatically in Figure 1.
Not all units are continuous; and facies changes are common. Nevertheless, the g
stratigraphy can be traced laterally from areas of detailed analysis, adjacent to the B-1/B-3 and B-2 shears, to that portion of the trench closest to the GETR.
B-2 O
9 9
9 9
9 6
4 4
9 9
9 8-1/8-3 Sheer 4
Strong Paleosol (Stage 5) s
,, -*-,,,'. 8 Stoneline & Colluvium (Stage 21 m i.
B 2 Shear p
Modern Soil (Stage 1) 1 - *
- 7 * 'f'f77/
Week Paleosol (Stage /)
3
,,, _, _,,l g - {, s " * %'~/ /
,,,i f
/ / './ I.#
iiisia
.w:
,,,,,,,, p iiii i
_~f.__.*eQ:
l
%gf.//fL e x~~ *, -~ * * * ~ t. e *
.s
'O 2% Alluvium (Stage 4) ' -
~
~
Base of Trench Figure 1: Schematic diagram showing relationship of late Quaternary soils and sediments at GETR site. Not to scale; stages refer to marine isotope chronology. For detailed log see ESA,1979, Figure E-4.
O The youngest sediments are colluvial / alluvial piedmont fan and debris flows g
laid down mainly during the stage 2 epoch of landscape instability, some 17,000 to 20,000 years ago. The base of these deposits is frequently marked by a distinct stoneline (Fig.1) with clasts derived from adjacent outcrops of the Livermore g
Gravels. Minor colluviation continues today, mainly on steeper slopes near hill-fronts, particularly at the B-1/B-3 shear. In general, however, regional landscape stability now prevails, giving rise to the weak-to moderately-developed modern g
solum (isotope stage 1).
Beneath the stoneline and its downslope projection (between about stations 1+00 and 4+60; ESA trench log, Fig. E-4) are channel-like deposits scoured into 9
older alluvium (Fig.1). Because of their stratigraphic position and bearing of a weakly-developed palcosol(remnant B and C horizons), these deposits were 3
likely laid down during an epoch of landscape instability preceeding stage 2. Infer-9 entially, therefore, these gravels were deposited mainly during isotope stage 4, about 60,000 to 70,000 years ago. Accordingly, the capping remnant palcosol formed during stage 3, some 35,000 to 50,000 years before present.
The stage 4 gravels, in turn, are underlain by intercalated sandy silts and clayey sands, often characterized by a weathered, reddish-brown matrix. Opposite the GETR these sediments do not bear the strongly-developed buried paleosol G
of stage 5 age (approximately 70,000-125,000 years old) so clearly visible adjacent to the B-1/B-3 and B-2 shears (Fig.1). Field inspection, however, indicates that these basal sediments continue beneath the stage 4 gravels to a depth of almost O
15 feet at the base of the trench. Thus these lowermost deposits, at their point nearest the GETR, were most likely laid down during a yet earlier epoch of regional landscape instability; namely, during isotope stage 6, about 128,000 to 195,000 9
years ago.
B-3 e
4
SUMMARY
9 The soil-stratigraphic section in Trench B-1 exposes a particularly complete seqJence of alternating depositional and weathering (soil) units. The section is dated relatively, mainly by degree of soil profile development and by association 9
with alternating epochs of regional landscape stability and instability, wrought by late Quaternary climatic change.
Maximum sediment production in the GETR area occurred during times of 9
landscape instability; from younger to older equated to marine isotope stages 2, 4, and 6, respectively. Epochs of landscape stability and soil formation dominated during stages 1,3, and 5, giving rise to the modern solum (stage 1), a weakly devel-9 oped buried soil (stage 3), and an underlying strongly-developed palcosol (stage 5).
By association with the marine isotope chronology, the oldest sediments exposed in Trench B-1, directly opposite the GETR, pertain to stage 6, and thus most likely were laid down between about 128,000 and 195,000 years ago.
49 89 9
9 B-4 S