ML20213G318

Safety Evaluation Accepting local-to-bulk Temp Difference of 12 F.Draft Technical Evaluation Rept Encl

ML20213G318
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Site:	LaSalle
Issue date:	11/12/1986
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8611180036
Download: ML20213G318 (11)

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NUCLEAR REGULATORY COMMISSION

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! l g**e s ,e ENCLOSURE 'l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THERMAL ADE00ACY OF SUPPRESSION POOL COMMONWEALTH EDISON LASALLE COUNTY STATION UNIT I DOCKET NO. 50-373

1.0 INTRODUCTION

I In our SER (Section 6.2.1.1 of NUREG-0519 ) we identified the licensee's

commitment to perform comprehensive safety relief valve (SRV) in-plant testing.

These tests were perfomed at the LaSalle County Nuclear Power Station Unit 1 31n December,1982 in accordance with g dance in NUREG-0763 usfr:g the test plan that had been approved by the staff The SRV in-plant tests were performed to confim the adequacy of the piping system design to safely accomodate the hydrodynamic loads and themal effects associatedwighSRVactivationfortransientswithintheplantdesignbases.

The Staff SER concluded that this objective was met. In addition, the test program was to determine plant unique thermal effects. Specifically 1) the maximum local-to-bulk pool temperature difference during SRV discharge; and

2) the adequacy of the suppression pool temperature monitoring system (SPTMS) to provide a conservative indication of bulk temperature. The test results covering these objectives are discussed below.

Local pool temperature is defined as the fluid temperature in the vicinity of ths quencher device during steam discharge. This temperature will differ from both the temperature of water in contact with steam, as well as the bulk i temperature of entire wetwell pool. The bulk temperature is a calculated value based on the total energy and w; ass released into the pool. The pool is assumed to act as a uniform heat sink. Since bulk temperature is used in plant i

transient analysis, the difference between the local-to-bulk value must be specified in order that the analysis can demonstrate operation within the prescribed transient limits.

2.0 EVALUATION To monitor the spatial and temporal variation of suppression pool temperatures during extended SRV discharge tests, an array of 34 temperature sensors were installed. In addition to the test instrumentation, the LaSalle suppression poolhag'34duelelementtemperaturesensorsthatmakeuptheSPTMS. Test results show that the average local-to-bulk pool temperature difference is 8.1*F. The corresponding 95 percent confidence level non-exceedance temperature 8611180036 861112 PDR ADOCK 05000373 P PDR

difference is about 12 F. This local-to-bulk pool temperature difference of 12*F should be used for plant transient analyses involving SRV discharges -

in Mark II plants that utilize Mark II T-quenchers equipped with end cap holes similar to the LaSalle Unit I design.

The second test objective concerning the perfomance of the SPTMS was accom-plished. For all the seven SRV blowdowns valves from the SPTMS temperature sensors exceeded the bulk temperature. Therefore, it can be concluded that the SPTMS provides a conservative estimate of bulk pool temperature. The response of the SPTMS was fairly unifom for all sensors, indicating good themal mixing around the pool. A large number of sensors provided a conser-vative indication of bulk pool temperature and, thus, the distribution and number of sensgrs was adequate and meets the acceptance criteria in NUREG-0487 and NUREG-0783

3.0 CONCLUSION

On the basis of the above evaluation and the attached Technical Evaluation Report by the Brookhaven National Laboratory, which we have found acceptable, we conclude that:

a) A local-to-bulk temperature difference of 12*F is acceptable for use during plant transients involving SRV discharges in Mark II plants that utilize Mark II quenchers equipped with end cap holes similar to the LaSalle Unit I design.

b) The LaSalle Unit 1 SPTMS is adequate in terms of the number of sensors and their spatial arrangement.

REFERENCES I - NUREG-0519, " Safety Evaluation Report related to the operation of LaSalle County Station Units 1 and 2, " March 1981.

2 - NUREG-0763, " Guidelines for confirmatory in-plant tests of safety-relief valve discharges for BWR plants, "May 1981.

3 - Sargent and Lundy Engineers, "LaSalle County Unit 1 in-plant SRV Test Plan, " Revision 4, October 27, 1980.

4 - Tin, C.C. et. al., "BNL's Evaluation of LaSalle County in-plant SRV test plan, Revision 4. April 15, 1981.

5 - Safety Evaluation Report, LaSalle Unit 1. In-plant SRV Test Evaluation Report, May 19, 1986.

6 - Sargent and Lundy Engineers, " Extended Blowdown Test Evaluation of Suppression Pool Temperatures Measurements," August 1,1983.

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3-7 - Attachment (2) to Comonwealth Edison letter dated October 16, 1984 from -

J. G. Marshall to H. R. Denton. i 1

8 - NUREG-0487, " Mark II Containment Lead Plant Program Load Evaluation and Acceptance Criteria", Noven:ber 1978.

9 - NUREG-0783, " Suppression Pool Temperature Limits for BWR Containments".

November 1981 4

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ENCLOSURE 2 Technical Evaluation Report (TER) for the LaSalle Unit 1 S/RV Extended Blowdown Tests -

by C. Economos April 1986 Department of Nuclear Energy Brookhaven National Laboratory Upton, New York 11973 ABSTRACT A detailed technical evaluation of the LaSalle Unit 1 Safety / Relief Valve Ex-

' tended Blowdown Tests is presented. The objective of these tests was to de-termine maximum local-to-bulk pool temperature difference during such valve discharges and to demonstrate the performance of the plants suppression pool temperature monitoring system in terms of its ability to indicate bulk pool temperature. Based on the results of these tests, the applicant proposes a local to bulk pool temperature difference of 12*F. From our evaluation of the

- information supplied, BNL concludes that the proposed temperature difference is acceptable. We also conclude that the plant temperature monitoring system will perform as required. The detailed basis for these conclusions is pre-4 sInted.

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1. BACKGROUND The objectives in these tests were to determine the maximum local-to-bulk sup-pression pool temperature difference (at) during safety / relief valve (S/RV) discharges and to demonstrate that the plant's suppression pool temperature

monitoring system' (SPTMS) provides a conservative indication of bulk pool tem-perature. These objectives are consistent with NRC staff requirements rela-tive to BWR suppression pool temperature limits as stated in the relevant Acceptance Criteria (Section 11.3 of Aopendix D of NUREG-0487 and Section 5.0 i

of NUREG-0783).

2. PLANT DESCRIPTION The LaSalle Unit 1 is a BWR plant with a Mark 11 type containment. For pres-sure relief of the reactor vessel during transients, the plant is equipped with a total of 18 S/RVs. The steam discharged from these valves is routed into the suppression pool via individual pipes, each of which is provided with a Mark 11 type T-quencher at the submerged end. This device consists of a l

length of horizontal perforated pipe about 10 feet in length. It is designed to reduce the dynamic loads caused by discharge of non-condensibles through the pipint 5ystem and to preclude the occurrence of steam condensation insta-bilities associated with low subcooling at elevated pool temperatures. The T-quenchers are distributed in two concentric rings around the periphery of the suppression pool with the axis of the ouencher arms aligned circumferen-tially. In addition to the perforations on the quencher arms, a number of holes are drilled into one of the end caps. The high pressure steam issuing through these holes is intended to induce a bulk swirling motion of the sup-pression pool relative to the plant vertical axis potentially resulting in re-duced thermal stratification. The rotation that would be induced is in the clockwise direction as viewed from above.

i The LaSalle plant is also equipped with an RHR system that consists of two in-dependent loops. These are designated as A and B. Suction for the individual loops are at the 32' and 250* azimuths, with the returns located at the 163' and 320' azimuths respectively. The RHR returns are apparently directed ver-i I

tically downward.I The applicant states that with loop A in ' operation, the bulk pool motion is as indicated in Fig. I which has been taken from Reference

2. Note that the swirling motion induced by the end cap holes (clockwise) can either reinforce or oppose that which is caused by RHR loop A, depending on the azimuthal location of the T-quencher. .

3. TEST DESCRIPTION The overall test program consisted of seven extended valve discharges involv-ing two different plant configurations. For five of the tests the T-quencher designated C and located in the outer ring at azimuth 230* was employed. In the two remaining tests. T-quencher G located in the inner ring at azimuth '

210' was utilized. Fw the five tests using quencher C, no RHR operation was employed and care was taken to insure a quiescent pool prior to the onset of the blowdowns. For the two remaining tests, RHR operation in the pool cooling mode using loop A was initiated coincident with S/RV' actuation. RHR operation

'was continued throughout the blowdown and beyond. No other parameter varia-

. tion was attempted, although the initial pool temperature exhibited some

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Lasalle Suppression Pool Showing RHR Ioop A in Operation

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variability (59' to 76*F) as did recctor power levels (46 to 51%). The. total

. discharge' time of the blowdowns varied from 9 to 17 minutes.

4. INSTRUMENTATION '

To monitor the spatial and temporal variation of su'ppression pool temperatures during installed.the extended blowdown tests, an array.of 34 temperature sensors were Most of these sensors were located on the submerged boundaries (containment wall, basemat, pedestal). However, a few were located on columns and two, sensors T32 and T33, were mounted on the discharge lines of'the two 3 tested quenchers (G and C, respectively) at an elevation 26 inches abwe the quencher arm centerline and on the windward (upstream) side of the lines rela- ,

tive to the bulk pool motion induced by the end cap steam flow.3 This repre-sents the closest proximity of a sensor to the steam discharge. A corglet_e -

description of this instrumentation was' given in Reference 4. BNL's evalu'a tion was presented in Reference 5. -

It was concluded that the temperature sen-During the extended blowdown tests, all but 2 of the 34 temperature s were operative.

In addition to the test instrumentation, the LaSalle suppression pool is equipped up the SPTMS. with an array of permanently installed-temperature sensors that make This system consists of 14 dual element temperature sensors which ment. are spaced more or less equally around the periphery of the contain-mainderTen of the on the sensors are mounted near the containment wall with the re-pedestal. All are at the same elevation, about 8-1 the suppression the test program. pool low water level. All SPTMS sensors were opera /2"during tive below 5 TEST RESULTS 5.1 Local Pool Temperature Response - The temperature histories indicated by all operative test sensors are presented in graphical form in the test reports and. its supplement.7 ing bulk pool temperature history is^ also provided.6For certain of these, a compar

,l The pool sensors selected for this treatment are clained.to best represent local temperature.

T4 and T32 for quencher G.These include sensors T2 and'T33,for quencher C and sensors haviour suggestive of large scale turbulent mixing.The A conservative

• time-traces exhibit a pronounced averaged value of AT was developed for these traces and is presented in tabular form. The peak value for all runs and all sensors is reported to be '

9.1*F.

The average over all runs with a given quencher is 8.1*F for the inboard quencher G with RHR (2 runs), and 6.6*F for the outboard quencher C without RHR (5 runs). The corresponding 95/9B confidence level non-exceedance temperature differences are given as 12*F and 11*F. respectively.

5.2 ~

SPTMS Performance - The performance of the SPTMS is characterized graph-  :

ically in the test report by exhibiting the spatial (circumferential) tempera-ture distribution at selected times (0, 4, 8 minutes and final time) after , ,

initiation of the blowdown transient. This data is supplied for each of the

• The non-conservative bias due to the zero AT that prevails at S/RV actuation was removed. .

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seven blowdowns performed during the test program. Almost without exception, the indicated temperatures from all sensors, at all times and for all tests exceeded, by a considerable margin, the current value of bulk temperatu're. At valve closure, in particular, the temperature indicated by all SPTMS sensors '

ranged from a minimum of 2*F to as much as 14*F above the corresponding bulk 1 temperature.

6. EVALUATION 0F TEST RESULTS 6;,1 Local Pool Temperature Response - Two features of the data presentation for AT require discussion; time averaging of the temperature histories and the choice of temperature e,ensors' that are used to characterize " local" pool temp-erature.

We consider time averaging acceptable and consistent with the turbulent char-acter of the physical process involved. We would expect the stability of the steam condensation process to which AT relates to depend on some suitable mean value rather than the instantaneous fluctuating values of temperature. In any case, the use of the 95-95 non-exceedance value which is derived virtually bounds all peak fluctuations. As for how to characterize " local" pool temper-ature, we note that in Section II.3 of Appendix D of NUREG-0487 an attempt is made to provide a practical definition. The following statement appears on i Page D-9.

Local temperature is defined as the water temperature in the vicinity of the quencher device.- For practical purpose, measurement from the temper-ature sensors,.which are located on the containment wall in the sector containing the discharge-device and at the same elevation of the dis-charge device, can be used Es local temperature."

Additional clarification is provided in Section 5.3 of NUREG-0783; viz:

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"The local pool temperature is defined as the fluid temperature in the vicinity of the quencher device during steam discharge. For practical

, purposes, the average water temperature observed within the region sub-tended by the quencher arms on the reactor side of the containment and at i the same elevation as the quencher device can be considered the local temperature. For-plants for which credit is to be taken for the effec-tiveness of the RHR to mix the pool, local temperature can be defined by using the average temperatures measured by only the sensors downstream of the quencher (relative to the RHR flow). However, in this case also, temperatures directly above the downstream quencher arm shall be included in the averaging,"

The sensors selected by the, applicant to characterize local pool temperature do not satisfy these reqJirements. First, they are insufficient in number to provide a reasonable spatial average. Also, a nonconservative bias could, in '

[ ' principle, be introduced by the use of sensors T2 a nd T4 which are located on the basemat 4-1/2 feet telow the quencher elevation and about 2 feet upstream of the quencher center relative to the bulk pool motion induced. by steam flow through the ead cap holes, and, in the case of quen:her G, by RHR operation.

t The other sensors (T32 and 133) are somewhat more favorably positioned in that they are located above .a quencher arm. As noted in Section 4, however, they are not on the downstream side of the quencher center.

..,, 6 - -...w.a ';..., x w s w h .,. m n p u;.q;. a c e .: y ;. g . ,.; g e-t Despite these deficiencies, it is still possible to conclude that the reported values of AT are reasonable and, indeed, conservative. This is because of the data plots that were provided via Reference 7. This information, since it in-cludes the temperature history recorded by all operating sensors, allows us to '

develop what we would consider a reasonable spatial average based on the def-initions cited above. We have done this and find that the temperature sensors that were selected by the applicant to develop AT are appropriate. In gener-al, they tend to indicate values that are somewhat higher than the average, except at, locations that are clearly inappropriate for development of AT.

Here again, this statement is based on the definitions given by the NRC staff in NUREG-0487 and NUREG-0783.

Finally, some comments on the effect of the LaSalle RHR system on AT. The way '

in which this aspect of the test program was implemented (See Section 3) was inconsistent with the applicant's commitment to the NRC staff, as implied by their response to question 6 of Reference 2 and question B.8 of Reference 8.

By varying two parameters or features of the tests simultaneously, any possi-bility of learning anything about RHR effects was precluded. Based on the available data, the combined effect of using an inboard quencher with RHR appears to have a negligible (and deleterious) effect on AT relative to an outboard quencher without RHR. It can be speculated that aT tends to be some-what greater for an inboard quencher without RHR operation; or, one can spe(G) culateand thatwould the AThave been in the even of vicinity greater l~ quencher G is about the same as that in the vicinity of quencher C and con-clude that the ability of the LaSalle RHR system to induce any mixing in the circumferential direction is negligible.

Wa tend to favor the latter speculation for two reasons. First, as indicated

' in Section 2, the LaSalle RHR system is not equipped with what have sometimes bten referred to as RHR elbows. That is, the RHR return is not directed in the motion.

circumferential direction to enhance its ability to induce a swirling bulk In such a circumstance, we have data to show (Monticello, 1977 tests ,

- Reference 9) that the effect of RHR operation on AT is nil. Second, we have the temperature data from the LaSalle SPTMS. A careful examination of this data indicates that the rate at which energy is transported fr'om the vicinity of the active quencher to circumferentially remote points differs very little in tests with and without RHR operation. It needs to be noted however that, in iboth cases, the swirling induced by flow through the end cap holes was present.

These arguments do.not. totally eliminate the uncertainty that exists. None-theless, we feel confident enough in their validity to judge that the AT's de-i rived from this test program are applicable for both inboard and outboard quenchers even in the absence of RHR operation. We also anticipate that suf-ficient margin between expected pool temperatures and .the temperature limits imposed by the NRC staff will be demonstrated to further add to this confi-d:;nce. >

6.2 SPTMS Performance - Insofar as the performance 6f the LaSalle SPTMS is ccncerned, there is no question that the system is acceptable. Any reasonable i

averaging of the temperature data output by this array, or even some subset of this. array, will clearly provide a conservative estimate of bulk pool tempera-ture. This is in great part due to positioning of the sensors well up in the suppression pool where thermal stratification tends to bias the measurements

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to the high side.

Also contributing to the generally favorable performance of the SPTMS cap holes. is the bulk mixing induced by the steam flow out of the quencher end

7. OTHER CONSIDERATIONS _ .

The test report 6 technical specifications is examined. includes a section in which the possibility of modify In another section, a detailed descri p-tion sented. of operator actions relative to the SPTMS and plant transients is pre This material seems inappropriate in a test report. BNL has neither reviewed of the argumentsor evaluated this material presented therein.and has formed no judgement on the merits

8. CONCLUSTIONS Based we concludeon ourthat:

review of the test reports and the various other documents cited 5

a) a local-to-bulk pool temperature difference 12*F is acceptable for use during plant transients involving S/RV discharges in Mark 11 plants that utilize Mark II T-quenchers similar in design to those installed in the LaSallo Unit 1 plant. In particular, this value of AT is acceptable only fw T-quenchers equipped with end cap holes.

b)

The LaSalle Unit 1 SPTMS is acceptable in terms of the number of sen-sors and their spatial arrangement.

9. REFERENCES 1.

Chapters 4 and 8 of "Susquehanna Steam Electric Station, Units 1 and 2 Design Assessment Report," Rev. 1, March 1979.

2.

Letter of NRC,from L.O.27, October De1 George of Commonwealth Edison Co. to B.J. Youngblood 1980.

3 Letter of NRC,from L.0. DelGeorge February 3, 1981. of Commonwealth Edison Co. to B.J. Youngblood 4.

Sargent vision 4, and Lundy October 27,Engineers, 1980. "LaSalle County 1 In-Plant SRV Test Plan" Re-5.

Lin, C[C.,

Report, et al.,

Revision 4, "BNL's October Evaluation of LaSalle County In-Plant SRV Test 27, 1980," April 15, 1981.

6 Sargent and Lundy Engineers, " Extended Blowdown Test Evaluation of Sup-pression Pool Temperature Measurements," August 1, 1983.

7. .

Attachment J.G. Marshall (2) to Commonwealth to H.R. Denton, NRC. Edison Letter Dated16,October 1984 from 8.

Letter of NRC,from AugustL.O. De1 George of Commonwealth Edison Co. to B.J. Youngblood 1980.

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