Conditional active redundancy

Conditional active redundancy involves a voting system and is used in applications such as digital/analog data processing when there is no simple way of identifying a failure. 

The MSF model (NUREG/CR-3837) is used principally to determine the level of dependence between safety systems introduced by maintenance, testing, and calibration activities. It is a mathematical model which modifies the independent failure probability of any single component by considering that a component with which it is redundant has already failed. This allows the conditional failure probabilities of redundant components to be calculated to determine the overall system failure probability. Documentation requirements are given in Table 4.5-6. 

Performance on test set 2 is shown in Table X. The A s and C s are combined in these statistics for a comparison of performance on all the actives. So far the BASIC keys are not as discriminating as the NCI keys. Improvement was expected by the use of conditional probabilities among linear sequences to cut down redundancy, but early results show no such improvement. This is being worked on by Paul Blower of CAS, who is also investigating the use of co-occurrences. 

Proof testing is executed periodically, and can be implemented using several strategies. The strategy establishes how the tests of the redundant components are scheduled with respect to one another. lEC 61508 (1998-2005) defines proof test as a periodic test performed to detect failures in a safety-related systems so that the system can be restored to an as new" condition or as close as practical to this condition". This standard establishes the need of routine maintenance action in order to detect unrevealed failures, being proof test one of these activities. It thus has an important role in the achievement of safety integrity. 

A two-channel scheme with the internal redundancy of active elements such as valves and pumps is provided for the majority of the KLT-20 safety systems. The use of a two-channel scheme of safety systems under specific conditions typical of floating structures (and resulting in the necessity to save room space and equipment weight compared with land-based NPPs) makes it possible to reduce the number of bulky equipment such as tanks and heat exchangers. 

---