Large-break design basis accident

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. 

Large pipes are eliminated by design, thus eliminating the large-break loss of coolant accident (LOCA) design basis accident. 

The low-pressure scenario which is initiated by a large-sized break in the primary circuit, that is an event similar to that of the loss-of-coolant design basis accident described in Section 6.2.1.. In the severe accident scenario it is additionally postulated that, after the action of the accumulators and the borated water storage tanks, the sump water recirculation pumps will fail to operate. Thus, the decay heat cannot be removed from the reactor core vnth the consequence that the water volume present inside the reactor pressure vessel (RPV) begins to boil off at about atmospheric pressure. The AB sequence of WASH-1400 describes such a large-break scenario. In this low-pressure scenario, the treatment of fission product behavior inside the primary circuit is comparatively simple the probability of occurrence of such an accident, however, is extremely small. 

The steel containment will not leak during any Design Basis accident and following a large break LOCA, and there should be no need to vent the containment post accident. 

While the large break LOCA presents the most significant design-basis accident pressure challenge for containment designers, there are other types of loads that must be considered in the design. These loads include ... 

The design basis accidents for the ECS are a loss of coolant accident from a double-ended guillotine break (DEGB) of a large process water line (D20) and a loss of pumping accident from a cooling water line break. Section 6.2.3.3.1.1 of the Safety Analysis Report (SAR) states that the ECS addition rate for a... 

Thus, the China syndrome led to a shift in emphasis from containment to prevention. As time passed, accident initiators other than the traditional large pipe break were identified as potentially leading to core melt. In particular, scenarios involving anticipated transients without scram, station blackout, other transients, and containment bypass would be evaluated, and regulated to reduce the probability of core meltdown. However, over the next decade, the emphasis was on the traditional design-basis... 

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... 

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