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Regulatory Guide 3.35, Revision 1

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Regulatory Guide 3.35, Revision 1 Revision 1 U.S. NUCLEAR REGULATORY COMMISSION July 1979 :*REGULATORY GUIDE OFFICE OF STANDARDS DEVELOPMENT REGULATORY GUIDE 335 ASSUMPTIONS USED FOR EVALUATING THE POTENTIAL RADIOLOGICAL CONSEQUENCES OF ACCIDENTAL NUCLEAR CRITICALITY IN A PLUTONIUM PROCESSING AND FUEL FABRICATION PLANT A. INTRODUCTION will review the proposal and approve its use, if found acceptable. Section 70.22, "Contents of Applications," of 10 CFR Part 70, "Domestic Licensing of Special B. DISCUSSION Nuclear Materials," requires, that each appli- cation for a license to possess and use special In the process of reviewing applications for nuclear material in a plutonium processing and permits and licenses authorizing the construc- fuel fabrication plant contain a description and tion or operation of plutonium processing and safety assessment of the design bases of the fuel fabrication plants, the NRC staff has principal structures, systems, and components developed a number of appropriately conser- of the plant. Section 70.23(a)(3) states that vative assumptions that are used by the staff applications will be approved if the Commission to evaluate an estimate of the radiological determines that, among other factors, the consequences of various postulated accidents. applicant's proposed equipment and facilities These assumptions are based on previous are adequate to protect health and minimize accident experience, engineering judgment, danger to life and property, and Sec- and on the analysis of applicable experimental tion 70.23(b) states that the Commission will results from safety research programs. This approve construction of the principal struc- guide lists assumptions used by the staff to tures, systems, and components of the plant evaluate the magnitude and radiological conse- when the Commission has determined that the quences of a criticality accident in a plutonium design bases of the principal structures, sys- processing and fuel fabrication plant. tems, and components and the quality assurance program provide reasonable A criticality accident is an accident resulting assurance of protection against the in the uncontrolled release of energy from an consequences of potential accidents. assemblage of fissile material. The cir- cumstances of a criticality accident are difficult In plutonium processing and fuel fabrication to predict. However, the most serious plants, a criticality accident is one of the criticality accident would be expected to occur postulated accidents used to evaluate the ade- when the reactivity (the extent of the deviation quacy of an applicant's proposed activities with from criticality of a nuclear chain reacting respect to the public health and safety. This medium) could increase most rapidly and guide describes methods used by the NRC staff without control in the fissile accumulation of in the analysis of such accidents. These the largest credible mass. In plutonium pro- methods result from review and action on a cessing and fuel fabrication plants where con- number of specific cases and, as such, reflect ditions that might lead to criticality are the lates~t general NRC-approved approaches to carefully avoided because of the potential for the problem. If an applicant desires to employ adverse physical and radiological effects, such new information that may be developed in the an accident is extremely uncommon. However, future or to use an alternative method, NRC experience with these and related facilities has demonstrated that criticality accidents may *Lines indicate substantive changes from previous issue. occur. USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Commission, Washington, D.C. 20566, Attention: Docketing and Regulatory Guides are issued to describe and make available to the public Service Branch. methods acceptable to the NRC staff of implementing specific parts of the Commission's regulations, to delineate techniques used by the staff in evalu- The guides are issued in the following ten broad divisions: sting specific problems or postulated accidents, or to provide guidance to applicants. Regulatory Guides are not substitutes for regulations, and com- 1. Power Reactors 6. Products phiance with them is not required. Methods and solutions different from tdose 2. Research and Test Reactors 7. Transportation set out in the guides will be acceptable if they provide a basis for the findings 3. Fuels and Materials Facilities 8. Occupational Health requisite to the issuance or continuance of a permit or license by the 4. Environmental and Siting 9. Antitrust and Financial Review Commission. 5. Materials and Plant Protection 10. General Requests for single copies of issued guides (which may be reproduced) or for Comments and suggestions for improvements in thease guides are encouraged at placement on an automatic distribution list for single copies of future guides all times, and guides will be revised, as appropriate, to accommodate comments in specific divsions should be made in writing to the U.S. Nuclear Regulatory and to reflect new information or experience. This guide was revised as a result Commission, Washington, D.C. 20555, Attention: Director, Division of of substantive comments received from the public and additional staff review. Technical Information and Document Control. In plutonium processing and fuel fabrication and dense layers, loss of water moderator by plants, such an accident might be initiated by boiling, or expulsion of part of the mass. (1) the inadvertent transfer or leakage of a solution of fissile material from a geometrically Generally, the criticality incidents were safe containing vessel into an area or vessel characterized by an initial burst or spike in not so designed, (2) introduction of excess the curve of fission rate versus time followed fissile material solution to a vessel, (3) intro- by a rapid but incomplete decay of the fission duction of excess fissile material to a solution, rate as the shutoff mechanism was initiated. As (4) overconcentration of a solution, (5) preci- more than one shutdown mechanism may affect pitation of fissile solids from a solution and the reactivity of the system and the effect of a their retention in a vessel, (6) introduction of particular mechanism may be counteracted, the neutron moderators or reflectors (e.g., initial burst was frequently succeeded by a entrance of water to a higly under-moderated plateau period of varying length. This plateau system), (7) deformation of or failure to was characterized by a lesser and declining maintain safe storage arrays, or (8) similar fission rate and finally by a further dropoff as actions which can lead to increases in the shutdown was completed. The magnitude of the reactivity of fissile systems. Some acceptable initial burst was directly related to the rate of means for minimizing the likelihood of such increase of reactivity and its magnitude above accidents are described in Regulatory the just-critical value but was inversely related Guide 3.4, "Nuclear Criticality Safety in to the background neutron flux, which is much Operations with Fissionable Materials Outside "1 greater for plutonium than for uranium Reactors. systems. I. CRITICALITY ACCIDENT EXPERIENCE IN RELATION TO Those systems consisting only of solid THE ESTIMATION OF THE MOST SEVERE ACCIDENT fissile, reflector, or moderator materials exhibited little or no plateau period, whereas Stratton (Ref. 1) has reviewed in detail 34 occasions prior to 1966 when the power level solution systems had well developed plateaus. For solution systems, the energy release of a fissile system increased without control as a result of unplanned or unexpected changes in during the plateau period, because of its dura- tion, provided the major portion of total energy its reactivity. Although only six of these released. For purposes of the planning neces- incidents occurred in processing operations, sary to deal adequately with criticality and the remainder occurred mostly in facilities for obtaining criticality data or in experimental incidents in experimental and production-type nuclear facilities, Woodcock (Ref. 3) made use reactors, the information obtained and its of these data to estimate possible fission yields correlation with the characteristics of each from excursions in various types of systems. system have been of considerable value for use For example, spike yields of 1E+17 and 1E+18 in estimating the consequences of accidental and total yields of 3E+18 and 3E+19 fissions criticality in process systems. The incidents were suggested for criticality accidents occurred in aqueous solutions of uranium or occurring in solution systems of 100 gallons or plutonium (10), in metallic uranium or less and more than 100 gallons, respectively. plutonium in air (9), in inhomogeneous water- Little or no mechanical damage was predicted at moderated systems (9), and in miscellaneous these levels. solid uranium systems (6). Five occurred in plutonium systems, including reactors and 2. METHODS DEVELOPED FOR PREDICTING THE MAGNITUDE criticality studies, of which three were in OF CRITICALITY ACCIDENTS solutions. The nuclear excursion behavior of solu- The estimated total number of fissions per 2 tions of enriched uranium has been studied incident ranged from 1E+15 to 1E+20 with a extensively both theoretically and experi- median of about 2E+17. More recently, another mentally. A summary by Dunenfeld and Stitt incident in a plutonium processing facility at (Ref. 4) of the kinetic experiments on water Windscale
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