Patent Number: 050911430
Section: claims

1. A natural circulation reactor providing natural circulation of coolant through a core without an external circulation force during normal operation of the reactor, comprising a reactor pressure vessel having the core housed therein and a reactor container having a wetwell and a drywell, said pressure vessel being located in a portion of the drywell centered in the wetwell, wherein the core is disposed at a position below a predetermined level of coolant in said pressure vessel so that a top portion of the core is maintained to be submerged below the coolant even in the event that any pipe connected to said pressure vessel is broken and a coolant level in said reactor pressure vessel is then lowered to the predetermined level by flushing as a result of pipe breakage. 2. A natural circulation reactor according to claim 1, wherein said pressure vessel is metallic, the top portion of said core being arranged at a position lower than passage holes between said portion of the drywell and vent pipes so that a level of the coolant outside of said pressure vessel is higher than the top portion of said core, and heat exchange between inside and outside of said pressure vessel at a level corresponding to that of the position of said core is performed between inner coolant and outer coolant of said pressure vessel through said metallic pressure vessel due to transmission of heat between the inner coolant and the outer coolant so that said core is cooled efficiently from outside of said pressure vessel. 3. A natural circulation reactor according to claim 2, further including at least one coolant injection system for preventing an accident of decrease in a level of the coolant due to breakage of a pipe, said system having a volume of coolant sufficient to maintain coolant in both the wetwell and a lower portion of the drywell at a level of the passage holes in the vent pipes so as to enable cooling of said core in said pressure vessel. 4. A natural circulation reactor according to claim 1, wherein the predetermined level of coolant in said pressure vessel is a minimized level of coolant when the level of coolant in said pressure vessel is lower by flushing due to breakage of at least one pipe connected to said pressure vessel. 5. A natural circulation reactor for enabling natural circulation of coolant through a core without an external circulation force during a normal operation of the reactor, comprising a reactor container having a wetwell and a drywell and a reactor pressure vessel having the core housed therein, the pressure vessel being located in a portion of the drywell centered in the wetwell and being disposed and configured so as to maintain a level of coolant in the pressure vessel above a top portion of the core even when a level of coolant in the pressure vessel is lowered by flushing due to breakage of at least one pipe connected to the pressure vessel such that the top portion of the core remains submerged beneath the level of coolant in the pressure vessel when the coolant level is lowered by flushing. 6. A natural circulation reactor according to claim 5, wherein the pressure vessel has a metallic wall, the top portion of said core being disposed at a position within the pressure vessel lower than passage holes between the portion of the drywell and vent pipes so that a level of the coolant outside of the pressure vessel is higher than the top portion of the core, the metallic wall of the pressure vessel together with the coolant inside and outside of the pressure vessel at a level corresponding to that of the position of the core enabling heat exchange between the coolant inside and outside of the pressure vessel through the metallic wall, thereby effecting cooling of the core from the outside of the pressure vessel. 7. A natural circulation reactor according to claim 6, further comprising at least one emergency system for being activated in response to an abnormal operation including breakage of a pipe and including at least one coolant injection means for preventing an accident of decrease in a level of coolant within the pressure vessel due to breakage of the pipe, the coolant injection means having a volume of coolant sufficient to maintain coolant in both the wetwell and a lower portion of the drywell at a level of the passage holes in the vent pipes so as to enable cooling of the core in the pressure vessel.