ISO/TC 85/SC 5/WG 8 - Nuclear criticality safety

This document specifies minimum nuclear criticality safety training requirements for operations staff, operations supervisors, and management. This document is applicable to areas, processes or facilities containing quantities of fissile material for which nuclear criticality safety assessment is required as defined in ISO 1709. This document is not applicable to the transport of fissile materials outside the boundaries of nuclear establishments.

This document provides specific requirements and guidance on the nuclear criticality safety of waste containing fissile nuclides, generated during normal operations. This document is intended to be used along-side and in addition to ISO 1709. This document applies specifically to the nuclear criticality safety of solid nuclear wastes. It also applies to residual quantities of liquids and/or slurries which are either intimately associated with the solid nuclear waste materials or arise as a result of processing or handling the waste. This document does not apply to bulk or process liquids (including higher concentration process solutions) or irradiated or un-irradiated fuel elements. NOTE The term fuel element is sometimes applied to fuel assembly, fuel bundle, fuel cluster, fuel rod, fuel plate, etc. All these terms are based on one or more fuel elements. A cylindrical fuel rod (sometimes referred to as a fuel pin) for a light-water-reactor is an example of a fuel element. All stages of the waste life cycle are within the scope of the document. This document can also be applied to the transport of solid nuclear waste outside the boundaries of nuclear establishments.

This document provides guidance, requirements and recommendations related to determination of limits on subcriticality dimensions and to their compliance with: — geometrical dimensions specified in the design (design dimensions), or, — actual dimensions. This document is applicable to nuclear facilities containing fissile materials, except nuclear power reactor cores. This document can also be applied to the transport of fissile materials outside the boundaries of nuclear establishments. Subcriticality dimension control based on dimensions and layout of fuel assembly, fuel rods and fuel pellets are not covered by this document. This document does not specify requirements related to the control of fissile and non-fissile material compositions. The Quality Assurance associated with the fabrication and layout of the unit based on specifications (e.g. drawings elaborated during design) is a prerequisite of this document. The Quality Assurance is important to ensure the consistency between the unit geometry, its general purpose and its intended function.

ISO 1709:2018 specifies the basic principles and limitations which govern operations with fissile materials. It discusses general nuclear criticality safety criteria for equipment design and for the development of operating controls, while providing guidance for the assessment of procedures, equipment, and operations. It does not cover specific quality assurance requirements or details of equipment or operational procedures. ISO 1709:2018 does not deal with the issues associated with administrative criteria relating to nuclear criticality safety. These issues are covered by ISO 14943. It does not cover the effects of radiation on man or materials, unless the material properties affect nuclear criticality safety. These criteria apply to operations with fissile materials outside nuclear reactors but within the boundaries of nuclear establishments. They are concerned with the limitations which are imposed on operations because of the properties of these materials which permit them to support nuclear chain reactions. These principles apply to quantities of fissile nuclides in which nuclear criticality safety is required to be established. ISO 1709:2018 can also be applied to the transport of fissile materials outside the boundaries of nuclear establishments.

ISO 16117:2013 provides a methodology to estimate a reasonably maximal value of the number of fissions of a postulated criticality accident. The fission number estimate, associated with its postulated criticality accident, impacts the accident emergency planning and response because it is used for the estimation of radiation doses and of radioactive materials release. ISO 16117:2013 does apply to nuclear facilities, plants, laboratories, storage, and transportation of fissile material (but not to nuclear power reactor cores) where a credible criticality accident may occur.

ISO 11320:2011 provides criteria for emergency preparedness and response to minimize consequences due to a nuclear criticality accident. The criticality safety of operations are evaluated in accordance with ISO 1709. ISO 11320:2011 applies to a site with one or more facilities which might contain significant quantities and concentrations of fissile material. The extent to which ISO 11320:2011 needs to be applied depends on the overall criticality risk presented by the facilities at the site. ISO 11320:2011 does not apply to off-site transport and transit storage of packages with fissile material. ISO 11320:2011 does not apply to sites with operating nuclear power plants or to facilities with research reactors which are licensed to become critical or near-critical, provided that there are no operations with fissile material external to the reactor for which a credible criticality accident risk exists. ISO 11320:2011 can be applied to such sites and facilities in specific cases, if supported by site management and by licensing authorities.

ISO 11311:2011 specifies common reference critical values (of which the effective neutron multiplication factor, keff, is equal to 1) for homogeneous water-moderated plutonium-uranium oxide mixtures based on an inter-code comparison of calculated critical values. It is applicable to operations with unirradiated mixed uranium-plutonium oxide (MOX) outside nuclear reactors. A classical validation approach for these systems is difficult because of the paucity of critical experiments for MOX fuel. Various reference systems, in terms of isotopic compositions, thicknesses of water reflection, and densities of oxide are evaluated by different combinations of calculation codes and nuclear data libraries (i.e. different calculation schemes). The critical values defined in ISO 11311:2011 are the lowest of those calculated by each of these calculation schemes and accepted as credible. The values in ISO 11311:2011 are reference values and not absolute critical values.

ISO 27468:2011 establishes an evaluation methodology 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 history considered in calculations provides a bounding value of the effective neutron multiplication factor, keff. ISO 27468:2011 is applicable to transport, storage, disposal or reprocessing units implying irradiated fissile material from pressurized water reactor (PWR) fuels that initially 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 ISO 27468:2011. ISO 27468:2011 does not specify requirements related to overall criticality safety evaluation or eventual implementation of burnup credit.

ISO 27467:2008 specifies areas that it is important to study when analyzing potential criticality accidents. It is important that a criticality accident analysis be performed each time a criticality accident is considered credible, due either to criticality contingencies (double batching, procedural violations, etc.) or to the failure of safety provisions (effectiveness of neutron absorber reduced by fire, etc.). It is important that the criticality safety specialist be mindful that the process of evaluation developed in ISO 27467:2008 does not cater to the unforeseen, since any actually occurring criticality accident will probably result from a scenario not envisioned or from failure to comply with prevailing regulations. ISO 27467:2008 does not address detailed administrative measures, for which the responsibility lies with the public authorities, nor does it deal with criteria used to justify the accident criticality analysis of a nuclear facility. ISO 27467:2008 does not apply to nuclear power plants.

ISO 14943:2004 provides criteria for the administration of nuclear criticality safety-related activities for operations which take place outside of reactors and for which there exists a potential for criticality accidents.

Is applicable to all operations with plutonium, uranium 233, uranium enriched in the 235 isotope, and other fissionable materials. Does not require separate additional instrumentation when the operating instrumentation of the facilities meets its requirements. Is principally concerned with gamma-radiation rate-sensing systems. Annex A refers to the specification of a minimum accident of concern, annex B provides examples of application, and annex C provides guidance for development of emergency plans. Does not include details of administrative steps or specific design and description of instrumentation.

Specifies the basic principles and limitations which govern operations with fissile materials. Describes general criticality safety criteria for equipment design and for the development of operating controls. Does not cover quality assurance requirements or details of equipment or operational procedures, does not deal with the effects of radiation on man or materials.These criteria apply to operations with fissile materials outside nuclear reactors but within the boundaries of nuclear facilities. Replaces the first edition, which has been technically revised.