Barrier approach

The first problems on the list are not specific to radon control but are encountered on nearly every construction job. In spite of quality control and communication problems and the understandable wariness builders show when asked to build something in a different way, the residential construction industry has responded to new techniques, materials, and public demands. The average house being built today is very different from a home built 20 years ago. If a product or a method can be demonstrated to reliably keep radon out without presenting significant problems with cost, scheduling, or installation, many builders would learn to use it. The major difficulty faced by mechanical barrier approaches is the thoroughness that seems to be required to ensure that no radon problem will occur. 

FIGURE 31.16 Summary of mechanical barrier approach for basement foundations. (Adapted from U.S. EPA, Radon-Resistant Construction Techniques for New Residential Construction—Technical Guidance, EPA/625/2-91/032, U.S. Environmental Protection Agency, Washington, DC, February 1991.)... 

Thermal calculations suggest that the char barrier approach can be highly efficient if optimized. Funt and Magill (8) showed that a 1 mm layer would keep an underlying substrate from reaching ignition temperature when the external fire atmosphere was at 743 C, and a 2.7 mm layer would suffice when the fire atmosphere was at 1500 C (Table II). 

Ideally, in a perfect world, all chemical facilities would be secured in a layered fashion (aka the barrier approach). Layered security systems are vital. Using the protection in-depth principle, requiring that an adversary defeat several protective barriers or security layers to accomplish its goal, chemical industry infrastructure can be made more secure. Protection in depth is a term commonly used by the military to describe security measures that reinforce one another, masking the defense mechanisms from the view of intruders, and allowing the defender time to respond to intrusion or attack. 

In particular, membrane bioreactors (MBRs) are today robust, simple to operate, and ever more affordable. They take up little space, need modest technical support, and can remove many contaminants in one step. These advantages make it practical, for the first time, to protect public health and safely reuse water for non-potable uses. Membranes can also be a component of a multi-barrier approach to supplement potable water resources. Finally, decentralization, which overcomes some of the sustainability limits of centralized systems, becomes more feasible with membrane treatment. Because membrane processes make sanitation, reuse, and decentralization possible, water sustainability can become an achievable goal for the developed and developing worlds. 

The Ground Water Rule (GWR) is a targeted strategy to identify ground water systems at high risk for fecal contamination. The proposed rule establishes a multiple barrier approach to identify and provide corrective measures for public ground water systems at risk of fecal contamination. The GWR will be issued as a final regulation in 2006. 

A more expensive barrier approach can be seen on bridges where the reinforced concrete has been clad in a brick or masomy finish, usually for cosmetic reasons. The salt rarely penetrates to the concrete surface. However, this option is rarely available for reasons of cost. Encasement of building elements can often be undertaken as part of a refurbishment programme, such as with a change of use. External cladding can be added which encloses concrete elements at risk of corrosion. Once in a warm, dry, indoor environment the risk of corrosion is significantly reduced. 

A more expensive barrier approach can be seen on bridges where the... 

The new Bayview Terminal had a design operating pressure of 740 pounds per square inch (50 atmospheres or 50 bar) and the accident pipeline was designed for much higher pressures, so it was necessary to install pressure-reduction equipment where the pipeline joined the new terminal. There were three layers of control and safety devices (i) a control valve to throttle the incoming flow, (ii) a pilot-operated spring-loaded relief valve, and (iii) three motor-operated isolation valves. This triple-barrier approach is sound design - a control system, backed-up by two protection systems (the relief valve and the isolation valves). In the event of a problem with the control valve, the relief valve should operate quickly, and only in the event of a fault with the relief valve should the isolation valves ever have been required to operate. 

The process, as described above, has been applied to three large systems, and a number of systems for chemical plant control. In all cases, the safety barrier approach allowed searches to be limited to just a few tens of hazardous signal paths - evidence of good safety design. 

The energy-barrier approach is described more in detail in Chapter 5. We will make extensive use of this approach in our modelling of the accident sequence in Chapter 6. It also provides us with essential criteria for the identification and assessment of hazards and the selection of remedial actions (see Parts III and V). 

A safety engineer, representing the barrier approach according to Haddon (Section 5.4) ... 

Recommendations for the improvements are given on the basis of assessment of the barriers available. Technical barriers are considered more reliable than administrative/human barriers. A consideration within the Barrier approach is that all suggestions for safety improvement should conceive the cost effectiveness of each improvement. 

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