1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationBS ISO 27468:2011Nuclear criticality safety Evaluation of systemscontaining PWR UOX fuels Bounding burnup creditapproachBS ISO 27468:2011 BRITISH STANDARDNational forewordThis Br
2、itish Standard is the UK implementation of ISO 27468:2011.The UK participation in its preparation was entrusted to TechnicalCommittee NCE/9, Nuclear fuel cycle technology.A list of organizations represented on this committee can beobtained on request to its secretary.This publication does not purpor
3、t to include all the necessaryprovisions of a contract. Users are responsible for its correctapplication. BSI 2011ISBN 978 0 580 65210 3ICS 27.120.30Compliance with a British Standard cannot confer immunity fromlegal obligations.This British Standard was published under the authority of theStandards
4、 Policy and Strategy Committee on 31 July 2011.Amendments issued since publicationDate Text affectedBS ISO 27468:2011Reference numberISO 27468:2011(E)ISO 2011INTERNATIONAL STANDARD ISO27468First edition2011-07-01Nuclear criticality safety Evaluation of systems containing PWR UOX fuels Bounding burnu
5、p credit approach Sret-criticit valuation des systmes mettant en uvre des combustibles REP UOX Approche conservative de crdit burnup BS ISO 27468:2011ISO 27468:2011(E) COPYRIGHT PROTECTED DOCUMENT ISO 2011 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
6、 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 address below or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01
7、 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2011 All rights reservedBS ISO 27468:2011ISO 27468:2011(E) ISO 2011 All rights reserved iiiContents Page Foreword iv Introduction.v 1 Scope1 2 Normative references1 3 Terms and definitions .1 4 Methodol
8、ogy for criticality safety evaluations considering burnup of the fuel3 5 Implementation of criticality safety evaluations considering burnup of the fuel.6 Annex A (informative) Validation of the depletion codes against post-irradiation examination data .7 Annex B (informative) Operational implementa
9、tion of a burnup credit application 8 Bibliography9 BS ISO 27468:2011ISO 27468:2011(E) iv ISO 2011 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Stan
10、dards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also t
11、ake part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of 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
12、is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibil
13、ity that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 27468 was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies, and radiological protection, Subcommitte
14、e SC 5, Nuclear fuel cycle. BS ISO 27468:2011ISO 27468:2011(E) ISO 2011 All rights reserved vIntroduction For many years, criticality evaluations involving irradiated uranium oxide (UOX) fuels in pressurized water reactor (PWR) considered the fuel as un-irradiated. Information on and consideration o
15、f the fuel properties after irradiation could usually have resulted in considerable criticality safety margins. The use of PWR UOX fuel with increased enrichment of 235U motivates evaluation of burnup credit in existing and new applications for storage, reprocessing or transport of irradiated fuel.
16、A more realistic estimation of the actual effective neutron multiplication factor, keff, of a system involving irradiated fuel is possible with methods available to nuclear criticality safety specialists. Thus, the maximum estimated keffvalue during normal conditions and incidents can be reduced com
17、pared with the assumption of an un-irradiated fuel. Moreover, the safe use of burnup credit can reduce the overall risk (fewer cask moves, etc.). Therefore, for the safe use of the burnup credit, this International Standard highlights the need to consider new parameters in addition to those that nee
18、d evaluation for un-irradiated fuel. It presents the different issues that should be addressed to support evaluations of burnup credit for systems with PWR fuels that are initially containing uranium oxides and then irradiated in a PWR. This International Standard identifies a bounding approach in t
19、erms of keffcalculation. Other approaches may be used (e.g. calculation of the average configuration with keffcriteria covering credible variations/bias/uncertainties) especially if there are additional mechanisms to control the subcriticality (e.g. use of boron, gadolinium or dry transport). Overal
20、l criticality safety evaluation and eventual implementation of burnup credit are not covered by this International Standard. However, the burnup credit evaluation in this International Standard should support use of burnup credit in the overall criticality safety evaluation and an eventual implement
21、ation of burnup credit. BS ISO 27468:2011BS ISO 27468:2011INTERNATIONAL STANDARD ISO 27468:2011(E) ISO 2011 All rights reserved 1Nuclear criticality safety Evaluation of systems containing PWR UOX fuels Bounding burnup credit approach 1 Scope This International Standard establishes an evaluation met
22、hodology for nuclear criticality safety with burnup credit. It identifies important parameters and specifies requirements, recommendations, and precautions to be taken into account in the evaluations. It also highlights the main important technical fields to ensure that the fuel composition or histo
23、ry considered in calculations provides a bounding value of the effective neutron multiplication factor, keff. This International Standard is applicable to transport, storage, disposal or reprocessing units implying irradiated fissile material from pressurized water reactor (PWR) fuels that initially
24、 contain uranium oxide (UOX). Fuels irradiated in other reactors (e.g. boiling water reactors) and fuels that initially contain mixed uranium-plutonium oxide are not covered in this International Standard. This International Standard does not specify requirements related to overall criticality safet
25、y evaluation or eventual implementation of burnup credit. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (inc
26、luding any amendments) applies. ISO 1709, Nuclear energy Fissile materials Principles of criticality safety in storing, handling and processing ISO 14943, Nuclear fuel technology Administrative criteria related to nuclear criticality safety 3 Terms and definitions For the purposes of this document,
27、the following terms and definitions apply. 3.1 actinide element with atomic number in the range from 90 to 103 NOTE Many actinides are produced during the irradiation due to neutron capture on other actinides and/or decay of other actinides and/or by (n,2n) reactions, etc. The corresponding nuclides
28、 are all neutron producers and some are net (considering neutron production and absorption) neutron producers in a slow neutron energy spectrum. BS ISO 27468:2011ISO 27468:2011(E) 2 ISO 2011 All rights reserved3.2 axial burnup profile real or modelled axial distribution of the burnup in the fuel ass
29、embly NOTE The axial distribution of the burnup is caused by axial neutron leakage, axial variations in the fuel enrichment, moderator temperature rise through the core, non-full length burnable poison and partial insertion of control rods. 3.3 burnable poison nuclide neutron absorber added to the f
30、uel assembly to control reactor reactivity and power distribution NOTE 1 As the reactor operation progresses, the amount of neutron absorbing material is depleted, or burned. Then, if the presence of burnable poisons (fixed or removable) is considered in a criticality safety evaluation, the most rea
31、ctive condition may not be for the fresh fuel. NOTE 2 See also ISO 921:1997, entry 135. 3.4 burnup average energy released by a defined region of the fuel during its irradiation NOTE 1 This region could be a complete fuel assembly or some part of the assembly. Burnup is commonly expressed as energy
32、released per mass of Initial fissionable actinides (uranium only for this International Standard). Units commonly used are expressed in megawatt day per metric tonne of initial uranium (MWd/t) or gigawatt day per metric tonne of initial uranium (GWd/t). NOTE 2 See also ISO 921:1997, entry 1156. 3.5
33、burnup credit margin of reduced kefffor an evaluated system, due to the irradiation of fuel in a reactor, as determined with the use of a structured evaluation process 3.6 cooling time time following the final irradiation of the fuel in a reactor NOTE During this period, the radioactive decay result
34、s in changes in the fuel nuclide concentrations. 3.7 depletion calculation calculation performed to determine the concentrations of individual nuclides in the fuel at the end of irradiation in a reactor; that is a cooling time equal to zero NOTE 1 Other fuel properties can usually be determined by d
35、epletion calculations (e.g. flux-weighted macroscopic cross-sections or lattice cell k). NOTE 2 Radioactive decay between reactor irradiation periods and after final shutdown is usually included in the same calculation procedure. 3.8 end effect impact on keffof the less irradiated parts of the fuel
36、assembly (upper and lower ends of the assembly) NOTE The end effect is commonly defined as the difference between the kefffor the two following systems: a system containing irradiated fuel assemblies having a constant fuel composition corresponding to the average burnup and irradiation energy spectr
37、um of the fuel, the same system containing irradiated fuel assemblies having an axially varying fuel composition corresponding to the modelled axial burnup profile, with consideration of the neutron energy spectrum during irradiation. BS ISO 27468:2011ISO 27468:2011(E) ISO 2011 All rights reserved 3
38、3.9 fission product nuclide produced from nuclear fission NOTE 1 During this reaction two or more fission products are produced together with neutrons and radiations (gamma, etc.). The fission products can be a direct result of the fissions or can be created after the decay of (or neutron absorption
39、 with) other fission products. Often only a selection of fission products is accounted for as neutron absorbers in burnup credit, but consideration of all fission products absorption is required to simulate fuel irradiation during reactor operation. NOTE 2 See also ISO 921:1997, entry 478. 3.10 loos
40、ely coupled system system in which two or more regions with high “local” values of keffare separated by regions with low keffimportance NOTE Convergence problems can occur when a Monte Carlo method is used for the keffcalculation of such systems where neutron interaction between the highly fissile r
41、egions is weak. 3.11 validation documented determination that the combination of models, methods and data as embodied in a computer code methodology is an appropriate representation of the process or system for which it is intended NOTE This documented determination is accomplished by comparing code
42、 results to benchmark experimental results to define code bias and areas of applicability of a calculation method. 4 Methodology for criticality safety evaluations considering burnup of the fuel IMPORTANT The application of this clause requires evaluators to know the initial composition of each fuel
43、 and its history of irradiation. 4.1 General The bounding approach identified in this International Standard consists of the main following steps, for a given application (e.g. a given transport, storage, reprocessing, disposal) and for a given range of irradiated fuels: to choose and justify a burn
44、up distribution to model in the fuel assemblies (see 4.2); to calculate the irradiated fuel nuclide concentrations for each burnup assessed, with considerations for the cooling time (see 4.3); to select the nuclides to be included in the evaluation of kefffor the application (see 4.4); to perform th
45、e criticality calculations of the evaluated application (see 4.5). For each step where a calculation code is used, the validation of these calculation tools shall be justified and documented. Such validation may consist of a global validation of the resulting keff. 4.2 Distribution of burnup 4.2.1 T
46、he burnup distribution of the irradiated fuel assembly shall be evaluated because of its impact on keff(see References 1, 2, 9, 15 and 16). The axial and radial/horizontal burnup gradients, due to the neutron flux distribution during the irradiation, are mainly related to: neutron leakage at the top
47、 and the bottom of the fuel assembly; neutron absorption within partially inserted control rods at the top of the fuel assembly; BS ISO 27468:2011ISO 27468:2011(E) 4 ISO 2011 All rights reserved the moderator density change from the bottom to the top of the core; radial leakage of the neutrons, whic
48、h depend on the environment of the assembly, on its position in the reactor during irradiation and on the presence of burnable poisons; radial absorption of the neutrons. WARNING The axial burnup distribution is not sufficient to determine the axial variation of the composition of the irradiated fue
49、l: the neutron spectrum of the irradiation flux also varies axially and has an impact on the fuel nuclide concentrations that are determined from the depletion calculation. Guidelines on the effect on fuel nuclides concentration of the fuel depletion parameters are given in 4.3. 4.2.2 Each fuel assembly may be divided into regions or zones in which the burnup is assumed to be uniform. The division into such regions or zones shall be justified for each application and may be different to
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