«Safety of Conversion Facilities and Uranium Enrichment Facilities Specific Safety Guide No. SSG-5 IAEA SAFETY RELATED PUBLICATIONS IAEA SAFETY ...»
4.17. The aim of the criticality analysis is to demonstrate that the design of equipment is such that the values of controlled parameters are always maintained in the subcritical range. This is generally achieved by determining the effective multiplication factor (keff), which depends on the mass, the distribution and the nuclear properties of uranium and all other materials with which it is associated.
The calculated value of keff is then compared with the value specified by the design limit.
4.18. The methods of calculation vary widely in basis and form, and each has its place in the broad range of situations encountered in the field of nuclear criticality
safety. The criticality analysis should involve:
— The use of a conservative approach (with account taken of uncertainties in physical parameters and of the physical possibility of worst case moderation conditions);
— The use of appropriate and qualified computer codes that are validated within their applicable range and of appropriate data libraries of nuclear reaction cross-sections.
4.19. The following are recommendations for conducting a criticality analysis for a conversion facility or an enrichment facility to meet the safety requirements
established in para. III.6 of Appendix III of Ref. :
— Mass. The mass margin should be 100% of the maximum value attained in normal operation (to compensate for possible ‘double batching’, i.e. the transfer of two batches of fissile material instead of one batch in a fuel fabrication process) or equal to the maximum physical mass that could be present in the equipment.
— Geometry of processing equipment. “The potential for changes in dimensions during operation shall be considered” (e.g. bulging of slab tanks or slab hoppers).
— Neutron interaction. Preference should be given to engineered spacing over spacing achieved by administrative means.
— Moderation. Hydrogenous substances (e.g. water and oil) are common moderators that are present in conversion facilities and enrichment facilities or that may be present in accident conditions (e.g. water from firefighting);
the subcriticality of a UF6 cylinder should rely only on moderation control.
— Reflection. Full water reflection should be assumed in the criticality analysis unless it is demonstrated that the worst case conditions relating to neutron reflection (e.g. by human beings, organic materials, wood, concrete, steel of the container) result in a lower degree of reflection. The degree of reflection in interacting arrays should be carefully considered since the assumption of full water reflection may provide a degree of neutronic isolation from interacting items. Moderation control should ensure criticality safety for an individual UF6 cylinder or an array of UF6 cylinders for any conditions of reflection.
— Neutron absorbers. “When taken into account in the safety analysis, and if there is a risk of degradation, the presence and the integrity of neutron absorbers shall be verifiable during periodic testing. Uncertainties in absorber parameters shall be considered in the criticality calculations.” The neutron absorbers that may be used in conversion facilities and enrichment facilities include cadmium, gadolinium or boron in annular storage vessels or transfer vessels for liquids. Absorber parameters include thickness, density and isotopic concentration.
Confinement to protect against internal exposure and chemical hazards
4.20. As far as possible, the following parameters should be minimized:
— The amount of liquid UF6 in process areas, e.g. by limiting the size of crystallization (desublimation) vessels in both conversion and enrichment facilities;
— The amount of nuclear material unaccounted for in the process vessels;
— The duration of operation when UF6 is at a pressure above atmospheric pressure;
— The capacity for storage of HF, NH3 and H2.
4.21. Conversion facilities and enrichment facilities should be designed to minimize, to the extent practicable, contamination of the facility and releases of radioactive material to the environment, and to facilitate decontamination and eventual decommissioning. Especially in the working areas where liquid UF6 is processed, two static barriers should be installed. Particular consideration should also be given to minimizing the use of flexible hoses and to ensuring their maintenance and periodic checking.
4.22. Use of an appropriate containment system should be the primary method for protection against the spreading of dust contamination from areas where significant quantities of either powder of uranium compounds or hazardous substances in a gaseous form are held. To improve the effectiveness of static containment, a dynamic containment system providing negative pressure should be used when practicable, through the creation of airflow towards the more contaminated parts of equipment or an area. The speed of the airflow should be sufficient to prevent the migration of radioactive material back to areas that are less contaminated. A cascade of reducing absolute pressures can thus be established between the environment outside the building and the hazardous material inside.
4.23. In the design of the ventilation and containment systems for areas that may contain elevated levels of airborne radioactive material during operation, account should be taken of criteria such as: (i) the desired pressure difference between different parts of the premises; (ii) the air replacement ratio in the facility; (iii) the types of filters to be used; (iv) the maximum differential pressure across filters;
(v) the appropriate flow velocity at the openings in the ventilation and containment systems (e.g. the acceptable range of air speeds at the opening of a hood); and (vi) the dose rate at the filters.
Protection of workers
4.24. The ventilation system should be used as one of the means of minimizing the radiation exposure of workers and exposure to hazardous material that could become airborne and so could be inhaled by workers. Conversion facilities and enrichment facilities should be designed with appropriately sized ventilation and containment systems in areas of the facility identified as having potential for giving rise to significant concentrations of airborne radioactive material and other hazardous material. Wherever possible, the layout of ventilation equipment should be such that the flow of air is from the operation gallery towards the equipment.
4.25. The need for the use of protective respiratory equipment should be minimized through careful design of the containment and ventilation systems.
For example, a glovebox, hood or special device should be used to ensure the continuity of the first containment barrier when changing a valve to remove the need for respiratory protection.
4.26. In areas that may contain airborne uranium in particulate form, primary filters should be located as close to the source of contamination as practicable unless it can be shown that the design of the ventilation ducts and the air velocity are sufficient to prevent unwanted deposition of uranium powder in the ducts.
Multiple filters in series should be used to avoid reliance on a single filter. In addition, duty and standby filters and/or fans should be provided to ensure the continuous functioning of ventilation systems. If this is not the case, it should be ensured that failure of the duty fan or filter will result in the safe shutdown of equipment in the affected area.
4.27. Monitoring equipment such as differential pressure gauges (on filters, between rooms or between a glovebox and the room in which it is located) and devices for measuring uranium or gas concentrations in ventilation systems should be installed as necessary. Alarm systems should be installed to alert operators to fan failure or high or low differential pressures. At the design stage, provision should also be made for the installation of equipment for monitoring airborne radioactive material and/or gas monitoring equipment. Monitoring points should be chosen that would correspond most accurately to the exposure of workers and would minimize the time for detection of any leakage (see para. 6.39 of Ref. ).
4.28. To prevent the propagation of a fire through ventilation ducts and to maintain the integrity of firewalls, and as practicable in view of the potential of corrosion by HF, ventilation systems should be equipped with fire dampers and should be constructed from non-flammable materials.
4.29. If fume hoods and gloveboxes are used (e.g. in laboratories), their design should be commensurate with the specific local hazards in the conversion facility or enrichment facility.
4.30. To facilitate decontamination and the eventual decommissioning of the facility, the walls, floors and ceilings in areas of the conversion facilities and enrichment facilities where contamination is likely to exist should be made non-porous and easy to clean. This may be done by applying special coatings, such as epoxy, to such surfaces and ensuring that no areas are difficult to access.
In addition, all surfaces that could become contaminated should be made readily accessible to allow for periodic decontamination as necessary.
Protection of the environment
4.31. The uncontrolled dispersion of radioactive or chemical substances to the environment as a result of an accident can occur if all the containment barriers are impaired. Barriers may comprise the process equipment itself, or the room or building structure. The number of physical barriers for containment should be adapted to the safety significance of the hazard. The minimum number of barriers is two, in accordance with the principle of redundancy (see para. II-1 of Annex II of Ref. ). The optimum number of barriers is often three. In addition, ventilation of the containment systems, by the discharge of exhaust gases through a stack via gas cleaning mechanisms such as wet scrubbers in conversion facilities, or cold traps and dry chemical absorbers in enrichment facilities, reduces the normal environmental discharges of radioactive or chemical (mainly HF) material to very low levels. In such cases, the ventilation system may also be regarded as a containment barrier. The design should also provide for the monitoring of the environment around the facility and the identification of breaches to the containment barriers.
Protection against external exposure
4.32. External exposure can be controlled by means of an appropriate combination of requirements on distance, time and shielding. Owing to the low specific activity of naturally sourced material, the shielding provided by the vessels and pipe work of a conversion facility or an enrichment facility will normally be sufficient to control adequately occupational exposure. However, in areas that are in close proximity to newly emptied UF6 cylinders or bulk storage areas, installation of shielding or restrictions on occupancy should be considered.
Additional shielding or automation may also be required for the handling of reprocessed uranium.
4.33. When reprocessed uranium is processed, shielding should be strengthened for protection of the workers, because of the higher gamma dose rates from 232U daughters and fission products.
4.34. In selecting the areas for storage of tailings, requirements on distance, occupancy time and shielding should be considered to minimize the direct exposure of members of the public to gamma and neutron radiation. In estimating the exposure, ‘sky shine’ (scattered gamma radiation in air) should also be taken into account.
POSTULATED INITIATING EVENTSInternal initiating events Fire and explosion
4.35. Conversion facilities and enrichment facilities, like all industrial facilities, have to be designed to control fire hazards in order to protect workers, the public and the environment. Fire in conversion facilities and enrichment facilities can lead to the dispersion of radioactive material and/or toxic material by breaching the containment barriers or may cause a criticality accident by affecting the system of the parameters used for the control of criticality (e.g. the moderation control system or the dimensions of processing equipment).