ML20209E996
| ML20209E996 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 09/02/1986 |
| From: | ALABAMA POWER CO. |
| To: | |
| Shared Package | |
| ML20209E994 | List: |
| References | |
| NUDOCS 8609120012 | |
| Download: ML20209E996 (24) | |
Text
- ._
PLANT SYSTEMS 3/4.7.9 SNUBBERS LIMITING CONDITION FOR OPERATION 3.7.9 All snubbers shall be OPERABLE. The only snubbers excluded from this requirement are those installed on nonsafety-related systems and then only if their failure or the failure of the system on which they are installed would have no adverse effect on any safety-related system.
APPLICABILITY: MODES 1, 2, 3 and 4. (MODES 5 and 6 for snubbers located on systems required OPERABLE in those MODES).
ACTION:
With one or more snubbers inoperable, within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> replace or restore the inoperable snubber (s) to 0PERABLE status and perform an engineering evaluation per Specification 4.7.9.c on the supported component or declare the supported system inoperable and follow the appropriate ACTION statement for that system.
SURVEILLANCE REQUIREMENTS 2
4.7.9 Each snubber shall be demonstrated OPERABLE by performance of the following augmented inservice inspection program and the requirements of Specification 4.0.5.
- a. Visual Inspections Snubber visual inspections shall be performed in accordance with the schedule provided in Table 4.7-3. This table is applicable for populations of at least 200 snubbers. In determining the subsequent visual inspection period, the percent of inoperable snubbers, r, shall be used. The snubbers nay be categorized into two groups: those accessible and those inaccessible during reactor operation. Each group may be inspected independently in accordance with the schedule provided in Table 4.7-3.
FARLEY - UNIT 1 3/4 7-20 AMENDMENT NO.
8609120012 860902 8-DR ADOCK 05000
TABLE 4.7-3 Snubber Visual Inspection Schedule Percent of Snubbers Found Current Visual Inoperable, r, per Current Next Visual Inspection Period
- Visual Inspection Period Inspection Period #
24 Months + 25% -
r < 0.76% 24 Months + 25%
0.76% T r < 0.99% 18 Months iT 25%
0.99% <r < 1.40% 12 Months + 25%
1.40% <r T 2.43% 6 Months 17 25%
r > 2.47% 3 Months}[25%
18 Months + 25% -
r < 0.62% 24 Months + 25%
0.62% i r < 0.76% 18 Months 17 25%
0.76% <r i 1.09%
12 Months 77 25%
1.09% <r i 1.96%
6 Months iT 25%
r > 1.93% 3 Months 1[25%
12 Months + 25% -
r < 0.58% 18 Months + 25%
0.58% T r < 0.76% 12 Months 17 25%
0.76% <r T 1.40%
6 Months 77 25%
r > 1.4D% 3 Months}[25%
6 Months + 25%
r=0 12 Months + 25%
0<r < 0.76% 6 Months IT 25%
r > 0.73% 3 Months}[25%
3 Months + 25%
r=0 6 Months + 25%
r>0 3 Months}[25%
- The provisiras of Specification 4.0.2 are not applicable.
- Earlier visual inspections than required may be utilized. If this option is chosen, the criteria for determining the next visual inspection period shall be the criteria associated with the earlier visual inspection period selected.
4 FARLEY - UNIT 1 3/4 7-20a AMENDMENT NO.
T PLANT SYSTEMS BASES 3/4.7.9 SNUBBERS All snubbers are required OPERABLE to ensure that the structural integrity of the reactor coolant system and all other safety related systems is maintained during and following a seismic or other event initiating dynamic loads.
Snubbers excluded from this inspection program are those installed on nonsafety related systems and then only if their failure or failure of the system on which they are installed would have no adverse effect on any safety related system. A manual, which contains a tabulation of the hydraulic and mechanical snubbers which are within the scope of Technical Specification 3/4.7.9, is maintained as a comprehensive list of snubbers which are to be included in the snubber inspection and test program. The manual includes, as a baseline, the snubbers identified in Technical Specification Tables 3.7-4a and 3.7-4b as of License Amendment No. 43. The manual will be controlled in accordance with 10CFR50.59.
The visual inspection frequency is based upon maintaining a constant level of snubber protection to systems. Therefore, the required inspection interval varies inversely with the observed snubber failures and is determined by the number of inoperable snubbers found during an inspection. Inspections performed before that interval has elapsed may be used as a new reference point to determine the next inspection. However, the results of such early inspections performed before the original required time interval has elapsed (nominal time less 25%) may not be used to lengthen the required inspection interval. Any inspection whose results require a shorter inspection interval will override the previous schedule. Adherence to the visual inspection schedule will provide a 95% confidence level that at least 90% of the snubbers in the plant are operable.
When the cause of the rejection of a snubber is clearly established and remedied for that snubber and for any other snubbers that ray be generically susceptible, and verified by inservice functional testing, that snubber may be exempted from being counted as inoperable. Generically susceptible snubbers are those which are of a specific make or model and have the same design features directly related to rejection of the snubber by visual inspection, or are similarly located or exposed to the same environmental conditions such as temperature, radiation, and vibration.
When a snubber is found inoperable, an engineering evaluation is performed.
The engineering evaluation shall determine whether or not the snubber mode of failure has imparted a significant effect or degradation on the attached component.
FARLEY - UNIT 1 B 3/4 7-5 Af1ENDMENT NO.
PLANT SYSTEMS BASES
=
3/4.7.9 SNUBBERS To provide assurance of snubber functional reliability, a representative sample of the installed snubbers will be functionally tested during plant shutdowns at 18 month intervals. Selection of a representative sample according to the expression 35(1+C/2) provides a confidence level of approximately 95%
that 90% to 100% of ths snubbers in the plant will be OPERABLE within acceptance limits, where c is the allowable number of snubbers not meeting the acceptance criteria. Observed failures of these sample snubbers shall require functional testing of additional units.
Hydraulic snubbers and mechanical snubbers may each be treated as a different entity for the above surveillance programs.
l FARLEY - UNIT 1 B 3/4 7-Sa AMENDMENT NO.
PLANT SYSTEMS 3/4.7.9 SNUBBERS LIMITING CONDITION FOR OPERATION 3.7.9 All snubbers shall be OPERABLE. The only snubbers excluded from this requirement are those installed on nonsafety-related systems and then only if their failure or the failure of the system on which they are installed would have no adverse effect on any safety-related system.
APPLICABILITY: MODES 1, 2, 3 and 4. (MODES 5 and 6 for snubbers located on systems required OPERABLE in those MODES).
ACTION:
With one or more snubbers inoperable, within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> replace or restore the inoperable snubber (s) to OPERABLE status and perform an engineering evaluation per Specification 4.7.9.c on the supported component or declare the supported system inoperable and follow the appropriate ACTION statement for that system.
SURVEILLANCE REQUIREMENTS 4.7.9 Each snubber shall be demonstrated OPERABLE by performance of the 4
following augmented inservice inspection program and the requirements of Specification 4.0.5.
- a. Visual Inspections Snubber visual inspections shall be performed in accordance with the schedule provided in Table 4.7-3. This table is applicable for populations of at least 200 snubbers. In determining the subsequent visual inspection period, the percent of inoperable snubbers, r, shall be used. The snubbers may be categorized into two groups:
those accessible and those inaccessible during reactor operation.
Each group may be inspected independently in accordance with the schedule provided in Table 4.7-3.
FARLEY - UNIT 2 3/4 7-20 AMENDMENT N0.
1
,-a < -------y - , - g. ~--. m -
TABLE 4.7-3 Snubber Visual Inspection Schedule Percent of Snubbers Found Current Visual Inoperable, r, per Current Next Visual Inspection Period
- Visual ~ Inspection Period Inspection Period #
24 Months + 25%
r < 0.76% 24 Months + 25%
0.76% T r < 0.99% 18 Months T 25%
0.99% <r T 1.40%
12 Months T 25%
1.40% <r T 2.43%
6 Months T 25%
r > 2.43% 3 Months][25%
18 Months + 25%
r < 0.62% 24 Months + 25%
0.62% T r < 0.76% 18 Months T 25%
0.76% <r T 1.09% 12 Months T 25%
1.09% <r i 1.96% 6 Months T 25%
r > 1.93% 3 Months][25%
12 Months + 25%
r < 0.58% 18 Months + 25%
0.58% T r < 0.76% 12 Months T 25%
0.76% <r i 1.40%
6 Months T 25%
r > 1.40% 3 Months][25%
6 Months + 25%
r=0 12 Months + 25%
0<r < 0.76% 6 Months T 25%
r > 0.76% 3 Months][25%
3 Months + 25%
r=0 6 Months + 25% ~
r>0 3 Months][25%
- The provisions of Specification 4.0.2 are not applicable.
- Earlier visual inspections than required may be utilized. If this option is chosen, the criteria for determining the next visual inspection period shall be the criteria associated with the earlier visual inspection period selected.
FARLEY - UNIT 2 3/4 7-20a AMENDMENT NO.
l
PLANT SYSTEMS.
BASES 3/4.7.9 SNUBBERS All snubbers are required OPERABLE to ensure that the structural integrity of the reactor coolant system and all other safety related systems is maintained during and following a seismic or other event initiating dynamic loads.
Snubbers excluded from this inspection program are those installed on nonsafety related syste.ns and then only if their failure or failure of the system on which they are installed would have no adverse effect on any safety relateo system. A manual, which contains a tabulation of the hydraulic and mechanical snubbers which are within the scope of Technical Specification 3/4.7.9, is maintained as a comprehensive list of snubbers which are to be included in the snubber inspection and test program. The manual includes, as a baseline, the snubbers identified in Technical Specification Tables 3.7-4a and 3.7-4b as of License knendment No. 34 and also includes the changes identified in Alabama Power Company letter dated December 8,1983 entitled " Safety-Related Snubber Technical Specification Table Changes". The manual will be controlled in accordance with 10CFR50.59.
The visual inspection frequency is based upon maintaining a constant level of snubber protection to systems. Therefore, the required inspection interval varies inversely with the observed snubber failures and is determined by the number of inoperable snubbers found during an inspection. Inspections performed before that interval has elapsed may be used as a new reference point to determine the next inspection. However, the results of such early inspections performed before the original required time interval has elapsed (nominal time less 25%) may not be used to lengthen the required inspection interval. Any inspection whose results require a shorter inspection interval will override the previous schedule. Adherence to the visual inspection schedule will provide a 95% confidence level that at least 90% of the snubbers in the plant are operable.
When the cause of the rejection of a snubber is clearly established and remedied for that snubber and for any other snubbers that may be generically susceptible, and verified by inservice functional testing, that snubber may be exempted from being counted as inoperable. Generically susceptible snubbers are those which are of a specific make or model and have the same design features directly related to rejection of the snubber by visual inspection, or are similarly located or exposed to the same environmental conditions such as temperature, radiation, and vibration.
When a snubber is found inoperable, an engineering evaluation is performed.
The engineering evaluation shall determine whether or not the snubber mode of failure has imparted a significant effect or degradation on the attached component.
FARLEY - UNIT 2 B 3/4 7-5 AMENDMENT N0.
PLANT SYSTEMS BASES 3
3/4.7.9 SNUBBERS To provide assurance of snubber functional reliability, a representative r sample of the installed snubbers will be functionally tested during plant I shutdowns at 18 month intervals. Selection of a representative sample according to the expression 35(1+c/2) provides a confidence level of approximately 95%
that 90% to 100% of the snubbers in the plant will be OPERABLE within acceptance limits, where c is the allowable number of snubbers not meeting the acceptance criteria. Observed failures of these sample snubbers shall require functional testing of additional units.
Hydraulic snubbers and mechanical snubbers may each be treated as a different entity for the above surveillance programs.
FARLEY - UNIT 2 B 3/4 7-Sa AMENDMENT N0.
l
ATTACHMENT 2 SIGNIFICANT HAZARDS EVALUATION PURSUANT TO 10CFR50.92 FOR THE PROPOSED CHANGE TO THE SNUBBER SURVEILLANCE REQUIREMENTS TECHNICAL SPECIFICATION Proposed Change This proposed change revises the visual inspection requirements of Technical Specification 4.7.9 and the accompanying Bases. Also included in the proposed change is the removal of instructions for visual snubber surveillance following initial power operation. These instructions are no longer applicable for Farley Nuclear Plant Units 1 and 2.
Background
The Technical Specifications for Farley Nuclear Plant Units 1 and f urrently incorporate a visual inspection frequency table for surveillance 01 afety-related snubbers that was developed by the NRC as part of the Standard Technical Specifications in 1975. This table is based on tne assumptions and mathematics cited in the " Note to Dennis L. Ziemann" from Mr. Joseph Sapir, Operating Reactors Branch
- 2, Division of Reactor Licensing, dated March 1,1975. This document establishes specific statistical goals which were combined with an assumption of small population to generate the existing Technical Specifications. Farley Nuclear Plant utilizes a larger population which results in an overly conservative achievement of the NRC goal. Therefore, Alabama Power Company feels that current Technical Specification requirements for visual inspection of safety-related snubbers are overly conservative and result in excessive surveillance requirements.
The Farley Nuclear Plant Technical Specification Bases state that functional testing requirements for safety-related snubbers are based on providing a confidence level of 95% that at least 90% of the safety-related snubbers in the plant will be operable.
The proposed nodification to the Technical Specification snubber visual inspection surveillance requirements is based on the application of statistical methodology to determine visual inspection intervals which meet the same acceptance confidence level as the Technical Specification snubber functional testing requirements.
The statistical methodology utilized employs a mathematical model which is used to determine the appropriate visual inspection interval. A chi-square distribution is used to estimate the individual snubber failure rate. The exponential distribution is used to predict the probability of a single snubber being operable. A binomial distribution can then be applied to the entire snubber population to determine the percent of failed snubbers which corresponds to particular current and next visual inspection intervals. The details and results of this analysis are provided in .
Attachment 2 Significant Hazards Evaluation Pursuant to 10CFR50.92 for the Proposed Change to the Snubber Surveillance Requirements Technical Specification Page 2 This proposed Technical Specification change involves only visual surveillance requirements and does not alter the currant Limiting Condition for Operation or the acccmpanying Action Statement for the snubber system. The statistical methods employed as the bases for the proposed Technical Specification change will not be used to alter the current Technical Specification requirement that all safety-related snubbers be operable or as justification to allow a snubber to remain in an inoperable condition. Furthermore, the conservative Technical Specification requirement to visually inspect 100% of the safety-related snubbers will not be altered.
Analysis Alabama Power Company has reviewed the requirements of 10CFR50.92 as they relate to the proposed change to the snubber visual inspection requirements and considers the proposed change not to involve a significant hazards consideration. In support of this conclusion the following analysis is provided:
(1) The proposed change will not significantly increase the probability or consequences of an accident previously evaluated because the existing snubber operability requirements will remain intact and the proposed visual inspection requirements will effectively verify snubber system reliability.
(2) The proposed change will not create the possibility of a new or different kind of accident from any accident previously evaluated because the change will not alter plant configuration or mode of operation.
(3) The proposed change will not involve a significant reduction to the margin of safety because the combination of visual inspection intervals which maintain a 95% confidence that at least 90% of all safety-related snubbers are operable at i all times along with the required functional testing of safety-related snubbers '
will provide adequate assurance that the snubber system will adequately perform its intended function.
Conclusion Based upon the analysis provided herewith, Alabama Power Company has determined that the proposed Technical Specification change will not significantly increase the probability or consequences of an accident previously evaluated, create the possibility of a new or different kind of accident from any accident previously evaluated, or involve a significant reduction in a margin of safety. Therefore, Alabama Power Company has determined that the proposed change meets the requirements of 10CFR50.92(c) and does not involve a significant hazards consideration.
s
9 Attachment 3
VISUAL INSPECTION PROGRAM FOR SNUBBER OPERABILITY SURVEILLANCE TABLE E CONTENTS
- 1. INTRODUCTION
- 2. DERIVATION OF PROBABILISTIC MODEL FOR RELIABILITY ANALYSIS
- 3. ESTIMATION OF PARAMETER
- 4. DETERMINATION OF VISUAL INSPECTION PERIODS
- 5. VISUAL INSPECTION PROGRAM -
- 6. CONCLUSION
- 7. REFERENCES TABLE 1 VISUAL INSPECTION SCHEDULE FOR ANY GROUP (CATEGORY)
, . , _ _ . . , . - _ - . . - . - - - -- ..n,., . , _-,..-- , , , , , _ _ _ - , _ _ _ , , - _ , . - _ . - - _ . _ - . -.n,, , ,_._ - - - - _ , ,, , _ , , _ . , , - -
VISUAL INSPECTION PROGRAM FOR SNUBBER OPERABILITY SURVEILLANCE
- 1. INTRODUCTION The objective of this study is to establish, based on probabilistic and statistical methodologies,'a visual inspection program to maintain the reliability of the snubber operability at a required level for the Joseph M.
Farley Nuclear Power Plant (FNP) Units 1 and 2. The
- required reliability level is specified by the following
! reliability criterion:
"To establish a visual inspection schedule to verify at a 95% confidence level that a minimum of 90% of the snubbers in a group or category are operable all the time".
Note that the above reliability criterion is based on maintaining a minimum level of snubber operability at all times end conservatively does not take the probabilistic nature' of the unlikely occurrence of a design basis event j , (DBE) into account.
- 2. DERIVATION OF PROBABILISTIC MODEL FOR RELIABILITY ANALYSIS Consider a group of snubbers. Let n denote the total number of snubbers in the group. Let X denote the random variable representing the time-to-failure (or operable life span) of any individual snubber in the group.
4 In this study', the random variable, X, will be assumed to follow the exponential distribution. Owing to all categories of the snubbers in both of the FNP Units 1 and 2 having long been periodically inspected and maintained, their hazard (time) functions (as defined by Eq.3-40 on Pg.103 of Reference 1) will virtually not fluctuate with the time between two consecutive visual inspection periods.
Because the hazard function of the exponential distribution 1
also does not fluctuate with time (as exhibited by Eq.3-42 on Pg.105 of Reference 1), it significantly indicates that the exponential distribution is appropriate for representing the time-to-failure of the snubbers.
2
Furthermore, per Pg.105 of Referenca 1, the exponential distribution is the most commonly used to represent the time-to-failure of a system. It plays a central role in reliability, comparable to that of the normal distribution in other applications.
since X follows the exponential distribution, its probabilitydensityfunction,g4(x)andprobability distribution, Fg(x) are
_ex rte , x>0,e>0
= '
((8) 0 ,
other wis e (1) x
-ex 1.- e ,x>o i
FX 4) = P{ X ( x} = f f (pdp . '
. (2)
~~ K 0 , otherwise 1
where the parameter e is physically referred to as the failure rate (number of failures per unit of time). Per Pg.107 of Reference 1, the mean or expected value of X, mg can be expressed as I
4 m y= (3) g Physically, mg is referred to as the the mean time-to-failure.
Let Ps be the probability that any individual snubber in the group fails before or at (any) time, t. Hence per Eq.2, P3 can be expressed as Pg = P(X < t } = l-e' ( 4')
i Let Z be the random variable denoting the number of failed snubbers in the group before or at time, t. Since the time-to-failure of each individual snubber in the group takes place independently of the other, the random variable Z follows the binomial distribution, namely, i
i 3
s n-1 P( Z 2) = ( n ,) P, ( 1 - P, ) (5) where rn , n! .
. (z )
- 2!(a-2)! (6)
Let P(Z < S} be the probability that there are no more than S anubbers in the group inoperable before and at time,
- t. Because Z is a binomial random variable, P(Z 1S}
can be expressed as S z P{ Z ( S } = g ( n,) P, ( 1 - P,)n- 2 (7)
The snubber reliability criterion (specified in.
Section 1) of having a 95% confidence level that there are a minimum of 90% of the snubbers (in the group) operable before and at (any) time, t, is equivalent to having a 95%
confidence level that there are less than 10% of the snubbers (in the group) inoperable befcre and at (any) time, t. Therefore, the latter (or equivalent) reliability criterion will be employed for the advantage of reducing numerical computation effort needed later. In order to obtain at 95% confidence level that there are less than 10%
of the snubbers (in the group) inoperable before and at (any) time, t; (a) set P(Z$S} in Eq.7 equal to 0.95, i.e.,
s x n-z p{z ( s) = ,1 ( n ) 2P, ( 1 - P,)
= o.95 (g) and (b) set the value of S in Eq.8 equal to the largest integer that is less than 0.In.
By substituting Pt from Eq.4 into Eq.8, Eq.8 becomes
$ r nI -et 1 -et n-z P(Z 4 S) = 22. o s r / (1 - 8 ) (8 ) 0 95 (9) i =
e 4
l
_ _ . . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ l
Eq.9 vill be the basis for the reliability analysis to develop the required visual inspection schedule in Sections 3 and 4.
- 3. ESTIMATION OF PARAMETER .
In order to be able to evaluat e the probability, P(Z < S} in Eq.9, the value ot the parameter o must be first estimated.
Let T denote a visual inspection period for the group (with n snubbers), and let I denote the total number of inoperable snubbers found in,the group per visual inspection period T. Also let an denote the lower confidence bound estimate of ex (the mean time-to-4 failure) at the (1-4)l00 percent confidence level. Per Eq.3-46 on Pg.107 of Reference 1, og can be expressed as
- 27
" * (10) x j x1-E,2X t 2 2
whereTi..,tz+2 is the Chi-Square value at (1-d)l00 th percentile at 21+2 degree of freedom. Per Eq.3-43 on Pg.107 of Reference 1, 7 is the sum of the actual operable time from every snubber in the group per inspection period and can be conservatively expressed as l , JI= (n-I)T (11)
Eq.11 provides the conservative 7 because all the inoperable snubbers are assumed to fail at the beginning of the visual inspection period, T. In reality, an inoperable snubber may become inoperable at any time between 0 and T. Note thatX*i.w,trtz can be obtained from Table IV on Pgs.314 and 315 of Reference 1 for 2I+2 6 100.
By substituting 7 from Eq.ll into Eq.10, Eq.10 becomes v 2 (n- 1)T (12) m .
s
~
X x l-1,2142 5
r----- >cwe,--e_ ,- ,-~n--,--,---,a-~ m-ao m,w o mm .nm,r-mm,.e_.-,,- were -men-o-nw-e-, ,- wwow -ww.m--ve-sa,-, ww-
Let e denote the upperatconfidence the (1-4)l00 bound estimate percent confidence of e (the snubberPer failure Eqs.3rate) and 12, & can be expressed as level.
a
- t-a. a2 + 2 (13) g, 2(n-3)T By substituting e in Eq.9 by S from Eq.13, Eq.9 becomes
-a,as+2 Kf., s + 2
" *(" "'15 (14)
(8 j P{I 4 S) =s=0 h $ U-8 Eq.14 can be used to established the snubber inspection schedule (corresponding to the reliability criterion specified in Section 1) as described in Section 4.
- 4. DETERMINATION OF VISUAL INSPECTION PERIODS Eq.14 in Section 3forcan be used to determine the visu a group of snubbers. For a specified in Section 1) given group of n snubbers, at the end of each visual inspection period, T (which will be referred to as the current visual inspection period and denoted byI,Tcisfrom found.now on), the total number of inoperable snubbers, I
Hence, the statistical estimate, e, for the current inspection period can be found remainspervirtually Eq.13.. unchanged Assume thatuntil the snubber failure rate (e) Substitute the the end of next visual inspection period.Then solve for t from values Eq.14.
of n, T=Tc 'and I into Eq.14.The value ofsuch t obtained that will be the period (which will be denoted by ta from now on) minimum of 90% of snubbers in the group are operabl the time is satisfied.
In this study, the upper confidence bound estimate (e) of the snubber failure rate (e) at the d =0.05. By 95%
substituting confidence level will be used, i.e.,
and t=tw into Eq.14, Eq.14 becomes o(=0.05, T=Tc 6
3 g S "'
-l P.{ I 6 S } = 2 a=o(n)(I-t-[8("97'.N)Z z (e f(n-07c
]t.f-rzo.15 (15)
As mentioned before for Eq.8 in Section 2, S is the largest integer that is less than 0.1n.
Note that Eq.15 shows that the next visual inspection period satisfying the reliability criterion for a
' snubber group depends on both the group size (n) and the total number of inoperable snubbers found (I) by the end of the current visual inspection period.
Therefore, in accordance with Reference 2 the existing 4
procedure specifying the next visual inspection period based on the number of inoperable snubbers found per the current visual inspection period alone is overly conservative for the group with large snubber populations and results in excessive surveillance.
' In order to facilitate establishing a visual inspection schedule table, the following properties of Eq.15 can be utilized.
1 (a) By fixing the current visual inspection period (Tc),
the percent of inoperable snubbers found (r) which is (I/n)l00, the maximum. percent or number (s) of the anubbers (in the group) allowed to be inoperable at any time, and the confidence level (c) of estimate of .
the maximum percent of inoperable snubbers (in the group) at any time, the larger the snubber group size (n), the longer is the next visual inspection period, tn. This is because per Eq.13, the larger the value of n, the smaller is the statistical estimate of failure rate, s.
(b) By fixing n, r, s and c as defined in Item (a) above, the longer the current visual inspection period (Tc),
the longer is the next visual inspection period (ts).
This is because per Eq.13 the larger the valge of Tg, the smaller is the estimate of failure rate e.
l 7
l 5. VISUAL INSPECTION PROGRAM By employing Eq.15 and its Properties (a) and (b) in Section 4, a snubber visual inspection schedule is i
established for any group (category) of the snubbers of the FNP Units 1 and 2 and is provided.as Table 1. As per Eq.15, the visual inspection schedule wi.11 ensure at
- _ a 95% confidence level that a minimum of 90% of the snubbers in each group is operable (or equivalently less than 10% of the snubbers in each group is allowed to be inoperable) at any time.
l It is emphasized that the following conditions have been i
conservatively impsed in the derivation of the visual inspection schedule (Table 1):
(a) In order to obtain the shortest tg for a percent of inoperable snubbers found (r), picked for trials in the process of constructing Table 1 and based on Property (a) i' of Eq.15, the smallest snubber group size (n')
associated with each value (picked for the trials) of r l aust be used. Such smallest group size (n')
corresponds to I = 1; namely, n' is to be computed from the equation, rn'/100=I=1. -
(b) For a specified current visual inspection period (T);
based on Property (b) of Eq.15, the lower bound of T is used in the process of constructing Table 1. For example T = 18 months + 25%, the value of T = 18-I 0.25x18 = 13.5 months Is used.
(c) For a specified next visual inspection period (t), the e
upper bound of t is used to calculate the confidence
, level (l'.e., the left hand side of Eq.15). For example t = 12 months + 25%, the value of t to be used for computing the confidence level is t = 12 + 0.25x12
= 15 months. Note that the higher the value of t, the lower is the confidence level.
In addition, in order to avoid the influence of a short inspection period effect as discussed in Reference 2, the next visual inspection period in Table 1 is conservatively allowed to increase only one step above the current visual inspection period regardless how small the percent of i
inoperable snubbers found per the current visual inspection period may be.
i 8
i
! _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ - - - - ~
As discussed in Item (a) in Section 5, for each upper bound value of r in Table 1, a corresponding minimum snubber group size is required in order to obtain the shortest next visual inspection period. For all the upper bound values of r in Table 1, the corresponding values of the required minimum group size range from 31 to 200 (except for r=0, the smaller the r value, the larger is the required minimum group size). Hence, it is conservatively required that Table 1 is not applicable for the snubbers with group size (n) smaller than 200.
In meeting the reliability criterion as specified in Section 1, the safety related snubber population may be treated as a single group or may be categorized into two groups: Those accessible and those inaccessible during reactor operation. Each group may be inspected independently in accordance with the determined schedule.
- 6. CONCLUSION l
In this study, a set of visual inspection schedQles to ensure at a 95% confidence level that there are a minimum of 90% of the snubbers in each snubber group or category of the FNP being operable all the time has been developed as exhibited in Table 1. This visual inspection schedule has been derived based on the probabilistic and statistical methodologies as described in Sections 2 and 3 including justification for all assumptions used. Additionally, the conservatisms used in the course of developing the visual inspection schedule are pointed out as they are applied.
In particular, the snubber reliability criterion (specified in Section 1) is based on providing a minimum level of snubber operability at all times and conservatively does not take the probabilistic nature of the unlikely occurrence of a design basis event into account.
It is emphasized that based on the reliability analysis in this study, the next visual inspection period satisfying reliability criterion (specified in Section 1) for a snubber group depends on both the group size and the total number of inoperable snubbers found by the end of the current visual inspection period. Hence, the existing FNP procedure specifying the next (subsequent) visual inspection period based on the number of inoperable snubbers found per the current visual inspection period alone is overly conservative for snubber groups with large populations, results in excessive surveillance, and should be replaced by the visual inspection schedule (Table 1) developed in this study.
9
1 t
)
l The details for the derivation and utilization of the proposed visual inspection schedule listed in Table 1 (for each snubber group of the FNP Units 1 and 2) have been !
provided in Section 5. .
In conclusion, a visual inspection schedule to verify at a 95% confidence level that a minimum of 90% of the snubbers in a group or category are operable all the time has been developed on the bases of rigorous statistical analysis and assumptions including various conservatisms. This snubber visual inspection program is proposed to replace the existing program for the FNP Units 1 and 2.
I D
?
.l lo
l I
i
- 7. REFERENCES 1.
Nahn, G. J. and Shapiro, S.S., " Statistical Models in l Engineering", John Wiley and Sons, Inc., New York, New York, 1967.
- 2. " Technical Specification for Snubber Operability and Surveillance", Note dated March 1, 1975 from Joseph Sapir, Operating Reactors Branch #2, Division of Reactor Licensing, the U.S. Nuclear Regulatory Commission (U.S. NRC) to Dennis L. Ziem, ann.
l i
i 11 4
,.- - , - _ . . - . . , - - . _ . - _ - - , , , _ ----.--,---,,-----,-v-- -- - ,-=
r 1
~
l i
TABLE .1_
l Visual inspection schedule for any group (category) of snubbers of the Joseph M. Farley Nuclear Power Plant Units 1.and 2.
Current Visual l Percent of Inoperable Snubbers -l Next Visual Inspection found (r) per Current Visual l Inspection Period
- Inspection Period Period i
24 months i 25% r 1 0.76% 24 months i 25%
24 months i 25% 1 0.76% < r 1 0.99% q 18 months i 25%
p 24 months i 25% O.99% < r 1 1.40%
12 months 1 25%
24 months i 25% 1.40% < r 1 2.43% 1 6 months i 25%
24 months 1 25% ; r > 2.43% 3 months i 25%
18 months i 25% r 1 0.62% l 24 months i 25%
i 18 months i 25% 1 0.62% < r < 0.76% 18 months + 25%
18 months 1 25% 0.76% < r i 1.09% 12 months i 25%
18 months i 25% 1.09% < r 1.1.96% 6 months i 25%
18 months i 25% r > 1.96%
3 months 1 25%
12 months 1 25% r1 0.58% l 18 months 1 25%
i i
12 months i 25% 0.58% < r 1 0.76% l 12 months i 25%
12 months i 25% 0.76% < r i 1.40% 6 months i 25%
12 months i 25% r > 1.40%
[ 3 months 1 25%
6 months i 25% r=0 l 12 months i 25%
0 < r 1 0.76% 6 months 1 25%
6 months 1 25%
6 months 1 25% r > 0.76% ,
3 months i 25%
O e
12
. l l
Table 1 (Cont'd)
Current Visual Percent of Inoperable Snubbers ! ext Visual Inspection found (r) per Current Visual '
Inspection Period * ! Inspection Period Period
__________________ __________________________________ l____________________
3 months i 25% r=0 6 months i 25%
3 months 1 25% ! r>0 3 months
__________________________________________________________________i________ 25%
Notes:
- 1. The above table is generated based on 95% confidence that at least 90% of the snubbers (in a group) being operable all the time.
- 2.
- Earlier visual inspection periods than required may be utilized. If this option is chosen, the criteria for determining the next visual inspection period shall be the criteria associated with the earlier visual insepetion period selected.
- 3. The above table is not applicable for the snubber group size (n) smaller than 200.
l 13 1 ,