Patent Number: 050193287
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a natural circulation type boiling light-water reactor according to an embodiment of the present invention has a pressure vessel 1 which is divided into a steam/water chamber and a steam chamber. A reactor core 2 provided with a fuel assembly including a plurality of fuel elements is disposed in the steam/water chamber of the pressure vessel 1. A core shroud 3 encircles the reactor core 2. A tubular chimney 4 extends from an upper end portion of the core shroud 3 towards the steam/water chamber. The chimney 4 is filled with light water as coolant. The inside diameter of the chimney 4 is greater than the outside diameter of the core shroud 3. In addition, an end 41 of the chimney 4, that is, a water surface of the coolant, is opened towards a steam dryer assembly 5 which will be described later, and therebetween there exists nothing to restrict the effective open area of the water surface. The steam dryer assembly 5 is disposed above the chimney 4, through which the steam/water chamber and the steam chamber of the pressure vessel 1 are communicated with each other. The dryer assembly 5 is mounted on a circumferential flange 13 projected radially inwards from the wall of the pressure vessel 1. A tubular steam guide 6 is attached to an outlet of the steam dryer assembly 5. The steam guide 6 is fixed to a top head portion 12 of the pressure vessel 1 through a stay 61. The steam guide 6 extends upwards within the steam chamber. When the top head portion 12 is assembled into the pressure vessel 1, projections 61 provided in the steam guide 6 are abutted onto eyenuts 51 provided in a top end of the dryer assembly 5 to press it against the projection 13 so as to fixedly retain it with respect to the projection 13. A steam outlet 7 is provided in a portion of the wall of the pressure vessel 1 corresponding to the steam chamber. An operation of the reactor having the abovedescribed arrangement will be described hereinunder. First, when the reactor is driven, the reactor core heats and boils the light water so as to generate steam. The steam generated comes up as main steam within the chimney 4. The main steam further comes up from the water surface of the coolant in a gas-liquid two-phase state (that is, steam and water droplets) toward the steam dryer assembly 5. As a result, the light water overflows from the chimney end 41 to flow down through a space defined between the pressure vessel 1 and both the chimney 4 and the shroud 3 towards the bottom of the reactor core, and then, it is heated again by the reactor core. In this way, a natural circulation of the coolant is accomplished. While the main steam passes through the steam dryer assembly 5, the wetness fraction thereof is reduced, and the main steam then comes up within the steam chamber along the steam guide 6 while being guided by the same. Subsequently, the main steam flows down toward the steam outlet 7 so as to be supplied to the turbine system through the steam outlet 7. Thus, a steam path is so formed that it extends from the shroud 3 to the steam outlet 7. In the present embodiment, the end 41 of the chimney 4, that is, the water surface of the coolant, is opened towards the steam dryer assembly 5. In other words, between the water surface of the coolant and the steam dryer assembly 5, there exists nothing to restrict the effective open area thereof. To the contrary, in a conventional forced circulation boiling light-water reactor shown in FIG. 2, a shroud 3 is closed at the end portion thereof, and a steam separator assembly 8 which comprises a plurality of steam separators each having a reduced sectional area is communicated with the closed end portion of the shroud. (Elements acting in the same manner as the corresponding ones shown in FIG. 1 are designated by the same reference numerals, respectively, and description of the operation thereof will be omitted. It is noted, however, that the dimensions of the elements designated by the same reference numeral are substantially equal with each other.) In the reactor shown in FIG. 2, since the effective area of the steam path is decreased by the steam separator assembly 8, the velocity of the main steam flowing within the steam separator assembly 8 is increased. Therefore, it takes the main steam about five seconds to flow from the shroud 3 to the steam outlet 7, that is, through the steam path. On the other hand, in the present embodiment, since no steam separator assembly 8 is provided, the effective area of the steam path is never decreased. In consequence, the main steam comes up slowly from the chimney end 41 at a reduced velocity towards the steam dryer assembly 5, thus flowing through the steam path taking a time period longer than five seconds. In addition, since the inside diameter of the chimney 4 is made greater than the outside diameter of the core shroud 3, the velocity of the main steam flowing within the chimney 4 is reduced less than the velocity of the main steam in the core shroud 3. Further, in the present embodiment, the provision of the steam guide 6 makes it possible to prevent the main steam from taking a short cut from the steam dryer assembly 5 to the steam outlet 7. Namely, the steam path is prolonged. In consequence, the time period while the main steam flows through the steam path is made further longer. By referring to FIGS. 3A and 3B, description will be given of the relationship between the length of the steam path and the velocity of flow of the main steam in detail making a comparison between the reactor of the present embodiment shown in FIG. 1 and the reactor shown in FIG. 2. A velocity V1 of the main steam flowing within the chimney 4 is lower than a velocity v1 of the main steam flowing within an extended part of the core shroud which corresponds to the chimney (V1&lt;v1). This is because, on the assumption that the amounts of the main steam generated in the reactor cores per unit time are identical, the velocity of the main steam flowing through the extended part of the shroud, the effective sectional area of which is equal to the sectional area of the core shroud 3, is unchanged and identical to that flowing through the core shroud 3, while the velocity of the main steam flowing within the chimney 4, the effective sectional area of which is increased as compared with the sectional area of the core shroud 3, is reduced. In addition, a velocity V2 of the main steam flowing through the space defined between the chimney 4 and the steam dryer assembly 5 is lower than a velocity v2 of the main steam flowing within the steam separator assembly 8 which corresponds to the space referred above (V2&lt;v2). This is because the velocity of flow of the main steam flowing within the steam separator assembly 8, the effective sectional area of which is decreased, is increased, while the velocity of the main steam flowing through the open space defined between the chimney 4 and the steam dryer assembly 5 is reduced. In this way, since the main steam flows through substantially the same distance (L1+L2=l1+l2) at a reduced velocity according to the present embodiment, it takes a longer time to flow from the core shroud 3 to the steam dryer assembly 5 in comparison with the conventional one. Subsequently, in the steam dryer assembly 5, the velocity of the main steam is reduced at a predetermined rate in either case (V3&lt;v3 and L3=l3). The main steam coming out of the steam dryer assembly 5 then flows within the steam chamber at a velocity V4 which is smaller than a velocity v4 obtained in the conventional one (V4&lt;v4). Furthermore, in the present embodiment, since the steam guide 6 prevents the main steam from taking a short cut to the steam outlet 7 that is, since the steam path is prolonged (L4&gt;l4), it takes the main steam a longer time period to flow from the steam guide 6 to the steam outlet 7, as compared with the conventional one. As clearly seen from the foregoing description, according to the reactor of the present embodiment, the time period while the main steam flows within the pressure vessel is made longer as compared with the conventional reactor. That time period is about five seconds in the conventional reactor, so that the time period referred above can be made sufficiently longer than a half-life of .sup.16 N (about seven seconds) provided that the dimensions of the elements are unchanged. As a result, the amount of .sup.16 N contained in the main steam can be remarkably reduced within the pressure vessel 1. In consequence, the amount of .sup.16 N in the main steam to be supplied to the turbine system is small so that a shield structure which is reduced in size and weight in comparison with conventional ones can be safely applied to the turbine and the piping systems. Comparison will be made between the reactors shown in FIGS. 1 and 2 in terms of the relationship between the .sup.16 N inventory index (inventory of .sup.16 N in the main steam/inventory of .sup.16 N in the main steam in the conventional core shroud (shown in FIG. 2)) at various spots of the power plants and the time period for the main steam to travel from the core shroud 3 to the respective spots, with reference to FIGS. 4A to 4C. Incidentally, reference numerals 9, 10 and 11 designate a reactor housing vessel, a high pressure turbine, and a low pressure turbine, respectively. As understood from the drawings, in the conventional power plant, the time period to travel to the steam dryer assembly 5 is about one second and that to the steam outlet 7 is about five seconds. On the other hand, according to the present embodiment, the time period to travel to the steam dryer assembly 5 is prolonged to about eight seconds and that to the steam outlet 7 is prolonged to about fourteen seconds (or twice of the half-life of .sup.16 N). This results in that although the inventory of .sup.16 N at the steam outlet 7 in the conventional power plant is 60% of that in the core shroud, the inventory of .sup.16 N at the steam outlet 7 in the power plant employing the present embodiment is reduced to 25% of that in the core shroud. That is to say, the inventory of .sup.16 N at the steam outlet 7 according to the present embodiment is reduced to about 40% (=25/60) in comparison with that according to the conventional art. This means that the thickness of the shield structure for the turbine system can be reduced by about 15 cm upon calculation in terms of concrete. According to the present invention, the above-mentioned effects can be obtained by reducing the velocity of the main steam within the pressure vessel and/or increasing the length of the flow path for the main steam. In consequence, other embodiments than the above-described one are also practicable; the one in which axial dimension of the chimney 4 is increased, the one in which a resistance through a passage of the steam dryer assembly is increased, and the like. Further, the above-described guide 6 is not limited to the illustrated one employed in the present embodiment and it may be replaced by the one which is inclined toward the direction opposite to the steam outlet or the one which spirals.