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«Safety of Conversion Facilities and Uranium Enrichment Facilities Specific Safety Guide No. SSG-5 IAEA SAFETY RELATED PUBLICATIONS IAEA SAFETY ...»

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— Releases of large amounts of nitrogen may result in a reduction of the oxygen concentration in breathing air in the work areas of the facility.

— Loss of steam or hot water supply may result in the solidification of UF6 in the piping and equipment in a diffusion facility.

— Failure of the air supply may result in the failure of safety related air operated valves.

Mechanical failure

4.61. Particular consideration should be given to the confinement of highly corrosive and hazardous materials such as UF6, F2 and HF in vessels, pipes and pumps and to powder transfer lines where abrasive powder will cause erosion.

4.62. The design should minimize the potential for mechanical impacts to containers of hazardous material caused by moving devices such as vehicles and cranes. The design should ensure that the movement of heavy loads by cranes above vessels and piping containing large amounts of hazardous and/or radioactive material is minimized, as a major release of hazardous or radioactive material could occur if the load were accidentally dropped.

4.63. Failure due to fatigue or chemical corrosion or lack of mechanical strength should be considered in the design of containment systems for hazardous and/or radioactive material.

External initiating events Earthquakes

4.64. A conversion facility or enrichment facility should be designed for the design basis earthquake to ensure that the ground motion during an earthquake at the site would not induce a loss of confinement capability (especially for confinement of UF6 and HF) or a criticality accident (i.e. a seismically induced loss of criticality safety functions, such as geometry and moderation) with possible significant consequences for site personnel or members of the public.

4.65. To define the design basis earthquake for the facility, the main characteristics of the disturbance (intensity, magnitude and focal distance) and the distinctive geological features of the local ground should be determined. The approach should ideally evaluate the seismological factors on the basis of historical data for the site. Where historical data are inadequate or yield large uncertainties, an attempt should be made to gather palaeoseismic data to enable the determination of the most intense earthquake affecting the site to have occurred over the period of historical record. The different approaches can be combined since the regulatory body generally takes into account the results of scenarios based on historical data and those based on palaeoseismic data in the approval of the design.

4.66. One means of specifying the design basis earthquake is to consider the historically most intense earthquake, but increased in intensity and magnitude, for the purpose of obtaining the design response spectrum (the relationship between frequencies and ground accelerations) used in designing the facility. Another way of specifying the design basis earthquake is to perform a geological review, to determine the existence of capable faults and to estimate the ground motion that such faults might cause at the location of the facility.

4.67. An adequately conservative spectrum should be used for calculating the structural response to guarantee the stability of buildings and to ensure the integrity of the ultimate means of confinement in the event of an earthquake.

Certain structures, systems and components important to safety will require seismic qualification. This will apply mainly to equipment used for storage and vessels that will contain significant amounts of fissile or toxic chemical materials.

Design calculations for the buildings and equipment should be made to verify that, in the event of an earthquake, no unacceptable release of fissile or toxic material to the environment would occur and the risk of a criticality accident would be very low.

External fires and explosions

4.68. Hazards from external fires and explosions could arise from various sources in the vicinity of conversion facilities or enrichment facilities, such as petrochemical installations, forests, pipelines and road, rail or sea routes used for the transport of flammable material such as gas or oil.

4.69. To demonstrate that the risks associated with such external hazards are below acceptable levels, the operating organization should first identify all potential sources of hazards and then estimate the associated event sequences affecting the facility. The radiological or associated chemical consequences of any damage should be evaluated and it should be verified that they are within acceptance criteria. Toxic hazards should be assessed to verify that specific gas concentrations meet the acceptance criteria. It should be ensured that external toxic hazards would not adversely affect the control of the facility. The operating organization should carry out a survey of potentially hazardous installations and transport operations for hazardous material in the vicinity of the facility. In the case of explosions, risks should be assessed for compliance with overpressure criteria. To evaluate the possible effects of flammable liquids falling objects (such as chimneys) and missiles resulting from explosions, their distance from the facility and hence their potential to cause physical damage should be assessed.

Extreme weather conditions





4.70. Typically, extreme weather conditions assumed in the design and in the evaluation of the response of a conversion facility or an enrichment facility are wind loading, tornadoes, tsunamis, extreme rainfall, extreme snowfall, extreme temperatures and flooding.

4.71. The general approach is to use a deterministic design basis value for the extreme weather condition and to assess the effects of such an event on the safety of the facility. The rules for obtaining the design basis values for use in the assessment may be specified by local regulations.

4.72. The design provisions will vary according to the type of hazard and its effects on the safety of the facility. For example extreme wind loading is associated with rapid structural loading and thus design provisions for an event involving extreme wind loading should be the same as those for other events with potentially rapid structural loading such as earthquakes. However, effects of extreme precipitation or extreme temperatures would take time to develop and hence there would be time for operational actions to be taken to limit the consequences of such events.

4.73. A conversion facility or an enrichment facility should be protected against extreme weather conditions by means of appropriate design provisions. These

should generally include:

— The ability of structures important to safety to withstand extreme weather loads;

— The prevention of flooding of the facility;

— The guarantee of safe state for the facility in accordance with the operational limits and conditions.

Tornadoes

4.74. Measures for the protection of the facility against tornadoes will depend on the meteorological conditions for the area in which the facility is located. The design of buildings and ventilation systems should be in compliance with specific regulations relating to hazards from tornadoes.

4.75. High winds are capable of lifting and propelling objects as large as automobiles or telephone poles. The possibility of impacts of missiles such as these should be taken into consideration in the design stage for the facility, as regards both the initial impact and the effects of secondary fragments arising from collisions with and spallation of concrete walls or from other types of transfer of momentum.

Extreme temperatures

4.76. The potential duration of extreme low or high temperatures should be taken into account in the design of the main process equipment and support system

equipment to prevent adverse effects such as:

— The crystallization of uranium nitrate solutions, or liquid or gaseous UF6;

— The freezing of the cooling system used in desublimers (cold traps) such as those used in off-gas systems;

— The freezing of emergency oil used to blanket concentrated HF solutions after a breach of a vessel;

— The liquefaction of solid UF6 in piping.

4.77. If safety limits for humidity or temperature are specified in a building or a compartment, the air conditioning system should be designed to perform efficiently also under extreme hot or wet weather conditions.

Snowfall

4.78. The occurrence of snowfall and its effects should be taken into account in the design and safety analysis. Snow is generally taken into account as an additional load on the roofs of buildings. The neutron reflecting effect and/or the interspersed moderation effect of the snow, if relevant, should be considered (e.g. for product cylinder storage areas).

Floods

4.79. Flooding should be taken into account in the design of a facility. Two

approaches to dealing with flooding hazards have been put forward:

— In some States the highest flood levels recorded over the period of historical record are taken into account and nuclear facilities are sited at specific locations or at a sufficient elevation to avoid major damage.

— In other States, in which the use of dams is widespread and where a dam has been built upstream of a potential or existing site for a nuclear facility, the hazard posed by a breach of the dam is taken into account. The buildings of the facility are designed to withstand the water wave released from the breach. In such cases the equipment — especially that used for the storage of fissile material — should be designed to prevent any criticality accident.

Accidental aircraft crash hazards

4.80. The likelihood and possible consequences of impacts onto a facility should be calculated by assessing the number of aircraft that come close to the facility and their flight paths, and by evaluating the areas vulnerable to impact, i.e. areas where hazardous material is processed or stored. If the risk is acceptably low, no further evaluations are necessary. See also para. 5.5 (item (h)) of Ref. [1].

4.81. For evaluating the consequences of impacts or the adequacy of the design to resist aircraft impacts, only realistic crash scenarios should be considered, which may require the knowledge of such factors as the possible angle of impact or the potential for fire and explosion due to the aviation fuel load. In general, fire cannot be ruled out following an aircraft crash and so the establishment of specific requirements for fire protection and for emergency preparedness and response will be necessary.

INSTRUMENTATION AND CONTROL (I&C)

Instrumentation

4.82. Instrumentation should be provided to monitor the variables and systems of the facility over their respective ranges for: (1) normal operation; (2) anticipated operational occurrences; and (3) design basis accidents, to ensure that adequate information can be obtained on the status of the facility and proper actions can be undertaken in accordance with the operating procedures or in support of automatic systems.

4.83. Instrumentation should be provided for measuring all the main variables whose variation may affect the processes, for monitoring for safety purposes general conditions at the facility (such as radiation doses due to internal and external exposure, releases of effluents and ventilation conditions), and for obtaining any other information about the facility necessary for its reliable and safe operation. Provision should be made for the automatic measurement and recording of values of parameters that are important to safety.

Control systems

4.84. Passive and active engineering controls are more reliable than administrative control and should be preferred for control in normal operational states and in accident conditions. Automatic systems should be designed to maintain process parameters within the operational limits and conditions or to bring the process to a safe state.



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