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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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4.85. Appropriate information should be made available to the operator for monitoring the effects of automatic actions. The layout of instrumentation and the manner of presentation of information should provide the operating personnel with an adequate impression of the status and performance of the facility. Devices should be installed that provide in an efficient manner visual and, as appropriate, audible indications of operational states that have deviated from normal conditions and that could affect safety.

Control rooms

4.86. Control rooms should be provided to centralize the main data displays, controls and alarms for general conditions at the facility. Occupational exposure should be minimized by locating the control rooms in parts of the facility where the levels of radiation are low. For specific processes, it may be useful to have dedicated control rooms to allow the remote monitoring of operations, thereby reducing exposures and risks to operators. Particular consideration should be paid to identifying those events, both internal and external to the control rooms, that may pose a direct threat to the operators and to the operation of control rooms.

Ergonomic factors should be taken into account in the design of control rooms.

Safety related I&C systems for normal operation

4.87. Safety related I&C systems for normal operation of a conversion facility or

an enrichment facility should include:

(1) Instrumentation and control relating to the process, e.g. indicating temperatures, pressures, flow rates, concentrations of chemicals and/or radioactive material, tank levels, cylinder weights.

The corresponding requirements are established in paras III.13 and III.14 of Appendix III of Ref. [1]. In filling UF6 cylinders, the weight should be monitored by appropriate and reliable devices to ensure that the fill limit is not exceeded.

(2) I&C relating to criticality safety.

For enrichment facilities, in-line devices for enrichment measurement should be used to monitor the enrichment levels of products. For diffusion enrichment facilities, the ratio of hydrogen to uranium should be monitored by an in-line analyser (e.g. an HF infrared analyser).

(3) I&C relating to the chemical purity of UF6 (for diffusion enrichment facilities).

The corresponding requirement is established in para. III.15 of Appendix III of Ref. [1].

(4) I&C relating to ventilation, i.e. mainly devices for measuring differential pressures across high efficiency particulate air (HEPA) filters and airflows.

(5) I&C relating to gaseous and liquid effluents.

Real time measurements should be provided if there is a risk of exceeding regulatory limits; otherwise, retrospective measurements on continuously sampled filters and/or probes will generally be sufficient.

(6) I&C relating to the prevention of explosive mixtures.

Real time measurements, controls and alarms are necessary if there is a risk of exceeding regulatory and safety limits, e.g. devices for measuring the concentration of O2 in the reduction kiln in conversion facilities.

Safety related I&C systems for anticipated operational occurrences

4.88. In addition to the listing provided in para. 4.87, I&C systems for use in

anticipated operational occurrences should include the following provisions:

(1) All rooms with both fire loads and significant amounts of fissile and/or toxic chemical material should be equipped with fire alarms (except where the permanent presence of operators is sufficient).

(2) Gas detectors should be used in areas where a leakage of gases (e.g. H2) could produce an explosive atmosphere.

(3) Real time detection and alarm systems should be used in the process areas and/or laboratories where HF and UF6 above atmospheric pressure is present.

(4) The devices used for monitoring releases of gaseous and liquid effluents in operational states should also be capable of measuring releases during anticipated operational occurrences. If the amounts released are significant, the recommendation presented in item (3) of para. 4.89 should be followed.

Safety related I&C systems for design basis accident conditions

4.89. I&C systems relating to design basis accidents should, in addition to the

previous listings, include provisions to address the following situations:

(1) Criticality.

The requirement on I&C systems relating to criticality control is established in para. III.16 of Appendix III of Ref. [1].

(2) Chemical release.

The requirement on I&C systems relating to monitoring for chemical releases is established in para. III.17 of Appendix III of Ref. [1].

(3) Release of effluents.

The devices used for measuring releases of gaseous and liquid effluents in operational states should also be capable of measuring such releases in the case of a design basis accident. If the measurement devices used in operational states become saturated in accident conditions, resulting in unmonitored releases of effluents, environment sampling should be used to estimate the releases of gaseous and liquid effluents.

HUMAN FACTOR CONSIDERATIONS

4.90. The requirements relating to human factor considerations are established in paras 6.15 and 6.16 of Ref. [1].

4.91. Human factors in operation, inspection, periodic testing, and maintenance





should be considered at the design stage. Human factors to be considered include:

— Possible effects on safety of unauthorized human actions (with account taken of ease of intervention by the operator and tolerance of human error);

— The potential for occupational exposure.

4.92. Design of the facility to take account of human factors is a specialist area.

Experts and experienced operators should be involved from the earliest stages of

design. Areas that should be considered include:

(a) Design of working conditions to ergonomic principles.

— The operator–process interface, e.g. electronic control panels displaying all the necessary information and no more;

— The working environment, e.g. good accessibility of and adequate space around equipment and suitable finishes to surfaces for ease of cleaning.

(b) Choice of location and clear labelling of equipment so as to facilitate maintenance, testing, cleaning and replacement;

(c) Provision of fail-safe equipment and automatic control systems for accident sequences for which reliable and rapid protection is required;

(d) Good task design and job organization, particularly during maintenance work, when automated control systems may be disabled;

(e) Minimization of the need to use additional means of personal radiation protection.

SAFETY ANALYSIS

4.93. Safety analysis for conversion facilities and enrichment facilities should be

performed in two major steps:

— The assessment of occupational exposure and public exposure for operational states of the facility and comparison with authorized limits for operational states;

— Determination of the radiological and associated chemical consequences of design basis accidents (or the equivalent) for the public and verification that they are within the acceptable limits specified for accident conditions.

4.94. The results of these two steps should be reviewed for identification of the possible need for additional operational limits and conditions.

Safety analysis for operational states Occupational exposure and exposure of the public

4.95. A facility specific, realistic, enveloping and robust (i.e. conservative) assessment of internal and external occupational exposure and exposure of the

public should be performed on the basis of the following assumptions:

(1) Calculations of the source term should use: (i) the material with the highest specific activity; (ii) the licensed inventory of the facility; and (iii) the maximum material throughput that can be processed by the facility. The poorest performances of barriers in normal operation should be used in the calculations. A best estimate approach may also be used.

(2) Calculations of the estimated doses due to occupational exposure should be made on the basis of the conditions at the most exposed workplaces and should use maximum annual working times. On the basis of data on dose rates collected during commissioning runs and as necessary, the operational limits and conditions may include maximum annual working times for particular workplaces.

(3) Calculations of the estimated doses to the public (i.e. a ‘critical group’ of people living in the vicinity of the facility) should be made on the basis of maximum estimated releases of radioactive material to the air and to water and maximum depositions to the ground. Conservative models and parameters should be used to calculate the estimated doses to the public.

Releases of hazardous chemical material

4.96. Facility specific, realistic, robust (i.e. conservative) estimations of chemical hazards to workers and releases of hazardous chemicals to the environment should be performed in accordance with the standards applied in the chemical industry.

Safety analysis for accident conditions Methods and assumptions for safety analysis for accident conditions

4.97. For conversion facilities and enrichment facilities, there is no general agreement on the best approach to the safety analysis for design basis accidents and the associated acceptance criteria. However, there is a tendency for the following or similar criteria to be adopted for new advanced facility designs.

4.98. For a conversion facility or an enrichment facility, consequences of design basis accidents would be limited to consequences for individuals on the site and close to the location of the accident. The consequences depend on various factors such as the amount and rate of the release of radioactive material and hazardous chemicals, the distance between the individuals exposed or affected and the source of the release, pathways for the transport of material to the individuals and the exposure times.

4.99. To estimate the on-site and off-site consequences of an accident, the wide range of physical processes that could lead to a release of radioactive material to the environment should be modelled in the accident analysis and the enveloping cases encompassing the worst consequences should be determined.

4.100. The following approaches should be considered in the assessment:

(1) An approach using the enveloping case (the worst case approach, e.g. the release of liquid UF6 from a cylinder filled to the maximum fill limit), with account taken only of those safety features that mitigate the consequences of accidents and/or that reduce their likelihood. If necessary, a more realistic case can be considered that includes the use of some safety features and some non-safety-related features beyond their originally intended range of functions to reduce the consequences of accidents (the best estimate approach).

(2) The worst case approach, with no account taken of any safety feature that may reduce the consequences or the likelihood of accidents. This assessment is followed by a review of the possible accident sequences, with account taken of the emergency procedures and the means planned for mitigating the consequences of the accident.

Assessment of possible radiological or associated chemical consequences

4.101. Safety assessment should address the consequences associated with possible accidents. The main steps in the development and analysis of accident

scenarios should include:

(a) Analysis of actual site conditions and conditions expected in the future.

(b) Identification of workers and members of the public who could possibly be affected by accidents; i.e. a ‘critical group’ of people living in the vicinity of the facility.

(c) Specification of facility configurations, with the corresponding operating procedures and administrative controls for operations.

(d) Identification and analysis of conditions at the facility, including internal and external initiating events that could lead to a release of material or energy with the potential for adverse effects, the time frame for emissions and the exposure time, in accordance with reasonable scenarios.



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