Материал: Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

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Fig. 10.3. An example of NM storage facility: A – c ontainer racks; B – NM safes

Storage facilities offer the best NM control solutions. The major elements of the storage facility structure (floors, walls, roofing and doors) normally satisfy to top-level material localization requirements, including reinforced-concrete floor, ceiling, roofing and walls of the thickness not less 20 cm. Any openings in storage facilities that permit unauthorized intrusion thereto are equipped with barriers. It is forbidden to process material and handle same other than for the purposes of NM storage (say, for experiments involving NM) within storage facilities. It is also forbidden to lay routes to other areas of operations through storage facilities.

NM access controls

An important feature of current accounting and control systems is the maximum use of access controls. All developed countries, including Russia, have their NM accounting and control systems relying on measured material balance. Furthermore, a measurement of NM inventory is in fact a

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serious physical experiment. Still there is more to that. Measurements of nuclear materials as such are rather a time-taking and hazardous procedure (especially when it comes to plutonium). So it is required to make so that NM measurements be reduced to as small scope as it can be. In this, the continuity of knowledge about NM is ensured through using NM access controls.

Access controls include two types of provisions known as surveillance and containment measures.

Surveillance. The simplest and the most commonly used surveillance type is visual surveillance. This requires continuous presence of a number of persons. A two-person rule is used for NM handling when critical operations with nuclear material are done by not less than two persons. A three-person rule can be used to handle weapon-grade material.

For NM control, many countries make an extensive use of optoelectronic surveillance (OES) via TV monitors. Fig. 10.4 shows an option of the OES camera installation in a spent fuel pool.

For years, optical surveillance of NM has been used successfully in the context of international safeguards. The IAEA uses optical surveillance to control NM movements, survey spent fuel in storage facilities and for other purposes. Photographic surveillance systems (MINOLTA) were employed by the IAEA until 1988 when these were replaced for video systems of a greater efficiency. Table 3.2 (Part I, Chapter 3) gives examples of video systems used now by IAEA, which demonstrate the increasingly great capabilities of this nuclear material control technique.

Containment. Containment ensures continuity of the knowledge about NM at rest. Containment, first of all, means use of all kinds of sensors that detect unauthorized access (tamper indicating devices or TIDs) or respond to changes in the condition or the positions of nuclear material (hybrid NM containment techniques). Thus, if a container shift is detected, the sensor generates the message of the container having been moved.

TIDs represent seals of different types.

Seals have the role of detecting unauthorized access to NM (seals are used for containers, item packages, doors and so on).

There are three properties that characterize the efficiency of seals:

∙weakness: means that seals are easily breakable, serving so to forestall unauthorized activities with NM;

∙tamper indication: if broken, seals are hard to replace without traces of the tamper left;

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∙ uniqueness: seals should possess unique identifiers (preferably concealed).

Fig. 10.4. An option of the OES camera installation in a spent fuel pool

Fig. 10.5 – 10.9 show examples of various seal type s used in practical NM handling: Е–cup, PS360 loop seal, Multi–lock, Cobra and Vacoss .

Fig. 10.5. Cup-Еseal design

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Fig. 10.6. A plastic loop seal (PS360 model)

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Fig. 10.7. Multi–Lock seal design: 1 – spring; 2 – aluminum ball; 3 – steel ball

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Fig. 10.8. Cobra seal design:

1 – compression rod; 2 – shearing razor; 3 – fiber optic cable; 4 – seal body

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Fig. 10.9. Reusable Vacoss seal design by Aquila, the USA

Adhesive seals (stamps). These seals have a paper, vinyl or lavsan base with an adhesive applied to it on the inside. Adhesive seals may contain one or more base layers.

Paper or vinyl adhesive seals have a single-layer base. After placement, these seals are hard to remove without traces of the tamper left because:

∙paper seals are based on rice paper so they can be broken easily when removed;

∙vinyl seals have cross cuts made along the entire base length, which makes it much more difficult to remove a seal without breaking it.

A lavsan seal has a transparent outer layer and a colored metallized inner layer. The inner layer bears a drawing and the ordinal number. The colored base layer of a lavsan seal has a seal breaking sign (the words Void or Open for example) preprinted across the base length on the adhesive side. When a seal is removed, this sign appears on the container surface and the stretched colored base fails to match the image left on the container.

Adhesive seals are used to seal containers with NM.

Hybrid containment techniques

Access control techniques include also US-developed hybrid methods used now also in Russia. These are gamma image memorization techniques. A detector is placed beneath a container with NM (e.g. HEU) for continuous gamma ray detection. The gamma ray spectrum comes further to the computer and is compared to the reference spectrum, i.e. the spectrum that was recorded when the material was placed under control. If the comparison fails to confirm the identity of the spectra, an alarm is on. A gamma image technology is used, which also ensures the confidence of knowledge about the status of nuclear material.

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Источник: https://studfile.net/preview/16708779/

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