Patent Number: 043572979
Section: description

For the purposes of explanation, the preferred embodiment is described herein as it is incorporated into a pool-type liquid metal cooled fast breeder reactor. A side elevational view of such a reactor is illustrated in section in FIG. 1. It should be understood, however, that the present invention has application in both loop and pool-type nuclear reactors as well as reactors which are cooled by water, gas and liquid metal (sodium). The nuclear reactor illustrated in FIG. 1 includes a reactor core 4, a primary tank 6, a coolant circulating pump 8, and an intermediate heat exchanger 11. In the actual plant there are a plurality of circulating pumps and intermediate heat exchangers but only one of each is illustrated in FIG. 1. The sodium coolant is contained in the reactor in a cold pool 9 and in a hot pool 10. The hot and cold pools are separated by a horizontal structure 12 which is a structural and thermal barrier that extends horizontally across the reactor and contains a plurality of cylindrical pump wells 16. These pump wells shroud each pump from the heat of the hot pool. The free surface of the sodium in the hot pool is indicated by numeral 13 and the free surface of the sodium in the cold pool 17, FIG. 2 is located within the pump wells 16. Referring to FIG. 2, the primary containment for the sodium coolant and the cover gas is the primary tank 6. Since the reactor is a pool-type reactor, the primary tank is generally cup-shaped and has no penetrations through its side and bottom walls. The fission products are thereby contained within the boundaries of the primary tank at all times. The upper portion of the primary tank terminates in an upper cylindrical support skirt 20. The support skirt and in turn the entire reactor are supported at point 7 from the ledge in the reactor cavity wall. The cavity wall is fabricated from concrete and the reactor cooling system is designed so that the point of support 7 never exceeds 150.degree. F. The thermal insulating apparatus of the present invention includes a cylindrical metal liner 22, FIG. 2 located vertically along the inner side wall of the primary tank. The liner is the first thermal barrier to the hot sodium and is in direct fluid contact with the sodium in the hot pool. The purpose of the liner is to form, in combination with the reactor vessel wall, an impermeable boundary to the gas-filled annular space and thus provide a satisfactory gaseous environment for the reflective insulation. The liner also forms the side wall of a trapped gas space as described below. The liner is supported from the upper support skirt 20 by a cylindrically shaped member 23 having an L-shaped cross section as illustrated in FIGS. 2 and 5. The liner extends from the bottom of the L-shaped member 23 down to the supporting structure 26, FIG. 2, located below the horizontal structure 12. The configuration of the L-shaped member is determined by fabrication and assembly considerations. Referring to FIG. 2, between the liner 22 and the primary tank side wall 6 and a plurality of vertically oriented, reflective metal plates 28. These plates are radially spaced apart around the outside of the core of the reactor and each has an arcuate cross-section. These plates are illustrated in detail in FIGS. 5 and 9. The purpose of these reflective metal plates is to thermally insulate the side walls of the primary tank from the temperatures generated by the reactor. Referring to FIG. 9, the plates are vertically segmented into arcuate sections to permit the lateral thermal expansion. In addition, the plates each contain a plurality of indentations 29, FIG. 5 which preserve the stand-off spacing between adjacent plates. The indentations also reduce convective heat transfer between the plates by minimizing the circulation of inert gas and sodium vapor in the trapped gas space which is described in detail below. The vertically segmented cylindrical plates are circumferentially staggered as illustrated in FIG. 9 in order to prevent direct radiative heat transfer from the liner 22 to the primary tank through the vertical slots between the arcuate sections. The metal plates 28, FIG. 2 reduce the radiation heat transfer from the hot pool in the radial direction by presenting a large vertical surface area. The plates are dimensioned to minimize their thickness and to incorporate as many plates as possible into the space between the liner 22 and the wall of the primary tank 6. In one embodiment there are twenty-three reflective plates each 0.1" thick, and spaced apart 0.5". The plates have a 0.06" cold clearance and each arcuate section has a 4' vertical length. The resulting arrangement gives an effective thermal conductivity of between 0.3 to 0.6 BTU/H-ft-.degree.F. Referring to FIGS. 5 and 6, each reflective plate 28 has a rigid supporting member attached along its upper margin. The supporting members engage two T-shaped brackets 33. The T-shaped brackets are radially spaced around the reactor and are welded to the bottom of the L-shaped member 23. The supporting members along with the reflective plates hang from the T-shaped brackets and the spacing between the reflective plates is maintained by the thickness of the supporting members 31 and the indentations 29. Referring to FIG. 2, the primary tank 6, the upper support skirt 20, the L-shaped member 23, and the liner 22 together form a trapped gas annulus. The trapped gas annulus is open at the bottom, hermetically sealed at the top, and has an inverted U-shaped cross section. When the primary tank is filled with sodium, the trapped gas annulus contains within its boundary a bubble of inert gas which is supplemented during filling via the makeup line 35 described below. The bubble prevents the reflective plates 28 which are located within this annulus from coming into fluid contact with the liquid sodium. The purpose of this construction is to eliminate the conductive heat transfer between the reflective plates and the primary tank which would occur if the plates were submerged in sodium. The gas annulus is open at the bottom so that in the event that in-leakage of liquid coolant occurs, it will readily drain. The level of sodium within the trapped gas annulus varies with power level. The level is illustrated in FIG. 2 at 40% power 38 and at 100% power 38'. The free surface 17 of the sodium in the cold pool 10 is located within the pump well 16, FIG. 2 and a hydrostatic head is developed between the cold pool free surface 17 and the level of sodium in the bottom of the trapped gas annulus. The level of sodium in the annulus falls as the reactor power level increases because as the power level increases, the level of the free surface in the pump well decreases. The top of the reactor is sealed by a cover 14, FIGS. 1 and 2. The cover includes both insulation and radiation shielding and forms no part of the present invention. The open space between the bottom of the cover and the hot pool operating level 13 is the cover gas space which is filled with helium. Located along the side wall of the primary tank in the cover gas space is a series of vertically oriented, reflective metal plates 40. These plates are arranged in a triangular shaped array which is shown in detail in FIG. 7. These reflective plates are fabricated and operate in the same manner as the reflective plates 28, FIG. 5 described above. The reflective plates 40 are suspended by a series of T-shaped brackets 33', 33" which are welded to the bottom of the cover 14 as illustrated in FIG. 8. The primary purpose of these plates is to control the axial temperature gradient in the liner 22. The reflective plates 40 also supplement the larger reflective plates 28 in passively insulating the primary tank side wall from the temperatures generated by the reactor. These plates are positioned to insure that the temperature gradient from the operating level 13 to the point of support 7 is smooth and without excessive thermal stresses. The reflective plates 28, 40 in combination with the cover insulation establish the temperature profile illustrated in FIG. 3. There is an additional triangular array of metal plates 44 located adjacent to the liner 22 between the hot pool 10 and the cool pool 9. These metal plates, which are vertically oriented and extend around the circumference of the reactor, provide a variable resistance to thermal conduction and so reduce the temperature gradient between the hot pool and the cold pool. The plates in this set are fully immersed in the sodium in the cold pool and are therefore considerably less efficient thermal barriers than the reflective plates. Referring to FIGS. 1 and 2, the primary tank 6 is positioned within a guard tank 42. The guard tank is an emergency boundary to protect against any sodium leakage from the primary tank. The annular space between the primary tank and the guard tank is filled with an inert gas and is used for in-vessel inspection of the primary tank. The outside of the guard tank is insulated with conventional ceramic fibrous insulation and is cooled by a cavity heat-removing system (not shown). The trapped gas annulus formed by the side walls of primary tank 6, the bottom of the L-shaped member 23 and the liner 28 is placed into operation by purging the oxygen from the primary tank and filling the primary tank with an inert gas such as helium. Thereafter, sodium is added to the primary tank and it is filled to the operational level 13. During the process of filling the primary tank, the sodium traps a bubble of helium in the annulus. After filling the primary tank, the level of sodium in the annulus is adjusted by adding helium through a make-up line 35, FIGS. 2, 4. During operation, the level of sodium in the annulus is monitored by measuring its pressure through the line 36. To minimize the number of penetrations through the reactor cover 14, the monitoring line 36 is located inside of and coaxial with the make-up line 35. If the level of sodium in the trapped gas annulus falls excessively, the helium in the annulus is vented through a vent line 37. The vent line directs the helium up the side wall of the primary tank to a point located just below the hot pool operating level 13. The vent line prevents bubbles of helium from being drawn into the circulating pump 8, FIG. 1, and thereafter flowing through the reactor and causing reactivity perturbations. It should be understood that the apparatus described above constitutes a passive thermal insulating system. The apparatus does not require any energy input from the reactor and does not degrade the efficiency and performance of the reactor. It should further be understood that this apparatus is used in combination with the insulation on the outside side walls of guard tank 42, FIG. 1 and a reactor cavity heat removing system (not shown). The temperature gradient from the sodium hot pool 10 to the guard tank 42 can be altered by adjusting both the amount of insulation used inside of the primary tank and on the outside of the guard tank. By proportioning this insulation any primary tank temperature or temperature gradient along the side wall of the primary tank can be attained. Thus, although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.