Nuclear fission
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- 11.4CrosS connection between different units
11.3Multiple initiatiorsIn the multi-unit context, an external hazard can induce initiating events which can impact only one unit (single unit IE) or more than one unit (multi-unit IE). Most of the external hazards may affect more than one unit, but not always a multi-unit IE is induced, due to inherent existing differences between the site units (position, design). Between the causes that could induce multiple IE, the following should be analysed [58]:
A shared system means multiple initiation points for the sequence of events, but the interactions can affect the entire sequence (not just the initiator). The unavailability of site shared fire protection could occur, along with the ventilations vulnerability in case of external explosion.
According to their impact on the multi-unit context, the hazards could be categorized in two large classes:
The external fires could have the capacity to affect multiple units when occurring. Aircraft crash and external explosions can affect multiple units under specific conditions, like in the case of proximity dependencies. Some hazards affect multiple units simultaneously, but we have also cases when an accident that initially impact only one unit can cascade or propagate to others on the site. The analysis need to consider whether the external hazard affects all the units of a site, and in case of a positive answer, if the magnitude of the hazard varies with the units. From the IE grouping point of view, events which affect one unit or multiple units should not be grouped together. Events which may be propagated from other units (including cascading events) (directly or via shared systems - as missile from one unit affecting the other units, fire spreading) should be considered, so given the unit IE, the conditional probability to face a multiple unit IE needs to be estimated. The existing Level 1 PSAs for multi-unit stations are mostly developed on a single unit basis, where one unit is selected as the representative reference model unit. However, in some cases, the initiating events that can affect the selected model unit include events that originate outside of the selected reference model unit [58], [59]. 11.4CrosS connection between different unitsFor sites with multiple units, their appropriate independence should be ensured. The possibility of one unit supporting another could be considered as far as this is not detrimental for safety [60] [61]. One of the post-Fukushima requirements was related to the use of alternative systems and cross-connections between units [19]. Sharing systems between units is influenced by four factors: safety, operability, costs, and licensing considerations. There are different types of sharing, as follows [62]:
A similar case is when full capacity, independent systems at each unit share standby or spare equipment (an installed spare pump that is shared between units). Even if the actual issue was to reduce costs, while maintaining the safety, many plants have found that providing additional capabilities to cross-tie and back-up systems can be beneficial, particularly for systems like electric power (capability to cross-tie ac and/or dc power supplies) and cooling systems. For example, crediting the use of a cross-connection between the units’ service water systems (additional protection when the service water cooling was lost at one unit) resulted in a 25% reduction in the CDF for each unit; provision of a cross-tie capability between the 4kV electrical safety buses at two units resulted in a 35% reduction in CDF [62]. In Japan “cross-tie of electric power supplies” among units at the multi-unit sites is taken into account in the internal events PSA because the cross-tie is one of the accident management measures. Based on the above mentioned, cross-connection of EDGs between units could be beneficial and worthy to be examined for a multi-unit site (as an accident management measure). A system could be shared:
Within a multi-unit, many systems, functions, and physical facilities are shared, including the control room, fuel handling system, fuel pool cooling and clean-up system, pump houses, radioactive waste treatment systems, fire protection system, potable and sanitary water system and switchyard. In case of systems sharing, there is the potential for inadequate operations when they are shared (the potential to have an inadequate number of available components or inadequate flow rates when are shared) [62]. Directory: wp-content -> uploads -> 2016 2016 -> 2017 afoCo Landmark Scholarship Program 2016 -> Instructions for the Preparation of Camera-Ready Contributions to the Conference Proceedings 2016 -> Step student Scholarships for Year 10-13 2016 -> The united church of canada 2016 -> Idan raichel biography – May 2016 Brief Producer, keyboardist, Lyricist, composer and Performer Idan Raichel 2016 -> An introduction to centre’s interventions expanding access to justice 2016 -> Sanchar Shakti 2016 -> Unit analysis should help here. We want number of bananas 2016 -> The Autobiography of Elder Joseph Bates 2016 -> The Great Heat and the Rhode Island Deep Water Bay Scallop (Argopecten irradians) Fishery 1875-1905 Download 1 Mb. Share with your friends:
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