ML20207L210
| ML20207L210 | |
| Person / Time | |
|---|---|
| Issue date: | 12/22/1986 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Sniezek J NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML20204J261 | List: |
| References | |
| FOIA-87-714 NUDOCS 8701120011 | |
| Download: ML20207L210 (30) | |
Text
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N G $ @'8 M N h (p17 W% :T bM.2Q[ gy DEC 2 21986 PEMORANDUM FOR:
James H. Sniezek, Deputy Executive Director for Regional Operations and Generic Requirements FROM: Harold R. Denton, Director Office of Nuclear Reactor Regulation
SUBJECT:
PROPOSED NEW STANDARD REVIEW PLAN SECTION 6.5.5, "PRESSURE SUPPRESSION POOLS AS FISSION PRODUCT CLEANUP SYSTEM:"
The enclosed generic reouirements review package contains a proposed new standard review p'9n section which, if approved, would add a procedure for establishing the fission product retention capabilities of BWR pressure suppression pools. This is a category 2 action.
The proposed item is one of the near-tem items discussed and scheduled in an infomation paper transmitted to the Connission on February 28, 1986'by the EDO entitled "Implementation Plan for the Severe Accident Policy State-ment and the Regulatory Use of New Source-Tem Infomation (SECY 86-76).
The proposed section does not place any requirement upon licensees, since no credit for fission product retention has previously been allowed in any operating license review. Licensees may opt for such credit, however, by appropriate license amendments. Its acceptance criteria and review pro-cedures contain three features: (1) stated values for pool decontemination factors, such that licensees or applicants claiming minimal credit need not perform cnmputer calculations, (2) technical specificaticn limits on drywell.
lea bge which are reviewed under SRP 6.2.1.1.C are also used to establish pool bypass rates, and (3) when the proposed new section is used to setThis retention last -
credit, accept ance criterion 5 of SRP E.5.1 is not to be applied.
pudtion is needed to prevent the use of SRP 6.5.1 to downgrade an existing standby oas treatment filtration system from being an effective engineered safety feature as defined under the testirg guidance of Regulatory Guide 1.M.
The net effect of the proposed new section for existing licensees is a possible relaxation in current staff positions which has no significant detrimental effect on safety, but which provides more flexibility and some potential cast savings to the industry in meeting the regulations. Since fission product cleanup credit for BWR suppression pools was reviewed and approved by the staff for the GESSAR application, the effect of this SRP section will also be to provice uniform and consistent guidance to the staff for the review of this area.
An earlier draft of the attached package has been reviewed by the ACRS, and their contents have been accommodated.
A 4er any further changes arisin from CRGR consideratio m _tbe nrop3 sed new section will be published for pub ic co MS2p/// M dpj o,,ic s > .......... ....... .. ~um u u ............m,u m r................. ..................... ,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,,,,
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E 22 E James H. Sniezek The proposed new section refers to the SPARC code, whicn This1s code a parthas of the only source-term code package developed by HES contractors. A recently oeen updated to treat iodine vapor in addition to aerosols.
Brookhaven National Laboratory report is enclosed as a technical finding document. A reterence appearing in the proposed section has not yet been printed, and a preprint has also been enclosed. These enclosures are intended to assist CRGR and ACRS memoers in their consideration of the general subject of pool retention, and contain no new guidance or criteria. A regulatory If the analysis, as specified in NUREG/ex-0058, Rev. 1 , is also encicsed.
proposed section were applied using current Regulatory Guioe 1.3 source On the term basis assumptions, there would be some net reduction in requirements.
o? the assessment of this reduction in the enclosed regulatory analysis, we concluce that public health and safety would be adequately protected if the The cost savings to an operating proposed new section were implementea.
plant made possible by this net reduction in requirements, nowever, would be small.
Committee .:onsideration of this matter by January 15, 1987, is requestea.
ort,3%al etsred ts.:
med n. voum=
Harold R. Denton, Director Office of Nuclear Keactor Regulation
Enclosures:
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- 1 Proposed New Standard Review Plan Section
- 6.5.5 PRESSURE SUPPRESSION POOLS AS FISSION PRODU ,
?
REVIEW RESPONSIBILITIES _ ,
I.
Primary - Plant Systems Branch Secondary - Reactor Systems' ,
1 AREAS OF REVIEW _ 'I i
i Pressure suppression pools are reviewed under this plan only wh applicant claims credit for fission product scrubbing and reten The pressure suppression pool and the drywell, the suppression pool.
, d re when considered as a barrior to the release of fission pro ucts, a -
reviewed to assess the degree to which fission products released during postulated reactnr accidents will be retained in the Leekage' paths which allow fission products to i suppression pool. j bypass the pool are identified and reviewed, and the maximum l fractional bypass leakage is obtained, for use in the evaluation o !
~
radiological dose consequences.
t I
1.) Fission Product Control Requirement _
Sections of the SAR related to accident analysis, dose !
calculations, and fission product control are reviewed to ,
6 l
.4 2
establish whether or not fission product. scrubbing of the l
drywell or reactor compartment' atmosphere is claimed or required for mitigation of oft-site consequences following a postulated accident. l 2.) Design Bases A compr.rison is made to establish that the design bases for the suppression poo. and the drywell or reactor compartment are consistent with the assumptions made in the accident
\
^
evaluations of SAR Chapter 15. ;
3.) System Design The internation concerning the suppression pool is reviewed to 4
familiarize the reviewer with the expected tegoerature histories, depth of fission product entry expected during postulated accidents and potential leakaoe paths through drywell penetrations.
4.) Testino and Technical Specifications The details of the applicant's proposed preoperational tests.
and, at the operating license stage, the surveillance l
The requirements, are reviewed under section 6.2.1.1.C.
results et that review are examined to assure that pool depth
' and amount of leakage bypassing the pool are maintained con-sistent with the assumptions used in assessing the pool's effectiveness in fission product cleanup.
- l i
l .
3 11 ACCEPTANCE CRITERIA The acceptance criteria for the fission product clean-up function of the suppression pool are based on the following requirements from
' Appendix A of 10 CFR 50:
A. General Design Criterion 41 (Ref.1) as related to the control of fission products following potential accidents.
B. beneral Design Criterion 42 (Ref.1) es related to the periodic inspection of engineered safety features.
C.
General Design Criterion 43 (Ref.1) as related to the periodic functional testing of engineered safety features.
Where they can De shown to be in compliance with these criteria, suppression pools may be given appropriate credit for fission product !
l scrubbing and retention (except for noble gases, for which no pool retention is allowe- ) in the staff's evaluation of the i adiological consequences of design basis accidents.
Specific criteria which must De met to receive credit are as follows:
l
- 1. The drywell ano its penetrations must be designed to assure 6
that, even witn a single active failure, all releases from tne core must pass into the suppression pool, except for small s
bypass leakage. !
- !,Y 4
- 2. The bypass leakage assumed for purposes of evaluating fission product retention must be no less than that accepted in the review under section 6.2.1.1.C. ana must be demonstrated in periodic tests by the license technical specifications also reviewed under that section.
?. For plants wnich have already received a construction permit, ,
the iodine retention calculated using this section must not l
' be used to justify removal of the standby gas treatment or other filtered exhaust system from status as engineered safety features. For such reviews, criterion 11.5 of SRP 6.b.1 shall not be applied, ana tne charcoal absorbers must be at least maintained to the minimum level of Table 2 in Regulatory Guide 1.52, Revision 2.
2 Acceptable methods for computir.g fission product retention by the suppression pool are given in Subsection III, "Review Procedures."
111 Heview Procedures The first step in the review is to determine whether or not the suppression pool is to be used for accident dose mitigstion purposes. If no fission product removal cred.t is claimed in the accident analyses appearing in chapter 15 of the dAK, no further review is required.
5 If the suppression pool is intended as an engineered safety feature for the mitigation of off-site doses, then the reviewer estimates its effectiveness in removing fission products from fluids expelled from the crywell or directly from the pressure vessel through the depressurization system. L i
- 1. Pool decontamination factor i The decontamination factor (DF) of the pool is defined as the ratio of the amount of a contaminant entering the pool to the amount leaving, vecontamination factors for each fission product form as ' unctions of time can be calculated Dy the SPARC code (Ref.2), and this calculation should be performed whenever the pool design is juaged by the reviewer to differ significantly from those found acceptable as fission product :
cleanup systems in past reviews. If, however, the time-inte-grated DF values claimed by the applicant are 10 or less for particulates and 100 er less for iodine vapor the applicant's values may oe accepted without any need to perform calculations (Ref. 3). A DF value of 1 (no retention) should be used for i noble gases and, unless the applicant demonstrates otherwise. l for organic 1odides as well.
If calculation of fission product cecontamination is done using the SPARC code, the review should be cooroinated with the Reactor dystems Branch, which is responsible for establishing the J
accident assumptions needeo to assemble the input for the calcu-lations. ;
(
6
- 2. Pool bypass fraction The traction of the drywell atmosphere bypassing the suppression pool by leaking through drywell penetrations is obtained as a product of the review under section 6.2.1.1.C. !
l If B is the bypass fraction and DF is the time-f r.tegrated pool I'
decontamination factor, then the overall decontamination, D, to be reported to the Reactor bystems Branch for use in chapter 15 dose calculations may oe taken as:
DF -
Q=- 1 t t 6 (DF-()
or 1 6_
' _.L. , G+
0 OF ,
The reviewer should clearly distinguish that fraction of B which l
may be further treated by the standby gas treatment system from that fraction of B which also bypasses secondary containment.
i
- 3. Otner conteinment atmosphere clean-up systems _
Plarts having drywell or containment spray systems for which vission product cleanup credit is claimed are reviewed separately under section 6.5.2, and credit for both suppression pool and spray cleanup can be given as a result of the separate reviews.
3,
- c- - - - . _ - _ _
o 7
- 4. Technical Specifications The technical specifications are reviewed to_ assure that they require periodic inspection to confirm suppression poo' depth and surveillance tests to confirm drywell leak tightness consistent with~the bypass-fraction used in computing the overall decontamination. Technical specification review is coordinated J with the Facility Operations Branen as provided in NRR Office Letter No. 51.
Av tvALUATION FINDINGS The reviewer verifies that sufficient infort.1ation has bee'n provided by the applicant and that the review and any calculations support conclusions of the following type, to be included in the staff's Safety Evaluation Report:
We have reviewed the fission product scrubbing function of the pressure suppression pool and find that the pool will reduce the fission product content of the steam-gas mixture flowing through the pool following accioents which blow down througn the suppression pool. We estimate the pool will decontaminate the flow by a factor of for molecular iodine vapor and by a factor of for particulate ,
fission products. No significant pool decontamination from 4
noble gases or organic iodides will occur. The system is largely passive in nature, and the active components are suitably redundant such that its fission product attenuation
8 The Tunction can be accomplished; assuming a single tailure.
applicant's proposed program for preoperational and surveillance tests will assure a continued state of readi- l ness, and that bypass of the pool is unlikely to exceed the l I
assumptions used in the dose assessments of Chapter 15. -
The staff concludes that the suppression pool is i
acceptable as a fission product cleanup system, and meets r
the requirements of General Design Criteria 41, 42 and 43. ,
V IPPLEMENTATION 5 t
Except in those cases in which the applicant proposes an ;
acceptable alternative method for complying with the specified portions of the Commission's regulations, the methods described nere in are to be used by the steff in its evaluation of (
conformance with Comissions regulations, implementation of the acceptance criteria of subsection 11 of l this plan is as follows:
f i
(a.) Operating plants and OL applicants need not comply with !
the provision of this review plan section.
(o.) CP applicants will be required to comply with the provisions !
of this revision, i
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1
)
9-VI- REFERENLEd j
- 1. 10 CFR Part 50. Appendix A, General Design Criteria 41, ,
"Containment Atmosphere Clean-up", 42, "Inspection of i
Containment Atmosphere Cleanup Systems", and 43, "Testing ;
of' Containment Atmosphere C1e'anup System *'. ,
- 2. P.C. Owczarski, R.I. Shreck and A.K. Postma, "Technical Bases and Users Manual for the Prototype of a Suppression Pool Aerosol Removal Code (SPARC)', NUREG/CR-3317,1985.
- 3. P.C. Owczarski and W.K. Winegardner, "Capture of lodine i in Suppression Pools", 19th DOL /NRC Nuclear. Air Cleaning Conference, Seattle, 1986.
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REGULATORY ANALYSIS OF THE REVIEW OF.
SUPPRESSION POOLS AS FISSION PRODUCT CLEANUP SYSTEMS
- 1. Statement of the Problem Regulatory Guide 1.3, "Assumptions Used for' Evaluating the Potential Radiological Corsequences of a Loss of Coolant Accident for Boiling Water Reactors," states, as Regulatory Position C.I.f. that "ho credit is given r
for retention of iodine in'the suppression pool." Before the time this guide was first published, November 2, 1970, experiments had demonstrated
' the efficacy of suppression pools in removing iodine in several chemical The adoption of Regulatory Position C.I.f.
- forms from air-steam mixtures.
therefore, was deliberately conservative. Factors which may have influenced its adoption are:
f (1) drywells are generally leaky, permitting significant bypass of the suppression pool.
f (2) suppression pool retention of fission products varies markedly with conditions pertaining during the accident, and would have G
i required more complicated models than any being used in 1970.
(3) because of heavy reliance on standby gas treatment systems, suppression pool credit was not needed by boiling water reactors to j
meet the dose guidelines of 10 CFR 100.
l
2 Standard Review Plan 6.5.3, "Fission Product Control Systems and Structures," contradicts Regulatory Guide 1.3 by stating that suppression pools may be considered as fission product control systems, although no i
guidance or reference is supplied as to methods to be used in their review. In NUREG-0979, supplement 4, "Safety Evaluation Report related to the final design approval of the GESSAR II BWR/6 Nuclear Island Design,"
the staif agreed to consider suppression pool retention in any application referencing the approved design. Revisions prompted oy new source tena information and the replacement of TIO-14844 by more realistic accident assumptions will result also in the revision of Regulatory Guide 1.3.
Regardless of whether en accidental release is assumed using the current Regulatory Guide 1.3 or u:ing the most modern methods, it is an undue conservatism to ignore the capability of the suppression pool to mitigate off-site dose consequences, provided that recogr2f tf on of such capability
. does not degrade safety.
1 l The effectiveness of suppression pools in retaining gaseous 1odine and particulate matter varies markedly with the conditions under which these
! materials are swept into the pool. While the overall effects of such l- variation can be calculated for any given postulated accident, this
! calculation would be uncertain in its predictions of the relevant
! It would be conditions and would be very expensive to perform.
inappropriate to solve the problem of ignoring suppression pool j
effectiveness by replacing it with a required set of calculations that are l
l l
u -
3 To avcid this further impractical for use in assessing effectiveness.
prcblem, the present proposal takes a narrow interpretation by replacing the undue conservatism of omitting credit in favor of moderately conser- .
vative simplifications.
- 2. Objectives The objective of the preposed action is to establish the degree to which suppression pools can be considered as fission product cleanup systems an by revising the Standard Review Plan (SRP) to include procedures and
- criteria for suppression pool design evaluation.
- 3. Alternatives The existing SRP 6.5.3, "Fission Product Control Systems and Structures,"
in 11.5, states that "Fission product retention credit assumed by the applicant for other systens, e.g., pressure suppression pools, may be acceptable provided that justification is supplied by the applicant."
This provisdon has been applied, so far, only in the review of the The existing SRP, however, contains no procedures GESSAR-II application.
One alternative to the proposed for reviewing pressure suppression pools.
new section, therefore, would be to continue to review pools on a This course would not consistently apply computer case-by-case basis.
code and model valioation experiments which have been devised for pur-poses of developing a means of calculating pool retention of fission products.
L
4
.SRP 6.5.3 could be revised to remove the statement allowing pool credit, and the GESSAR !! SER could be amended to retract the earlier position.
This course would remove the inconsistency between the SRP and Regulatory Guide but, in addition to ignoring the large volume of research data supporting pool credit, would provide an undue degree of conservation to-tne staff's review and be contrary to commission policy. (Goal 2.4 NUREG-0885, Policy and Planning Guidance, 1986)
The altern]tive selected is to propose an additional review plan section which would provide a consistent use of the available data without de-gradation of safety.
In proposing review procedures, two decisions were made concerning the means by which the review could be simplified.
(a) time-averaged decontamination factors (DF) were introduced, (b) minimum DF values were stated, such that only applications claiming larger DF's would require plant specific computer runs.
These decisions were prompted by practical considerations in conducting 1
reviews; not taking the proposed course would have required great computer l
expense in any review. If a novel suppression pool feature were propend, such as, for example, a chemi'al additive or increased submergence depth of the downcomers or quenchers, the Source Tertn Code Package computer codes could be run to quantify the effectiveness of the pool. Use of I
the Source Term Code Package costs about $25,000 per accident sequence.
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o 5
The minimum DF values chosen are designed to be sufficiently small that no accident sequence is likely to be found to have a smaller tin,e-averaged value, even allowing a margin of safety for uncertainties.
- 4. Consequences _
By resolving the contradiction between Regulatory Guide 1.3 and SRP 6.5.3 '
in favor of th. tormer, the staff would be denying a large body of evidence proving the efficacy of suppression pools in retaining fission products. This might be defensible on the grounds of being conservative, but would not permit the realistic consideration of core melt accidents as they are currently being modeled to be used in licensing decisions.
By continuing the present situation, i.e., taking no action, the con-1 Licensees could reque.st suppression pool credit, tradiction would remain.
based on the staff's GESSAR Il statement, but the staff would have no j
eensistent guidance for performing the review, and would be reduced to either accepting or rejecting the licensecs' submittal, or running the source term code package repeatedly, i
e The consequences of the proposed new section would be the effect that ,
increased pool credit would have upon the efficiency required of other fission product control systems in order to meet the dose guidelines of 10 CFR Part 100.
A licensee could request pool credit to justify a relaxation of the maintenance and surveillance requirements placed on other systems.
w p.- .. - e = .-. -n - - -- ---- -.+
w, -.w asu k . j 6
Apart from the r.ontainment buildings themselves, the most important acci- ,
t dent off-site dose mitigation features of boiling water reactor plants under the SRP are the standby gas treatment systems (SGTS). These filtered 1 exhaust systems are designed to have maximum effectiveness against the foms !
of fission product iodine assumed to be released by TIO-14844. When reviewed i
against the fission product releases predicted by the new source term code !
E e package, however, suppression pools art capable of a high degree of retention i of fission products. The proposed change will focus attention on sup- ,
t pression pools as dose mitigation features, and as a means of providing '
l defense-in-depth in fission product nitigation capability. l The development of regulatory requirements for suppression pools might ll f
I lead existing licensees to upgrade the quality of drywell penetrations, as f Drywell pene- !
I' part of measures to minimize pathways bypassing the pools. f trations are already subject to leak testing at each refueling under SRP [
f 6.2.1.1.C. ,
P
.I j
l At present, Regulatory Guide 1.3 assumes that 22.75% of the core iodine i inventory as molecular iodine, 1.25% as particulate and 1% as organic iodide arr available for release from containment. Typical standby gas treatment systens (SGTS) serving BWR secondary containments as filtered exhaust systems are maintained at 99% efficiency against all of these i
[
} forms, i.e., after one-hundred-fold decontamination U.25% of the core I
iodine is exhausted into the environment as the sum of the primary I
containment and main steam line isolation valve leak rates following a !
- l OBA-LOCA.
i P-
7 .
Against the same release to containment, the minimum decontamination factors in the proposed SRP 6.5.5 would reduce the 25% of the core iodine inventory available for primary containment leakage to 3.5%, assuming 10%
suppression pool bypass leakage. For if, pool bypass leakage,1.6% of the core iodine inventory would be computed as available, i
J Using the assumed release in Regulatory Guide 1.3, and obtaining sup-4 pression pool scrubbing credit with 10% bypass, a typical BWR could meet ,
i 10 CFR Part 100 thyroid dose guidelines and still reduce the plant SGTS It should not, however, be assumed that by l efficiency from 99% to 95%.
reducing bypass leakage and claiming suppression pool credit a licensee could greatly reduce the su, 'il'vance testing requirements of their SGTS.
Other design basis accidents, for examr ie the fuel handling and instrument f line break accidents, also require the use of the SGTS to meet the
- a:ceptance criteria of 54P 15.6.2, 15.6.5 Appendix B, and 15.7.4.
I3I l would lead to dose
! For a typical plant, the release of 3000 Ci of i
consequences in excess of the guideltres of 10 CFR Part 100. This amount i
I i
t 13 equivalent to only a few parts per million of the enre inventory of ,
For a typical BWR, a million-fold reduction in iodine fission products. i' iodine 15 mostly achieved by a low leakage containment (0.5% per ^ n. ?
1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> leaks 4X10-4) and to a lesser extent by the SGTS (10^ penetratst I
byiodine). Since suppression pools are virtually useless against organic l I
l iodide, and since it is not feasible to eliminate bypass completely, overall decontamination factors of more than about 15 cannot be practically )
! achieved using the current iodine chemistry assumptions in Regulatory l r
, i f
i
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l ?
( - . - - -_ _
j
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8 i
I Guide 1.3, even if moleculap and particulate iodine forms are totally 1
absorbed by the pool. !
i I
a A 15-fold reduction in SGTS effectiveness, i.e., from a penetration test of 1% or less to one of 15% or less, would reduce its organic iodide absorption efficiency from 99% to 85%. Unfortunately, the SRP section dealing with SGTS review, 6.5.1, states that systems requiring iodine j
aben ption efficiencies of less than 90% may be reviewed under SRP 11.3.
Charcoal absorbers reviewed under SRP 11.3 may follow Regulatory Guide f l
1.140 rather than 1.52, and are not built or maintained to engineered safety teature standards. To prevent use of suppression pool'eredit to justify not maintaining and testing SGTS absorbers to Regulatory Guide
! 1.52 criteria, prior to revision of SRP 6.5.1, explicit mentior of SGTS
,i surveillance tests has been added as a criterion.
A typical SGTS contains about $20,000 of irrpregnated charcoal per train, l i
with a comparable additional labor cost for renewing and testing if filter replacement i:5 n'eoed to pass a surveillance test.
If the surveillance test criteria of a Sri" .+ e relaxed, charcoal change out would be l 4 required less ofter, 5
- t- reducing maintenance by of the order of 10 i
dollars per year.
I
' It is also possible that a licensee might wish both suppression pool and maximum SGTS credits, white requesting an increase in allowable containment leakage. Again, a very large saving in the costs of containment integrated leak rate tests would not be expected, since the i
large degree of iodine fission product retention would not be associa,-4 i
,u 9
with any change in the postulated noble gas releases during a LOCA.
incensees electing this course would be limited by the 10 CFR Part 100 guideline for whole body doses at the low population zone boundary over
. the course of the accident. For mest BWRs, a doubling of the containment leak rate would bring the roble gas *elease consectences to the guideline,
'. although for some plants having favorab'e meteorological parameters and large low population zones several-folo increases would still meet the
- guidelines.
i While granting s erait for suppression pool scrubbing, as proposed, would allow the determ..ts'ic licensing calculations of accident dose to'bo i vore easily met, the primary thrust of the change will be to n11 w 1
- gieater BWR cortainment leak rates and more noncondensible accident fission products past SGTS filters. That is, existing BWR containment l
' leak rates of about 0.5 volume percent per day maybe increased to as much
! as 5 volume percent per day, and 99 percent elemental iodine filter 4
effteiencies maybe reduced to 90 percent. The change, therefore, may
! result in increases in the quantities of fission products postulated to
- 1. However, regulatory I be released during design basis accidents.
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guidelines would still be met and the change in risk is expected to be very small since the bulk of publi: risk is attributed to accidents in
)
l which the containment fails or is bypassed (i e., severe accidents not
- design basis accidents).
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_ _ _ _ . _ _ , . _ . . _ _ . , _ , _.,_ _ _ _ _ ,. ~.____ _ __ _ _ , _ . _ _ ._.
v -
10
.; i Se Oecision Rationale c F
Strategic goal 2.4 of the NRC Policy and Planning Guidance, 1986, lists as.
objectives the corrpletion of the reassessment of source terms and the implementation of' appropriate revisions in staff practices. Th? source term revisions will involve mary relatto changes to the SRP and regulatory
~
guidant.e and may also include rulemaking and revision of existing regulations. The propostd action is perceived as an early step in'this l l
process, since it will out in place the review procedures and criteria i necessary for considering the mitigatien of new source terms by suppression '
t pools, t The proposed section is equally applicable to the Jource term assumptions contained in Regilatery Guide 1.3 and to the fission product r; leases cal-
.ulated oy the ',ource Term Code Package. For both applications the pro- !
posed action offers the following advantages: I j
1.) Supprersien pool fission product retention can be assumed to be descr' bed by conservatively chosen decontamination tactors.
The f f
use <,f these factors avoids tne large expense of computer analysis !
neeyed to quantify suppression pool response using the available ;
cuaputer codes. As discussed earlier, very large decontamination f
fJctors can be calculated, but the net effective decontamination !
l achievable is limited by the possibility of pool bypass leakage. !
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{
G . . - . - . - - . - .- N
11 2.) Existing plants have the possibility of reducing maintenance costs 1 for their chsrcoal absorbers by being able to retain the absorbent for longer periods of time between changes.
i The cost savings of these advantages would vary with the degree to which licensees and aplicants elected to c1cim suppression pool i fission product cleanup .redit, and the number and diversity of accident sequences necessarj to represent the effectiveness of l the pool, f I While releases of fission products as assumed in Regulatory Guide 1.3 are effectively reduced by filtered exhaust systems, the releases calculated for many accident sequences by the Source Term Code Pachge are morr effectively reduced by suppression pool scrubbing. l By adding guidance (or the review of suppression pools as fission product cleanup systems in the form proposed, conservative but ;
appropriately realistic credit would be assessed without significant lost of the safety afforded by existing filtered exhaust systems, i
j
- 6. Implenientation ,
l j l
The proposed action requires no action of existing licensees, except as 7 l i they might voluntarily elect to reanalyze the accident consequences and }
l submit an FSAR amendment to reduce reported todine doses. This action j would take effect upon publication of the proposed revision, j f
c f
o 74 19th DOE /NRC NUCLEAR AIR CLEANING CONFErsENCE ,
- CAPTURE OF , IODINE IN SUPPRISSION POOLS P. C. Owczarski and W.. K. Winegardner Pacific Northwest Laboratory *, Richland, Washington Abstract ,
The ef fectiveness of suppression pools in capturing airborne iodine species was investigated. A computer code was used to simulate
.the scrubbing of particulate iodide, vapor elemental iodine, and vapor .
organic lodides. For a typical postulated severe core damage accident i
sequence, suppression pools were effective scrubbers of elemental iodine if the pool was alkaline or dilute in iodine and of particles
>1.5 um mass median diameter. Little scrubbing of organic lodida
, species occurred. An absorption model shows that elemental lodine can i be absorbed by wet alkaline droplets before the droplets encounter the suppression pool. Thus, the iodine removal effectiveneas of the pools l ;
I is likely to be controlled by particle s: rubbing. ;
a i I. Introduction ;
i i
The estimation of airborna source terms in postulated severe core melt accidents required the evaluation of the, responses of nuclear I
reactor Engineered Safety Features (EST) under accident conditions. .
As part of this evaluation, the Pacific Northwest Laboratory (PNL) has !
been studying the aerosol capture effectiveness of soillag Water
- i
' Reactor (BWR) pressure suppression pools.** The initial work assumed *
' that fission product iodine would exist as Cs! in the aerosol leaving the reactor primary system. Concern remains that other chemical spec-j I
~
les of iodine might exist, notably 12 and organic iodides (tepresented by CH 3I) . Continuing work reported here shows that the sc of the pool i
effectiveness quantified by decontamination f actors (dis)yubb ng ;
varies dramatically for the three chemical species. -
To estimate the pool l
developed the SPARC',code (gqrubbing effectivenessdLon I' particles .
l with existing publishad data'.(3hich has beer, The SPAPC partially code was v.slidate then modified to j include I 2 An additional function of SPARC com- i putes the absorption of 22 and CH 3 I scrubbing.by particles containing deliquescent CsoH.
Technical Mases summary II. f This section summarizes the technical bases for the models in !
SPARC. The bases for 9 previously discussed (Rost
'1 and of briefly will be the particle repeated nern. Then scrubbing the models have; s
j bases for iodine scrubbing will be discussed. l l [
i
- Pacific Northwest Laboratory is operated f or the U.S. Depart-ei.t of Energy by Battelle Memorial Institute. -
- Work supported by the U.S. Nuclear Regulatory Commission under
( Centract DE-AC06-7 6RLO 1830, NRC FIN B2444. '
+DT
- mass flev rate of a fission eroduct into coolLass flow rate of snat fissio
~
19th DOEINRC NUCLEAR AIR CLEANING CONFERENCE The SPARC simulation of pool scrubbing first relies on a descrip-tion of the hydrodynamics of gases entering a water pool at a sub-merged depth. The hydrodynamics of the vent exit region are very important as are the hydrodynamics of the bubble swarm rise to the pool surface.
Particle Scrubbing A number of phenomena have been identified as contributors to the particle scrubbing process. Theso are:
e particle inertia at the vent exit
,e bubblo inertia at the vent exit e steam condensation at the vent exit e temperature gradient at the vent exit e steam formation during bubble rise
- e particlo growth
( e bubble circulation during swarm rise e bubble coalescence /redispetsion during swarm rise.
The above phenomena are quantitatively modeled in SPARC for their [*
roles in the particle capture mechanisms at the vent exits (centrifu-gal scrubbing, inertial deposition, steam condensation and thermo-phoresis) and during swarm rise (centrifugal scrubbing, gravity set-tling and Brownisn dif fusion) . The centrifugal scrubbing here refers
, to deposition of particles at curved gas-liquid surf acas caused by the acceleration of particles in the radial direction as a result of tan-gential surface velocities.
Iodine Behavior .
A number of aspects of iodine behavior are related to its capture in suppression pools. These aspects can be identified in'three regions of the flow of gases. The first region is the flow of iodine species in the core-melt off gases in the reactor primary system. The second is the vent exit region in the pool and the third is the bubble swarm rise region in the pool.
In the prinary system, where gases are hydrogen' and steam and iodine species can be I2, organic lodiden, HI, and particulate iodides such as Cs!, conditions can exist that f avor the complete removal of the volatile inorganic species f rom the gas phase. 'These f avorable conditions consist of a suf ficiently lov temperature so that alkaline aerosol particles can exist as a liquid or partially liquid phase.
Alkglg.
300 C hydroxides This liquid such as CsoH phase can be have highly thisreactive propertywith in the thevicinity volatileof species HI and 1 2 We speculate that solid Cs0H can be reactive with these species as well. The SPARC code has a subroutine that allows the user to switch on this iodine absorption process in the primary system. The proc 6.ss is modeled as a continuous plug-flow rer.ctor where spherical aerosol particles absorb elemental iodine at a rate contrclied by the dif fusion of I2 in the gas phase around the partic-les. Although not modeled, HI would behave similarly to I2, but with a slignely higher dif fusion coefficient. The results of using this subroutine are discussed in III. Accident Secuence Results.
19th 00E/NRC NUCLEAR AIR CLEANING CONFERENCE As tho gasos lcavo tho primary system, they water the pool ct o depth through a specific vent type. In the region of this vent, the .
gases try to equilibrate with the thermodynamic conditions of the pool at the vent depth. This equilibration process frequently results in steam condensation and scrubbing of particles. In SPARC, this conden-sation results in some deposition of I 2 and CH 3 I, but is limited by the species solubility at the interf ace.
Af ter the initial gas globules at the vent break up into the ris-ing bubble swarm, the SPARC code assumes that bubble circulation con-tinually renews the bubble interf ace and that the film theory of mass
. transfer resistance holds on both sides of the interf ace. The equili-brium boundary conditions at the interf ace for the two volatile ..odine species are:
(TI2(aq))1 = H(I2)(I2(gas))1
- and (CH3 i(aq))1 = M(CH3 7)(CH3 f(gas))1 where (TI2(aq))1 = total liquid molar concentration of iodine at the interf ace as 12-s . .
(12(gas))1 = interf acial ger, molar concentration of 1 2 and H(I2) = iodine partition coef ficient.
Similar definitions hold for CH I 3 1
controlled by the f ast reactions:(5)The aqueous chemistry of iodine is I2(gas) = T2(aq)
I2(aq) + I~ = I 3 '
2 2 (aq) + H2 O = H+ + I~ + HIO 12(aq) + H2 O = H 2 OI* + I' M2 0 = H+ + OH" .
By using the quilibrium constants for the above five reactions, the partition coefficient is quantitatively defined if mass balances of all iodine species and H+ and OR~ are maintained. The value of H(CH 3 sure data.
I) is(gtained in a simpu. vay using solubility and vapor prus-
, SPARC Validation The particle capture model .n SPARC vs been ' partially vali-dated virh data as they become available.g! The iodine capture ,
are validated 5.y small-scale t.asts. The data of Dif fey et modg al. compare f avorably with SPARC calculations for both I2 and CH3 I i
19th DOE /NRC NUCLEAR AIR CLEANING CONFERENCE scrubbing. .No p :ans exist to measure volatile iodine scrubbing in large-scale experiments.
Accident Parameters ,
SPARC can be used to analyze pool scrubbing during the course of an accident scenario. A number of accident parameters must be d2 fined for each time step when pool scrubbing is important. The most impor-tant set of parameters is the particle size distribution. The SPARC input parameters are listed below:
Pool e concondensable gas flow rate into pool e noncondensable gas composition -
e steam flow rate into pool e pool depth, temperature e pool size, configuration e pressure above pool e pool composition (surf actants) e vent exit configuration Aerosol Particles e mass flow rate o size distribution e density / shape f actors e solubility in wuter (and fraction of soluble alkaline materials) , ,
Iodine e nass flow rates of each lodine species e temperature and pressure of primary system These parameters are defined for an example accident scenario in the next section. -
g e
III. Accident Secuence Results To examine the behavior of iodine species in the pool, we used a si ecific pcstulated accident sequence to establish the pool, f1 [d)and ssion product characteristics for this study. The Tc sequence for a Mark I BWR was' selected as a representative accident. In this accident, a ter.nsient event was followed by control rod insert. ion f ailure, but emergency core cooling systems operated. However, the reactor power level exceeded the cooling capability of the suppression pool, overpressure f ailure of the containment occurred followed by stoppage of reactor vessel coolant flow. The core heated up and melted, releasing fission products into outflowing steam and hydrogen.
During this melt release, these gases and fission produ f ts flow f rom the core through the primary system and suppreusion pool. It is for this period, f rom 134 to 168 min af ter the initiating transient, that we have analyzed the pool scrubbing ef fectiveness of iodine species as
- - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _____ i
19th DOE /NRC NUCLEAR AIR CLEANING CONFERENCE well as any prior reaction of vapor elemental iodine with alkaline
) aerosol droplets in the primary system upctream of ti.e pool, a
The data pertinent to the SPARC analysis are summarized here: 8 z
Aurosol flow rates ranged from 110 g/s at the beginning of vt%
coreofn.clt to less than 1 g/s at the end. CsOH ranged from 60 Ninety-nine to 10 this per-aerosol. Iodine flow rates ranged from 9 to 1 g/s.
cent of this iodine was examined as either I2 vapor orParticle ao CsI sizes parti-3 cles and 1% of the iodine Vas assumed to be as CH I. Geomst-began at 1.5 pm mass median diamater' and finished at 2.7 um.ric stand
[ particle density remained constant at 1.7 and 3 g/cm , respectively. g Gases f rom the steadily depressurizing primagy system Steam (2 to 1.3 atm) flow began at 5 maintained a temperature range of 340 to 360 C. Hydrogen flow began at 170 g/s and a 1300 at 110g/sg/sand. ended at 8 g/s.These gas and aerosol flows entered the suppression pool through 13 t-quenchers at 12 f t submergence.
b With the above aerosol and flow specifications, the SPARC code was run as three independent cases: Case 1 where the elemental iodine was allowed to be absorbed by alkaline aerosol droplets in the primary system, Case 2 where the alkaline materials were absent in the aero- in sol, and Case 3 where the CsoH was not allowed to react with the I 2 the primary system. In Case 1, iodine was present as CsI in Also, the par-the ticulate mass. In Case 2,*the iodine remained as vapor 12 pool did not have the benefit of becoming alkaline f rom Cs0H parti-cles, eo Ig scrubbing was af fected by an initially neutral pool (pH =
6.5 at 100 C) that became slightly acidic at the end of core melt (pH
= 's . 9 ) .
In Case 3, the iodine also remained ao 12 vapor, but the pool became alkaline during core melt and reached pH = 8.3.
The first of the SPARC results examined is the behavior of ele-cental iodine in the primary system in case 1. Here,I 2 was absorbed by wet alkalins particles in the short, once-through pass of gases throu.gh the primary system. The SPARC subroutine for these calcula-tions computes the instantaneous abcorption rate for the entire aero- l sol clouc Theingas termsresidence of the half-life time inof the I I Mark primar:rexistence systems, whighas the elementa form.
splits the flow into two parallel streams, has a value of 2 x 10 /Q s where Q is the total primary system exit s olumetric flow rate secong/s.
in cm Figure 1 compares the half-lif e with tie' residence time for the melt release period of the Tc sequence., Here it is evident that suf ficient residence time exists from the beginring of the melt to I nearly its end to absorb virtually all of the I 9 Only at the end of f uel melt does the residence time equal the ha'.f-life of I 2, which
' indicates that only one half the I 2 vapor is ebsorbed by droplets at that time. The centration lodine half-life of particles decreased increased with time and particle sizesbecause increased thethereby con-decreasing the area available for absorption. The gas flow rate dra-matica11y increased at 163 min resulting in the insuf ficient residence time f or reaction.
Use of the primary system absorption model, Case 1, was done is not likely solely f or demonstrating that elemental iodine (or HI) to exist as a species in the presence of particles containing CsoH in a moist environment. ,
h
~ - _ _ _ _ _ _ _ _ ___ _ _ _ . .
19th DOEINRC NUCl. EAR AIR CLEANING CONFERENCE 108 i j i I l F Residence Time i
< . l
- l l I e l . I 1e r C. -
l I
l m g I ,
? .
3 . . 1 i l l 1 10' 1
- ai it
't :. mI is !
,y,. .
yI lx e l5 dl lilu 1* i go** - gI I . I 17 j Half Life l" l !
V !
I - 1 l
a
- go 8 :.
i .
l I ,
.2 -
l l t l l
- I
, I I i
' ' I I I ' '
10'* ,
130 135 140 145 150 155 160 165 J70 i Accident Time, min r
TIGURE 1 COMPARISON OF THE HALF-LIFE OF I2 EXISTENCE IN THE PRIMARY SYSTEtt WITH THE RESIDENCE TIME ,OF GASES IN THE PRIMARY. SYSTEM (CASE 1) ,
The scrubbing of iodine species is portrayed ink Figures 2 and 3.
In the first figure, the lastantaneous I CsI, and CH 31 DFs are plot-tad versus time during the melt release.2,In Case 1, where Cs! is the ,
lodine species, the scrubbing of Cs! generally increases in time because of the gradually increasing particle size until 154.5 min, ,
where particle size stabilizes until the end of the melt release.
However, steam and. hydrogen gas flow increase dramatically at this [
point, and as a result the inertial particle capture mechanism at the vent exit increases the Dr. In Case 2, where the iodine is elemental and the posi receives no alkaline particles, the iodine scrubbing is ,
represented by the 12 curve. Here the I2 flow rate is f airly high until 14s.5 min, then the rate (and incoming I2 concentration) decreases. These decreases cause the pool scrubbing to become less effective at the lodine concentrations of the pool. However, in Case 3 the pool was allowed to increase in pH from incoming CsoH particles, thetimehistoryofiodineDFsyasdifferent. Then the instantaneous DFs were never less than 3 x 10 during the accident. The CH 3 I curve a
19th DOE /NRC NUCLEAR AIR CLEANING CONFERENCE shows scrubbing initially and around 148 min, when incoming airborne concentra'. ans are sufficient to drive CH 31 into the pool. Otherwise, the pool is stripped of CH I3 (Drs (1) during periods of low CH 13 con-centration in the incoming gas. It should be noted that the 14 assumption does not affect the Drs for CH 3 1.
10'
~
l T
l
/
]l1 ,
I l
- 3 10' e- l I
- 1
- I l
- Cs1 l la Ni . !l l!
l
] 10' :r{ll =
l{
le 7 I" ll lt l
g .I l4 1
E ~ dl
- l3 i io' r gl 1
.$l l*
l l
~
l
~
l I to' r-I 1
I, l
1 l l
~ '
l C4al l i . l l 1 I
., il l I I I I I e 140- 145 165 170
. 130 135 150 155 160 Accident Time, min ')
FIGURE 2 8 INSTANTAFEQUS POOL DECONTAMINATION FACTORS FOR IODINE SPECIES DURING CORE MELT IN TC ACCIDENT SEQUENCE. CsI CURVE REPRESENTS CASE 1, I2 CURVE REPRESENTS CASE 2 AND THE CH 3 1 CURVE IS THE SAME IN ALL CASES Another representation of pool scrubbing is portraytd in Firare 3. Here, the time-integrated DF is portrayed ove. '.he core-nelt period. This Dr is defined (over the time period 6t) a.
Dr(time-integrated)1 =
total mass of seecies i_ entering the pool in ot, total mass of species 1 leaving the pool in at' a
19th DOE /NRC NUCLEAR AIR CLEANING CONFERENCE The integration period starts at the beginning of the core molt. The initial DFs in both rigures 2 and 3 are identical. Cases 1 and 2 are again represented by the CsI curve and the I2 curve, respectively.
The inportant observation here is that even though the case 2 pool is slightly acidic, the integrated DF is one order of magnitude greater than the Case 1 integrated DF. gefinalintegratedDFforI2 in Case 3 (alkaline pool) is 2 x 10 , which is more than seven orders of magnitude larger than the corresponding Case 2 DF.
10' e g
~ i l
l l
l g -
0* la i ! ~ l
.I I El I.
3 5 10 r !
I B i =l IE
, . E I $l Cst II
, 1 9
- 31 si 7 3 ia
.5 10' r *E l li
- I lw I
l
'l l 1 l , I
.10' { l l
~. l !
l I
-~
~ CHal i I l ; I o 1i _ t ' '
i
! t 130 135 140 145 150 '155 160 165 170 Accident Time, min FIGURE 3 DECONTAMINATION FACTORS FOR IODINE SPECIES INTEGRATED OVER THE CORE HELT PERIOD. THE Cs! CURVE REPRESENTS CASE 1, THE I 2 CURVE REPRESENTS CASE 2 AND THE CH3 I CURVE IS THE SAME IN ALL CASES Sorte general conclusions can be drawn f rom the results of'the core melt sequence above:
o Pool scrubbing of lodine can be very effective when lodine is I vapor if the pool iodine concentration is low or if the poci is 2 alkaline.
19th DOE /NRC NUCLEAR AIR CLEANING CONFERENCE e Pool scrubbing of CH3 I is poor.
- Pool scrubbing of lodine as particulate Cs! can be fairly effec-tive for large particles (>1.5 pm mass median diameter) e 12 vapor cannot exist long in the presence of large numbers of wet ahkaline droplets e Tbn limitiag pool Dr would b6 th.at of particulate CsI unless sig-nificant cora lodine (>0.1%) is converted to CH3 I.
. References (1) Owc arski, P. C. , A. K. Postma and R. I. Schreck. Technical Bases and User's Manual for SPARC - A Suppression Pool Aerosol Removal Coda. HURIG/CR-3317, PNL-4742, U.S. Nuclear Regulatory Conmis sion, Washington, D.C. ,19 85.
(2) Owczarski, P. C. and W. K. Winegardner. "Validation of SPARC, A Su spression Pool Aerosol capture Model." Paper I AEA-SM-381/29 presanted. at IAEA International Sym Evalue; ion for Accident Conditions,posium on Source October Term 1,1985, 28-November Columbus, Ohio,19 85. .
. (3) Cunana, J. C., M. R. Kuhlman and R. N. Oehlberg. "The Scrubbing I of Tission Product Aerosols in LWR Water Pools Under Severe Accident Conditions - Experimental Results.
- In Proceedings:
American Nuclear Society Meeting on rission Product Behavior and source Tern Research. NP-4113-SR, Electric Power Research Institute, Palo Alto, Calif ornia,1985.
(4) Rollet, A.-P. , R. Cohen-Adad and C. Ferlin. *Le systeme -
eau-hydro):yde de cesium. ' Comptes Rendu, Vol. 256, Pt. 6,
- p. 5580 (1963).
, (5) Eggleton, A. E. J. A Theoretical Examination of Iodine-Water Partition Coef ficients. AERE-R 4 8 8 7. Atomic Energy Research j Establishment, HarvelT, England,1967. g (6) Glow, D. N. and E. A. Noelwyn-Hughes. *Chemica'l Statics of the Methyl Halides in Water." Discussions of the raraday Soef ety, No. 15, pp. 150-161 (1953).
(7) Dif fey, M. R. et al. ' Iodine Clean-up in a Steam Suppression System.' In International Symoosium on rission Product Release and Transport Under Accident conditions. CONF-650407 (Vol. 2),
Oak Ridge National Laboratory, Oak Ridge, Tennessee,1965.
(8) Gieseke, J. A. et al. Radionuclide Release Under Specific LWR Accident Conditions, Volume II, BWR, MARK I Design. SMI-2104, Volume II, Battelle Columbus Lacoratories, Columbus, Ohio,1984.
.