ML19345B691
| ML19345B691 | |
| Person / Time | |
|---|---|
| Issue date: | 11/13/1980 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19345B680 | List: |
| References | |
| FOIA-81-16, REF-10CFR9.7, TASK-IR, TASK-SE SECY-80-504, NUDOCS 8012020353 | |
| Download: ML19345B691 (6) | |
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UNITED STATES
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flovember 13, 1980 SECY-80-504 NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 INFORMATION REPORT For:
The Connissioners From:
H. R. Denton Thru:
W. J. Dircks, E00 dM#
\\.;
Subject:
ICDINE RELEASE OURIttG REACTOR ACCIDEllTS
Purpose:
To inform the Co=nission of the staff's discussions with Drs. W. R. Stratton, A. P. Malinauskas, and D. O. Campbell; to surmiarize the potential impact of tneir hypotheses concern-ing lodine release during reactor accidents; and to propose an agenda for the Ccmmission meeting on this t::pic on Novemoer IS.
Background:
In a letter to Chairman Ahearne dated August 14, 1950 (enclosed as Attachment I) three scientists trom Los Alamos and Oak Ridge flational Laboratories expressed their belief that current t!RC models ano Regulatory Guides do not correctly cescribe the release or iodine during nuclear reactor accidents.
The staff invited the three scientists to ciscuss the technical bases of tneir letter.
On October 22, 1980 Drs. Stratton, Malinauskas and Campoell met with acout 40 memoers of the staff.
The minutes of this meeting anc tne attendees list are enclosed (Attachment 21 As a result of a request frca the three presenters at that meeting, the staff has suggested an agenda for the Comnission's meeting on this topic, whicn is enclosed as Attachment 4 Discussion:
10 CFR 100 requires the postulation of a Cesign Basis Accident,
" hypothesized for the purpose of site analysis... that woulc result in potential hazards not exceeded by thoss from any accident consid-ered crecible." Reg. Guides 1.3 and 1.4 define the assumptions to be made to meet this requirement, which include an assumed release from the fuel into the containment of 1007, of the core inventory of noble gases and 257. of the iodine.
Secause of its relative aoun-dance, its high radiotoxicity, and its assumed release and transport in vapor (elemental or organic compounds) form, racioiodine is nearly always the most significant fis ion product, from a personnel hazarc viewpoint, in design basis accident evaluations.
In their letter to Chairman Ahearne dated August 14, 1980, Ors. W. R.
Stratton, A. P. Malinauskas, anc D. O. Campbell reported their opinion that current flRC models and Regulatory Guides are in error, anc that behavior projected from these models grossly overestimates the public risk resulting from reactor accidents due to the assumption that significant amounts of raciolacine can be releaseo to the atmos::here
Contact:
Walter Pasedag, itRR 49-27193 or Jacaues Read, i1RR 80t20';O M 3 49-27845
. as elemental vapor. Their August 14 letter briefly outlines a variety of evidence that radiciodine would not be expected to exist during an accident as elemental vapor, and urges a reassess-ment of the safety criteria and effectiveness of and need for engineered safety features.
The assertions of Stratton, et al would have a major. impact on the assumptions concerning Design. Basis Accidents.
It should be noted, however, that studies of the risk to the public from nuclear power plants, (e.g. the Reactor Safety Study, WASH-1400) have demonstrated that the overall risk to the public is dominated by more severe acci-dents involving substantial releases of solid fission products and including a breach of containment. For these accidents, which sub-stantially exceed the Design Basis Accidents in severity of their consequences, the assumptions concerning iodine release do not control the calculated accident consequences, and a reduction of the assumed iodine release would not substantially reduce the overall risk to the public.
Should the assertions of Stratton, et al, be shown to be entirely correct, a large number of regulatory requirements and guidance documents related to Design Basis Accidents would be affected, although revisions of pertinent sections of the regulations (10 CFR 100.11, 10 CFR 50 - App. A) would not necessarily be required. A thorough review, and possible revision, of the following requirements and guidance documents would be warranted:
(a) Regulatory Guides 1.3 (1.4) Assumptions Used for Evaluating the Potential Radiological Consequences of a Loss-of-Coolant Accident for Boiling (Pressurized) Water Reactors.
These guides assume 25% of the iodine inventory to be instantaneously airborne within the containment atmos-phere for the Design Basis Accident (DBA). This per-centage could be substantially reduced.
1.5 Assumptions Used for Evaluating the Potential Radiological Consequences of a Steam Line Break Accident for Boiling Water Reactors.
This guide assumes dissolved iodine within reactor coolant to be completely volatilized.
This could be reduced to a small fraction or eliminated.
1.7 Control of Combustible Gas Concentrations in Containment Following a Loss of Coolant. Accident.
This guide assumes 507. of the core inventory iodine is dissolved in the coolant for purposes of computing water radiolysis. Ynis percentage could be increased.
. 1.2d Assumotions Used for Evaluating the Potential Rad...ogical Consequences of a Fuel Handling Accident in the Fuel Handling and Storage Facility for Boiling and Pressurized Water Reactors.
129 This guide assumes 30% of the I and 10% of other radioisotopes of iodine to be released as gas from damaged fuel. These percentages could be reduced or eliminated.
1.52 Design, Test.,9, and Maintenance Criteria for Post-Accident Engineered-Safety-Feature Atmosphere Cleanup System Air Filtration and Adsorption Units of Light-Water-Cooled Nuclear Power Plants.
This guide specifies adsorber efficiencies, iodine adsorption capacities, and radiciodine decay heat dissipation for iodine filters.
These design capa-cities could be greatly reduced.
1.77 Assumptions Used for Evaluating a Control Rod Ejection Accident for Pressurized Water Reactors.
This guide assumes 10% of the iodine in damaged fuel and 25% of the iodine in melted fuel to be released as gas. These fractions could be reduced or eliminated.
1.97 - Instrumentation for LWR Power Plants to Assess Plant Conditions During and Following an Accident.
The range and relative significance of instruments to measure iodine in the containment and off-gas streams could be adjusted downward.
(b)
Action Plan (NUREG-0660) items related to fission product release assumptions and radiation protection, including sections:
II.B.2 Plant Shielding to Provide Access to Vital Areas and Protect Safety Equipment for Post-Accident Operation.
Design review of airborne iodine protection equipment would be of reduced priority.
II.8.3 Post-Accident Sampling.
The amounts and chemical forms of iodine to be found in vapor and condensed phases would be changed.
II.B.6 Risk Reduction for Operating Reactors at Sites With High Population Densities.
A reexamination of the priorities of risk analyses would be appropriate.
. II.F.1-2 Sampling and Analysis of Plant Effluents.
In order to detect the much l'ower radiciodine concentrations in air streams, more sensitive and elaborate techniques will have to be used.
II.E.4 Containment Design.
The public risk from containment a'tmosphere leakage or purging would be less controlled by iodine emission.
III.A.3 Improving flRC Emergency Preparedness.
See item (c) below.
III.D.I.3 Ventilation System and Radiciodine Absorber Criteria.
Ventilation system iodine absorber criteria could be relaxed.
III.D.3.3 In-P1 ant Radiation Monitoring.
Less emphasis could be placed upon iodine vapor monitoring and filtration.
III.U.2.2 Radioiodine, Carbon-14, and Tritium Pathway Dose Analysis Studies of the transport of iodine and its dispersal pathways in the environment would be re-ordered to place emphasis upon ionic and organic.: forms.
(c) Emergency Preparedness Assumptions Protective measures for the 50 mile emergency planning zone for the ingestion pathway, including the staff's position on thyroid blocking,are based to a large extent on high iodine concentrations assumed to be released from containment. An elimination of che potential for large iodine releases would warrant a reexamination of the bases for these staff positions. For the plume exposure pathway, health effects from severe core melt accidents are controlled by other isotopes, and may not be affected significantly.
(d) Design of Engineered Safety Features The absence of substantial release of volatile iodine during reactor accidents would also re-direct the design, installation,
- and testing requirements, as reflected in the staff's Standard Review Plan, Regulatory Guides, as well as Industry Standards, of Engineered Safety Feature systems designed to mitigate the conse-quences of accidents.
In lieu of systems designed to absorb and contain iodine (e.g. iodine retaining containment spray additives, charcoal filtration systems) the emphasis would most likely shift to systems designed to retain liquid effluents and aerosols of solid fission products.
These major revisions in NRC assumptions and the re-design of engineered safety features suggested as realizable goals by Stratton et al would require that both of the following assertions be shown to be true:
(1) iodine released from fuel under wet, non-oxidizing conditions does not appear in volatile forms, and (2) accidents with dry, oxidizing environments either cannot occur in LWR's or their probabilitics are low enough not to be considered as design basis accidents.
The discussion at the October 22 meeting between the staff and Stratton et al left a consansus that the first of these assertions could be readily tested, while the second could prove more difficult.
A confimation of the first assertion would decrease computed public risk from some accidents, since the conditions it describes are present in the more likely postulated accidents. The proof of the second assertion would be required to alter the assumptions concerning Design Basis Accident evaluations.
An essential element of the assertions of Drs. Stratton, Malinauskas and Campbell, i.e. the identification of cesium iodide in the release from irradiated fuel was identified by research sponsored by NRC.
The staff is continuing research related to fission product behavior under accident conditions.
Current research includes programs in the area of fission product release from fuel under high temperature con-ditions, and the transport of fission products in the primary coolant system and the containment. These research programs are identified and sumarized in Attachment 3.
In addition, preliminary planning is in progress on confirmatory research concerning accident mitigation systems.
In order to provide direction to and monitor the progress of these research programs related to fission product release, a Research Review Group was reactivated. Although the group consists of NRC staff members only, the use of non-NRC expert consultants from the scientific comunity is anticipated. This group could function as the task force envisioned by Drs. Stratton, Malinauskas and Camobell, if the Commission so desires.
l 6-This paper has been prepared by the Office of Nuclear Reactor Regulation with the concurrence of the Office of Standards Development.(OSD) and the Office of Nuclear Regulatory Research.
050 notes that a change in the accident ludine source term could be important to the rulemakings on reactor siting and minimum engineered safety features.
$b n
Harold R. Denton, Director. l' j.'
"i Office of Nuclear Reactor Regulation
Enclosures:
1.
Letter from W. R. Stratton, A. P. Malinauskas, and O. O. Campbell 2.
Minutes of Meeting 3.
Note to T. E. Murley on Research Programs Related to Iodine Release and Transport 4.
Suggested Agenda for Nov.18 Commission Meeting DISTRIBUTION Commissioners Commission Staff Offices ACRS 4
Secretariat
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ATTACHMENT 1 a
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l August 14, 19E0 Chairman John Ahearne U.S. Nuclear Regulatory Com=ission 1717 H Street Washington, D.C.
20555
Dear Chair =an Ahearne:
We wish to bring to your attention a matter that may be a very i=portant devel-opment in reactor safety analysis.
We believe that sufficient evidence has accumulated to show that the behavior of iodine during nucicer reactor accidents is not correctly described by existing NRC models and Regulatory Guides.
Iodine volatility is 'rossly overestimated by these models for accidents in which sub-stantial amounts of water are present, and escape of iodine to the environment will be extremely small (as it was at Three Mile Island) as long as reasonable containment integrity is also maintained.
As a consequence, the risk to the general public presented by iodine is lower than estimated, perhaps by orders of magnitude.
Our concern with this issue originated with our involvement in the several Technical Staf f Analyses for the President's Commission on the Accident at Ihree Mile Island.
The mechanism for the behavior of iodine that we propose here was derived from those analyses, from further examiration of experimental and theoretical studies involving the chemistry of iodine and cesium fission pro-ducts in light water reactor fuel and systems, and from the observed behavior of iodine subsequent to fuel failures during accidents and incidents at other reac-tor sites.
We believe that the explanation presented her4 will change the pre-sent concepts of the hazards involved during and subsequent to reactor accidents and, therefore, will require a critical reexamination of how these hazards and risks are calculated, and the criteria to which engineered safeguards are designed and installed.
Although the Three Mile Island,(TMI) reactor core inventories of xenon-133 and iodine-131 were comparable, between 2.4 and 13 million curies of xenon escaped to the environment during the accident, while only 13 to 18 curies of iodine similarly escaped!
This great disparity was identified as a matter of crucial yf the President's Commission, and an 1.
It was clear that we could not clai=
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