ML20028F924
| ML20028F924 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Surry |
| Issue date: | 06/29/1981 |
| From: | Dircks W NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Chilk S NRC OFFICE OF THE SECRETARY (SECY) |
| Shared Package | |
| ML20027A699 | List: |
| References | |
| FOIA-82-530, RTR-NUREG-0771, RTR-NUREG-0772, RTR-NUREG-771, RTR-NUREG-772 NUDOCS 8302070148 | |
| Download: ML20028F924 (11) | |
Text
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6: 3 JUN 2 91531 U/6 L, v - Ro 7 R\\
L 2
H BORANDLC4 FOR: Samuel J. Chilk, Secretary FRGi:
William J. Dircks, Executive Director for Operations
SUBJECT:
RESPONSE TO CONISSIONERS' REQUESTS DURING BRIEFING ON SECY 81-240 DRAFT NUREG-0771 AHD 0772 Pcference:
Memorandum from S. J. Chilk, SECY, to W. J. Dircks, EDO, May 29,1981 f H810521A s
REQUEST: Corraissioner Ahearne requested a response from staff to identify those accident sequences and associated probabilities in Category III (Appendix A -
HUREG-0772) with sufficiently high probability that continue to dominate.
RESPONSE: The attached analysis addresses Ccomissioner Aheame's question regarding accident sequences that continue to dominate (risk) as a result of potential changes to the release-frcra-plant radiological source tem esti-mates identified in NUREG-0772.
The analyses"irI NUREG-0772 identified a mechanism, not accounted for in the Reactor Safety Stucty (HASH-1400), that might lead to substantial attenuation of fission products within the reactor coolant systs (RCS). This particular mechanism is the rapid aggimeration and deposition of extremely dense aerosols in the vicinity of the melting core. The analyses in NUREG-0772 related to this effect shoved, as expected, that the potential reduction in source tem frce the agglomeration effect was significant only for sequences characterized by low RCS steam flow rates and high acrosol generation rates. These sequences are the transient and small break LOCA initiated core cult cases.
There are ccrapeting effects which tend to reduce the attenuation potential of this rechanism. These are: high temperatures near the core that ce prevent the deposition of cesium, iodine, and other relatively volatile fission products on the aerosol particles; the desorption and regeneration of the aerosols as the core continues to melt; and rapid steam and hydrogen flow rates that occur during blowdown following reactor vessel crit-through, which may sweep aerosol materials out of the RCS. The uncertainties arising from these effects, plus the uncertainties in the calculations themselves, preclude imputing any precision to the estimates of the effect at this time.
An overall factor of ten reduction in the radiological source tem is con-sistent with the analyses presented in NUREG-0772 for these sequences.
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,. S muel J. Chilk JUN 2 9 1931 On this basis, the r.nalysis presented with this nemorandum includes the follwing ccaparisons - the relative contributions to risk frca the najor occident sequences identified in the Reactor Safety Study (RSS) when the source terms for small break LOCA and transient initiated sequences are reduced by a factor of approximately ten and the source tems for the large cnd intemediate break LOCA's and event Y are unchanged from RSS estimates (tthich is also consistent with the findings of NUREG-0772).
This analysis indicates that with the revised source tem for scall break and transient initiated sequences the relative inportance of the different types of accident sequences remains. unchanged from the RSS estimates.
Event Y remains the most important accident sequence when the RSS estimates for event V probability are used.
Small break LOCA and transient sequences are next in icportance, and large and intermediate break LOCA's are of lesser importance. With event V probability reduced by a factor of 100 (assumed to result fe En changes to surveillance requirements for RCS/ interfacing systems check valves), the relative fraportance of event Y is significantly less than for small break LOCA and transient sequences and is comparable to large rad intermediate break LOCA's. The results of the analysis are shown on Fin re 1.
The results obtained frce this analysis are strictly applicable only to the Surry plant.
Ccuparisons of the results from probabilistic risk assessment studies for different plants show order of magnitude differences in the probabilities for similar accident sequences. Consequently, the results i
obtained from a similar analysis on a different plant will be ntnerically different. The basic conclusions of this analysis (that small break LOCA and transient initiated sequences continue to contribute substantially mont to risk than large and intemediate' break LOCA's) appear, however, to be valid for all plants. A detailed description of the analysis is contained in the attachment.
REQUEST: Chairman Hendrie rectr.nended that the staff also seek comments frtaa knowledgeable individuals outside NRC; sources would include the National Academy of Sciences and the American Institute of Chemical Engineers.
RESPONSE: The staff will seek cocnents, on a continuing basis, frce knowledge-able individuals outside of NRC on research directed towards improved acci-dent source tems.' This will be implemented through the fomation of a peer review group which will include sources such as the American Institute of Chemical Engineers, the National Acadeqy of Sciences, and others similar to those used in the March 1981 peer review of Draft NUREG-0772. The peer review group will be fomed by September 1981, and an initial meeting is l
tentatively planned for January of 1982.
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OFFICIAL RECORD COPY j
s Sc:uel J. Chilk
_p JUN 2 91531 cc: Chainr.m Hendrie Comissioner Bradford l
Comissioner Ahearne l
Comissioner Gilinsky 03C OPE E
9 RECORD NOTE: This completes mail control RES-001964.
The attached analysis I
was reviewed by M. Taylor and M. Cunningham of DRA.
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l ATTACEMEtiT - RiALYSIS In liUREG-0772, a potentially major fission product attenuation mechanism, not considered in the Reactor Safety Study (RSS), was identif'ed.
l'ajor reduc-tions in the particulate source tem as a result of aggloieration and settl-ing of aerosols wit.hin the reactor coolant system are pre /icted for certain types of accident sequences.
This effect was most pronounced for those accident sequences which are characterized by low steam flow rates through the RCS and high aerosol generation rates which result in very high aerosol mass concentrations near the core.
These conditions would be expected to exist during transient and small break LOCA' initiated core melt sequences.
In order to estimate the effect of potential changes in the release-from-plant source terms for these sequence on, overall risk and, on the relative risk contribution of each type of accident sequence identified in the RSS; we assume that the particulate source tem, as estimated in the RSS, is reduced by approximately one order of magnitude for the small break LOCA (S,3) and transient '(T) initiated sequences.
Although there are currently laFge uncer-tainties as to the actual amount'of attentuation, this reduction would be consistent with the results of analyses in fiUREG-0772.
It is further assumed that iodine (in the fom of CsI) and cesium would be attenuated by approxi '
mately the same amount as the other particulate materials.
In order'to esti-mate the impact of such reductions, a simplified set of calculations are presented for the Surry PWR analyzed in the RSS.
With these reductions, all Surry 5,, and T sequences predicted to fall into RSS release category 2 (see encloseh RSS Table 5.1) would now fall into release category 4, and similarly all ' current category 3 releases would become cate of s' team explosion containment failure sequences) gory 5 (with the exception Category 1 releases are all in-vessel steam explosion sequences where containment failure results frod the steam explosion.
The steam explosion containment failure sequences are assumed unaffected by the source tem changes.
In addition, although stea'm explosion containment failure sequence probabilities are considered to be substantially lower (than RSS estimates) based on current research, their probabilities have not been changed in this analysis.
S and T sequences, which currently result in releases that fall into categofies 4 through 9 would also be reduced.
However, the revised releases will then fall into release categories (6-9) which have a negligible influence on the relative importance of different accident sequences and are consequently ignored in this analysis.
Enclosed is RSS Table 5-2 which shows the Surry dominant accident sequence probabilities vs. release category.
Table 1 shows the sum of the dominant accident sequence probabilities for each type of sequence (e.g., A S1, S2, etc.) without the probability smoothing that was done in.RSS Table 5-2.
Table 2 gives the same infomation after the changes (discussed above) were made.
In order to rigorously determine the change in calculated risk (e.g., number of latent cancers per year) associated with the changes in release category probabilities between Tables 1 and 2, one would have to recalculate the various health effects resulting from the new radioactive release probabil-ities using detailed consequence models such'as was done in the RSS.
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However, a reasonable first order approximation of the relative change in the importance of each type sequence can be obtained by the straightfoneard -
process of multiplying the sequence probabilities times the amount of radio-active materials released.
In this analysis, we chose to multiply the sequence probabilities times the fraction of core inventory of iodine released.
For example, to detemine the importance of large break LOCA initiated sequences we multiply the large-break sequence category 1 probability (Table 1) times the category 1 iodine release fraction (RSS Table 5.1).
We do the same for categories 2 through,5 and sum the resultant products.
This sum is then a measure of the risk importance of large break LOCA sequences.
We shall call this parameter the sequence risk index.
Table 3 shows the values for this parameter for all sequences using both the RSS release category probabilities (Table 1) and the revised probabilities (Table 2).
The percentage contribu-tion to the total risk index of each sequence type is also shown on Table 3.
The results from Table 3 a'e shown graphically on Figure 1.
As can be seen r
from Figure 1, the event V sequence remains the dominant accident sequence for this plant (with RSS estimates for event V probability).
However, even-with an order of magnitude reduction in the amount of radioactive material released, th'e small break LOCA's and transient initiated core melt sequences
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are still significantly larger contributors to the risk index than the large and intemediate break LOCA's.
Table 4 shows the relative importance of the various accident sequence types assuming that the V sequence p.robability has s
been reduced by a factor of 100.
(Event V has long been identified as a dominant sequence for many PWR's. Much has been done through licensing require-ments to institute surveillance procedures which should greatly reduce the probability of event V.)
In this case, we see that' with event V probability lowered, the small break and transient sequences contribute approximately 98 percent of the total risk index using RSS estimates for all other sequence release probabilities.
With the revised source tem (and with event V prob-ability reduced), the small breaks and transients are still dominant contrib-utors to the total risk index (86 percent).
Thus, it can be seen from Table 4 that this appraisal of accident source tems indicates about a potential five-fold reductign in one measure of risk (total risk index approximately 4 (10)~
vs. 2(10)~ ).
However, there is still the potential for large accident releases and, therefore, there is not a significant change in the range or character of potential radionuclide releases during severe LWR accidents.
This analysis indicates that with a revised radiological source. tem for small break and transient initiated sequences, which reduces the release magnitude by approximately one order of magnitude below RSS estimates, the relative importance of the accident sequences remains unchanged from the RSS estimates.
That is, event V (with the RSS estimated probability) is the most important accident sequence.
Small break LOCA's and transient sequences are next in importance and large and intemediate break LOCA's are of lesser importance.
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A number of caveats concerning limitations in the preceding analysis must be
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mentioned.
First of all, the results of this analysis pertain only to the PWR (Surry) examined in the RSS. The effect of the revised source term on the relative contributions to " risk" of the various sequence types may yield somewhat different results for plants with dominant accident sequences dif-ferent than those for the. RSS PWR.
However, in the broad comparison range of this analysis, we would not expect.other LWR's to be significantly different.
Second, this analysis used a simplified method for measuring relative risk.
. This method does not account for the increased importance of noble gases and volatile iodine forms (which would not be subject to the same amount of attenu-ation in the RCS as are the particulates).
Third, it was assumed in this, analysis that iodin'e and cesium are' attenuated to the same extent as the less volatile fission products by the agglomeration and settling mechanisms.
However, it is not clear that species with inter-mediate volatilities, such as Cs1 and Cs0H, will condense on (or coagglomerate with) the large aerosol source near the core.
These uncertainties exist because:
1.
A significant fraction of the iodine and cesium species may be released i
early in the core heat-up phase prior to significant generation of less volatile fission product and structural material aerosols (which form the bulk of the aerosol mass).
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2.
The gas temperatures in the vicinity of the core may be above the boiling points of the cesium and iodine species and consequently they may remainlin the gas phase as vapors until they are transported out of the region of high aerosol concentration.
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a TABLE 5-1 SL V.ARY OF ACCIDW TS INVOLVING CORE DURA 10w wuwIwc E12 VAT 2 cus N
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E E-'A..a* DN P T.
CA I. -:E7 Pa a rt er-Tr (MI)
(Hr)
(MT)
(heters) (10 Stu/Mr) Re.Kr Org. I 3
Cs-Rb Te.!b Ba.5r ku la '.
M1 910' 2.5 0.5 1.0 25 520IOI 0.9 640'3 0.7 0.4 0.4 0.05 0.4
- 3 10'3 rwn 2 sa20 2.5 0.5 1.0 0
170 0.9 7a30" 0.7 0.5 0.3 0.06
,0.02 dalf' twR 3 4 x20 5.0 1.5 2.0 0
6 0.8 6 10~
0.2 0.2 0.3 0.02 0.03 3 10'3 rwn 4 5 10' 2.0 3.0 2.0 0
1 0.6 2x10'3 0.09 0.04 0.03' 5 10"'
3:10 4110
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0.3 0.3 2a10'I 0.03 9:20"I 5x10' la10'3 610 7m10 Pvn 6 Ea20 12.0 10.0
, 1.0 0
N/A 0.3 2 10 Ba10
8:10 la10'3 9 a10" 7a20' 1x10'
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20.0 10.0 1.0 0
N/A
&a10' 2 30' 2x10' 3:10' 2:10' 1x10 1 10i 2x10' Fwn a d a2 0' O.5 0.5 N/A O
N/A 2a10' Sa10' 1x10' 5x10'* 1x10 1x10
O O
Fwn D 4x10'#
0.5 0.5 N/A 0
N/A 3x10 Talc 1x10' 6a30' 1x10'3 1x10' O
O twR 1 1x20 2'. 0 2.0 1.5 25 ~
' 130 1.0 7 10"3 0.40 0.40 0.70
'O.05 0.5 5:10~3 M2 6a10 30.0 3.0 2.0 0
30 1.0 7x10'3 0.90 0.50 0.30 0.10 0.03 4x10
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4 ft) A discussion of the imetcyes used in the study is fcess in A;;=ndia VI.
Backgroun6 cm the isetcpe groups and release mecha.tisms ia fo md in 4 pendia VII.
(b) Includes Mo, Rh, Tc, Co.
(c) 2.ncludes sed, T. Ce, Pr, 14, Nb Aa, Ca, Pu. pp, Er.
(d)
A ac.-er energy release rate than this value applies to part of the gerlod over which the reeleertivity is 1eing seleased.
"he ef fect of Icwer energy release rates on consequences is found in 4;endia V2.
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El '20 TABLE 5-2 ON TOLLC4 To PAGE o ~
L TABLE.1.
SEQUENCE PROBABILITY ESTIMATES VS. RELEASE CATEGORY Category Sequences
'l 2
3 4
5 A
2.5(10)-10 1.6(10)-10 4.9(10)-8 1(10)-II 7(10)'
51 7.0(10)-10 5.6(10)-10 1.2(10)~
1(10) 1.1(10)-
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V 4(10)-0 T.
3(10)-8
' 2.7(10)-6 1(10)-7 6(10)-10 TABLE 2.
SEQUENCE PRD'BA'BILITY ESTIMATES VS. RELEASE CATEGORY N
Category Sequences 1
2 3
4 5
A 2.5(10)-10 1.S(10)-10 4.9(10)-8 1(10)-II 7(10)-9 51 7(10)-10 5.6(10)-10 1.2(10)-I 1(10)-II 1.1(10)-8 52 2.2(10)-
[ 0
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[1.5(10)-7]
[1.6(10)-9] [2.2(10)-6 -3 R
2(10)-12 4.1(10)-II 1(10)-9 V
4(10)-6 T
3(10)-8
[ 0 ']'
1(10)~
[2.7(10)-6]
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I TABLE 3.
RISK IMPORTANCE OF ACCIDENT Sequence Types Sequence Type RSS Revised 1(10)-8 0.2%
1(10)-8 0.3%
A S1 2.5(10)-B' O.5%
2.5(10)-8 0.7%
S2 4.8(10)-7 9.2%
1.1(10)f7' 3.4%
R 2.3(10)-10 2.3(10)-10 V
2.8(10)-6 53.8%
2.8(10)-6 86.8%
T 1.9(10)-6.
36.5%
7.8(10)-7 8.7%
TOTAL 5.2(10)-6 3.2(10)-6 CHANGE IN TOTALS = 38%
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RISK IMPORTANCE OF ACCIDENT SEQUENCES TYPES WITH EVENT V PROBABILITY REDUCED s.
t Sequence. Type RSS i
Revised A
1(10)-8 OMt 1(10)-8 2.2%
r S1 2.5(10)-8 1.05 2.5(10)-8 5.5%
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S2' 4.8(10)-7 19.6%,
1.1(10)-7 24.3%
.R 2.3(10)-10 2.3(10)-10 Y
2.8(10)-
1.1%
2.8(10)~
6.2%
T 1.9(10)-6 77.8%
2.8(10)-7
- 61. 8 %
TOTAL 2.4(10)-6 4.5(10)-7 Approximate 5-Fold Decrease In Total e
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NUCLEAR REGULATORY COMMISSION W ASH IN GT ON, D.C. 20b55 ACTION - Minogue ys:
D s
=
May 29, 1981 c
))
orrect or ins Rehm
. SECRETARY yg Pasedag, NRR Silberberg, RES Stello NEMORANDdMFOR: William J. Dircks, Executive Director for Operations p
Shapar l
M FROM:
Samuel J. Chilk, Secret r p
SUBJECT:
STAFF REQUIREMENTS - BRI F4G ON SELY-81-240, DRAFT NUREG REPORTS 0771 AND 0772 REL TING TO ACCIDENT SOURCE TERM ASSUMPTIONS, 2:05 P.M., THURSDAY, MAY 21, 1981, COMMISSIONERS' CONFERENCE ROOM, D.C. OFFICE (OPEN TO PUBLIC ATTENDANCE)
The Commission was briefed by staff on draft reports on accident source term assumptions.
i,
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6,Comiss!oner Ahearne requested a response from staff to identify those lE accident sequences and associated probabilities in Category III (Appendix A--
NUREG-0772) with sufficiently high probability that continue to dominate.
(RES)- (SECY Suspense: 6/15/81) i
.S, Chairman Hendrie recommended that staff also seek comcnts from knowledgeable individuals outside NRC; sources would include the National Academy of Sciences and the American Institute of Chemical Engineers.
(RES)
(SECY Suspense: ToBeDetermined) cc:
Chairman Hendrie Commissioner Gilinsky Commissioner Bradford Commissioner Ahearne Commission Staff Offices Public Document Room g
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