The Design-Basis LOCA

CFR 100 stipulates that neither an individual located at any point on the outer boundary of the exclusion area for two hours immediately following onset of the postulated fission product release nor an individual located at any point on the outer boundary of the low population zone for the duration of the accident should receive a total radiation dose in excess of 25 rem to the whole body or 300 rem to the thyroid. Thus, the design-basis LOCA, whose consequences were not to be exceeded by any other credible accident, became the focus of siting evaluations. 10 CFR 100 also stipulates that the... 

One of the most significant barriers to the release of fission products to the environment from a postulated loss-of-coolant accident is the containment building. This structure is designed to have a very low leakage rate even given the peak internal pressure that would result from the design-basis LOCA. This peak pressure would rapidly decrease as heat was absorbed by the internal structures nd lost by conduction to the... 

A small power reactor would have a separate containment strocture acting as an additional, leak-tight barrier to the release of fission products to die environment and capable of withstanding the excess pressure that would develop during a design-basis LOCA event in the primary system. 

Luxat, J.C. 2003. Historical (perspective of the design basis in Canada. In Proceedings of the OECD/ CSNI/CNRA Workshop on Redefining Large Break LOCA, June 2, Ziuich, Switzerland. 

Passive safety characteristics allow the SCOR to respond safely to all incidents within the design basis with few operator actions. Except LOCA, where low electric power is needed at... 

This maximum credible accident has, at various times, also been referred to as the design basis accident (DBA), the design-basis loss of coolant accident (LOCA), and the siting-basis LOCA. Given the rather prescriptive assumptions that evolved for demonstrating compliance with 10 CFR 100, the term design-basis LOCA is adopted here. This hypothetical accident is invariably initiated by the reactor-coolant system pipe break that would yield the highest containment pressure. 

Many of the action items addressed small-break and transient initiated accidents. Their significance had previously been identified by WASH-1400 and its reviews. Traditionally, historical attention had been on the design-basis large break LOCA. The emphasis on small breaks and transients was immensely affected by the TMI-2 accident. Many procedural, software, and hardware modifications were implemented... 

Note that the decay heat generated in this time interval is significant, amounting to almost one third of the energy added to the containment atmosphere. As indicated in Table 2.1-3, decay heat is conservatively (usually 20%) above best estimate values in such design-basis LOCA calculations. Of course, this is also conservative with respect to the calculated peak clad temperature. 

LOCAs may be further subdivided into large, intermediate, small, and very small depending on the injection systems required to successfully respond to the LOCA. Transients initiators are usually events related to the balance of plant (BOP). Some typical transient initiators are listed for BWRs and PWRs in Tables 2.2-1 and 2.2-2. These transients are explicitly considered in probabilistic risk assessments, as discussed in Section 2.6. Note that these initiators are somewhat more specific than the design-basis initiators presented in Table 2.1-2 and include more events, although there is some overlap in the respective lists. 

WSRC-RP-89-1287, "Flow Instability Analysis of a Design Basis LOCA for the K-14.1 Subcycle" (U), December 1989. 

The enveloping nature of the design basis event relative to other events will be confirmed. The design basis event shall be a double-ended guillotine break (DEGB) loss-of-coolant accident (LOCA), unless an alternative basis is demonstrated to be... 

Electric wires and cables used in nuclear power plant are exposed by low dose rate irradiation during the life time of the plant. In addition, loss of coolant accident (LOCA) that is a design basis accident brings about the degradation by heat and radiation on the electric wire and cables. The dose varies from plant to plant. IEEE std.323-1974 (/) estimated the dose 0.5 MGy for the period of the life time and 1.5 MGy for LOCA as one of the example. The standard estimated 40years and one year, for the lifetime of plant and duration of LOCA respectively. 

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 PCCS maintains the containment within its pressure limits for design basis accidents such as a LOCA. The system is passive with no components that move. The PCCS consists of three low pressure totally independent loops, each containing a steam condenser (passive... 

The analysis of all design basis accidents has been performed. The analysis shows that the consequences of all design basis accidents are well below NRC limits. The peak clad temperature following a large-break (LOCA) is about 1080°C (< NRC limit 1204°C). For small LOCA, the peak clad temperature is about 530°C, due to the large capacity of the high head injection system. 

Unresolved Safety Issue (USI) A-02 in NUREG-0933 (Reference 1), addresses asymmetric blowdown loads imposed on the reactor vessel (RV) as a result of a design basis loss of coolant accident (LOCA). The resultant forces from these loads could affect reactor vessel support integrity, thus jeopardizing plant safety. 

In the System 80+ Standard Design, Engineered Safety Features (ESF) are incorporated to mitigate design basis events (DBS s), including a loss of coolant accident (LOCA). 

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