ML20125C097
| ML20125C097 | |
| Person / Time | |
|---|---|
| Issue date: | 09/20/1979 |
| From: | Frank Akstulewicz Office of Nuclear Reactor Regulation |
| To: | Strosnider J Office of Nuclear Reactor Regulation |
| References | |
| REF-GTECI-A-03, REF-GTECI-A-04, REF-GTECI-A-05, REF-GTECI-SG, TASK-A-03, TASK-A-04, TASK-A-05, TASK-A-3, TASK-A-4, TASK-A-5, TASK-OR NUDOCS 8001030562 | |
| Download: ML20125C097 (13) | |
Text
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'% , , ,', , + SEP 2 01979 MEMORANDUM FOR: Jack Strosnider, Task Manager A-3, 4, & 5 THRU: R. Wayne Houston, Chief l Accident Analysis Branch, DSE I
FROM: Francis Akstulewicz, Nuclear Engineer l I
Accident Analysis Branch, DSE
SUBJECT:
STEAM GENERATOR TUBE INTEGRITY (TASK A-3, 4, & 5) 1 Attached is the text for proposed section IV.C. entitled Main Steamline Break Outside Containment to be included in the staff report on steam generator tube integrity. The text describes the scenario of the postulated accident, provides the assumptions used to calculate the doses and states conclusions about steam generator tube damage based upon radiological considerations. If there are any questions concerning the text or the analyses, please contact me at X27598.
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43.;(c ( 2kt O-FrancisAkstulewicz,NuclearJnyineer Accident Analysis Branch Division of Site Safety and Environmental Analysis
Attachment:
As stated cc: W.Jeeger
- g. Hanauer G. Knighton M. Wohl M. Tokar L. Soffer R. Myer 90007e 27 F. Odar
! L. Bell l
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-.aAe-IV. C. Main Steamline Break Outside Centainment h }O The initiating event is by definition a non-mechanistic, double-ended rupture of a main steamline. The break is assumed to occur between the containment and the MSIV, such that steam is directly released to the environment via the ruptured steamline and cannot be terminated by the operation of the inboard MSIV (single failure). With the pressure.on the secondary side of the affected steam generator dropping rapidly to near atmospheric pressure, the affected steam generator is expected to empty out the entire secondary side (i.e., dryout) in a matter of a couple of minutes. The other steam generators, through the common steamline header,will also lose a significant fraction of their secondary side contents before MSIV closures !
of the intact steamlines terminates this release.
The reactor protection system instrumentation will detect unusual renctor conditions, and cause the reactor to trip by
-;pid insertion of the control rods. The reactor trip terminates
'ne fission chain reaction. However, the increased heat i moval capability of the affected steam gererator as well as
<e steam dump to atmosphere from the uncffected steam generator (s)
.ause a rapid cooling of the primary systc! water. This has
.e net effect of inserting positive re3ctivity; so much that she reactor core may experience a returr m power condit ion.
it is during this reactivity insertion period that a tremendous thermal stress transient occurs in the fuel pellets and fuel cladding. Depending upon the conditicr. cf the core and the severity of the transient, substantial amounts of fuel cladding
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could experience perforation and the most reactive rods may experience some centerline melting. This process would release substantial amounts of fission products into the primary system coolant. ye a q.
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A second effect of this accident is a very large primary to secondary pressure differential across the steam generator tubes. Following the steamline break, the sect .iary side of the affected steam generator will be operating ,t atmospheric pressure conditions. Coupled with the primary system response, a maximum LP across the steam generator tube walls as high as
- 2500 psi could be anticipated. While new steam generator tubes are not expected to rupture under these pressure conditions, tubes which have been extensively degraded by the effects of wall thinning or tube denting could rupture and increase the primary to secondary leakage considerably.
Another pheno e i' which would be expected to occur under these accider. r.fitions is known as iodine spiking. Whenever the primary e experiences a reactivity, temperature or pressure trar.: , the fuel experiences tremendous stress conditions . n ireatly enhance the ability of radiciodine to escape from 15. rel to the fuel rod gaps. Since some fu;l clad damage c::J s even under normal operating conditions, the stress transin; results in sizeable amounts of radiciod$.u being released into the primary system over the course of the accident.
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- It is important to note that the effect of fuel clad perforation or fuel melt is more severe than that due to iodine spiking phenomena. !
Gen 6 rally, a fuel clad perforation event would release more activity into the coolant system than an iodine spike. For example, even using " realistic" assumptions for gap activities, clad perforation of about 0.11 percent of the fuel rods in the core would result in a primary coolant activity greater than 60 pCi/ gram dose equivalent l
I-131, which is the short term operational limit defined in the staff ,
standard technical specifications for operating plants. For this reason a consequence analysis for an accident sequence involving a pre-existing iodine spike plus fuel cladding failures or fuel melting does not provide much more information, from the radiological consequence point of view, than a similar analysis which considers only the effects of fuel cladding failure and therefore will not be 3'Y
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considered in this report.
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a The previous paragraphs presentii the accident scenario and effects which the staff analyzes to de -ine whether or not the radiological consequences from such a pos : d accident are within acceptable limits. The current staff r. ? is to assume that it would take a reector operator 30 minutes li ~ectly recognize the accident, depressurize the primary sv' -
eflood and isolate the steam generator with thc tube failures, and te cte any further release. Fcrr those 30 minutes the activity leat ., .om the primary to secondary side via the primary coolant is assi"ed directly released to the environment.
The staff has evaluated three situations to determine the total number of steam generator tubes which could rupture without producing l 90007;30 i
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, consequences exceeding the 10 CFR Part 100 reference dose guidelines.
These situations are discussed belm: as Case 1 - Fuel Clad Failures, Case 2 - Pre-Existing Iodine Spike and Case 3 - Time Dependent Iodine Spike.
Case 1. - Fuel Cladding Failures and Fuel Melting This case represents the worst case conditions which could be experienced given a MSLB accident. Depending upon the severity of the reactivity, !
l temperature and pressure transients in the core fuel," realistic" values as large as 10% of the fuel pellet activity in the rod gap could be expected. We have assumed in this analyses that 10% of the fuel rod iodine activity and 10!! of the fuel rod noble gas activity is released into the reactor coolant system for each perforated' rod.
Since a large primary to secondary leak would exist because of the steam generator tube failures, this activity would be transported from the primary side to the secondary side where it would leak to the environment. ~
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}' g The potential consequences for Case 1 were c, Ed using the assumptions shown in Table 1. The analysis was perform ~ rning that all the activity released from the perforated and M - ced fuel would be instantaneously mixed throughout the prims,y lant system. Because of the dry steam generator secondary side ar :h effects as e volatilizeition and atomization of the primare ilant at elevated temperatures and 1arge system AP's (Ref. 1), nii primary coolant activity leaked to the secondary side was assumed released directly to the environment for that time period prior to steam generator isolation.
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In calculating the doses, the staff assumed that steam was
.ekncd at ground level and did not experience any ground deposition, plume depletion or radioactive decay during transit.
The staff also used a realistic atmospheric dispersion factor (Ref. 3) for the course of the accident. ,
Based on calculations using the 300 rem thyroid dose limit suggested by 10 CFR Part 100, Figure 1 shows the percentage of fuel clad damage for a given number of assumed steam generator tube failures for ;wo different reactor power levels. This figure shows that relatively little fuel (less than 1.5 percent of the core) could experience fuel clad perforations without exceeding the reference thyroid dose of 300 rem for even a single steam generator tube failure. :
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Case 2. - Pre-Existing Iodine Spike For the reasons discussed earlier, the case of fuel failures and a pre-existing iodine spike is not analyzed. Tnerefore precludin; fuel melting or cladding failures, the most serious case to analyzc would be that of a pre-existing iodine spite at tne time of the accident. Plants are currently allowed by the standard technica' specifications to operate at radiciodine activity levels >) uCi/g.;
but no more than 60 ' gram dose equivalent I-131 for < 107 of all operating time. This case assumes that the plant has experienced sufficient power, temperature, and/or pressure changes such that the l
postulated steamline break occurs at full power with the primary coolant activity at the maximum concentration allowed by the plant 90007232
, technical specifications (60 pCi/ gram dose equivalent I-131).
The maximum permissible number of steam generator tubes wnich could rupture and still result in a dose hess than the Part 100 guidelines was calculated using the conmon assumptions presented for Case 1 above and also those listed in Table 1. The total maximum number of steam generator tube failures' calculated for Case 2 was 9 tubes or a total combined primary coolant leak rate of less than 1125 gpm for the accident.
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Case 3. - Time - Dependent Iodine Spike
" c Ju w _1.1 a This case is considered the most realistic of the three cases.
It combines a large steam generator leak rate with the main steamline break accident, but does not assume any additional fuel clad perforations. It also assumes that the plant operations history would not have resulted in a pre-existing primary coolant spike, but would have a primary coolant concentration at the time of the postulated MSLB no greater than the maximum steady state concentration permitted by the plant technical specifications.
(1 pCi/ gram dose equivalent I-131). The case assumes that due to the power, temperature, and pressura effects that the postulated accident has on the primary sys a. the nominal fission product release rate for iodine from th; p eviously perforated fuel will be enhanced by a factor of 500 (1:er . 2). The normal radio-iodine cleanup of the primary coolent system is assumed not available and the iodine concentration continues to increase during the course of the accident at a conservative linear rate (if the loss term through the ruptured tubes is neglected).
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The maximum number of steam generator tubes which could rupture and still result in a thyroid dose less than the 10 CFR Part 100 guidelines was calculated using some of the common assumptions presented for Case 1 above and also those listed in Table 1.
The maximum pennitted number of steam generator tube failures was calculated to be 24 or a total combined primary coolant leak rate of less than 3000 gom. D *D 9' f
& a J X} m Conclusions The three case analyses provided valuable insights into the maximum permitted leak rate of primary coolant to the secondary side under main steamline break conditions. Case 1 shows very clearly that even with " realistic fuel gap activities and
" realistic" meteorology, small amounts of fuel clad perforation (less than 1.5 percent) could result in radiological consequences exceeding the 300 rem value. This suggests that unless fuel clad failure can be virtually precluded during the MSLB, the radiological consequence of a single stear emerator tube rupture would not be acceptable.
A point of argument can be made that pler ' lower thermal power would not experience consequences ^o - '; severe as those calculated for this report. Howe.er .:nts of lower thermal power typically have smaller pria clant systeme volumes. The result is that while indeed t t primary coolant iodine concentration (iCi/ gram) for lower tnumal power plants is smaller than that for a 3800 MWt plant, it is not significant enough to change the conclusion stated in the paragraph above for the case of fuel clad failures greater than about 2% of 90007;34
0 the core. For values of clad failure less than 2% of the I core, lower thermal power cores could indeed stand more primary coolant leakage to the secondary side than that calculated in this analysis and still not exceed the rem thyroid dose criterion. l I
I Case 2 is independent of reactor power since it is dependent upon the technical specification that permits plants to operate at the 60 pCi/ gram dose equivalent I-131 level for small periods of time. Therefore, the results for this case would provide for all plants not experiencing cladding failure, the ;
l limiting number of steam generator tube failures that could occur and still result in doses less than the reference value,.
Using realistic meteorology, the calculations for Case 2 show that a maximum number of 9 steam generator tubes could fail before a 300 rem thyroid dose would be exceeded.
Case 3 shows that if the primary coolant concentration is carefully controlled, and no fuel failures occur from the postulated accident, a larger number of steam generator tubes (24) could fr.il before the 300 rem consequence criterion would be exceeded. Again for plants with lower thermal power ratings, the number of steam generator tubes which could fail and still not exceed 9 9 300 rem thyroid dose value would be larger than the number for a 3800 MWt thermal plant calculated in Case 3.
In summary, the analyses of these three cases lead to two I principal conclusions. First if no fuel clad failures occur as a result of the accident, the results frcr Case 2 would 90007;35 t
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dictate that no more than nine steam generator tubes could fail and still result in consequences less than 10 CFR Part 100 ruidelines values. Second, if fuel failures occur, any
'r fuel failures amounting to greater than about 1.5% of the core would produce consequences greater than the 300 rem thyroid limit with even a single steam generator tube failure. It is clear that since fuel clad perforations can detennine the degree of steam generator tube damage that can be tolerated, criteria should be established to preclude or minimize fuel failure clad for the main steamline break accident.
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TABLE I ASSUMPTIONS USED IN THE CALCULATION OF THE RADIOLOGICAL CONSEQUENCES
- 1. Reactor Power, Mw ' 3800
- 2. Reactor Coolant Liquid Volume, ft 3 12060
- 3. Iodine Escape Rate Coefficient, sec -I 1.3 x 10 -8
- 4. SteadyStatePrjmaryCoolantSpecific Activity, pC1 gram 1.0
- 5. Time to Reflood and 1solate Affected Steam Generator, Minutes 30
- 6. Primary to Secondary Leak Rate for One Ruptured Steam Generator Tube, GPM 125
- 7. Iodine DF for Leakage in Steam Generator Until Reflo;d 1.0
- 8. Realistic Atmospheric Dispersion Factor, sec/m3 1.0 x 10-4 (Reference 3) 90007337
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REFERENCES
- 1. Postma, A. K Tam, P.S., " Iodine Behavior In A PWR Cooling System Following a Postulated Steam Generator Tube Rupture Accident",
NUREG-0409, January 1978.
- 2. NUREG 75/087, Standard Review Plan Section 15.1.5, " Radiological Consequences of Main Steamline Failures Outside Containment (PWR)",
Revision 1, August 1978. .
- 3. " Anticipated Transients Without Scram for Light Water Reactors",
NUREG-0460, Volume 2,Section VI, April 1978.
- 4. NUREG 75/087, Standard Review Plan Section 15.6.3, " Radiological Consequences of Steam Generator Tube Failure (PWR)", Revision 1, August 1978.
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