Local shielding

Figure Bl.11.7. chemical shifts in [10]-paracyclophane. They have values on either side of the 1.38 ppm found for large polymethylene rings and, thus, map the local shielding and deshielding near the aromatic moiety, as depicted in the upper part of the figure.

It is occasionally necessary for personnel to enter some of these facilities. This requires considerable time for decontamination and placement of local shielding. The operations that will be performed are carefully planned and rehearsed in order to get the entry over with as swiftly as possible. A special group of employees, the health physicists, are responsible for radiation safety throughout the plant, and they provide close monitoring of these entries. 

When the box is used for productions of radiopharmaceuticals incorporating a-or P-emitting radionuclides, closed box units without any lead coating may be sufficient. When handling radionuclides with mixed emitting properties, a possibility is to concentrate the shielding to critical parts of the process. This can be done by use of local shielding inside the production unit. However, for aseptic production, one must keep in mind a potential disturbance of the airflow inside the box. 

Radiation. The external radiation hazard from uranium is caused chiefly by beta radiation and is significant only if the uranium surface to which people are exposed is large. The problem presents only minor restraints to the designer, localized shielding normally proving adequate without the extreme measures necessary in fission-product-handling plants. 

High y activity 1-100 curies t Entry possible after draining plant and a limited amount of decontamination, particularly if some local shielding is used Careful use of some conventional plant items may be permissible... 

IjOw y activity <1 curiet Entry may be possible during operation, particularly if local shielding is used Conventional plant items can be used... 

Boron has been included as a neutron absorber in various materials in addition to concrete. For example, borated graphite, a mixture of elemental boron and graphite, has been used in fast-reactor shields. Boral, consisting of boron carbide (B4C) and aluminum, and epoxy resins and resin-impregnated wood laminates incorporating boron have been used for local shielding purposes. Boron has also been added to steel for shield structures to reduce secondary gamma-ray production. In special situations, where a shield has consisted of a heavy metal and water, it has been beneficial to add a soluble boron compound to the water. 

Below the CPP, CT cannot displace adsorbed oxygen so long as the passive film remains intact hence, pitting is predicted not to occur. Should passivity break down because of factors other than those described [e.g., reduced oxygen or depolarizer concentration at a crevice (crevice corrosion), or cathodic polarization of local shielded areas], pitting could then initiate independent of whether the overall prevailing potential is above or below the critical value. But under conditions of uniform passivity for the entire metal surface, application of cathodic protection to avoid pitting corrosion need only shift the potential of the metal below the critical value. This is in contrast to the usual procedure of cathodic protection, which requires polarization of a metal to the much more active open-circuit anode potential. 

More complete trades of reactor and local shielding may reduce overall mass... 

The external radiation environment is described in detail in the Environmental Requirements Document (982-00029). The internal radiation environment will be influenced by position relative to the reactor and shield, by local shielding, and by the external radiation levels. Solar, galactic and Jovian radiation sources, coupled with reactor radiation, must be considered during electronics selection, shielding trades and material evaluation. A detailed description of the internal radiation environments relative to the different zones of the reactor module can be found in Section 10.5,... 

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