Patent Number: 050911430
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a natural circulation reactor and, more particularly, to a natural circulation reactor suitable for maintaining a reactor core submerged under coolant even on the supposition of breakage of any pipe connected to a pressure vessel of the reactor. As described in "Annual Meeting of 1987", Japanese Nuclear Society, Corp., (at Nagoya University, Apr. 1-3, 1987), E44, Conceptual Study of Natural Circulation BWR - (1) Plant Outline, a conventional natural circulation reactor has been designed to inject light water from a tank of an accumulated coolant injection system on the supposition of breakage of any pipe connected to a pressure vessel of the reactor. SUMMARY OF THE INVENTION On the supposition of breakage of any pipe connected to the reactor pressure vessel in the aforementioned reactor of prior art, a coolant, i.e., light water, within the reactor pressure vessel would be flushed in response to abrupt decompression caused by blowdown upon breakage of any pipe of large diameter, or in response to abrupt decompression caused by startup of an automatic decompressing system (ADS) upon breakage of any pipe of small diameter, whereby a large amount of coolant is discharged from the reactor pressure vessel. Accordingly, there is a possibility of exposing a top portion of the reactor core temporarily during an intermediate period before actuation of the accumulated coolant injection system to start injecting of a coolant into the reactor core, because the coolant level in the reactor is lowered as voids fail to occur after the completion of flushing. Further, in long-term cooling situations when a residual heat removal system is actuated in a core cooling mode to inject a coolant within a pressure suppressing chamber into the reactor core after the reactor has been decompressed completely, the coolant level in the reactor is elevated with the coolant injected, and the coolant is flown out from the broken part. At this time, there is a possibility that a portion of the coolant may be flown out from the broken part directly as it remains at a low temperature without acting to cool the reactor core, and hence the decay heat generated from the reactor core may not be removed efficiently. Accordingly, the prior art has been required to set a flow rate of the residual heat removal system and the capacity of a heat exchanger as to leave a sufficient allowance. It is an object of the present invention to provide a natural circulation reactor which can maintain a core in a reactor pressure vessel submerged under coolant even on the supposition of breakage of any pipe connected to the reactor pressure vessel. To achieve the above object, the present invention provides a natural circulation reactor having a reactor pressure vessel with a core housed therein, the core being disposed in such a location that a top portion of the core is submerged under coolant even in the event that any pipe connected to said reactor pressure vessel is broken and then a coolant level in the reactor pressure vessel is lowered due to flushing. In accordance with the present invention as arranged above, the reactor core will be submerged under coolant and hence will never be exposed even during an intermediate period before actuation of the accumulated coolant injection system subsequent to flushing, although the coolant present in the reactor core would be flushed and lost from the reactor pressure vessel in response to abrupt decompression caused by blowdown through the broken part of any pipe or by start-up of an automatic decompressing system (ADS), thereby lowering a coolant level in the reactor after the completion of flushing, on the supposition of breakage of any pipe connected to the reactor pressure vessel. Furthermore, after the completion of flushing, the accumulated coolant injection system is actuated to inject a coolant from an accumulated coolant injection tank, and then a residual heat removal system starts its operation to inject a coolant in a pressure suppressing chamber. The coolant thus injected into the reactor pressure vessel is flown out of the reactor pressure vessel while keeping the reactor core submerged under coolant. The outflowing coolant is transferred to a lower drywell surrounding a lower portion of the reactor pressure vessel for filling the lower drywell with the coolant. Herein, the reactor core is disposed below a level of passage holes of the lower drywell, while as the amount of the coolant filled in the lower drywell is increased, the resulting level of the coolant is elevated beyond a position of the reactor core. Afterward, the coolant is flown out through the passage holes into the pressure suppressing chamber. Accordingly, although a portion of the coolant injected into the reactor pressure vessel is flown out of the reactor pressure vessel from the broken part directly as it remains at a low temperature without removing any decay heat of the reactor core, the coolant at a low temperature is pooled in the lower drywell and utilized to cool the wall surface of the reactor pressure vessel from its outside, thereby in turn cooling the reactor core. Thus, the decay heat generated from the reactor core is removed reliably and efficiently.