Safety cases operating conditions, changes

So far we have considered only the steady state critical reactor where the neutron flux does not vary with time. We now extend the discussion to cover the case where an initially critical reactor is rendered sub- or supercritical by some change such as the movement of a control rod. The dynamic response to changes in the multiplication factor is of obvious importance to the practical operation of the reactor and in particular to the assessment of its safety under operational conditions. 

II processes are subject to disturbances that tend to change operating conditions, compositions, and physical properties of the streams. In order to minimize the ill effects that could result from such disturbances, chemical plants are implemented with substantial amounts of instrumentation and automatic control equipment. In critical cases and in especially large plants, moreover, the instrumentation is computer monitored for convenience, safety, and optimization. 

Three main options from different possible combinations of process units have been previously studied (Khan and Amyotte, 2005, Palaniappan et al., 2002b). The first option is the base case with no additional inherent safety features. The second and third options are revised versions of the first one. Modifications can include some or all of the following a quench tower to reduce the temperature, change of solvent to lower the severity of the operating conditions, an extraction column, a solvent mixer to optimize the use of solvent and efficiency of acid extraction and the use of solvent recycle. 

Constant superimposed backpressure Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn t change appreciably under any conditions of operation. In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure (Figure 3.8). 

Ageing is defined as a general process in which characteristics of components, systems and structures gradually change with time or use. This process eventually leads to degradation of materials subjected to normal service conditions These include normal operation and transient conditions under which the component, system or structure is required to operate. Postulated accident and post-accident conditions are excluded [2] and should be evaluated on a case-by-case basis for the effect on the reactor safety and utilization. 

A form of authority that must be understood by all employees is that they have, under defined criteria, the authority to stop or cease doing something that is an obvious hazard. We have seen too often in weak safety cultures the mindset that an operation has to continue regardless of any potential perceived hazards. Hazardous Conditions in many cases may exist because of problems that were built into a process and may not be resolved without sigiuficant changes in a number of operational factors. 

II-3. The frequent changes in core loading referred to above affect the nuclear and thermal characteristics of the core. Care has to be taken to ensure that, in each case, these characteristics are correctly determined and that they are checked against the relevant conditions for nuclear and thermal safety before the reactor is put into operation. 

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