Key Safety Functions

The USA/GE advanced liquid metal cooled reactor (ALMR) was designed to provide high reliability for the key safety functions, including reactor shutdown, heat removal, and containment. These functions can be achieved by passive means (thermal expansion, temperature effects on neutron absorption, natural circulation of the sodium coolant, and natural air circulation). 

The Windscale Vitrification Plant vitrifies high level (highly active) liquid waste arising from reprocessing operations at Sellafield. The plant operates two identical vitrification lines with a current combined throughput of 350 product containers per year. A third line is currently under construction and will commence operation in the year 2000. The key safety function of the plant is to convert mobile material into a solid immobile form which can be more easily managed, stored, and transported. 

The Key Safety Functions (KSFs) are high level safety functions that fiandamentally need to be maintained to provide assurance of nuclear and radiological safety. If the Key Safety Functions can be maintained, the plant will be acceptably safe. 

The Key Safety Functions are open to a degree of choice different selections reflect the aims and ethos of the operating organisation. Typically, only a few Key Safety Functions are required to make sure that nuclear safety is maintained. The following Key Safety Functions are identified for the APIOOO ... 

Each Key Safety Function can be broken down into subordinate safety functions this process identifies which plant SSCs provide are contributing to the delivery of particular Key Safety Functions. This process has been used to inform the design requirements on plant SSCs to maintain safety. 

However, it is UK nuclear safety case practice to consider dose to the operator separately from dose to the public. It can be seen that the third of the IAEA s fundamental safety fiuictions can easily be decomposed into third and fourth of the Key Safety Functions proposed fertile APIOOO. 

The technical scope of the safety case is effectively defined by the Key Safety Functions presented in the previous sub-section. Consideration is given, within the nuclear safety case, to faults, operator errors and hazards that could challenge the Key Safety Functions. 

The plant Key Safety Functions can be challenged by internal and external hazards therefore it is necessary to identify the claims on SSCs to withstand these hazards or respond such that the KSFs are protected. 

Each internal hazard has been reviewed separately in the Internal Hazards Topic Report (Reference 4.3). The Internal Hazards Topic Report reviews each internal hazard to identify the requirements from the SSCs that ensure that the delivery of the key safety functions (KSFs) is not compromised. In the topic report these are termed safety fimctional requirements (SFRs) and are summarised against each internal hazard. The Internal Hazards Topic Report provides the evidence that these SFRs can be substantiated. 

Environmental qualification of the safety significant SSCs within the Containment/Shield Building ensure that key safety function delivery will not be affected by extreme ambient temperature. 

The fault schedule (Reference 5.1) is a key document in the DBA. The fault schedule presents the postulated faults, and the identified protection for each of the Key Safety Functions (KSFs). The fault schedule describes the response of the APIOOO to initiating events in the following categories ... 

The consolidated categories for initiating events reflect their potential to affect delivery of the Key Safety Functions spurious reactor trip, increase in heat from the primary systems, decrease in heat removal by the secondary system, decrease in reactor coolant flow rate, reactivity and power distribution anomalies, increase in reactor coolant inventory, decrease in reactor coolant inventory and anticipated transients without scram. 

The following pressuriser design requirements ensure control over the key safety functions during normal operation ... 

The Internal Hazards Topic Report (Reference 7.1) identifies safety functional requirements that represent the design requirements on systems, structures and components (SSCs) that enable the key safety functions to be maintained or their loss to be protected against when challenged by internal hazards. 

The Level 3 features are the safety measures that intervene to protect against loss of a key safety function. On the APIOOO, the following SSCs are in this category ... 

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