Safety Case operating conditions

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. 

Clock Spring design required a safety factor of 2 based on a maximum anticipated load of 10 ksi (69 MPa). The composite was designed to withstand 20 ksi (138 MPa) loads at 50 years of exposure to worst case operating conditions. Based on this equation the Clock Spring 20 ksi (138 MPa) design life is 67 years. This equation shows the minimum new (t= 1) tensile strength of the composite should exceed 42 ksi (290 MPa) (GRI, 1998). 

Cathodic protection of water power turbines is characterized by wide variations in protection current requirements. This is due to the operating conditions (flow velocity, water level) and in the case of the Werra River, the salt content. For this reason potential-controlled rectifiers must be used. This is also necessary to avoid overprotection and thereby damage to the coating (see Sections 5.2.1.4 and 5.2.1.5 as well as Refs. 4 and 5). Safety measures must be addressed for the reasons stated in Section 20.1.5. Notices were fixed to the turbine and the external access to the box headers which warned of the danger of explosion from hydrogen and included the regulations for the avoidance of accidents (see Ref. 4). 

In the example the manufacturer has been specified from available performance curves, and the details of construction must be obtained. The pump is selected to operate at 22 GPM and 196 to 200 feet head of fluid, and must also perform at good efficiency at 18 GPM and a head which has not been calculated, but w hich will be close to 196 to 200 feet, say about 185 feet. Ordinarily the pump is rated as shown on the specification sheet. This insures adequate capacity and head at conditions somewhat in excess of normal. In this case the design GPM w as determined by adding 10 percent to the capacity and allowing for operation at 90 percent of the rated efficiency. Often this latter condition is not considered, although factors of safety of 20 percent are not unusual. However, the efficiency must be noted and the increase in horsepower recognized as factors w hich are mounted onto normal operating 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. 

Moreover, reactor safety also requires fulfilling a more ambitious objective, that is, to design a reactor that will remain stable in case of mal-operation. The result will be a robust process towards deviations from normal operating conditions. This goal can be reached if the accumulation of non-converted reactants is controlled and maintained at a safe level during the course of reaction. The concept of maximum temperature of synthesis reaction (MTSR) was introduced for this purpose. This point will be described in the second section. In the... 

Safety aspects for two different situations are considered. The first is for nominal operating conditions, where the objective is to maintain stable control of the reactor temperature. The second is for deviations of these operating conditions, especially in the case of a cooling failure, where the objective is to design a reactor that behaves safely even under adiabatic conditions. 

The vapor velocity is essential information for the assessment of reactor safety at boiling point. This is particularly the case when cooling by evaporation, as normal operating conditions or if the boiling point is reached after a failure. 

Calculation results of radiation factors analysis show that maximum annual effective radiation dose for population in case of the design-basis accident during Victor II dismantling will not exceed 0.1 mSv. It is considerably less than dose limit for population under normal operational conditions given in Radiation Safety Standards (NRB -99). 

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. 

The process requirements for both temperature and pressure should also be evaluated against the production equipment capabilities as part of the production equipment assessment. Normal operating conditions are used as a base case, but upset conditions should also be included as part of the evaluation. If venting is chosen to control unintended reactions, vent sizing calculations must be performed and peripheral equipment selected as needed. Experiments and simulations to determine consequences of unintended reactions and the interpretation of these experiments are documented as part of the production process safety review. 

Safety management has to protect the battery against critical operation conditions and against failure of the BMS or its associated sensors and wiring. In the case of failure, substitute functions may be necessary in order to continue battery operation, perhaps at a lower performance. 

Testing can be more than manual operation of the system under simulated conditions. Inspection of key artefacts such as requirements, designs and specifications by snitable experts is a widely used technique in other safety critical industries and offers valuable evidence for the safety case. 

BNFL is permitted to carry out its operations at Sellafield under the conditions contained within its Nuclear Site License granted by the HSE under the Nuclear Installations Act 1965. This license contains a number of conditions which relate directly to the safe operation of nuclear plants and which have to be met in order for a plant to operate. One requirement is for the production and assessment of safety cases to justify safety during the design, construction, manufacture, commissioning, operation, and decommissioning phases. 

The Safety Case produced for the Windscale Vitrification Plant in 1994 included a detailed and comprehensive assessment of fault conditions in the plant using HAZOP and Probabilistic Risk Assessment techniques. The Safety Case identified a number of major hazards. These major hazards, along with the protective measures, Operating Rules, and Safety Mechanisms designed to prevent these hazards or to mitigate them are briefly described below. 

---