«Safety of Conversion Facilities and Uranium Enrichment Facilities Specific Safety Guide No. SSG-5 IAEA SAFETY RELATED PUBLICATIONS IAEA SAFETY ...»
7.32. Preference should be given to estimating the internal dose received by members of the public using environmental monitoring data. However, internal doses may also be estimated by using qualified dispersion and dose models in conjunction with reliable data on effluents.
Control of external exposure
7.33. There are only limited operations in a conversion facility or an enrichment facility where specific measures for controlling external exposure are required.
Typically these will be areas where the following activities take place:
— Operations involving recently emptied cylinders;
— Storage of bulk quantities of uranium;
— Handling of UF6 cylinders;
— Handling of ashes from fluorination.
7.34. Moreover, it should be noted that much more extensive controls for limiting external exposure will be required in the processing of reprocessed uranium than in the processing of natural uranium.
7.35. Radioactive sources are also used in a conversion facility or an enrichment facility for specific purposes, e.g. radioactive sources are used for checking uranium enrichment.
7.36. External exposure should be controlled by:
— Ensuring that significant amounts of uranium and recently emptied cylinders are remote or appropriately shielded from areas of high occupancy;
— Ensuring that sources are changed by suitably qualified and experienced persons;
— Performing routine surveys of radiation dose rates.
7.37. Additional controls should be considered if reprocessed uranium is used as a feedstock at the facility. Such material has a higher specific activity than uranium from natural sources and thus has the potential to increase substantially both external and internal exposures. It could also introduce additional radionuclides into the waste streams. A comprehensive assessment of doses due to occupational exposure and exposure of the public should be carried out before the first introduction of uranium from other than natural sources.
7.38. In conversion facilities and enrichment facilities that process uranium with a 235U concentration of more than 1%, it is particularly important that the procedures for controlling criticality hazard are strictly applied (paras 9.49 and
9.50 of Ref. ).
7.39. In addition to the requirement established in para. III.23 of Appendix III of Ref. , operational aspects of the control of criticality hazards in conversion
facilities and enrichment facilities should include:
— Anticipation of unexpected changes in conditions that could increase the risk of a criticality accident; for example, unplanned accumulation of uranium fluorides (e.g. in ventilation ducting), inadvertent precipitation of material containing uranium in storage vessels or loss of neutron absorbers;
— Management of the moderating materials; for example, before a new product cylinder is used in the facility, checks should be undertaken to ensure that no hydrogenous material is present in the cylinder (e.g. water or oil);
— Management of mass in transfer of uranium (procedures, mass measurement, systems and records) for which safe mass control is used;
— Reliable methods for detecting the onset of any of the foregoing conditions;
— Periodic calibration or testing of systems for the control of criticality hazards;
— Evacuation drills to prepare for the occurrence of a criticality and/or the actuation of an alarm.
7.40. The tools used for the purposes of accounting for and control of nuclear material, such as instruments used to carry out measurements of mass, volume or isotopic compositions and software used for accounting purposes, may also have application in the area of criticality safety. However, if there is any uncertainty about the characteristics of material containing uranium, conservative values should be used for parameters such as the level of enrichment and the density.
This arises in particular in connection with floor sweepings and similar waste material.
7.41. Criticality hazards may be encountered when carrying out maintenance work. For example, if “uranium has to be removed from vessels or pipe work, only approved containers shall be used” (para III.24 of Appendix III of Ref. ).
Waste and residues arising from decontamination activities should be collected in containers with a favourable geometry.
INDUSTRIAL AND CHEMICAL SAFETYSee also para. 7.3.
7.42. The chemical hazards found in conversion facilities and enrichment
facilities may be summarized as follows:
— Chemical hazards due to the presence of HF (e.g from UF6), F2, HNO3, NH3 and uranium compounds;
— Explosion hazards due to H2, NH3 ammonium nitrate, methanol, solvents and oxidants present in diffusion cascades;
— Asphyxiation hazards due to the presence of nitrogen or carbon dioxide.
7.43. The threshold of HF that a human can detect by smelling is lower than the occupational exposure level. As a consequence, specific routine occupational measurements for HF need not be implemented. In addition, releases of UF6 generate a visible white cloud of UO2F2 particulates and HF that can easily be seen, leading to the requirement established in para. III.19 of Ref. : “see, evacuate or shelter, and report”.
7.44. A health surveillance programme should be set up, in accordance with national regulations, for routinely monitoring the health of workers who may be exposed to uranium and associated chemicals, e.g. HF, F2 and HNO3. Both the radiological and the chemical effects of uranium should be considered, as necessary, as part of the health surveillance programme.
7.45. Fire hazard analyses should be repeated periodically to incorporate changes that may affect the potential for fires (see para. 4.38).
RISK OF OVERFILLING OF CYLINDERS
7.46. The corresponding requirement is established in para. III.27 of Ref. . Fill limits for cylinders should be established to ensure that, when UF6 expands (by around 35%) on liquefaction, hydraulic rupture does not occur. Further, heating after liquefaction is required for hydraulic rupture to occur.
7.47. In a conversion facility or an enrichment facility, the weight of a cylinder being filled should be monitored, generally by means of weighing scales.
RISK OF OVERHEATING OF CYLINDERS
7.48. The requirements on the risk of overheating of cylinders are established in paras III.28 and III.29 of Appendix III of Ref. . “In the event of an overfilled cylinder, UF6 in excess shall be transferred by sublimation only” (e.g. by evacuation to a cooled low pressure receiving vessel).
HANDLING OF CYLINDERS CONTAINING LIQUID UF6
7.49. Movement of cylinders containing liquid UF6 should be minimized.
Cylinders containing liquid UF6 should be moved only using appropriately qualified apparatus that has been designated as important to safety. Relevant administrative operational limits and conditions should be established for the movement and storage of cylinders containing liquid UF6, e.g. maximum allowed heights, speeds and distances during movement, dedicated storage areas, minimum cooling times, use of valve protectors and restrictions on load movement above hot cylinders.
ON-SITE HANDLING OF SOLID UF6
7.50. The length of time required for the cooling of a cylinder containing liquid UF6 should be sufficient to ensure that all of the liquid UF6 has solidified.
7.51. Cylinders containing solid UF6 should be moved only using appropriately qualified apparatus that has been designated as important to safety.
7.52. Consideration should be given to the impact of a fire on a cylinder containing solid UF6 (e.g. a fire involving a transporter for UF6 cylinders).
7.53. Site licences generally define a site limit for the total amount of tailings of UF6 (depleted uranium hexafluoride) that may be stored. Therefore, a plan for disposition of tailings should be prepared well before this limit is reached, to ensure that future arisings of tailings do not exceed the site limit. Tailings of UF6 may be deconverted to a chemically more stable form of uranium, e.g. an oxide of uranium.
7.54. A recording and tracking system should be used to make periodic inspections of uranium accounting and ensure cylinder integrity.
7.55. Periodic inspections of the tailings storage area should be conducted to check standards of housekeeping and ensure that there is no fire load in the storage area.
MANAGEMENT OF RADIOACTIVE WASTE AND EFFLUENTS
7.56. The requirements relating to the management of radioactive waste and effluents in operation are established in paras 9.54–9.57 of Ref. .
7.57. Gaseous radioactive and chemical discharges should be treated, where appropriate, by means of HEPA filters and chemical scrubbing systems.
Performance standards should be set that specify performance levels at which filters or scrubber media are to be changed. After filter changes, tests should be carried out to ensure that new filters are correctly seated.
7.58. Liquid discharges should be treated effectively. Chemicals should be recovered and reused where possible. This is particularly important for HF produced in the deconversion process. Care should be taken to ensure that HF is suitable for reuse externally.
7.59. One easy way to minimize the generation of solid radioactive waste is to remove as much outer packing as possible before material is transferred to contamination areas. Processes such as incineration, metal melting and compaction can be used to reduce the volume of wastes. As far as reasonably practicable and in accordance with national regulations, waste material should be treated to allow its further use. Cleaning methods should be adopted at the facility that minimize the generation of waste.
7.60. In conversion facilities, unburnt ashes resulting from the fluorination of uranium should be treated to recover the uranium content. The remaining material (oxides of 234Th, 230Th and 228Th if reprocessed uranium is used) should be stored safely. To limit exposure, the treatment of ashes should be postponed to benefit from the decay of 234Th and 228Th.
7.61. Information on the management of waste and effluents can also be found in Refs [8, 9].
EMERGENCY PLANNING AND PREPAREDNESS
7.62. The requirements for emergency planning and preparedness are established in paras 9.62–9.67 and paras III.31 and III.32 of Appendix III of Ref. . The conditions for declaration of an off-site emergency at a conversion facility or an enrichment facility may include large releases of UF6, HF, F2 and NH3 and also, depending on national requirements and facility specific considerations, criticality accidents, large fires (e.g. in the solvent extraction units of a conversion facility) or explosions.
8.1. Requirements for the safe decommissioning of a conversion facility or an enrichment facility are established in Section 10 and paras III.33–III.35 of Appendix III of Ref. . Recommendations on decommissioning of nuclear fuel cycle facilities, including conversion facilities and enrichment facilities, are provided in Ref. .
8.2. Owing to the low specific activity of the depleted, natural and LEU that is processed at conversion facilities and enrichment facilities, most of the waste resulting from the decommissioning of such facilities will be in the low level waste category.
8.3. The preparatory steps for the decommissioning process should include the
— A post-operational cleanout should be performed to remove all the gaseous UF6 and the bulk amounts of uranium compounds and other hazardous materials from the process equipment. The corresponding requirement is established in para. III.33 of Appendix III of Ref. .