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Evaluation methodology for seismic base isolation of nuclear equipments K. E b i s a w a a n d T. U g a

Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, lbaraki-ken, 319-11, Japan Received 7 September 1992

An evaluation methodology for seismic base isolated nuclear equipments is proposed. The evaluation can be classified into two steps. In the first step, the seismic functional failure probability during the lifetime of equipment without base isolation devices is quantified in order to decide the applicability of the base isolated structure. The second step is comparative and calculates the ratio of the seismic failure frequency of the equipment without base isolation devices to that with them in order to evaluate the effectiveness of the base isolated structure. The sample evaluation considers the case of high voltage type emergency transformer with ceramic tubes.

1. Introduction The past countermeasure to preclude the failure of important safety related equipments of nuclear power plants due to severe seismic loading has been secured by upgrading both their rigidity and their damping characteristics. Recently, application of the seismic base isolation to ntle!ear power plants is considered for and adopted to the design of the reactor building and the floor of the control room in order to reduce seismic loading. In the case that seismically fragile and safety related equipments can be identified and that it is easy to apply seismic base isolation devices, the seismic base isolation to the equipments is very economical and attractive. This paper proposes a methodology to evaluate the applicability and effectiveness of the seismic base isolation to the equipments in order to reduce core damage due to destructive earthquakes. Further, a sample evaluation to a certain type of equipment in a nuclear power plant is given.

2. Selection of the equipments and technical requirements to apply seismic base isolation devices to them

2.1. Selection of the equipments In order to effectively apply seismic base isolation, an appropriate selection of equipments has to be con-

ducted by referring to the activities and the results of seismic PSA studies, [1]-[4], i.e. essentially by examining the components that are seismically fragile and might be difficult to secure countermeasures against seismic loads, and safety related components that might serve as either initiators of or as mitigators in core damage event scenarios. Figure 1 shows cumulative probabilities for functional failure of safety related equipments as a function of the acceleration level at bed rock on a Japanese site, which was calculated based on LaSalle seismic fragility

1.0 ic insulator o

~

1- /

A.

Motor operated valve

I/ /Ractor 0.5

a

If

//S.

N

r•

0

1000

2

00

Maximum acceleration at bed Rock (Gal) Fig. 1. Examples of cumulative probabilities for functional failure of safety related equipments.

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K. Ebisawa, Z Uga / EL'aluationmethodologyfor seismicbase isolation

data [5]. As can be seen from this figure, the electrical equipments with ceramic tubes and the reactor protection panel have been found to be seismically fragile. In our example, the high voltage emergency transformer with ceramic tubes is selected in our case, because many types of the electrical equipments with ceramic tubes were observed to have failed in past intense ground motions such as during the Miyagioki earthquake, which occurred on 12th of June, 1978 in Japan [6], and during the Loma Prieta earthquake on 17th of October, 1989 in U S A [7].

2.2. Technical requirements to apply seismic base isolation devices to the equipments When we presume that at least four base isolation devices are placed on square corners between the equipment foundation and the bottom surface of the selected equipment, the following technical requirements can be necessary: (1) to maintain the fixed clearance, a few decade centimeters distance in order to avoid to touch the neighboring equipments or structures, (2) to avoid resonance with the seismic waves that passed through the isolation devices, the natural period of the equipment without seismic base isolation devices should be limited to below 1 / 3 times that of the equipment with the isolation devices, (3) to prevent a turn over as well as to prevent the transfer of the excessive weight from one side of the base isolation device to the others, the height-to-width ratio should be limited, and, (4) concerning the type of the highdamping rubber bearings, a certain weight will be necessary to allow the specified displacement.

3. Concept of evaluation methodology for seismic base isolated structure The evaluation process for seismic base isolated structures can be classified into a quantitative part, deciding its applicability, and into a comparative part, evaluating its effectiveness. The former part's objective can be accomplished by estimating the seismic failure probability during the equipment life without base isolation devices. This probability can be calculated in the same way as in standard reliability engineering, i.e. in the form of a functional relationship between the cumulative probability for failure F(t) in a life interval (0, t), and a failure rate Ac, assumed to be constant during the entire lifetime, according to the formula:

F ( t ) = 1 - exp( - A c t ).

(1)

Parameter h c can be calculated using the density function of the ground motion occurrence frequency, f(a), and the failure probability of equipment under a ground motion, p(a), as follows:

ac = fDf(~ ) p(~) da,

(2)

where D~ is the domain of ground motion. When the seismic load that affects to equipment without devices, l(a, y), and the seismic resistance of the equipment, R(y), are independent from each other, p(a) can be calculated as follows:

p ( a ) = fD/(a, y) R ( y ) dy,

(3)

where l(a, y) is the probability density function, R(y) the cumulative distribution function and Dy the domain of failure. In the case that F(t) is significant in the context of safety and replacement cost considerations, the comparative evaluation can be introduced by a ratio of the failure frequency without base isolation devices to that with them. After examining whether or not the technical requirements for seismic base isolation as mentioned in section 2.2 are met, the effectiveness of seismic base isolated structures can be expressed by the value of this ratio. A graphical presentation of the above described evaluation procedure is shown in Fig. 2. From this figure, it can be seen that both failure mode and vulnerable part of equipment are identified using the records of earthquake disaster and the design specification. Based on these identification results the limit value of functional failure (hereafter referred to as functional failure value) corresponding to the seismic resistance is estimated. The seismic response that is derived from the seismic load at vulnerable parts is also estimated. Then the failure probability of equipment can be described as conditional failure probability when the response exceeds the limit functional failure. Next, the failure rate represented as frequency per year is described by integrating the failure frequency of each ground motion level. This is achieved by multiplying the failure probability and the seismic load frequency, which corresponds to the occurrence frequency of ground motion at the equipment foundation soil. This frequency per year is estimated for two cases, the evaluation with and without seismic base isolation devices.

K. Ebisawa, T. Uga / Evaluation methodology for seismic base isolation

iSeismicresistanceofequipment i

i

Seismicload

i

Conditionalfailureprobability

321

Then due to former evaluation, the failure probability during life is calculated by multiplying the failure frequency per year without devices and the duration of the lifetime. Similarly due to latter evaluation, the ratio of the failure frequency per year without devices to that with them is calculated.

-~-~ Seismicloadfrequency ] 4. Sample evaluation for applicability and effectiveness of methodology

I

Failurefrequency

Quantitativeevaluation I

I

J

Failureprobabilityduringlifeof equipment withoutbaseisolationdevices

Comparativeevaluation

1

I Failurerateas ratioof failurefrequency withoutbaseisolationdevicesto that withthem Fig. 2. Outline of evaluation procedure.

4.1. General

A sample evaluation is presented for high voltage type emergency transformers of 275 kV with ceramic tubes which are usually used in Japanese commercial nuclear power plants. The corresponding location is supposed to be at the Japan Atomic Energy Research Institute (JAERI) Tokai site and the transformers are presumed to be in use for 40 years as a design life. Although it is difficult to fix the functional failure value related parameters easily since they will be influenced by various failure modes, the functional failure

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Inertiaforce .._...~ t,{ ~,~,~eramic tube by groundmotion ~l~T~,,,,.~.FIange V~

Bindingspring

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: 490 mm )

Leakagepart of isolationoil

"ll l l JIIIIIIIIIIIIIIIlil l l l l Jl l l ~ Schematic sketch of relationship between inertia force and binding force

Configuration of ceramic tube

Fig. 3. Schematic sketch of relationship between inertia force by ground motion and binding force by spring and configurationof ceramic tube.

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K. Ebisawa, T. Uga / Evaluation methodology for seismic base isolation

value is usually represented by a single parameter such as the acceleration in order to be able to estimate the failure probability in a simple way. In our paper, an acceleration parameter will be used for this purpose. Therefore the seismic response must also be described by the same parameter corresponding to the functional failure value. Such response acceleration can be described by multiplying the transmission function by the acceleration of ground motion as an input to equipment. The ground motion occurrence frequency can be represented by utilizing the seismic hazard which is used for evaluating seismic risk and is defined as the relationship of ground motion intensity and its respective occurrence frequency. In the case of our sample evaluation, the abovementioned functional failure value, seismic response and hazard must be estimated. Concerning the seismic hazard at J A E R I Tokai site, a value recently estimated by one of the authors, [8], will be used (see also section 5.1). Below, the former two items will be described in detail.

4.2. Functional failure value of transformer The emergency transformer of three phases alternating current consists of three ceramic tubes and its electrical isolation is guaranteed by the isolation oil in three tubes. Usually, the functional failure mode of this transformer is considered to be the leakage of isolation oil from at least one tube from the records of the disaster earthquake. Such leakages can be prevented by combining the lower-side of the upper ceramic tube and sleeve by means of the binding force of a spring as shown in Fig. 3. Therefore the vulnerable part of a transformer of this type has been considered to be the area at the flange between the tube and the sleeve. The leakage of oil at this part will result in the functional failure of the transformer. Therefore the functional failure value herein is defined as a limit maximum acceleration resulting in a leakage, O/Lira" The values of O/Liracan be calculated from an equation describing the relation between the momentum by the inertia force at the flange and that by the binding force of spring, T. Since T has generally been adjusted to the range of 13.2 to 17.5 tons, the median of T was adopted for the estimation and O/Lim,CaI was calculated to be about 0.62 G. Further a literature survey for the functional failure of 275 kV transformer with ceramic tubes based on the vibration test was performed. It was found that - under the condition that T is in the range of 13.2 to 17.5 tons - aLim,Te s is in the range of 0.6 to

0.75 G [9]. Since aLim,CaI is within the range of OtLim,Tes and considering that the range from 0.6 to 0.75 G is two logarithmic standard deviations, the statistical treatment of these values resulted in the median of 650 Gal and the logarithmic standard deviation of 0.1.

4.3. Seismic response of transformer In failure probability estimation, the seismic response is represented not to be the design response but the realistic response. Since this realistic response must be estimated at each level of acceleration of ground motion, it is usually very difficult. Therefore, since the so-called "response factor method" [10], which is one of the available method for estimating the realistic response in the seismic risks evaluation, involves a relatively small estimation effort, this method has been used. In this method, the realistic response to the design ground motion level O/D, qR(O/O, y) is estimated by dividing the design response to O/D, qO, by the response factor F R. Further, assuming that qR(O/D, y) is in proportion to a acceleration level a, the realistic response to O/, qR(O/, y), is estimated as follows: QD q R ( a ' Y)

O/

(4)

FR O/D'

where F R is introduced as a measure of conservatism enclosed in response calculations in seismic design analysis. It is defined as a ratio of conservative design response to realistic response and has a probability density function represented by the logarithmic normal distribution of the median ]e-aR and the logarithmic standard deviation fiR" Using ~R and fiR, q R(a, Y) can be described as follows: 1

qR(a, Y)-- 2 ~ f l R y q DO/

2

x e x p [ - (In

(5) 2/32

J

The response factor of the transformer as recently estimated by one of the authors, [11], is used (~R = 1.21, /3R = 0.58). In our paper, the qD mentioned above is calculated by the direct integration method using the time history of design ground motion and multi-lumped mass vibration model. In the following, these items will be described in detail.

K. Ebisawa, T. Uga / Evaluation methodologyfor seismic base isolation 4.3.1. Design ground motion Whether the technical requirements to apply seismic base isolation devices are satisfied or not must be evaluated with respect to the design ground motion. Therefore the design basis ground motion S 1 for the equipments of Japanese nuclear power plants, resulting from the maximum design earthquake in accordance with the Regulatory Guide [12], was used. Further in order to indicate the effect of the difference in frequency characteristics of ground motion to the responses of equipment, the design basis ground motion S 2 for the extreme design earthquake was also used. The former has the predominant frequency of about 2.5 and 6.3 Hz, and the maximum acceleration of 267 Gal, which shall here be called as S1N. The latter has that of about 2.5 and 3.5 Hz, and 407 Gal, which shall here be called as S2F. Since the realistic response is calculated by normalizing qD by a D as it is shown in eq. (4), the effect of difference in the maximum acceleration of these earthquake motions to the realistic response of equipment will not be large. 4.3.2. Multi-lumped mass vibration model The emergency transformer was seismically isolated by the high damping rubber bearings which have a

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323

beneficial effect on base isolated structure of heavy equipment such as the transformer. They were installed on each pedestal of square corners between the foundation and the body as shown in Fig. 4. The transformer with base isolation devices was designed for S1N to satisfy the technical requirements described in section 2.2. The main specifications of the transformer can be expressed as follows: The ceramic tube has a length of 3.22 m, the maximum outside diameter is 490 mm and the weight is 1.9 ton [13]. The bearing has a rated weight of 20 ton, the horizontal stiffness corresponds to the natural frequency of about 1 Hz and the mean damping is about 14%. Such a transformer was designed with respect to the multi-lumped mass vibration model with the base isolation devices as shown in Fig. 4. Now, the main specification of the model shall be described: The spring constants of sway and rocking represented the soiltransformer interaction are estimated by Tajimi theory [14] using the soil material value of the shear wave velocity of 500 m / s , and have values of 1.6 × 104 t o n / c m and 4.0 × 109 ton c m / r a d , respectively. The rocking spring constant between the sleeve and the ceramic tube is determined based on data of the Society of Electrical Co-operative Research and has a

Ceramic tube Mass number ~t

a , - " " Beam number (11

~

Rocking spring / ~ / / c ° n s t a n t at sleeve (4/ (51 (6)

(7// (8) ---------~

Foundation

Sway spring / constant

9

Rocking spring ""~ : ~ constant ~ ~

Base insolation spring

( 1 2) (1 3)

Foundation soil Fig. 4. Schematic sketch of structure of transformer with ceramic tube and high damping rubber bearing and multi-lumped mass vibration model.

?L.Ebisawa, T Uga / Evaluation methodologyfor seismicbase isolation

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value of 1.0 X lo5 ton cm/rad [9]. In this vibration model, the response of tube is considered to be dominated by the rocking spring between the sleeve and the ceramic tube. 4.4. Correlation of failure probability The functional failure of at least one tube within three tubes will result in the functional failure of the transformer. In the case that the functional failure probability of each tube equals and each correlation coefficient of failure between two tubes is p, the functional failure probability of at least one tube, p(E), can be calculated as follows: P(q=P(4)

(3-3P+P2)a

(6)

where p(E,) is the functional failure probability of only one tube. The value of p has been estimated to be about 0.8 based on the method proposed by M.P. Bohn et al. [15]. Using this value, the failure probability including failure correlations, p(E), has been estimated to be about 1.24 times p(E,). Therefore the failure frequency per year has also been estimated to be about 1.24 times that of only one tube.

5. Sample calculation results and their discussion 5.1. Sample calculation results Evaluations of S,N and S,F in the cases with and without base isolation devices have been carried out. In the following, the sample calculation results for these two cases are described. The design response at the ceramic tube of transformer without devices to S,N was calculated to be about 865 Gal. From this response value and the estimated others values, the cumulative probability of functional failure of transformer with only one tube as a function of the acceleration level at the transformer foundation soil was calculated as shown in Fig. 5. The frequency density of functional failure was calculated by multiplying this cumulative probability by the seismic hazard at JAERI Tokai site. This calculation result is also shown with the hazard at Tokai site in Fig. 5. By integrating this frequency density, the functional failure frequency per year with one tube was calculated to be about 2.0 X 10e3 (l/year). By multiplying this value by the value of 1.24 mentioned above in section 4.4, the functional failure frequency with three tubes taking

Maxinium acceleration at transformer foundation soil (Gal)

Fig. 5. Calculation results of cumulative probabilities and frequency densities of functional failure of transformer with one ceramic tube and without base isolation devices or with them by leakage of isolation oil.

account of the failure correlation, A, without,was calculated to be about 2.5 x lop3 (l/year). By multiplying and life of 40 years, the failure probability hC without during life was calculated to be about 0.1. On the other hand, the functional failure frequency with three tubes and with devices, hcwith, was calculated to be about 8.2 x lo-’ (l/year) in the same way as without devices. From hcwithout and A, with, the ratio of Actithout to A,-+, which represents the effectiveness of base isolated structure was calculated to be about 3050. The calculation results with devices are also shown in Fig. 5, respectively. Similarly the ratio to S,F was calculated to be about 770. 5.2. Discussion of sample calculation results The applicability and effectiveness of base isolation structures are discussed based on sample calculation results. The failure probability during life was calculated to be about 10 %. Whether this value is significant or not can be judged by comparison with a significance level to be determined by considering the aspects of safety and replacement costs. Therefore, the

K~Ebisawa, T. Uga / Evaluation methodologyfor seismic base isolation safety must be adequately examined by utilizing the evaluation results of seismic risk, and the replacement costs must also be estimated in detail, based on design specifications. Although this value gives only a rough estimate based on simple methods, it is considered to imply the overall measure of applicability for base isolation structures and therefore is recommend to be used as the tentative measure of applicability. The ratio of ~tCwithout to )tCwit h as the measure of base isolation structure effectiveness to SIN and $2 F were calculated to be about 3050 and 770, respectively. Whether these values are effective or not, will depend both upon safety and replacement costs alike as applicability. Further these values may change according to the difference in following items: (1) kinetics characteristics of base isolation device, (2) vibration characteristics of equipment, (3) frequency characteristics of ground motion, which will have a large influence on the response of equipment, and (4) the degree of seismic hazard leading to functional failures frequency. Therefore, these items should, for example, be examined in detail as follows: the realistic response of equipment should be estimated in detail taking into account the nonlinearity. Further, the functional failure value should also be estimated based on the data of vibration test of fixed equipment or the detail analysis results etc. Although the above-mentioned calculation results represent only rough estimates, they are - similar to the case of the applicability measure - considered to suggest the measure of effectiveness of base isolation structure and therefore is recommend to be used as the tentative measure of effectiveness.

325

estimated based on the response factor method. From this response and the functional failure value of equipment, the cumulative probability of functional failure can be calculated. Next, the frequency density can be calculated by multiplying the cumulative probability by the seismic hazard. By integrating this frequency density, the functional failure frequency per year without devices, /~'Cwithout, can be calculated. By multiplying ~'Cwithout with the lifetime, the failure probability during life can be calculated. In the case that this value is significant, after examining the technical requirements for base isolation and designing the base isolated structure, the comparative evaluation can be introduced. In the second step, to evaluate the effectiveness of the base isolated structure, the comparative ratio of ~tCwithout to ~'Cwith, which is the functional failure frequency with devices, shall be quantified. The effectiveness can then be judged by comparing this value

®

DI

Examination of technical requirements for base isolation )

+ Design of base isolated structure I

6. Proposed evaluation for applicability and effectiveness of base isolation structure

The sample evaluation for the applicability and the effectiveness of base isolation structures has been discussed by utilizing several methods which are used in seismic risk evaluation. The outline of evaluation based on such methods is described. The evaluation can be classified into quantitative evaluation and comparative evaluation. In the first step, to decide the applicability of base isolated structures, the functional failure probability during the life of equipment without base isolation devices is quantified. Both failure mode and vulnerable part of equipment are identified. The realistic response of equipment without devices at the vulnerable part is

t Functional failure frequency with base so at on dev ces ,~ , .~)

I

Judgment of effectiveness i < a c without /

,t ~ with < 10 : effectiveness is small

1 0 < ,I , without /

,I , w i t h < 10 2 : effectiveness is large,

i

10 2 < ), ~ w i t h o u t / / A • with effect veness s very arge,

Fig. 6. Proposed evaluation of applicability and effectiveness of seismic base isolated structure.

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K~ Ebtsawa, T. Uga / Evaluation methodology for seismic base isolation

with the suggested measure as the indication of three grades as follows: 1 < ,~.Cwithout/~.C with ~ 10: effectiveness is small, 10 < Ac without//}tC with --< 102: effectiveness is large, 10 2 < Ac without//AC with: effectiveness is very large. These values are the tentative measure which allow the formulation of a rough overall estimate, based on the sample calculation results. Therefore, these values themselves might change by the degree of the safety and the replacement costs. The authors have proposed the evaluation methodology for applicability and effectiveness of base isolation of safety as shown in Fig. 6. In order to conduct the beneficial evaluation, the methods which are related the estimation of realistic response, the functional failure value and the occurrence frequency of ground motion included in the evaluation m e n t i o n e d above should be determined reasonably, respectively.

Acknowledgements The authors gratefully acknowledge the contributions of Messrs. T. Iba and Y. Imazuka of Ohbayashi Corporation for helping the generation of vibration model used in sample evaluation and Messrs. H. Kameoka and M. Hoshino of C R C Research Institute, Inc for helping a set of sample evaluation related calculation.

References [1] C.A. Cornell and N.M. Newmark, On the seismic reliability of nuclear power plants, Proc. Topical Mtg. of ANS on Probabilistic Analysis of Nuclear Reactor Safety (1978). [2] J.W. Hickman et al., NUREG/CR-2300 (1981).

[3] Y.Id. Lee, Major safety issues of nuclear power plants in Taiwan/ Republic of China, 2nd international Topical Mtg. on Nuclear Power Plant Thermal Hydraulics and Operations (1986). [4] NRC, NUREG-1150 (1989). [5] J.E. Wells, Reliability data required for a seismic risk assessment, Proceedings of the 5th EUREDATA Conference (1986). [6] S. Shiomi, Earthquake resistant design of electric substation equipments considering dynamic soil-foundation interaction, Central Research Institute of Electric Power Industry (CRIEPI)-Civil Engineering Laboratory Report No. 303 (1990) * [7] K. Ohtomo et al., Investigation of seismic effect and restoration on electric power facilities due to the 1989 Loma Prieta earthquake, CRIEPI-Abiko Research Laboratory Report No. U90027 (1990) * [8] K. Ebisawa et al., A methodology of probabilistic seismic hazard evaluation and sensitivity study, Proceedings of Japan Society of Civil Engineers, Vol. 437/I-17 (1991) *. [9] Society of Electrical Co-operative Research, Seismic design of transformer with ceramic insulator, Electrical Co-operative Report, Vol. 38, No. 2 (1982) * [10] K. Ebisawa et al., Effect of nonlinearity in "Response Factors" on system unavailability in seismic probabilistic safety assessment, JCOSSAR'91 (1991) *. [11] K. Ebisawa et al., Evaluation of response factors for seismic risk analysis of nuclear power plants, SMiRT 11, Vol. M (1991). [12] Nuclear Safety Commission, Japan, Examination guide for seismic design of nuclear power reactor faculties (1981). [13] The Standard of Japan Electrical Manufactures's Association (JEM), Dimensions of hollow porcelain, JEM 1397 (1981) * [14] H. Tajimi, Basic theories on seismic design of structures, Report of the Institute of Industrial Science Univ. Tokyo, Vol. 8, No. 4 (1959) * [15] M.P. Bohn et al., Application of the SSMRP methodology to the seismic risk at the Zion nuclear power plant, NUREG/CR-3428 (1983).

* In Japanese.

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