Barriers, passive protection systems

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

This safety function is ensured by protecting and maintaining the integrity of the potential radioactivity release barriers (fuel, reactor system boundary and containment). These barriers are passive components as themselves in addition, several passive means are proposed in V-392 design for the protection of these barriers (some of them are reflected above). The V-392 design implies substantial improvement of the containment protection against different loads related to design basis and severe accidents, and various passive systems are important part of this protection. 

SSCs important to safety shall be designed and located, subject to compliance with other safety requirements, so as to minimize the effects of fires and explosions. A fire hazard analysis and an explosion hazard analysis shall be carried out for the research reactor facility to determine the necessary ratings of the fire barriers and means of passive protection and physical separation against fires and explosions. The design shall include provisions to prevent or limit the formation of explosive atmospheres. Fire detection systems and fire fighting systems of the necessary capability shall be provided. 

Self-protection means that the reactor plant is capable of preventing damage to physical protective barriers in emergency situations in the event of failure of active safety systems and non-intervention or errors by the plant personnel, by virtue of passive safety systems and inherent safety features based on the action of physical laws of nature which result in selflimitation of the reactor power and temperature. 

A.211. This section should describe the design requirements for fire protection inside the facility. It should include passive features, such as isolation, separation, selection of materials, the building layout and zoning, the location of Are barriers, and the safety system layout and protection (including separation of safety related redundant systems). Tne fire protection system is described in para. A. 1008. 

Low-permeability passive perimeter gas control systems (Figure 16.7) effectively block gas flow into the areas of concern by using barriers (such as synthetic membranes or natural clays) between the contaminated site and the area to be protected. In the low-permeability system, gases are not collected and therefore cannot be conveyed to a point of controlled release or treatment. The low-permeability system can also alter the paths of convective flow. 

Absorption Across the Skin. An aqueous carrier may be used for a variety of dermal products. In fact, carriers can be designed to limit the transportation of the penetration of the active ingredient (such as an insect repellent), if the desired effect is to keep the activity on the surface of the skin. Once again, however, only those materials that are dissolved will be available for penetration across the skin to gain access to the systemic circulation. For almost all chemicals in or about to enter commerce, dermal penetration is a passive process. The relative thickness of the skin makes absorption (into the systemic circulation) slower than the absorption across the GI or pulmonary barriers. This is compounded by the fact that the stratum comeum ftmction is to be impervious to the environment. One of the skin s major functions is protection from infection. Once a chemical penetrates into the dermis, it may partition into the subcutaneous fat. Essentially, absorption across the skin is a two-step process with the first being penetration and deposition into the skin and the second being release from the skin into the systemic circulation. The pattern of blood levels obtained via dermal penetration is generally one with a delayed... 

The term blood-brain barrier (BBB) refers to the special obstacle that drugs encounter when trying to enter the brain from the circulatory system. The difference between the brain and other tissues and organs is that the capillaries in the brain do not have pores for the free flow of small molecules in the interstitial fluid of the brain. To enter the interstitial fluid, all molecules must cross a membrane. This design is a protective measure to defend the brain from unwanted and potentially hazardous xenobiotics. Traditionally, drugs that target the brain or central nervous system (CNS) cross the BBB by passive diffusion. Transport by carrier proteins across the BBB is becoming better understood but remains an area of active research. 

Most body tissues are protected by lipophilic barriers that serve as the body s primary protection against absorption of chemicals. It is well established that lipophilic chemicals can penetrate lipophilic barriers (including mucous membranes) much more readily than can hydrophilic chemicals by passively diffusing across lipid-rich cell membranes. 2 4l The lipid-rich mucous membranes also serve as barriers to the absorption of hydrophilic species. Lipophilic chemicals, however, promote the permeation of hydrophilic chemicals that are dissolved in the lipophiles. Lipophiles are routinely used, for example, in drug delivery systems (see Section 3.3). 

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