Patent Number: 049833537
Section: description

Referring to FIG. 1 a sodium cooled nuclear reactor vessel V is illustrated. The particular vessel V shown is a prior art configuration. A steam generator M is illustrated operatively connected to the sodium reactor. The steam generator here shown is a preferred embodiment of a related art steam generator. This steam generator is not prior art. A complete description of this generator may be found in U.S. patent application Ser. No. 231,031 filed Aug. 11, 1988 and entitled Compact Intermediate Heat Transport System for Sodium Cooled Reactor, now U.S. Pat. No. 4,905,757, issued Mar. 6, 1990. Referring to the reactor V, a pool of sodium 14 is confined within an inner shroud vessel 16. Sodium pool 14 forms the so-called hot leg. Tracing the hot leg of the sodium cooled nuclear reactor, sodium from the sodium pool 14 passes upwardly from a core 12 where it receives heat. It thereafter passes downwardly through an intermediate heat exchanger H. In such passage it liberates its heat to the "cold leg" of the secondary loop. After the liberation of heat, the sodium of the primary loop then passes in its own "cold leg" to a bottom plenum 20. In bottom plenum 20 the sodium passes upwardly in a pumping leg at an annulus 22 into pump inlet 24 and through an electromagnetic pump P1. At electromagnetic pump P1, the sodium reverses at loop 26 passing through a discharge plenum 28 to the bottom of the core 12. At core 12 the sodium flows upwardly and to pool 14. The cycle endlessly repeats. It will be noted that sodium flow occurs within an inner shroud L. Shroud L provides an emergency heat outflow Such emergency heat outflow is not pertinent to this disclosure and will not be further discussed here. As is common in reactors, a control rod cavity 30 contains applicable control rods for the penetration into and out of the reactor to control the reaction Intermediate heat exchanger H interior of the sodium cooled reactor vessel constitutes the heat exchange interface between the primary and radioactive sodium loop and the secondary sodium loop. As here illustrated, lines 18, 20 provide for secondary sodium flow to and from the intermediate heat exchanger H. As here illustrated, the line 20 is a part of the cold sodium leg of the secondary loop. The line 18 is a part of the hot sodium leg of the secondary loop. Hot sodium flows in outer concentric pipe 18 into the steam generator M. Generator M constitutes a generally cylindrical vessel with dome closures at both ends and having an outer vessel 60 and an inner and concentric vessel 62. The interstitial volume between the outer vessel 60 and the inner concentric vessel 62 is filled with helically coiled tubes. These tubes begin at tube sheets placed within lower water inlets 71, 74. The tubes extend upwardly into the interstices between the outer vessel 60 and the inner vessel 62. Specifically, and in the area 78, the tubes coil helically about the inner vessel 62. In such helical coiling, the tubes coil until they reach the upper portion 78 of the steam generator M. At upper portion 78, the tubes pass directly vertically upward to tube terminating tube sheets within steam outlets 81, 84. The steam is generated by the heat transferred from the hot sodium during the upward passage of water through the helically coiled tubes. The hot leg of the secondary sodium loop continues at inlet pipe 40. Sodium counterflows the water in the helically coiled tubes 78. This counterflow includes passage from the inlet at 40 down to the plenum 64. At plenum 64, upward sodium flow occurs in two separate paths. First, a single electromagnetic pump Q' is located. Pump Q' takes suction at 201 and discharges high pressure, sodium at 200. The discharged high pressure, sodium passes into the inlet 210 of a jet pump located inside of the interior cylindrical vessel 62 and supported by struts 240 In the second flow path, sodium flows interior of the inner vessel 62 outside of the electromagnetic pump. Specifically, and as indicated at arrow 250, sodium flows in an annulus exterior of the electromagnetic pump and passes into the mixer section 210 of the jet pump. The sodium then exists at a diffuser 220 into an outlet 230. At outlet 230 the sodium is pumped to and towards the heat exchanger H. It will be seen at the bottom of the steam generator that there is provided a diaphragm D mounted to a protruding nozzle 270. Diaphragm D is designed to rupture in the case of a sodium water reaction. When the diaphragm D ruptures, sodium empties from the steam generator vessel. Having set forth the prior art sodium reactor vessel V and the related art steam generator M, the casualty scenario against which this invention guards may now be set forth. It is assumed for purposes of the discovered scenario that a tube rupture has occurred in the worse possible location. Specifically, such a location is shown at 300. It is further assumed that more than one tube is effected by the rupture and the pressure generated by the chemical reaction breaks the rupture diaphragm. Viewing FIG. 1, it can be seen that the sodium, hydrogen, steam and other compounds from the violent sodium water reaction at 300 have to pass along the entire length of intact tubes within the coiled helical tubes 78 to the rupture diaphragm. After such passage, the gases will find their way into plenum 64 and out diaphragm D at protruding cylindrical nozzle 270 at the bottom of the steam generator. It will be remembered that high pressure steam in lines 90 and high pressure feedwater in lines 91 is assumed to be present. This high pressure steam from the turbine side of the plant and feedwater from supply steam is presumed to flow to the site of the reaction at 300. Accordingly, region 300, the site of the tube breakage, will be presumed to be a high pressure violent reaction continuously supplied with the necessary sodium reactive steam and water to keep the reaction sufficiently long (terminated in the steam generator by Na expulsion thru the rupture disk) to cause a large number of tube ruptures at the site 300. This being the case, the present casualty scenario presumes that the continuing steam/water flow and associated pressure drop within the tube bundle will force the Na/steam interface along conduits 18, 20 and back into the intermediate heat exchanger H. It will be realized that as the steam sodium interface penetrates the specific conduits 18 and 20, the conduits will, in all likelihood, propagate the sodium water reaction into the main reactor vessel. Remembering that the sodium interior of the vessel is radioactive, complication of the disclosed casualty by penetration of the steam into the radioactive vessel is to be avoided. This being the case, the improvement of this invention can now be set forth. Referring to FIG. 2, a steam generator M having an outer cylindrical vessel 60 and an inner cylindrical vessel 62 is illustrated. Between inner cylindrical vessel 62 and the outer vessel 60 there is placed an intermediate cylindrical vessel 63. Intermediate vessel cylindrical 63 opens to the plenum 64 at the bottom. Likewise, intermediate cylindrical vessel 63 opens at the top to the cover gas region C. In the view illustrated in FIG. 2 normal reactor operation is assumed. It is instructive to understand this normal reactor operation so that a serendipitous advantage of this invention can be understood. Sodium typically flows in from the reactor along leg 18 and is distributed at a manifold 170 at the top of the reactor. The sodium in the hot leg flows downwardly over the helical tubes 78 down into plenum 64. At the plenum 64 the sodium flows inwardly to the inside of the interior cylindrical vessel 62. At this point, pump Q' acting as an electromagnetic pump, pumps a high pressure, low volume, flow of sodium into a jet pump inlet 210. The sodium discharged from the electromagnetic pump entrains sodium passing about the outside surface of the pump into the mixing section of the jet pump 210. The sodium passes to a diffuser 220 and outwardly on the cold leg 20. It will be appreciated that plenum 64 is the low pressure region of the secondary sodium loop. The sodium in the interstitial area between the inner cylindrical vessel 62 and the intermediate cylindrical vessel 63 is supported in its static head from the relatively low pressure plenum 64. Plenum 64 has a relative low pressure because it constitutes the suction side of the pump Q'. Consequently, it has a sodium/cover gas interface 80 adjacent the bottom of the interstitial volume between the inner cylindrical vessel 62 and the intermediate cylinder 63. It can be seen that the cover gas C penetrates downwardly almost the full length of the intermediate cylindrical vessel 63. There is thus placed between the inner cylindrical vessel 62 and its cold leg of sodium and the outer vessel 60 and its contained hot leg of sodium, a region of cover gas C. Insulation by the region of cover gas C occurs not unlike that insulation common in a Dewar flask. Stated in other terms, the intermediate cylinder 63 prevents heat being shunted directly from the hot leg to the cold leg of the steam generator M. Having set forth this serendipitous characteristic, operation of the steam generator in the casualty scenarios herein set forth can be understood with respect to FIG. 3. Assuming that a casualty has occurred in an area 300, the diaphragm D on cylindrical nozzle 270 at plenum 64 immediately ruptures. Liquid sodium from the secondary loop immediately drains to a sodium dissipation system including a holding tank and stack. These conventional prior art systems are not shown. Regarding the sodium in the interstitial volume between the inner cylindrical vessel 62 and the intermediate vessel 63, sodium likewise immediately drains. This draining of sodium opens a gas free path from the top of the steam generator C directly to the plenum 64. This can be seen to be almost direct from the site of the violent sodium water reaction 300. This may be easily understood. Assuming that a chemical reaction has occurred at 300 and steam is continuously being supplied, two flow paths will be present. First, steam can discharge from the site of the reaction down through the remaining intact tube 78 and out the diaphragm D. Since the remaining intact tubes constitute a considerable flow barrier, especially where the tube rupture is in the upper portion of the tube coils, this route for the outgassing of the components of the violent reaction will have only a minority of the total flow. An additional flow path is defined between the inner vessel 62 and the outer vessel 63. Specifically with all sodium expelled, gas can pass upwardly from the site of the 25 reaction into the now vacated cover gas region C' and in the top of the intermediate cylindrical vessel 63. From the intermediate vessel 63, a direct and free nonencumbered flow path out the diaphragm D is defined. Consequently, hot leg inlet 18 and cold leg outlet is 20 does not experience a large pressure differential. Specifically, steam/water from the site 300 cannot penetrate along the length of conduit 18 to effect the continuance of the casualty to and towards the reactor. It should be mentioned that because of the intermediate cylinder 63, the overall diameter of the steam generator vessel 60 is slightly increased. However that may be, the increase is not substantial. For example, whereas a prior art steam generator illustrated in FIG. 1 as a diameter of 8 feet, the disclosed generator with the intermediate vessel has a diameter of 9 feet. It will be understood that this invention can be operative in those types of steam generators which do not include a central contained pump. Such a steam generator is illustrated in FIG. 4 Referring to FIG. 4, a steam generator M' is illustrated having an outer vessel 60 and a single interior cylindrical vessel 63. Vessel 63 opens to a plenum 64 at the bottom and opens to the cover gas region C at the top. As before, helically coiled tubes conventionally run between feedwater inlets 71, 74 at the bottom and steam outlets 81, 84 at the top. In most steam generator constructions, it is not possible to helically coil the tubes 78 to occupy the entire inner diameter. Consequently, and in the prior art, an inner cylindrical vessel 63 has normally been a vacuous and closed area. Sodium is conventionally withdrawn from plenum 64 in the cold leg and passed to a relief nozzle (the relief nozzle not being shown in the view of FIG. 4). The reader can understand that the installed conduit 63 without the inner cylinder 62 functions precisely analogous to that illustrated in FIGS. 2 and 3. Specifically, and during normal operation (as shown in FIG. 2) the sodium level in the central duct will be at an elevation supported by the low pressure in the plenum 64. Upon a casualty occurring at the top of the coiled tubes, sodium will empty, and the gas and sodium from the site of the violent reaction will pass interiorly of the central cylinder 63 and out the bottom of the vessel. The reader will likewise appreciate that varying constructions may be used.