Design base accident

This is a design-based accident that could lead to the largest off-site release of activity and was originally referred to as the maximum credible accident. 

The second subsystem includes three motor-driven pumps of 10 m /h capacity and of 3.0 MPa pressure head. Its water inventory is 50 m. Each system has two channels. A special system for recirculation of water from the condensate collectors back into the reactor is provided using glandless motor pumps. A special structure excludes loss of coolant and ensures long-term passive heat removal from the core and reactor vessel during beyond design base accidents. 

To achieve competitive construction costs with a LWR, rationalization of the intermediate heat transport system is required. Considering the direct effect of a sodium-water reaction on the core, even if it is BDBA (Beyond Design Base Accident), it is difficult to secure general acceptance of eliminating the secondary heat transport system at an early stage of FBR practical application. 

BEYOND DESIGN BASES ACCIDENTS IN SPENT FUEL POOLS... 

The heat released in the containment vessel during the accident is absorbed in the suppression pool for one day further heat removal is possible over a longer period by evaporation of the water in the outside pool of the containment. This system gives sufficient time margins to cool the core for severe accidents as well as design base accidents. 

The capability of operator and system mitigative response to the design bases accident shall be ascertained to be within that assumed in the analysis based on a best estimate scenario for the accident, both with and without the worst single active component failure. WSRC shall ensure that all power-limits related systems will function in a predicted and uniform manner, (i.e., one system will not interfere with the function of another). 

The PSAR of the CEFR was finished in May of 1998, but the review lasted 2 years. The primary pipe rupture accident is the key accident and the most important topic in this review. The owners (us) think that the 0.25DT (D is the diameter of the pipe and T is the thickness of the pipe) size break initial event should be a DBA (Design Base Accident), but the double-ended guillotine break instantaneous should be a BDBA (Beyond DBA). However, the reviewer was of the opinion that the double-ended instantaneous guillotine break should be DBA. The owner should calculate the break position spectrum in order to find the most serious case. 

In design it is typical that the same mistakes are done again since the use of available information is not organized. The use of case-based reasoning enhances the reuse of available experience, which reduces the possibility that the same errors are done more than once. In this work CBR was used for the evaluation of the inherent safety of process structure. The casebase was collected from design standards, accident documents and good engineering practice. 

As an example, it s common in safety literature to designate as incidents or near-misses the events that do not result in harm or damage, and to designate as accidents those that do. Differentiation is based entirely on outcomes. 

A formal hazard analysis of the anticipated operations was conducted using Preliminary Hazard Assessment (PHA) and Failure Modes and Effects Analysis (FMEA) techniques to evaluate potential hazards associated with processing operations, waste handling and storage, quality control activities, and maintenance. This process included the identification of various features to control or mitigate the identified hazards. Based on the hazard analysis, a more limited set of accident scenarios was selected for quantitative evaiuation, which bound the risks to the public. These scenarios included radioactive material spills and fires and considered the effects of equipment failure, human error, and the potential effects of natural phenomena and other external events. The hazard analysis process led to the selection of eight design basis accidents (DBA s), which are summarized in Table E.4-1. 

Based on the results of the hazard analysis, the accidents posing the greatest risk involve direct exposure to radiation (workers) and airborne releases of radioactive material (the public). Accident analysis entails the formal quantification of a limited subset of accidents as design basis accidents (DBAs). These accidents are to represent a complete set of bounding conditions as noted in DOE 5480.23 (DOE 1994a). Further, as stated in DOE-STD-3009-94,... 

Based on a review of the hazard tables prepared for this SAR, the accidents identified as design basis accidents for more detailed quantitative evaluation in this SAR include ... 

One of the fundamental basic rules of the EOPs is to not contradict nor violate the design bases of the plant. The loop isolation using the loop isolation valves has not been considered in the Paks accident analyses nor in the Final Safety Report, and as such, is not documented as a plant design basis. There were investigated some LOCA cases - medium and small sizes -with MLIV closure (see analysis No. 1. in Appendix 4). After MLIV closure on the broken loop RCS was filled up by 3 HPIS injection and there is less than 5 minutes to avoid the opening of PRZR relief valve. In conclusion, it was decided not to close MLIV if any of the HPIS pumps are in operation. 

The plant conditions considered in the design basis analysis include anticipated operational occurrences and design basis accidents (DBAs). The division is based on the frequency of the occurrence. 

The initiation and operation of all special safety systems, if required, is fully automatic, based on diverse and redundant measurements. For example, two independent and diverse reactor trip (shutdown) signals are provided for each of the shutdown systems for every design basis accident requiring reactor shutdown. 

This dociunent describes a representative radionuclide source term to the containments of reactors for use in the traditional design basis accident analysis. The source term is based on the more mechanistic studies of radionuclide behaviour conducted since the accident at Three Mile Island. Although the source is representative of the magnitude and timing of severe accident source terms, it is not intended to represent any specific severe accident sequence nor is it intended to be a boimding source term. 

The containment isolation system (CIS) provides the means of isolating the various fluid systems passing through the containment walls as required to prevent the release of radioactivity to the outside environment. Design bases of the CIS consider several factors. Subsequent to an accident which may release radionuclides within the containment, there must be a barrier in all pipes or ducts that penetrate the containment. Leakage from the containment through these pipes or ducts which penetrate the containment is minimized by a double barrier. This double barrier ensures that failure of a single active... 

Either event based or symptom based procedures shall be developed for abnormal conditions and design basis accidents. Emergency operating procedures or guidance for managing severe accidents (beyond the design basis) shall be developed . ... 

Scenario dependent procedure is a general term apphcable to any event based (i.e. event specific) procedure. These procedures are developed and optimized to respond to a specific event or category of events, as would typically be the case for design basis accidents. It should be noted that the event specificity applies to individual procedures within the EOF package. 

Analysis of the design basis and beyond design basis accidents for NPPs based on a floating power unit with the KLT-20 reactor is being performed using a set of calculation codes developed by OKBM and proven in calculations of stationary and transient modes of ice-breaker reactor operation. 

Such an accident is never considered for the reactors with cores based on rod-type fuel elements, as core cooling with an acceptable temperature of zirconium claddings cannot be provided in this case. For VKR-MT, the rupture of reactor vessel bottom is considered as a beyond design basis accident. 

For anticipated operational occmrences and accident conditions, the OPs should provide instructions for the recovery. For design basis accidents (DBAs), these procedures to keep the plant state within specified limits may be event based or symptom based. For beyond design basis conditions, the instractions will be symptom based that is, they will use parameters indicating the plant state to identify optimum recovery routes for the operator without the need for accident diagnosis. 

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