«Safety of Conversion Facilities and Uranium Enrichment Facilities Specific Safety Guide No. SSG-5 IAEA SAFETY RELATED PUBLICATIONS IAEA SAFETY ...»
4.36. The fire hazards that are specifically encountered in a conversion facility such as from anhydrous ammonia (explosive and flammable), nitric acid (ignition if organic materials) and hydrogen should be given due consideration.
4.37. For gaseous diffusion enrichment facilities, fire prevention and mitigation should be given due consideration for the use and storage of lubricating oils or oxidants like ClF3.
Fire hazard analysis
4.38. As an important aspect of fire hazard analysis, areas of the facility that require special consideration should be identified. Special fire hazard analyses
should be carried out as follows:
(1) For conversion facilities:
(a) Processes involving H2, such as reduction of uranium oxide;
(b) Workshops using flammable liquids (e.g. dodecane), such as purification units and laboratories;
(c) The storage of reactive chemicals (e.g. NH3, H2, HNO3, dodecane);
(d) Areas with high fire loads, such as waste storage areas;
(e) Waste treatment areas, especially those where incineration is carried out;
(f) Rooms housing safety related equipment, e.g. items, such as air filtering systems, whose degradation may lead to radiological consequences that are considered to be unacceptable;
(g) Transformers and rooms housing battery chargers;
(h) Control rooms.
(2) For gaseous diffusion enrichment facilities:
(a) Areas with high fire loads, such as areas containing lubricating oil tanks and vessels containing degreasing or decontamination solvents;
(b) The storage areas for reactive chemicals (e.g. ClF3, F2);
(c) Diesel storage tanks;
(d) Transformers and rooms housing battery chargers;
(e) Areas storing combustible waste prior to its conditioning;
(f) Control rooms.
(3) For gas centrifuge enrichment facilities:
(a) Diesel storage tanks;
(b) Transformers and rooms housing battery chargers;
(c) The storage of solvents (e.g. methylene chloride CH2Cl2);
(d) Areas storing combustible waste prior to its conditioning;
(e) Control rooms.
4.39. Fire hazard analysis involves identification of the causes of fires, assessment of the potential consequences of a fire and, where appropriate, estimation of the frequency or probability of occurrence of fires. It is used to assess the inventory of fuels and initiation sources, and to determine the appropriateness and adequacy of measures for fire protection. Computer modelling of fires may sometimes be used in support of the fire hazard analysis.
4.40. The estimation of the likelihood of fires can be used as a basis for making decisions or for identifying weaknesses that might otherwise go undetected. Even if the estimated likelihood of fire may seem low, a fire might have significant consequences for safety and, as such, certain protective measures should be taken such as delineating small fire areas, to prevent or curtail the fire spreading.
4.41. The analysis of fire hazards should also involve a review of the provisions made at the design stage for preventing, detecting and fighting fires.
Fire prevention, detection and mitigation
4.42. Prevention is the most important aspect of fire protection. Facilities should be designed to limit fire risks by the incorporation of measures to ensure that fires do not break out. Mitigation measures should be put in place to minimize the consequences of a fire in the event that a fire breaks out despite preventive measures.
4.43. To accomplish the twofold aim of fire prevention and mitigation of the consequences of a fire, a number of general and specific measures should be
taken, including the following:
— Separation of the areas where non-radioactive hazardous material is stored from the process areas;
— Minimization of the fire load of individual rooms;
— Selection of materials, including those for civil structures and compartment walls, penetrations and cables associated with structures, systems and components important to safety, in accordance with functional criteria and fire resistance ratings;
— Compartmentalization of buildings and ventilation ducts as far as possible to prevent the spreading of fires. Buildings should be divided into fire zones. Measures should be put in place to prevent or severely curtail the capability of a fire to spread beyond the fire zone in which it breaks out. The higher the fire risk, the greater the number of fire zones a building should have;
— Suppression or limitation of the number of possible ignition sources such as open flames or electrical sparks.
4.44. Fire extinguishing devices, automatic or manually operated, with adequate extinguishing agent, should be installed in the areas where the outbreak of a fire is possible (see Ref. , Appendix III, para. III.10). In particular, “the installation of automatic firefighting devices with water sprays shall be assessed with care for areas where UF6 is present, with account taken of the potential risk of HF generation and criticality events for enriched material” (Ref. , Appendix III, para. III.11). Consideration should be given to minimizing the environmental impact of the water used to extinguish fires.
4.45. The design of ventilation systems should be given particular consideration with regard to fire prevention. Dynamic containment comprises ventilation ducts and filter units, which may constitute weak points in the fire protection system unless they are of suitable design. Fire dampers should be mounted in the ventilation system unless the likelihood of widespread fires is acceptably low.
They should close automatically on receipt of a signal from the fire detection system or by means of temperature sensitive fusible links. Spark arrestors should be used to protect the filters if necessary. The required operational performance of the ventilation system should be specified so as to comply with fire protection requirements.
4.46. Lines that cross the boundaries between fire areas or fire zones (e.g. electricity, gas and process lines) should be designed to ensure that fire does not spread.
4.47. An explosion can be induced by fire or it can be the initiating event that results in a fire. Explosions could breach the barriers providing confinement and/or could affect the safety measures that are in place for preventing a criticality accident.
4.48. In conversion facilities and enrichment facilities, the possible sources of
(a) Gases (in conversion facilities: e.g. H2 or NH3 used in the reduction process; in enrichment facilities: chemical oxidants such as F2, ClF3 or UF6). Design provisions should be implemented to prevent an explosive mixture of the above chemical oxidants and of hydrocarbons or halo-hydrocarbons. Where the prevention of such an explosive mixture cannot be ensured, consideration should be given to the use of an inert gas atmosphere or dilution systems.
(b) Solid chemical compounds (in conversion facilities only: ammonium nitrate when in a high temperature environment);
(c) Monitoring of possible deposits should be implemented to prevent any accumulation of ammonium nitrate.
4.49. Flooding in a conversion facility or an enrichment facility may lead to the dispersion of radioactive material if the radioactive material were not kept in a confined state (e.g. yellow cake, ammonium diuranate (ADU) in conversion). For UF6, which is always kept in a confined state, flooding would only result in a release of hazardous materials if there were a simultaneous loss of confinement.
4.50. In any case, flooding may lead to a change in criticality safety parameters such as reflection and/or moderation.
4.51. In facilities where vessels and/or pipes containing water are present, the criticality analyses should take into account the presence of the maximum amount of water that could be contained within the room under consideration as well as the maximum amount of water in any connected rooms.
4.52. Walls (and floors if necessary) of rooms where flooding could occur should be capable of withstanding the water load to avoid any ‘domino effect’ due to their failure.
Leaks and spills
4.53. Leaks from containment systems such as vessels, cylinders, pumps, valves and pipes can lead to the dispersion of radioactive material (e.g. uranium solutions and powders, gaseous or liquid UF6) and toxic chemicals (e.g. HF, F2, NH3, ClF3) and to the unnecessary generation of waste. Leaks of hydrogenous fluids (water, oil, etc.) can adversely affect criticality safety. Leaks of flammable gases (e.g. H2) or liquids can lead to explosions and/or fires. Leak detection systems should be deployed where leaks could occur.
4.54. For conversion and uranium recovery locations of enrichment facilities, vessels containing significant amounts of nuclear material in solution form should be equipped with level detectors and alarms to prevent overfilling and with secondary containment features such as bunds or drip trays of appropriate capacity and configuration to ensure criticality safety.
4.55. The surfaces of floors and walls should be chosen to facilitate their cleaning, in particular in wet process areas. This will also facilitate the minimization of waste from decommissioning.
Loss of support systems
4.56. To fulfil the requirement established in para. 6.28 of Ref. , an emergency
power supply should be provided for:
— Monitoring systems for radiation protection and environmental protection;
— Detection and alarm systems for leaks of hazardous materials;
— Fire detection and alarm systems;
— Criticality accident detection and alarm systems;
— Ventilation systems, if necessary for the confinement of hazardous material;
— Some process control components (e.g. heating elements and valves);
— Fire pumps, if fire water is dependent on off-site electrical power.
4.57. For enrichment facilities, a loss of electrical power may result in major operational consequences. In addition, there may be some safety implications from a loss of electrical power, such as the formation of solid uranium deposits.
— For the centrifuge process, a backup electrical power system should be provided for the removal of the UF6 from the cascade and its transfer to UF6 cylinders or chemical absorber traps.
— For the diffusion process, the inherent heat is sufficient to keep the UF6 in its gaseous form for several days in the process equipment. However, solidification of the UF6 may start beyond this period. A first potential safety issue involves the heating of solidified UF6 for sublimation within the process equipment and piping, which may lead to local liquefaction of the UF6 and a subsequent loss of confinement. A second potential safety issue is that a large quantity of solid enriched uranium could accumulate in an unsafe geometry such that a loss of moderation control could cause a criticality event.
4.58. The licensing documentation (safety case) should address the remedial actions necessary for the facility, including the items identified above to return to a safe operational state, unless the likelihood of an extended loss of power can be ruled out on probabilistic grounds.
4.59. The loss of general supplies such as gas for instrumentation and control, cooling water for process equipment and ventilation systems, heating water, breathing air and compressed air may have also some consequences for safety.
— Loss of gas supply to gas controlled safety valves and dampers: In accordance with the safety analysis, valves should be used that are ‘design to fail’ to a safe position;
— Loss of cooling or heating water: Adequate backup capacity or a redundant supply should be provided for in the design.
Loss or excess of process media
4.60. The following list gives examples of hazards to be considered during the
safety assessment as defined in para. 6.29 of Ref. :
— Incomplete chemical reactions in conversion facilities may lead to a release of hazardous chemicals.
— Overpressure in the equipment may cause an increase of the levels of airborne radioactive material and/or concentration of hazardous material in the working areas of the facility.
— Excess of F2 in the fluorination process in conversion facilities may result in its release.