Материал: Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

pulses in the range of counting rates differing by more than six orders of magnitude.

Passive systems of neutron coincidence counting allow determining the mass of plutonium by logging spontaneous-fission neutrons of mostly evennumbered isotopes (238Pu, 240Pu, 242Pu). With plutonium isotope composition known, the measurement result – 240Pueff mass – can be converted to the total Pu mass in the sample.

The fissionable isotope 235U does not go into spontaneous fission readily enough to be logged by passive systems. In this case, instruments will register secondary induced neutron radiation, which arises under exposure to neutrons of an AmLi source (active systems). For low-energy incident neutrons, the induced fission of 238U in a specimen makes an insignificant contribution to the measured rate of neutron coincidences.

Neutron coincidence counting systems have detectors of two major configurations: well-type detectors in which a specimen is fully enclosed and collar-type detectors embracing a specimen from the outside. The geometry of the former detectors is preferable as it is possible in this case to register all emitted neutrons. But the alternative collar-type geometry allows measuring specimens too large to be placed in a well-type detector (e.g., a fuel assembly). In its inspection activities, the IAEA employs neutron instruments of more than 20 types, which have various design features and are usable for specimens of certain sizes and forms and various Pu and U mass ranges. Some of them are presented in Table 3.5.

Table 3.5

Neutron coincidence logging instruments in use for determining the mass of fissionable material

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∙to verify the quality of working standards in use for calibration of nondestructive analysis instruments;

∙to carry out periodic verification of measurement systems at the inspected facility.

In general, destructive verification measurements carried out by the Agency include the following successive stages:

1)independent sampling;

2)in-situ conditioning of samples to ensure their integrity during transportation;

3)packaging, sealing and dispatch of samples to the IAEA laboratories;

4)statistical estimation of the analysis results.

The main analytical methods of destructive testing practiced by the Agency during inspections are listed in Table 3.7. This table presents the estimated random and systematic components of errors in measurements of materials of nuclear grade or similar chemical purity. It is obvious that the sampling process itself or the presence of impurities in material can tangibly affect the data of Table 3.7.

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a)excepting INF;

b)for ratios of the main isotopes.

Safeguards-related activities of the IAEA

The inspection activities of the IAEA depend on the scale of nuclear operations carried out by a state. These operations will be small-scale in states having, e.g., one small research reactor alone, but can be quite extensive in countries with many nuclear fuel cycle facilities. The scope of inspection activities is the greater, the larger is the number of NFC facilities in a country. Table 3.8 presents rough estimates of the nuclear material quantities covered by the IAEA safeguards. It may be seen that the NM quantities under control are steadily growing, and so is the number of facilities under the IAEA safeguards.

This being so, the inspection activities, including independent measurements of nuclear materials and their control by means of containment and surveillance techniques, depend largely on the type of nuclear facilities under the safeguards. Reactors and storage facilities, where materials appear as articles, such as fuel assemblies, call for less inspection effort than do facilities with materials found in bulk, where the greater part of NM is in motion or under processing. Table 3.9 shows the numbers of various facilities covered by the IAEA Safeguards.

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