Patent Number: 051961610
Section: summary

TECHNICAL FIELD OF THE INVENTION The present invention relates to a storage rack for the interim storage of hazardous nuclear fuel assemblies. More particularly, this invention relates to a fail-safe rack structure adapted for surviving catastrophic event storage of recoverable (reusable) irradiated fuel rod assemblies. BACKGROUND OF THE INVENTION Nuclear reactors consist of an array of fuel rods containing the radioactive nuclear fuel. The rods are commonly elongated slim metal tubes and are configured in groups, as in modular or unit fuel rod assemblies. After an extended period of reactor use, such irradiated or spent fuel assemblies must be lifted from the reactor proper and replaced while still retaining appreciable amounts of numerous fission products. Such irradiated fuel assemblies have generally been stored in special liquid filled pools until they are to be reprocessed. Racks for such interim storage of bundles removed from nuclear reactors are known where the assembly receiving tubes stand on a bottom plate of the main storage pool or tank. They are usually braced laterally by structures connected to both the bottom plate and the tank walls, in anticipation of surviving seismic rare disturbances. Recoverable Irradiated Fuel (RIF) assemblies are placed in storage and utilize pool water to fulfill four basic needs: (1) to provide nuclear isolation between RIF units; (2) to provide radioactive shielding to protect personnel; (3) to remove the heat buildup caused from radioactive decay of the fuel rods; and (4) to limit radioactive contamination to the storage pool and racks. The dependability of a liquid moderator, usually water, to meet these critical needs is contingent on the integrity of the storage pool. If the main reservoir level cannot maintain its functional safe level, e.g., because of a seismic event or piping system failure, then the RIF loaded rack will lose its liquid cover. Such a loss could lead to a critical configuration of the RIF units, leading to excessive heat buildup, possibly lethal radiation levels, and the consequent unregulated broadcast of radioactive contamination in the storage pool. Because of these ominous risks of moderator loss, commercial facilities must then resort to emergency remedial actions to restore the required liquid cover over the irradiated fuel assemblies. Such steps include but are not necessarily limited to: (1) closing gates to isolate the affected storage; (2) moving single RIF units to an alternate storage pool; (3) moving a loaded storage rack to a functioning pool should the inherent rack design so permit; and (4) rapid refilling the entire reservoir to regain the liquid level essential to proper operation. If such crash steps cannot be implemented prior to fuel rack exposure, site evacuation may be necessary due to rising radiation levels or the danger of reaching a critical fuel configuration. The dire consequences of even remote events like failure of the pool liquid level over nuclear fuel assemblies is a continuing concern in the nuclear industry, which has accordingly generated many corrective or protective approaches. Among the prior art approaches to remedial design of nuclear fuel storage assemblies is the storage rack of U.S. Pat. No. 4,400,344 to Wachter, et al., disclosing a rack consisting of a checkerboard array of square storage cells or tubes. Alternate cells in each row include a neutron-absorbing poison material (usually in the tube wall), while the other cells are used for storage of the spent fuel assemblies. For temporary storage, the poison cells contain a moderator like water, and the entire rack is encased in concrete for shielding. The resultant 50 percent loss of rod storage space and the complex construction make evident the expense and the probable inability to withstand a major seismic mishap because of the vulnerability of the concrete casing to fracture. Another approach to interim storage of rod assemblies is seen in U.S. Pat. No. 4,348,352 to M. Knecht, which discloses a rack installable in a water tank and which is designed to be earthquake proof, also receiving the fuel rod assemblies in close packing. The maintenance of the liquid level in the water tank is quite dependent upon the integrity of the reinforced concrete tank (reinforced with steel plate) for storing of the rack unit. Seismic safety is thus tied to massive steel sheet reinforced concrete storage pools, rather than to any improvement in rack configuration which would retain cooling water about irradiated rod assemblies despite unplanned liquid level loss about the rack. U.S. Pat. No. 4,187,433 to Zezza discloses a high storage density nuclear fuel assembly in a pool with each fuel cell being vertically movable in the storage rack. Circulation openings at the top and bottom of the cells permit pool water circulation through them; of course, this is effective only so long as ambient water is available in the pool. U.S. Pat. No. 4,318,492 to Peehs, et al., is directed to a fuel assembly storage capsule formed of a sleeve sealed at its bottom and having an upper opening closable by a cover that has a cross section matching that of the fuel assembly to be received therein. The sleeve and the cover rim define an annular space filled with air. No means for liquid circulation therethrough is taught. U.S. Pat. No. 4,788,029 to Kerjean, et al., employs an apparatus for storing fuel assemblies in a pool where they are separated by a water gap. The walls of the cells are externally covered with a neutrophage material for irradiation control. There is no teaching relating to use of circulating liquid for waste heat removal and control. OBJECTS OF THE INVENTION It is a principal object of the present invention to provide a storage rack for toxic waste materials, e.g., hazardous irradiated fuels, which has a passive fail-safe protection feature while the rack is storable in a large volume moderator fluid pool. It is another object of this inevention to provide a rack that requires no hasty and hazardous unit relocation to alternate moderator pools upon loss of liquid level in the main pool. Still another object of the invention is to provide a storage rack configuration that will maintain nuclear isolation between RIF units, adequate liquid shielding for personnel, and means for ongoing removal of decay fuel heat upon loss of moderator fluid level from the main storage pool. Yet another object of the invention is to provide a toxic waste storage system the use of which, in the event of liquid loss in the main storage pool, precludes having to invoke standard emergency fuel assembly safety actions usually initiated to maintain the existing liquid cover over resting assemblies. SUMMARY OF THE INVENTION In an effort to overcome the above-mentioned deficiencies of the prior art, the present invention provides a storage rack for retaining toxic waste materials, e.g., hazardous nuclear fuel assemblies, which are normally reposed in a liquid moderator filled pool. The rack of the present invention provides an important passive accident safety configuration, comprising an integral container with conjoined external sidewalls and a bottom plate adequate to support a liquid load as well as the weight of the stored assembly. A tailored cover plate contains insertion slots for the rod assemblies. By virtue of such a configuration, any unplanned loss of liquid level in the main storage pool does not inherently result in liquid level loss over the assemblies positioned within the rack. The outer container sidewalls are welded or otherwise securely affixed to an internal framework that supports the RIF assembly modules. The design and construction can thus accommodate the loss of hydrostatic head from the ambient water should the pool liquid level be lost. The liquid present within the rack walls is retained despite any degree of liquid level drop occurring outside in the main pool. Nuclear isolation is maintained by the interstitial liquid filling in between and within the storage tubes, as is radiation shielding of personnel from the retained liquid cover over the cells within the rack. Moreover, the removal of heat from radioactive rods at least partially continues because heat generated by the nuclear fuel is dissipated into the ambient liquid, which then circulates by convection flow. Lastly, spread of hazardous contamination is precluded by the continuing liquid cover over the RIF modules while they reside in the storage rack of the invention. The operational aspects of the passive storage rack are otherwise identical to prior racks in normal use, except for its unique fail-safe, interim protection function, as described herein. The storage rack is essentially, in its lateral configuration, a double walled shell. An integral inner rectangular shell (generally having no bottom plate) is interposed between the imperforate rack external sides and the periphery of the array of elongate receiving tubes adapted for containing irradiated fuel or similar toxic waste assemblies. The inner shell is sufficiently spaced from the sidewall to define a vertical peripheral passageway within the container sidewalls and is adapted to permit downward flow of the cooler moderator fluid entering from the ambient pool. Each of the elongate tubes has one or more ports disposed on its lower edge, which edge fixedly abuts the bottom plate. The ports are intended to permit bottom water inflow for absorbing the waste heat normally generated by the fuel assemblies loaded into the elongate tubes. Convection liquid flow moves the liquid through the rack while maintaining a more or less steady state temperature with cooling water flowthrough. Exit ports are preferably located in the upper planar member (preferably having at least one for each elongate tube) to permit the escape of warmer liquid from the rack confines back into the ambient pool. An important feature of the present invention is that this rack structure assures that the retained moderator fluid can circulate fully through the rack proper only while it is disposed wholly beneath the pool level. Should the pool liquid level be lost, until major remedial steps are taken, the trapped liquid will then recirculate solely within the rack container, warming up steadily yet still providing interim protection. In the presently preferred embodiment, the upper member is provided with a plurality of hinged squared edge covers having one or more ports in each to permit the escape of warmed liquid to the ambient pool. This hinging aspect facilitates the lifting out of the nuclear fuel assemblies, as desired. The hinge covers are conveniently aligned with the otherwise squared open ends of the elongate receiving tubes. The foregoing general description will serve to explain the mode of operation and principles of the invention in general terms so as to enable those of ordinary skill in the art to which this invention pertains to make and use the same. The accompanying drawings, together with the following detailed description of the invention, will serve to further disclose the best mode currently contemplated by the inventors of carrying out the invention; they are exemplary and explanatory but are not restrictive of the invention, the scope of which is defined solely by the claims.