Criticality safety considerations

Criticality Safety Considerations in the Fabrication of UO2 and PUO2-UO2 Fuels for Light-Water Reactors, C. L. Brown (PNL), Invited... 

At the Los Alamos Scientific Laboratory the need has been recognized for a program to provide an improved appreciation of criticality safety fundamentals to super visors of fissile material processes. Many of the senior people who have been with the Laboratory for many years are now retiring. These people, had the opportunity to develop their familiarity with criticality safety considerations as operations e3q>anded from the relatively small scale which was characteristic of the early days. Their successors are denied this opportunity, and some feel the need for an educational program having more. depth than is appropriate for every employee. 

Continuous monitoring is necessary on critical machines where problems can develop rapidly and have severe financial consequences. Typical machines in this category are unspared process compressors. Remotely located machinery such as pipeline gas compressors also require continuous monitoring. Also, continuous monitoring may be dictated by safety considerations. Even though the cost of a failure is small, machines should be continuously monitored if a failure will result in hazards to personnel. Figure 8-38 depicts a typical continuous monitoring system. 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

Safety considerations are paramount in any boron hydride synthesis. The energy yield from the oxidations of boron hydrides is too high for any cavalier treatment of boron hydrides. Exclusion of air is the critical consideration in diborane reactions. Decaborane(14) is less reactive, generally, in a kinetic sense, but the thermodynamic potential is comparable. In addition, all volatile boron hydrides are toxic. The procedures described in the latter two preparations are within our experience non-hazardous. These procedures should be followed in every detail improvisation is not recommended. 

It is to be noted that not every critical failure results in an accident. However, the question is not whether such consequences are inevitable, but whether they are possible. Safety considerations are conservative and rigid and they are assessed at the most conservative level. In the absence of proof that a failure cannot affect safety, it is considered by default as a critical issue that requires immediate consideration. 

Tetranitromethane is of explosion whose oxygen balance is 49 % with explosion heat of 1,892 J/g. The pure tetranitromethane is of poor sensitivity that cannot be detonated by even 10 g special as booster. However, it is violently explosive when it is mixed with combustible material at almost zero oxygen balance, and it has a small critical diameter (thickness) and significant detonation and shock sensitivity. Such explosives are generally prepared when they are needed and the quantity addresses the requirement exactly regarding the safety considerations. Table 7.14 lists the detonation properties of tetranitromethane and certain combustible compounds at zero oxygen balance. 

Considerable effort has been concentrated on the development of a plant design which has an acceptable criticality safety case, but which is not constrained in terms of its flexibility to process a wide variety of feed material and manufacture a wide range of fuel assemblies. The principal objective of the design was to make the plant inherently safe where practicable for normal operations and potential fault conditions. Only where this objective is not practicable are additional safety protection systems provided. This enhances the robustness of the safety of the plant and reduces the requirement for potentially complex safety protection systems. 

Gurpinar, A. (1997) A review of seismic safety considerations in the life cycle of critical facilities , Jourrml of Earthquake Engineering, 1(1). 

The ES H Manual Supplement also considers applicable ANSI standards, including the basis for criticality requirements, record keeping, assessments for potential criticality events, criticality safety control parameters, conducting criticality safety analyses, preparation of plans and procedures, requirements for criticality alarms, personnel training, posting, and operational considerations. 

The proper selection and assembly of components in a pressure system are critical safety factors. Considerations should include the materials used in manufacturing the components, compatibility with the materials to be under pressure, the tools used for assembly, and the reliability of the finished connections. No oil or lubricant of any kind should be used in a tubing system with oxygen because the combination produces an explosion hazard. 

Probably the most critical aspect of the introduction of a new process is that of the education and training of the personnel involved. It is vital that the people have a qualitatively correct overall picture of the importance of the different stages of the bonding operation, and of the health and safety considerations. The use of an untrained workforce is likely to cause problems sooner rather than later ... 

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