The reactor vessel constitutes the main part of pressure boundary of primarycoolant.
反应堆压力壳构成了主冷却剂压力边界的主体。
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When you have an event, the reactor is operating at full power, you blowdown the primarycoolant which is hot, you start injecting emergency core cooling water which is cold.
We also need to cool the reactor and the spent fuel in the spent fuel pool but that basically means you wanna maintain the coolant inventory which means water in the primary system.