Safety case regime assessment

The safety case regime of the United Kingdom and Australia is lypieal of the midpoint of the dimension whereby a process (risk assessment) is specified by which the goal of risk control is to be met, and acceptable methods of risk analysis are prescribed, but it is then left to the regulated company to convince the regulator of the way it is led to control risk by that risk assessment process. Further guidance is in the form of codes which have the status of acceptable translations of the goals, but leave the choice open to the company to comply in other ways. 

These models consider either the thermodynamic or mechanical non-equilibrium between the phases. The number of conservation equations in this case are either four or five. One of the most popular models which considers the mechanical non-equilibrium is the drift flux model. If thermal non-equilibrium between the phases is considered, constitutive laws for interfacial area and evaporation/condensation at the interface must be included. In this case, the number of conservation equations is five, and if thermodynamic equilibrium is assumed the number of equations can be four. Well-assessed models for drift velocity and distribution parameter depending on the flow regimes are required for this model in addition to the heat transfer and pressure drop relationships. The main advantage of the drift flux model is that it simplifies the numerical computation of the momentum equation in comparison to the multi-fluid models. Computer codes based on the four or five equation models are still used for safety and accident analyses in many countries. These models are also found to be useful in the analysis of the stability behaviour of BWRs belonging to both forced and natural circulation type. 

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