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1、INTERNATIONAL STANDARD Is0 1709 Second edition 1995-l l-01 Nuclear energy - Fissile materials - Principles of criticality safety in storing, handling and processing hergie nucl b) the mass of fissile material present and its distribution within the system being assessed; cl the mass and distribution
2、 of all other materials associated with the fissile material. The assessment should consider all processing con- ditions that can be foreseen as reasonable. 4.2 Methods of control Methods of control of criticality safety in any oper- ation include, but are not limited to, any one or a combination of
3、 the following: a) b) cl d) 4 f) limitation of the dimensions or shape of operational equipment; control of the mass of fissile material present in an operation; control of the concentration of fissile material in solutions; control of neutron moderation associated with the fissile material; the pre
4、sence of appropriate neutron absorbers; reliance on neutron absorbers requires assurance of their continued presence; control of the spacing between material and equipment. 4.3 Achievement of control The control of criticality safety by such methods as those indicated in 4.2 can be achieved by a) eq
5、uipment design; b) use of process control systems with associated instrumentation; cl administrative control of operations. Where practicable, the maintenance of control shall depend on safety features incorporated in the equip- ment, or instrumentation, rather than on adminis- trative control. It i
6、s recognized that some reliance on administrative control is inherent in any operation. 4.4 Factors affecting criticality A number of factors shall be considered singly and in combination for a proper assessment of criticality safety. Some of the more important factors are given in 4.4.1 to 4.4.5. 2
7、 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 03:25:13 MDTNo reproduction or networking permitted without license from IHS -,-,- Q IS0 4.4.1 Moderation IS0 1709: 1995(E) 4.4.5
8、 Geometry The presence of neutron-moderating material mixed with fissile material can substantially reduce the mass of fissile material necessary to achieve criticality. Water, oil and similar hydrogenous substances are the most common moderators present in the storage, handling and processing of fi
9、ssile material, and all rea- sonably forseeable modes of incorporation shall be considered. 4.4.2 Reflection The most effective neutron reflector commonly encountered in handling and in processing fissile material is water of thickness sufficient to yield maximum nuclear reactivity. However, careful
10、 consideration shall be given to systems where significant thicknesses of other common structural materials (for example wood, concrete, steel), which may be more effective neu- tron reflectors than water, may be present. For some situations, the reflection provided by personnel may be important. 4.
11、4.3 Interaction Consideration shall be given to neutron interaction between units when at least two units containing fissile material are present. It is possible to reduce neutron interaction to acceptable proportions either by spacing units, by insertion of suitable neutron- moderating and absorbin
12、g materials between units, or by some combination of these methods. 4.4.4 Neutron absorbers Equipment and processes can conform to the requirements of criticality safety by using neutron- absorbing materials, such as cadmium and boron, provided available data confirm that their suitability and their
13、 presence can be assured. Where practicable, the incorporation of solid neutron absorbers as permanent, integral parts of equipment is more desirable than the use of neutron absorbers in solution, because of the processing controls required to demonstrate the continued presence of dissolved absorber
14、s. Neutron-absorbing materials are most effective for neutrons of thermal energy and care shall be exer- cised to ensure that their effectiveness is not seriously reduced in operational or accident conditions, which might change the fissile assembly into one characterized by neutrons of intermediate
15、 or high energy. Criticality control can be provided by the use of processing or storage vessels that have a large neu- tron leakage. Cylinders or slabs of a suitable shape can be very reliable safety designs. Consideration shall be given to possible changes in vessel dimen- sions caused by over-pre
16、ssurization or corrosion. 4.5 Possible abnormalities The effect of the occurrence of reasonably fore- seeable abnormal conditions shall be considered in the assessment of safety. These include such factors as a) loss or introduction of moderating material into or between units of fissile material: f
17、or example, evaporation, precipitation, dilution and flooding; b) introduction of neutron-reflecting material near units of fissile material; cl change in shape of fissile material due to such occurrences as vessel leakage or breakage; d) change in operating conditions: for example, loss of flow, pr
18、ecipitation, excessive evaporation, violation of mass or volume limits; e) change in conditions of neutron interaction: for example, collapse or overturn of equipment; f) loss of neutron absorber, or reduction in absorber effectiveness as a result of moderator loss; g) double batching, or over batch
19、ing to the extent that the equipment does not preclude such an occurrence. 4.6 Bases of assessment Wherever possible, the specification for criticality safety shall be established on bases derived directly from experiments. In the absence of directly applicable experimental measurements, which is a
20、common situation, the results of calculations are acceptable, provided they are shown to compare favourably with exprimental data. However, calculated results shall be subjected to margins of safety sufficient to guarantee with confidence that the system will be subcritical. 4.7 Margin of safety In
21、all specifications, the margin of safety shall be commensurate with the uncertainty in the basis of as- sessment, the probability of its violation, and the seriousness of the consequences of a conceivable criticality accident. Copyright International Organization for Standardization Provided by IHS
22、under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 03:25:13 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 1709:1995(E) 8 IS0 As an example, operations should, in general, incorporate sufficient safety features so that two
23、unlikely, independent, concurrent changes must occur in the conditions originally specified as essential to criticality safety before the system may become criti- cal. The occurrence of one of these changes would indicate that the safety of the process should be re- evaluated. 7 Dispatch and receipt
24、 of material Appropriate arrangements shall be made between the consignor and consignee before fissile material is dispatched from an establishment. Provision shall be made for the receipt of damaged packages. 8 Monitoring of procedures 5 Equipment control Prior to starting a new or modified process
25、 or processing line, it shall be ascertained that all equipment is consistent in dimension and material with the assumptions on which the criticality safety assessment was based. Processing operations shall be reviewed by comparison with the applicable written procedures on a periodic basis. The rev
26、iew shall be conducted by persons not directly involved with operations, and a written report provided for management and super- visors. 6 Material control 10 Need for criticality alarms The movement of fissile material shall be controlled. Appropriate labelling of materials and marking of areas sha
27、ll be maintained, specifying material identification and all limits on parameters that are subjected to criticality control. The need for criticality accident alarms shall be evalu- ated in accordance with IS0 7753. Where alarm systems are considered necessary, emergency pro- cedures shall be prepar
28、ed. Guidance for the preparation of emergency procedures may be found in annex A of IS0 7753. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 03:25:13 MDTNo reproduction or netwo
29、rking permitted without license from IHS -,-,- IS0 1709:1995(E) Q IS0 ICS 27.120.30 Descriptors: nuclear energy, nuclear reactors, nuclear fuels, ftssionable materials, radloactwity, safety, safety requirements, fuel storage, materials handling, processing. Pnce based on 4 pages Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 03:25:13 MDTNo reproduction or networking permitted without license from IHS -,-,-
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