Patent Number: 048083690
Section: summary

BACKGROUND OF THE INVENTION This application is a continuation of application Ser. No. 026,842, filed Mar. 17, 1987. This invention relates to an emergency core cooling apparatus, and more particularly to an emergency core cooling apparatus having high-pressure systems and low-pressure systems. A boiling water nuclear power plant is provided with an emergency core cooling apparatus for dealing with a loss of coolant accident. An example of a conventional emergency core cooling apparatus for a boiling water reactor is discussed under "Current Status of Advanced Boiling Water Reactor (ABWR)" in the "Hitachi Review" (December 1984 issue, Vol. 33 - No. 6), pages 299-306. As will be described in detail later, this conventional emergency core cooling apparatus has two systems of high-pressure core spray apparatus (which will hereinafter be referred to as HPCS), one system of high-pressure coolant injection apparatus (which will hereinafter be referred to as HPCI) and three systems of low-pressure flooding apparatus (which will hereinafter be referred to as LPFL). The HPCS's and HPCI are high-pressure emergency core cooling apparatuses, and the LPFL's low-pressure emergency core cooling apparatuses. SUMMARY OF THE INVENTION An object of the present invention is to provide an emergency core cooling apparatus capable of preventing the effective heat generating portion of a core from being exposed no matter what kind of breakage occurs in a pipe for an emergency core cooling apparatus. Another object of the present invention is to provide an emergency core cooling apparatus capable of cooling a core when the nuclear reactor is stopped under normal conditions, by using compact coolers. The first characteristics of the present invention reside in that the elevation of the coolant discharge ports, which are in a reactor vessel, of a plurality of high-pressure emergency core cooling systems, which are adapted to supply a coolant to the interior of a core-surrounding shroud in the reactor vessel, is set higher than that of the coolant discharge ports, which are in the shroud, of a plurality of low-pressure emergency core cooling systems, which are adapted to supply a coolant to the interior of the reactor vessel. Since the positions of the coolant discharge ports, which are in the reactor vessel, of the high-pressure emergency core cooling systems are set higher than those of the coolant discharge ports, which are in the shroud, of the coolant discharge pipes for the lower-pressure emergency core cooling systems, a core cooling operation can be carried out effectively by the high-pressure emergency core cooling systems from a point in time earlier than the time of depressurization of the nuclear reactor in the case where the breakage of a pipe (breakage of a pipe in a low-pressure emergency core cooling system) occurs in a low position, at which the securing of a predetermined water level in the nuclear reactor must be done under severe conditions at the time of occurrence of the breakage of a pipe, in the nuclear reactor. When the breakage of a pipe (breakage of a pipe in a high-pressure emergency core cooling system) occurs in a high position, a large quantity of water resides in the nuclear reactor, and, moreover, the water level in the nuclear reactor rapidly reaches the elevation of the coolant discharge ports of the high-pressure emergency core cooling systems, so that the depressurization rate in the nuclear reactor due to the steam discharge becomes high. Consequently, the occurrence of an increase in the coolant injection rate of the remaining high-pressure core cooling systems and the early starting of the injection of a coolant by the low-pressure core cooling systems can be expected. As a result, a decrease in the water level in the nuclear reactor is suppressed. The second characteristics of the present invention reside in that the requirements, which are other than the requirements constituting the first characteristics mentioned above, are added thereto, the additional requirements consisting of providing the coolant discharge ports, which are in the shroud in the low-pressure emergency core cooling systems, at the portions of the interior of the shroud which are above the core, and providing the cooling means in the low-pressure emergency core cooling systems with a means for supplying the coolant, which is in the portion of the interior of the reactor pressure vessel which is below the core or between the shroud and reactor vessel, to the core when the nuclear reactor is stopped under normal conditions. According to such second characteristics, the low-pressure emergency core cooling systems are furnished with the functions of supplying a coolant to the core, and the coolant in the reactor vessel is supplied from the portion of the interior of the reactor vessel which is on the outer side of the shroud or the lower portion of the interior thereof to the cooling means in the low-pressure emergency core cooling systems. Accordingly, when the nuclear reactor is stopped under normal conditions, the coolant cooled by the cooling means passes through the core necessarily, so that the core can be cooled reliably. Since the high-temperature coolant, which has passed through the core, is necessarily supplied to the cooling means in the low-pressure emergency core cooling systems, the operation efficiency of these cooling means can be improved, and the capacity of these cooling means can be reduced.