Patent Number: 054992770
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The focus of the enhancements disclosed in U.S. Pat. Nos. 5,043,135 and 5,339,340 was the annular space in the air riser gap 6 between the outside surface of the containment vessel 7 and the inside surface of the collector cylinder 4, as shown in detail in FIG. 2. Heat is normally transferred to the air stream from these surfaces by convection. In the design of the conventional ALMR, these smooth surfaces have roughness that is characteristic of commercially available, nuclear-grade stainless steels. In accordance with the teaching of U.S. Pat. No. 5,043,135, the surface roughness was increased by creating surface protrusions or boundary layer trips 10 oriented in a direction essentially perpendicular to the air flow direction, e.g., circumferentially around the cylindrical surface of the containment vessel 7. The enhancement of U.S. Pat. No. 5,339,340 involves placing a perforated collector cylinder 11, having multiple openings or holes 12 as indicated in FIG. 2, in the hot air riser 6. The holes 12 can be of arbitrary shape, although a circular shape would be the most economical from a manufacturing standpoint. The degree of perforation, i.e., the total surface of the openings compared to the total perforated collector cylinder surface area, is a variable and can be selected to provide optimum thermal performance. The purpose of the holes 12 is to allow a fraction of the thermal radiative heat flow emanating from the containment vessel 7 to reach and be absorbed by the collector cylinder 4. The remainder of the radiative heat flux is absorbed by the perforated collector cylinder 11. Thus, the surfaces of both the collector cylinder 4 and the perforated collector cylinder 11 receive heat by radiative heat transfer. The fraction that each will receive can be controlled by the degree of perforation selected for the perforated collector cylinder 11. The degree of perforation will be based on an optimization study to achieve maximum overall convective heat transfer from all the heat transfer surfaces in the hot air riser 6. The convective heat transfer rate to the air (the heat sink) depends on the temperature difference between the steel surface and air, and the convective heat transfer coefficient, which in turn depend on the air flow velocities in the individual flow channels created by the perforated collector cylinder 11, namely, inner channel 13 and outer channel 14. The overall optimization process must consider the proper positioning of perforated collector cylinder 11 in relation to the adjacent walls of containment vessel 7 and collector cylinder 4 to achieve the desired air flow distribution between the inner and outer flow channels. The relative positioning of the perforated collector cylinder 11 will also depend on the boundary layer trip configuration if these are included in the heat transfer system. With the air-side RVACS enhancement features of U.S. Pat. Nos. 5,043,135 and 5,339,340 included in the present design, the heat transfer resistance in the inert-gas gap 16 between the reactor vessel 15 and the containment vessel 7 becomes controlling. Since almost all heat transfer in this gap is by thermal radiation, some improvement in the overall heat rejection capability of the RVACS might be achieved by improving the thermal emissivities of the vessel surfaces. However, significant further increase in the thermal emissivities of these surfaces is not possible because they have already been increased by applying carefully prepared oxide layers. Thus, to further improve the passive heat removal capability, other means must be adopted. In accordance with the present invention, enhancement of the passive heat removal capability in an AMLR is achieved by introducing means for removing heat directly from the inert gas in the gap space 16 and inducing significant natural convection flows in the gap space. The increased flow velocities in the gap space 16 result in higher convective heat transfer between the reactor vessel 15 and the containment vessel 7. In addition, RVACS performance is increased in an indirect manner because more draft head and associated RVACS air flow result, as explained hereinafter. Thus, the overall performance of the composite or dual RVACS in accordance with the invention is increased. The degree of increase depends to a large extent on the investment one is willing to make in the inert gas/RVACS air exchanger. The design and operation of the heat removal enhancement means in accordance with the preferred embodiment of the invention is explained with reference to FIGS. 3-6. In accordance with the invention, four inert gas inlet ducts 21 extend horizontally into the outlet plenum and are attached to the wall of the containment vessel 7, as indicated in FIGS. 3 and 4. The inlet ducts 21 are in flow communication with the inert gas-filled gap space 16 (see FIG. 2) via four inlet openings 22 (see FIG. 4). Similarly, four inert gas outlet ducts 23 are also attached to the containment vessel at approximately the same elevation as the inlet ducts 21 and communicate with the inert gas-filled gap space 16 via four outlet openings 24, each of which is positioned at an angular location which is essentially 90.degree. from the corresponding inlet opening 22, as best seen in FIG. 5. Each reactor assembly quadrants has one inlet opening 22 and one outlet opening 24. The design is further modified by including four vertical inert gas riser ducts 25 positioned adjacent to the four RVACS stacks 27 along the entire length of the stack located within the refueling enclosure 28. Each inert gas riser duct 25 is in flow communication at its bottom end with a corresponding one of the four inert gas outlet ducts 23, as shown in FIG. 3, and is covered with thermal insulation 26, as shown in FIG. 4. In addition, each riser duct 25 extends horizontally at the top end thereof through the wall of the associated RVACS stack 27, into the RVACS inlet ducts 29 and joining with one long side of the rectangular RVACS outlet ducts 30, as indicated in FIGS. 3 and 4. Inert gas downcomer ducts 31 are formed by one long side of the RVACS outlet duct 30-which now serves as an inert gas/RVACS air heat exchanger 32; part of the long wall of the RVACS inlet duct 29 adjacent to and opposing the heat exchanger 32; and side walls 33 formed by extending the two short side walls of the RVACS outlet ducts 30 until they join with the opposing wall of the RVACS inlet duct 29, as indicated in FIG. 4. Each downcomer duct 31 extends the entire length of the associated RVACS stack 27. The bottom end of each downcomer duct 31 joins in flow communication with the associated inert gas inlet duct 21, as indicated in FIG. 3. The preferred embodiment of the inert gas/RVACS air heat exchanger 32 described herein is but one of a large number of possible designs that could be considered and which would be acceptable from a structural point of view. However, the disclosed preferred embodiment is considered to be the best mode because it minimizes the modification needed to incorporate the inert gas/RVACS air heat exchanger of the present invention in a conventional plant. In accordance with the invention, modifications are made to the conventional reactor assembly design as indicated in FIGS. 5 and 6. Four flow baffles 34 are placed in the inert gas-filled gap space 16 at 90.degree. intervals along the circumference. These baffles extend from near the top of the reactor vessel 15 and the containment vessel 7 to near the bottom of the cylindrical portions of the vessels. Thus, these flow baffles define four quadrants, two of which are denoted as downflow zones 35 and two of which are denoted as upflow zones 36. The downflow zones 35 are located at circumferential positions corresponding to the inlet openings 22 and the upflow zones 36 are located at circumferential positions corresponding to the outlet openings 24. Note in FIG. 5 that the downflow zones 35 are positioned radially outside of the two sets of electromagnetic pumps 37 whereas the upflow zones 36 are positioned radially outside of the intermediate heat exchangers 38. [Pumps 37 and heat exchangers 38 are conventional components disclosed, for example, in U.S. Pat. No. 4,882,514 to Brynsvold et al.]The reason for this orientation is that the regions of the reactor vessel 15 outside of the intermediate heat exchangers 38 are normally hotter than the regions outside the electromagnetic pumps 37, which will tend to promote flow patterns of the inert gas around the bottom of each baffle as shown in FIG. 6. During operation of the heat removal enhancement means of the present invention, the inert gas in the two upflow zones 36 will rise because vessel surface temperatures are higher in these zones. The inert gas then proceeds through the four outlet openings 24 into the four inert gas outlet ducts 23 and thereafter into the four inert gas riser ducts 25. Each inert gas riser duct is positioned adjacent to a corresponding one of four RVACS stacks 27, as indicated in FIGS. 3 and 5. From there the hot inert gas is ducted into the four inert gas downcomer ducts 31, where the inert gas is cooled by the inert gas/RVACS air heat exchangers 32. The cooled inert gas then flows in sequence through the four inert gas inlet ducts 21, the two downflow zones 35 and then the four inlet openings 22. The cooled inert gas is heated as it is directed downward, but the upward buoyancy thus created is overcome by the much larger positive head created in the elevated inert gas/RVACS air heat exchangers 32. The inert gas flows laterally near the bottom of the reactor assembly, as indicated in FIG. 6, in the open space below the end points of the flow baffles 34 and then enters the two upflow zones 36. The inert gas is heated further as it flows upwards in the upflow zones and then repeats the entire inert gas flow path again. Operation of the dual RVACS increases the decay heat removal capability in three different ways. First, heat is removed directly from the reactor vessel outside surface by the circulating inert gas and transferred to the RVACS outlet air 39 in each inert gas/RVACS air heat exchanger 32. This contribution to the improvement in RVACS performance is by far the largest, perhaps being as much as 90% of the total when the heat exchanger surface area (A) is large. Second, heat is transferred to the containment vessel 7 by the vigorous natural convection flow created in the inert gas-filled gap space 16, which heat is in turn transferred to the conventional RVACS air stream. Finally, the RVACS air flow rate and thereby its performance are increased because heat is added to the RVACS outlet air 39 in the inert gas/RVACS air heat exchangers 32, which provides increased natural circulation head and RVACS flow rate, thus increasing air-side heat transfer coefficients as well as surface-to-air temperature differences. In specific preliminary analysis cases considered for the dual RVACS concept utilizing the enhancement methods described in U.S. Pat. Nos. 5,043,135 and 5,339,340, using a UA product parameter [UA is the product of the heat exchanger overall heat transfer coefficient U and the heat exchanger surface area A] value of 3320 Btu/hr-.degree.F corresponding to utilizing one long side of the RVACS air outlet duct 30 for the inert gas/RVACS air heat exchanger as shown in FIGS. 3 and 5, the overall performance of the dual RVACS increased by about 13%. The corresponding reactor core power that would be possible without reducing the RVACS temperature margin is about 950 MW.sub.t corresponding to an estimated net reduction in bussbar cost of 4 mills/kWh. Further increases are possible by simply providing more heat exchanger surface area. For the other analysis case considered, where all of the RVACS air outlet duct was used as inert gas/RVACS air heat exchanger area, it was determined that by using the dual RVACS of the present invention, the reactor core power can be increased from 840 to 985 MW.sub.t (i.e., a 17.5% increase), resulting in an estimated bussbar cost reduction of 5 mills/kWh. Such a power increase must be consistent with other design constraints that might exist in the current ALMR. However, if this power increase could be implemented, significant net reduction in the electric power generating cost could be realized. Thus, the basic concept of the invention is that heat is removed directly from the reactor vessel outside surface by circulating inert gas. The heated inert gas then circulates via multiple flow paths through heat exchangers which remove heat from the inert gas. The cooled inert gas then flows by natural circulation back to the annular space between the reactor vessel and the containment vessel. This concept has been illustrated by disclosure of the foregoing preferred embodiment. However, it is understood that this novel concept is subject to change following trade-off and detailed thermal performance evaluations without departing from the spirit and scope of the invention. Also, routine variations and modifications of the disclosed apparatus will be readily apparent to practitioners skilled in the art of passive air cooling systems in ALMRs. For example, the heat exchangers could also be arranged to reject heat directly to atmospheric air which is not a part of the RVACS air cooling stream. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.