1、BS ISO27467:2009ICS 27.120.30NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBRITISH STANDARDNuclear criticalitysafety Analysis of apostulated criticalityaccidentThis British Standardwas published under theauthority of the StandardsPolicy and StrategyCommittee on 31 March2009 B
2、SI 2009ISBN 978 0 580 57126 8Amendments/corrigenda issued since publicationDate CommentsBS ISO 27467:2009National forewordThis British Standard is the UK implementation of ISO 27467:2009.The UK participation in its preparation was entrusted to TechnicalCommittee NCE/9, Nuclear fuel cycle technology.
3、A list of organizations represented on this committee can be obtained onrequest to its secretary.This publication does not purport to include all the necessary provisionsof a contract. Users are responsible for its correct application.Compliance with a British Standard cannot confer immunityfrom leg
4、al obligations.BS ISO 27467:2009INTERNATIONALSTANDARDISO27467First edition2009-02-15Reference numberISO 27467:2009(E) ISO 2009Nuclear criticality safety Analysis of a postulated criticality accidentSret-criticit Analyse dun hypothtique accident de criticitBS ISO 27467:2009ISO 27467:2009(E)ii ISO 200
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8、below.COPYRIGHT PROTECTED DOCUMENT ISO 2009 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the addr
9、ess below orISOs member body in the country of the requester.ISO copyright officeCase postale 56 CH-1211 Geneva 20Tel. + 41 22 749 01 11Fax + 41 22 749 09 47E-mail copyrightiso.orgWeb www.iso.orgPublished in SwitzerlandBS ISO 27467:2009ISO 27467:2009(E) ISO 2009 All rights reserved iiiContents Page1
10、 Scope 12 Normative references 13 Terms and definitions 14 Criticality accident analysis objectives . 25 Components of a criticality accident analysis 2Annex A (informative) Flow diagram of a criticality accident analysis 6Bibliography . 7BS ISO 27467:2009ISO 27467:2009(E)iv ISO 2009 All rights rese
11、rvedForewordISO (the International Organization for Standardization) is a worldwide federation of national standards bodies(ISO member bodies). The work of preparing International Standards is normally carried out through ISOtechnical committees. Each member body interested in a subject for which a
12、technical committee has beenestablished has the right to be represented on that committee. International organizations, governmental andnon-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the InternationalElectrotechnical Commission (IEC) on all matters o
13、f electrotechnical standardization.International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.The main task of technical committees is to prepare International Standards. Draft International Standardsadopted by the technical committees are circulated to
14、the member bodies for voting. Publication as anInternational Standard requires approval by at least 75 % of the member bodies casting a vote.Attention is drawn to the possibility that some of the elements of this document may be the subject of patentrights. ISO shall not be held responsible for iden
15、tifying any or all such patent rights.ISO 27467 was prepared by Technical Committee ISO/TC 85, Nuclear energy, Subcommittee SC 5, Nuclearfuel technology.BS ISO 27467:2009ISO 27467:2009(E) ISO 2009 All rights reserved vIntroductionIn nuclear facilities processing or storing fissile material, provisio
16、ns are made to avert the risk of criticality. Thepurpose of a criticality safety analysis is to ensure that the measures taken are adequate to prevent criticalityaccidents. The risk contributions associated with a criticality accident arise from direct radiation from the fissionevents, the presence
17、of fission products, as well as from possible airborne radioactive gases and particulates.Worldwide criticality-accident experience shows that these are very rare events, yet the risk associated withfuture occurrences cannot be completely eliminated. It is difficult to contemplate all the scenarios
18、whoseconditions can lead to a criticality accident, and even more so to avoid them, particularly with solution mediawhere many of the past accidents have occurred. For this reason, an analysis based on postulated accidentscenarios, in any facility where a potential risk of criticality can still be e
19、xtant, can be the vehicle to understandthe expected consequences and provide for the appropriate provisions and protective actions.BS ISO 27467:2009.vi BS ISO 27467:2009INTERNATIONAL STANDARD ISO 27467:2009(E) ISO 2009 All rights reserved 1Nuclear criticality safety Analysis of a postulated critical
20、ity accident1ScopeThis International Standard specifies areas that are important to study when analysing potential criticalityaccidents.NOTE 1 It is important that a criticality accident analysis be performed each time a criticality accident is consideredcredible, due either to criticality contingen
21、cies (double batching, procedural violations, etc.) or to the failure of safetyprovisions (effectiveness of neutron absorber reduced by fire, etc.).NOTE 2 It is important that the criticality safety specialist be mindful that the process of evaluation developed in thisInternational Standard does not
22、 address the unforeseen, since any actually occurring criticality accident will probably resultfrom a scenario not envisioned or from failure to comply with prevailing regulations.This International Standard does not address detailed administrative measures, for which the responsibility lieswith the
23、 public authorities, nor does it deal with criteria used to justify the accident criticality analysis of a nuclearfacility.This International Standard does not apply to nuclear power plants.2 Normative referencesThe following referenced documents are indispensable for the application of this documen
24、t. For datedreferences, only the edition cited applies. For undated references, the latest edition of the referenced document(including any amendments) applies.ISO 7753, Nuclear energy Performance and testing requirements for criticality detection and alarm systems3 Terms and definitionsFor the purp
25、oses of this document, the following terms and definitions apply.3.1 accident scenarioset of credible, postulated conditions under which a fissile-material-containing facility/process develops one ormore fault conditions such that it is likely to exceed the critical state, and thus to result in a cr
26、iticality accident3.2 radiological consequencesindividual and collective radiation doses resulting from the combined effects of external exposure due to directradiation and internal exposure caused by inhaling or ingesting either volatile radionuclides released toenvironment or non-volatile and fiss
27、ile material suspended as particles in the facilityBS ISO 27467:2009ISO 27467:2009(E)2 ISO 2009 All rights reserved4 Criticality accident analysis objectives4.1 Criticality accident analysis shall be comprised of the following considerations (see Figure 1):a) determination of the credible accident s
28、cenario(s);b) estimation of the power history and the energy release;c) suitable means of detecting the accident and suitable siting of detectors;d) estimation of potential individual exposure and radiological impact of radionuclide releases on the generalpublic and the environment;e) suitable provi
29、sions for emergency preparedness and response to a criticality accident.4.2 These requirements may be facilitated in part or whole by the following considerations:a) criticality accident dynamics, i.e. understanding the mechanisms that govern accident progress in order toestimate energy releases;b)
30、accident-detection capabilities, including timely triggering of criticality alarms for immediate evacuation ofindividuals, as a means of limiting radiation exposure risk;c) analysis of “airborne releases” by measuring potentially releasable fission gases and aerosols, andsuspendable nuclear matter,
31、and estimating their impact on people and the environment;d) accident dosimetry, i.e. determination of the doses due to accident-induced neutron and gamma irradiation,for radiation risk analysis purposes;e) consideration of the overall risk of harm from measures considered in the emergency plan.NOTE
32、 Items of information in c) and d) are used to estimate radiological consequences.5 Components of a criticality accident analysis5.1 Criticality accident analysis topicsThe criticality accident analysis should address the following topics:a) definition of the accident scenarios that permit the analy
33、sis of accident dynamics (total energy release,duration, first power spike characteristics, etc.);b) study of adequate detector locations based on the analysis of accident phenomena, in particular the firstspike characteristics;c) estimated risk of individual exposure in the vicinity of the critical
34、ity accident and assessment of theenvironmental impact of airborne releases also served by accidents characteristics (fissile media,configurations, energies, etc.);d) emergency preparedness and response support necessary.NOTE Annex A illustrates how the components of an analysis of a postulated crit
35、icality accident, shown as differentsteps/stages of a study, can be addressed.5.2 Definition of a postulated accident and associated neutronics calculations5.2.1 The criticality safety specialist shall define the accident scenario according toa) the type of fissile material, type of process (high-te
36、mperature, i.e. risk of fire, aqueous, damp, mixtures, etc.)and affected equipment configurations (i.e. geometry, interaction with adjacent materials or moderators,etc.),b) the events leading to supercriticality, that is, definition and chronology of the accident (insertion of excessfissile material
37、 or moderator, loss of moderator, process malfunction, etc.).BS ISO 27467:2009ISO 27467:2009(E) ISO 2009 All rights reserved 35.2.2 Based on the scenario(s) as defined in 5.2.1, the following criticality parameters should be calculated:a) total reactivity insertion and insertion kinetics, to estimat
38、e the power excursion (in particular the first powerspike);b) system neutronics parameters.NOTE The relevant parameters in b) that can be needed for the analysis are the neutron lifetime, the prompt critical state,the delayed neutron importance (i.e. effective delayed neutron fraction), the material
39、 buckling, the infinite multiplicationfactor, the migration lengths, etc.5.3 Accident physics calculations5.3.1 Evaluating criticality accident physicsGiven the postulated accident scenario as defined in 5.2, the criticality safety specialist shall evaluate theaccident physics by using either or bot
40、h of the following:a) calculation codes,b) simplified models.WARNING The long-term effects of an accident can be driven mainly by heat exchanges in thevicinity of the supercritical system (cooling, ventilation). Consideration of the possible long-termaccident effects should be given when analysing a
41、 criticality accident.5.3.2 Using calculation codes5.3.2.1 These calculations require use of a criticality accident computer code that determines power andenergy changes in the transient and provides trends for other parameters (temperature, radiolysis gas andsteam bubbles, pressure, etc.) as a func
42、tion of time.WARNING Uncertainties determined by validation or justified by the inadequacy of the model torepresent the criticality accident scenario can exist. So care should be taken when using computercodes.5.3.2.2 The calculations shall specifically estimatea) the accident “envelope”, meaning th
43、e total number of fissions that can halt the accident by restoring thesystem to permanent subcriticality if no suitable means (e.g. neutron poisoning, robotic equipment) wereused to quench it,b) the first power-spike characteristics, which enable the optimal location of the criticality accident alar
44、msystem detectors.5.3.3 Using simplified models5.3.3.1 For accident physics, simplified options providing “order-of-magnitude” data should be considered.5.3.3.2 These entail the use ofa) simplified models devised from experimental data,b) results of experiments or past accidents that were adequately
45、 documented.BS ISO 27467:2009ISO 27467:2009(E)4 ISO 2009 All rights reserved5.4 Calculation of neutron and gamma radiation Criticality accident dosimetry5.4.1 Evaluating criticality accident dosimetryRadiation doses and the extent and distribution of gamma and neutron radiation fields shall be estim
46、ated basedon the results of the accident physics calculations described in 5.3. The criticality safety specialist shall evaluatethe criticality accident dosimetry by usinga) calculation codes,b) simplified models.5.4.2 Evaluation using calculation codes5.4.2.1 Calculation of radiation and associated
47、 doses shoulda) optimize the estimate of individual and collective exposure from a postulated accident and define the zonesfor evacuation, along with other topics related to criticality alarms and emergency response,b) optimize the determination of the locations for criticality detectors and demonst
48、rate that the criticality alarmswill respond to the postulated criticality accident(s).5.4.2.2 Computer codes should then be used to estimate the radiation produced by the accident source,together with its propagation in the locality of the accident.WARNING Uncertainties determined by validation or
49、justified by the inadequacy of the model torepresent the criticality accident scenario can exist. So care should be taken when using computercodes.5.4.3 Evaluation using experimental resultsExperimental dose data obtained for different critical assemblies should be compiled in the form of nomographsto show dose changes registered at various short distances from each radiation source.5.5 Demonstration of timely alarm actuation5.5.1 Adequate criticality
