Nuclear fission



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10.2Special provisions


There are additional means that can be undertaken in order to be better prepared in case of emergency caused by external fire, explosion or aircraft crash. It concerns the following issues:

  1. Emergency preparedness and response should also take into account accidents induced by external events; in particular the procedures and means should be predicted at the early phase of the development of emergency situation. In this respect layer of protection analysis (LOPA) is one of the possible techniques for risk assessment and optimization of emergency response.

  2. Proper localization of standard emergency equipment should be carefully analysed in order to minimize the possibility of the occurrence of common cause failures and further development of emergency situation.

  3. Early detection of external fire may be a key factor for successful response. Therefore fire monitoring and protection systems have to be designed, built and maintained in such a way that detection of fire should be as fast as possible. For existing NPP appropriate improvements of existing systems can be made after careful inspection.

  4. As external fires and explosions can be mostly caused by transportation accidents the risk coming from such events can be minimized by undertaking decisions on the prohibition of transport of dangerous materials in the vicinity of NPP.



10.3Preventive measures


Based on the results of the hazards scenarios analyses and the evaluation of emergency response actions the preventive measures may be identified and implemented as deemed practical. This is especially important for the scenarios for which emergency actions are insufficient or cannot be implemented in timely manner to reduce the consequences below the acceptable level. Some examples of preventive measures are listed below [57]:

  • optimization of the mobile equipment location and storage protection features;

  • arrangement of additional passageways to the plant site in order to reduce arrival time of mobile equipment;

  • preventive arrival and set up of mobile equipment (in the case of slow progressing external hazards);

  • reduction of combustible materials adjacent to or on the nuclear site (see ch.2.37 of NSG1.5 [29]), arrangement of exclusion zones in close proximity to the plant and along the electrical transmitting lines to prevent external fires propagation;

  • isolation of the air intake of the main control room in the event of toxic clouds (see ch.3.18 of NSG1.5 [29]);

  • reinforcing the elements and structures that can cause seismic induced fires or block important access paths due to local structural collapse.


11SOLUTION TO MODEL MULTI-UNIT FOR MAN-MADE HAZARDS AND AIRCRAFT CRASH PSA

11.1Accident sequences


Man-made hazards and aircraft crash accidental events can simultaneously affect all the units at a site: this requires appropriate interface arrangements to deal with as well as with the potential domino effects (as explosions resulting in pressure or shock wave propagating from one unit to another). These site initiating events create the potential for similar accident sequences due to the failure of common or shared mitigation systems as well as the potential for common cause failure of identical components across units or inter-unit common cause failures. In addition, a single-unit event can trigger a cascade sequence to impact the other units: for units with shared or connected structures, internal fires, for instance consequential to an aircraft crash, can propagate from the first-impacted unit to affect the second unit.

11.2CCF


The first step within the approach to assessing site integrated risk consists in the identification of the interactions between the units because of specific design features, operating practices, safety features and culture, economic considerations and construction layout: the multi-unit dependencies must be identified, accounted for and modelled within the PRA model of the site. These include principally the common elements shared by units in the site, including:

  • common physical location (that is, single site or regional site),

  • common or shared systems; examples of common or shared systems include e.g. switchyard, fire protection pumps/tanks, ultimate heat sink, where the risk issue is related to system failure impacting all the units and the system resources directed to one unit, not available to the second unit for instance,

  • proximity dependencies, where a common environment has the potential to affect multiple units: these apply to common or connected structures (like turbine building, auxiliary building, main control room); if there was an explosion with consequences on the site and two units were located very close together, the same explosion could affect both units,

  • human and organizational dependencies, addressing shared staff resources, like the shared operators and FLEX equipment, whose action can be challenged by the event occurrence,

  • unit interconnections in the form of cross-tie systems and swing equipment, such as emergency diesel generators,

  • identical components (that is components with same design and operation) with the potential of cross-unit common cause failures.

These dependencies are to be modelled somehow in the site PRA framework. In order to accomplish the task, for instance, the dependencies of all front-line systems are to be defined in a dependency matrix. This approach is already typically performed for single-unit PRAs and includes only hard physical connections, such as a motor-operated valve needing to have power from a predefined source. These matrices allow the PRA model developer to know what to consider when creating the system fault tree. Using the base PRA, the initiating events, shared connections, and identical components would be developed. As regards human and organizational dependencies, one can resort to human reliability analysis.




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