Water cooled reactor

A light water cooled reactor has a structure of injecting spray water from ECCS to the upper plenum upon loss of coolant accidents for cooling the reactor core. However, steams generated by boiling under a reduced pressure in the lower plenum and the reactor core may possively form an uprising stream to prevent the injected cooling water from falling into the reactor core. This is called as CCFL phenomenon. The feature of this invention resides in suppressing the pressure drop caused in a pressure vessel thereby preventing CCFL. That is, it has been noted to temporarily moderate the pressure drop due to the reactor core cooling. If the pressure drop is temporarily moderated to suppress the boiling under reduced pressure only in this period, generation of the uprising steams can be suppressed correspondingly. Accordingly, emergency core cooling water flows into the reactor core thereby contributing to the cooling of the reactor core. (I.S.)