Patent Number: 049922319
Section: summary

The present invention relates to an emergency core cooling system for a nuclear reactor which operates upon occurrence of an accident. An emergency core cooling system installed on a boiling water reactor has a large number of spray nozzles mounted on ring an annular headers. The ring headers are disposed at upper outer circumferential positions of a reactor core composed of a large number of fuel channels. Upon occurrence of a loss-of-coolant accident, emergency cooling water which is driven by an emergency core cooling system pump is conducted to the headers and sprayed onto the fuel channels through the nozzles. For safety purposes, a multiple protection arrangement is provided for the nuclear reactor wherein two, i.e., upper and lower stages of ring headers are disposed on the upper outer circumferences of the core and constructed so that the core is sufficiently cooled even when one of the two stages becomes inoperative. Additionally, a system for injecting water directly into the core is also provided. Generally, mounting angles of the nozzles mounted on the lower header stage are somewhat upward as compared with those mounted on the upper header stage. However, the nozzles mounted on both the lower and upper header stages extend downwardly with respect to the horizontal axes thereof. Here the "mounting angle o the nozzle" signifies the direction in which the center axis of the outlet hole of the nozzle extends. As stated above, the headers are arranged at the outer circumferential positions of the core so as to surround the core. A large number of spray nozzles are mounted on the upper and lower header stages in a manner so as to face the center of the core and the emergency cooling water is sprayed from the nozzles in a distribution pattern as discussed hereafter. The fuel channels arrayed at the peripheral part of the core are distributively supplied with the emergency cooling water by the contributions of only several adjacent spray nozzles, whereas the fuel channels at the central part of the core are supplied with quantities of emergency cooling water by the superposed contributions of substantially all of the spray nozzles. Accordingly, the prior arrangements provides for the mounting angles .theta. of the spray nozzles to be in a downward direction with respect to the horizontal axes as described before, in order that the spray water from each spray nozzle may be distributed more in a region nearer to the nozzle (in the peripheral part of the core) and less in a region far from the nozzle (in the central part of the core). When a loss-of-coolant accident occurs the emergency cooling water is sprayed into a stream atmosphere at the upper portion of the reactor through the spray nozzles. An experiment was accordingly conducted in which cooling water was sprayed into a steam atmosphere by the use of one spray nozzle. As a result, it has been revealed that the water distribution phenomena differ in a steam environment from that distribution considered to be provided by prior arrangements. That is, the emergency cooling water sprayed from the nozzle is usually at a low temperature and in a subcooling temperature state. Therefore, when the emergency cooling water is sprayed into the steam atmosphere, the water exchanges heat with the steam to condense the steam. The condensation takes place for the steam in both an inner space and outer space of a spray water annulus. It has been determined that in a case where the steam is condensed in the outer space of the spray water annulus, a pressure decrease due to the condensation is small because the space has a large volume. In contrast, in the inner space of the spray water annulus, a pressure decrease due to the steam condensation becomes large for the reasons that the inner space has a small volume and that the space is surrounded with the water screen of the cooling water or a large number of water drops and is supplied with a small amount of steam from the environment. Therefore, the pressure of the internal space becomes lower than that of the outer space. There has been observed the phenomenon of the collapse of the water annulus in which the difference of the pressures squeezes the spray water annulus toward the inner space. Thus, it has been revealed that the path length of the spray water shortens due to the collapsing phenomenon of the spray jet. An object of the present invention is to provide an emergency core cooling system for a nuclear reactor which is well-suited for use in steam environment which is the actual operating atmosphere of emergency core cooling systems. Another object of the present invention is to provide an emergency core cooling system for a nuclear reactor wherein at least a portion of the nozzles of at least one header stage are mounted with an upwardly directed angle with respect to the horizontal axis for providing an improved water distribution in a steam environment. In accordance with the present invention, an emergency core cooling system for a nuclear reactor includes at least one header stage disposed on an upper outer circumference of the reactor core, a plurality of nozzles mounted on the header stage, at least a portion of the nozzles having center axes mounted so as to extend in upward direction with respect to the horizontal axis and an arrangement for conducting cooling water to the header stage whereby an improved water distribution is obtained at the time of a loss-of-coolant accident. According to a feature of the present invention at least some of the spray nozzles installed on a lower header stage are mounted upwardly with respect to the horizontal axis so as to distribute into the central area of the core the spray water in an amount necessary for the cooling of the core, and the remaining spray nozzles are mounted so as to distribute the spray water into the peripheral area of the core whereby an optimum peformance of the emergency core cooling system is exhibited.