Revision I U.S. NUCLEAR REGULATORY COMMISSION July 1979 'PA' S t REGULATORY GUIDE "** OFFICE OF STANDARDS DEVELOPMENT REGULATORY GUIDE 334 ASSUMPTIONS USED FOR EVALUATING THE POTENTIAL RADIOLOGICAL CONSEQUENCES OF ACCIDENTAL NUCLEAR CRITICALITY IN A URANIUM FUEL FABRICATION PLANT A. INTRODUCTION B. DISCUSSION Section 70.23, "Requirements for the ap In the process of reviewing applications for proval of applications," of 10 CFR Part 70, licenses to operate uranium fuel fabrication "Domestic Licensing of Special Nuclear Mate plants, the NRC staff has developed appro rials," requires, among other things, that the priately conservative assumptions that are used applicant's proposed equipment and facilities be by the staff to evaluate an estimate of the adequate to protect health and minimize danger radiological consequences of various postulated to life or property. In order to demonstrate the accidents. These assumptions are based on adequacy of the facility, the applicant must previous accident experience, engineering provide an analysis and evaluation of the judgment, and the analysis of applicable design and performance* of structures, sys,' experimental results from safety research tems, and components of the facility. The programs. This guide lists assumptions used to objective of this analysis and evaluation is to evaluate the magnitude and radiological con assess the risk to public health and safety: sequences of a criticality accident in a uranium resulting from operation of the facility, includ fuel fabrication plant. ing determination of the adequacy of struc tures, systems, and components provided for A criticality accident is an accident resulting the prevention of accidents and the mitigation in the uncontrolled release of energy from an of the consequences of accidents. assemblage of fissile material. The circum stances of a criticality accident are difficult to In a uranium fuel fabrication plant, a criti predict. However, the most serious criticality "cality accident is one of the postulated acci accident would be expected to occur when the dents used to evaluate the adequacy of an reactivity (the extent of the deviation from applicant's proposed activities with respect to criticality of a nuclear chain reacting medium) public health and safety. This guide describes could increase most rapidly and without control methods used by the NRC staff in the analysis in the accumulation of the largest credible of such accidentS. These methods result from mass. In a uranium fuel fabrication plant where review and action on a number of specific cases conditions that might lead to criticality are and, as such, reflect the latest general NRC carefully avoided because of the potential for approved approaches to the problem. If an adverse physical and radiological effects, such applicant desires to employ new information an accident is extremely uncommon. However, that may be developed in the future or to use experience with these and related facilities has an alternative method, NRC will review the demonstrated that criticality accidents could proposal and approve its use, if found occur. acceptable. In a uranium fuel fabrication plant, such an accident might be initiated by (1) inadvertent Lines indicate substantive chances from previous issue. transfer or leakage of a solution of fissile USNRC REGULATORY GUIDES SComrments RGULTOYiGsRguly mitcosdn eobe sent wasnton. to " Secretary D.C. of2050 toe Comrn.iokn,Attention: Docketing US. Nuclear and Regulatory Guides am issued to describe Wnd hlae avalable Wotie public Service Dranci. methods ecceptable to ta NRC staff of inplernentsng spc•pfic parts of tie Comnmision's ugulations. to delneata tecahiques used by ft staff in evai.u- The gukdes are ssued in "etfollowing t broad divisons: ~~¶fcproblerns or postulated accidents. or 00 provide guidanca 10 Regukxy Guides we not substf••es for regulation, amndCorn- 1. PWm ReactorS 6. Products pliarce with diem k not required. Methods and solutksodfferentfhose 1uorn 2. Reserch and Test Reactao 7. Transporation Wt out In eOguds vin be acptable Iftoy provide a bashsfor f 3. Fuels and Materats Faciities . Occupational Health rquisite ID die Issuance or continuance Of a permit or license by toe 4. Enwonmentst and Siting S Antitrust and Financial Review Mataerals and Plant Protection 10. General SS, Conet an~d stigestion for limprvemewit inminse Wleu weariecouraed at ptacernentIiuqua for an single an SUMMW'a copies ofdsvibA- isued guides rw fowlwhch=Ve niy copies be reproduced) of fMur Wodeor for ail trne, and guides WO be revised, =appropriate.,,soccornmodate c.vnmens in spacft, divisions -hotd be mdeIn writing to t'e US. Nucear Regulatory 4W to 1`0l new information or expiene. This guide was revised ,a result Cornissicn. Washngton, D.C. 205. Attrntion: Director, Division of at substantive conirnentUecaived from te pubic and additional stiff review. Technical Information and Document Control. material from a geometrically safe containing mechanism may be counteracted, the initial vessel into an area or vessel not so designed, burst was frequently succeeded by a plateau (2) introduction of excess fissile material solu period of varying length. This plateau was tion to a vessel, (3) introduction of excess characterized by a lesser and declining fission fissile material to a solution, (4) overconcen rate and finally by a further dropoff as shut tration of a solution, (5) failure to maintain down was completed. The magnitude of the sufficient neutron absorbing materials in a initial burst was directly related to the rate vessel. (6) precipitation of of fissile solids from a increase of reactivity and its magnitude above solution and their retention in a vessel, the just-critical value but was inversely related (7) introduction of neutron moderators or to the background neutron flux. reflectors (e.g., by addition of water to a highly undermoderated system), (8) deforma Those systems consisting only of solid fis tion of or failure to maintain safe storage sile, reflector, or moderator materials exhibited arrays, or (9) similar actions that can lead to little or no plateau period, whereas solution increases in the reactivity of fissile systems. systems had well developed plateaus. For solu Some acceptable means for minimizing the likeli tion systems, the energy release during the hood of such accidents are described in Regu plateau period, because of its duration, latory Guides pro 3.4, ."Nuclear Criticality Safety vided the major portion of the total in Operations energy with Fissionable Material Outside released. For purposes of the planning Reactors,"' neces and 3.1, "Use of Borosilicate Glass sary to deal adequately with Raschig criticality Rings as a Neutron Absorber in incidents in experimental and production-type Solutions of Fissile Material."' nuclear facilities, Woodcock (Ref. 2) made use of these 1. CRITICALITY ACCIDENT data to estimate possible fission yields EXPERIENCE IN RELATION TO from excursions THE ESTIMATION OF THE MOST SEVERE ACCIDENT in various types of systems. For example, spike yields of IE÷i7 and 1E÷18 Stratton (Ref. 1) has reviewed in detail and total yields of 3E+18 and 3E+19 fissions 34 occasions prior, to 1966 when the power level were suggested for criticality accidents of a fissile system increased without control as occurring in solution systems of 100 gallons or a result of unplanned or unexpected changes in less and more than 100 gallons, respectively. its reactivity. Although only six of these Little or no mechanical damage was predicted at occurred in processing operations, and the these levels. remainder occurred mostly in facilities for obtaining criticality data or in experimental 2. METHODS DEVELOPED FOR PREDICTING THE MAGNITUDE reactors, the information obtained and its OF CRITICALITY ACCIDENTS correlation with the characteristics of each system have been of considerable value for use The nuclear excursion behavior of solu in estimating the consequences of accidental tions of enriched uranium has been studied criticality in process systems. The incidents extensively both theoretically and experi occurred in aqueous solutions of uranium or mentally. Dunenfeld and Stitt (Ref. 3) plutonium (10), in metallic uranium or summarize the kinetic experiments on water plutonium in air (9), in inhomogeneous water boilers using uranyl- sulfate solutions and moderated systems (9), and in miscellaneous describe the development of a kinetic model solid uranium systems (6). The estimated total that was confirmed by experiment. This model number of fissions per incident ranged defines the effects of thermal expansion and 2 from IE+15 to IE+20 with a median of about 2E÷17. radiolytic gas formation as power-limiting and In ten cases, the supercriticality was halted by shutdown mechanisms. an automatic control device. In the remainder, the shutdown was effected as a consequence of The results of a series of criticality excur the fission energy release that resulted in sion experiments resulting from the introduc thermal expansion, density reduction from the tion of uranyl nitrate solutions to vertical formation of very small bubbles, mixing of light cylindrical tanks at varying rates are and dense layers, loss of water moderator by summarized by Licorch6 and Seale (Ref. 4). boiling, or expulsion of part of the mass. This report confirms the applicability of the kinetics model for solutions, provides correla Generally, the criticality incidents were tions of peak power with reactivity addition characterized by an initial burst or spike in a rate, notes the importance of a strong neutron curve of fission rate versus time followed by a source in limiting peak power, and indicates rapid but incomplete decay as the shutoff the nature of the plateau following the peak. mechanism was initiated. As more than one shutdown mechanism may affect the reactivity Many operations with fissile materials in a of the system and the effect of a particular uranium fuel fabrication plant are conducted with aqueous (or organic solvent) solutions of fissile materials. Consequently, well-founded 'Copies may be obtained from the U.S. Nuclear Regulatory methods for the prediction of total fissions and Commission. Washington. D.C.