«Safety of Conversion Facilities and Uranium Enrichment Facilities Specific Safety Guide No. SSG-5 IAEA SAFETY RELATED PUBLICATIONS IAEA SAFETY ...»
— In conversion facilities, uranium exists in diverse chemical and physical forms and is used in conjunction with flammable or chemically reactive substances as part of the process.
— In enrichment facilities, most of the uranium is in the chemical form UF6.
The physical form of UF6 could be the gaseous, liquid or solid state.
2.2. In conversion facilities and enrichment facilities, the main hazards are HF and UF6. In addition, where uranium is processed that has a 235U concentration of more than 1%, criticality may be a significant hazard. Workers, the public and the environment must be protected from these hazards during commissioning, operation and decommissioning.
2.3. Generally, in a conversion facility or an enrichment facility, only natural uranium or LEU that has a 235U concentration of no more than 6% is processed.
The radiotoxicity of this uranium is low, and any potential off-site radiological consequences following an accident would be expected to be limited. However, the radiological consequences of an accidental release of reprocessed uranium would be likely to be greater, and this should be taken into account in the safety assessment if the licence held by the facility permits the processing of reprocessed uranium.
2.4. The chemical toxicity of uranium in a soluble form such as UF6 is more significant than its radiotoxicity. Along with UF6, large quantities of hazardous chemicals such as HF are present. Also, when UF6 is released, it reacts with the moisture in the air to produce HF and soluble uranyl fluoride (UO2F2), which present additional safety hazards. Therefore, safety analyses for conversion facilities and enrichment facilities should also address the potential hazards resulting from these chemicals.
2.5. Conversion facilities and enrichment facilities do not pose a potential radiation hazard with the capacity to cause an accident with a significant off-site release of radioactive material (in amounts equivalent to a release to the atmosphere of 131I with an activity of the order of thousands of terabecquerels).
However, deviations in processes may develop rapidly into dangerous situations involving hazardous chemicals.
2.6. For the application of the requirement that the concept of defence in depth be applied at the facility (see Section 2 of Ref. ), the first two levels of defence in depth are the most important, as risks can be reduced to insignificant levels by means of design and appropriate operating procedures (see Sections 4 and 7).
3.1. The site evaluation process for a conversion facility or an enrichment facility will depend on a large number of criteria, some of which are more important than others. At the earliest stage of planning a facility, a list of these criteria should be prepared and considered in accordance with their safety significance. In most cases, it is unlikely that all the desirable criteria can be met, and the risks posed by possible safety significant external initiating events (e.g.
earthquakes, aircraft crashes, fires and extreme weather conditions) will probably dominate in the site evaluation process. These risks should be compensated for by means of adequate design provisions and constraints on processes and operations as well as possible economic arrangements.
3.2. The density of population in the vicinity of a conversion facility or an enrichment facility and the direction of the prevailing wind at the site should be considered in the site evaluation process to minimize any possible health consequences for people in the event of a release of hazardous chemicals.
3.3. A full record should be kept of the decisions taken on the selection of a site for a conversion facility or an enrichment facility and the reasons behind those decisions.
GENERALSafety functions for conversion facilities and enrichment facilities
4.1. Safety functions (see Ref. , Appendix III, para. III.1), i.e. the functions the loss of which may lead to releases of radioactive material or chemical releases having possible radiological consequences for workers, the public or the
environment, are those designed for:
(1) Prevention of criticality;
(2) Confinement for the prevention of releases that might lead to internal exposure and for the prevention of chemical releases;
(3) Protection against external exposure.
4.2. For conversion facilities:
— The main hazard is the potential release of chemicals, especially HF and UF6. Controls to address this hazard will adequately protect also against internal radiation exposure.
— A criticality hazard exists only if the conversion facility processes uranium with a 235U concentration greater than 1%.
— External exposure is a concern for the handling of residues containing thorium and its daughter products produced in fluorination reactors.
External exposure is also a concern in the handling of recently emptied cylinders, especially those used as containers for reprocessed uranium, where there is a buildup of 232U.
4.3. For enrichment facilities:
— The main hazard is a potential release of UF6;
— A criticality hazard exists since the concentration of 235U present in enrichment facilities is greater than 1%;
— External exposure is a concern in the handling of recently emptied cylinders, especially those used as containers for reprocessed uranium, with buildup of 232U.
Specific engineering design requirements
4.4. The following requirements apply:
(1) The requirements on prevention of criticality are established in paras 6.43–6.51 and III.3–III.7 of Appendix III of Ref. ;
(2) The requirements on confinement for the prevention of releases that might lead to internal exposure and chemical hazards are established in paras 6.37–6.39, 6.54–6.55 and paras III.8 and III.9 of Appendix III of Ref. ;
(3) The requirements on protection against external exposure are established in paras 6.40–6.42 of Ref. . Shielding should be considered for processes or areas that could involve sources of high levels of external gamma radiation, such as reprocessed uranium or newly emptied cylinders (e.g. exposure to daughter products of 232U and 238U).
Design basis accidents and safety analysis
4.5. The definition of a design basis accident in the context of fuel cycle facilities can be found in para. III-10 of Annex III of Ref. . The safety requirements relating to design basis accidents are established in paras 6.4–6.9 of Ref. .
4.6. The specification of a design basis accident (or equivalent) will depend on the facility design and national criteria. However, particular consideration should be given to the following hazards in the specification of design basis accidents for
(a) A release of HF or ammonia (NH3) due to the rupture of a storage tank;
(b) A release of UF6 due to the rupture of a storage tank, piping or a hot cylinder;
(c) A large fire originating from H2 or solvents;
(d) An explosion of a reduction furnace (release of H2);
Natural phenomena such as earthquakes, flooding or tornadoes1;
(e) (f) An aircraft crash;
Nuclear criticality accidents, e.g. in a wet process area with a 235U content (g) of more than 1% (reprocessed uranium or unirradiated LEU).
4.7. The first four types of events ((a)–(d)) are of major safety significance as they might result in chemical and radiological consequences for on-site workers and may also result in some adverse off-site consequences for people or the environment. The last type of accident on the list would generally be expected to result in few or no off-site consequences unless the facility is very close to occupied areas.
4.8. The hazards listed in para. 4.6 may occur as a consequence of a postulated initiating event (PIE). Selected PIEs are listed in Annex I of Ref. .
4.9. The potential occurrence of a criticality accident should be considered for facilities that process uranium with a 235U concentration of more than 1%.
Particular consideration should be given to the potential occurrence of a For some facilities of older design, natural phenomena were not given consideration.
These phenomena should be taken into account for the design of new conversion and enrichment facilities.
criticality accident for facilities treating various feed products including reprocessed uranium.
4.10. The specification of a design basis accident (or equivalent) will depend on the facility design and national criteria. However, particular consideration should be given to the following hazards in the specification of design basis accidents for
(a) The rupture of an overfilled cylinder during heating (input area);
(b) The rupture of a cylinder containing liquid UF6 or the rupture of piping containing liquid UF6 (depending on the facility design for product take-off);
(c) A large fire, especially for diffusion facilities;
(d) Natural phenomena such as earthquakes, flooding or tornadoes (see footnote 1);
(e) An aircraft crash;
(f) A nuclear criticality accident.
4.11. These hazards would result primarily in radiological consequences for on-site workers, but might also result in some adverse off-site consequences for people or the environment. The last type of hazard on the list would generally be expected to result in few or no off-site consequences unless the facility is very close to populated areas.
4.12. The hazards listed in para. 4.10 may occur as a consequence of a PIE.
Selected PIEs are listed in Annex I of Ref. .
Structures, systems and components important to safety
4.13. The likelihood of design basis accidents (or equivalent) should be minimized, and any radiological and associated chemical consequences should be controlled by means of structures, systems and components important to safety and appropriate administrative measures (operational limits and conditions).
Annexes III and IV contain examples of structures, systems and components and representative events that may challenge the associated safety functions.
SAFETY FUNCTIONSPrevention of criticality 4.14. “For the prevention of criticality by means of design, the double contingency principle shall be the preferred approach” (Ref. , para 6.45).
Paragraph III.5 of Appendix III of Ref.  establishes requirements for the control of system parameters for the prevention of criticality in conversion facilities and enrichment facilities. Some examples of the parameters subject to
control are listed in the following:
— The mass and degree of enrichment of fissile material present in a process:
for conversion facilities, in vessels or mobile transfer tanks, or analytical laboratories; for enrichment facilities, in effluent treatment units or analytical laboratories;
— Geometry and/or interaction (limitation of the dimensions or shape) of processing equipment, e.g. by means of safe diameters for storage vessels, control of slabs and appropriate distances in and between storage vessels;
— Concentration of fissile material in solutions, e.g. in the wet process for recovering uranium or decontamination;
— Presence of appropriate neutron absorbers, e.g. neutron poisoning of cooling water in gaseous diffusion enrichment facilities;
— Degree of moderation, e.g. by means of control of the ratio of hydrogen to U in UF6 cylinders and in diffusion cascades.
4.15. Paragraph III.4 of Appendix III of Ref.  requires that preference be given to achieving criticality safety by design rather than by means of administrative measures. As an example, to the extent practicable, vessels which could contain fissile material should be made geometrically safe.
4.16. Several methods can be used to perform the criticality analysis, such as the use of experimental data, reference books or consensus standards, hand calculations and calculations by means of deterministic or probabilistic computer codes. Calculations should be suitably verified and validated and performed under a quality management system.