Patent Number: 041773853
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear fuel storage and in particular to a method and apparatus for storing fuel assemblies in a pool. Reactor fuel element assemblies are frequently stored in storage pools which can accommodate either new or spent fuel assemblies. The pool is filled with water which may be borated. This supplies cooling of the assemblies as well as moderator and also poison if the water is borated. It is of course essential that the stored mass not be permitted to assume a geometry which is either critical or supercritical. The storage pool must be provided during the initial construction of the plant so as to provide for storage of any fuel assemblies which would have to be removed from the reactor. The storage pool at this time need not be capable of storing its ultimate capacity. Investment in expensive materials as components of the storage assembly require a present investment if supplied with the initial storage rack. There is an obvious economic saving if such investment could be deferred. Most storage arrangements are designed for a particular fuel enrichment and are, therefore, completely inadequate should fuel of additional enrichment have to be stored at some time in the future. While borated water may be used in the pool to compensate for this additional enrichment it is considered an unsafe practice to completely rely on the boron content. In the event that the pool develops a leak and water must be replaced with fresh water, the boron content is depleted. Furthermore, there is always a potential for an operating errorwhereby the boron concentration is not maintained at the safe level. Storage racks have been designed utilizing the flux trap principle as illustrated in U.S. Pat. No. 4,004,154 issued to Frank Bevilacqua on Jan. 18, 1977. In such a device a stainless steel plate closely surrounds the fuel assembly being stored with water contained between the plates. Fast neutrons from the fuel pass through the plates and are slowed to thermal levels by the water. At the thermal level they are not able to return through the plates to the fuel. The required spacing for a particular fuel enrichment is calculated according to well known nuclear physics principles. There is an inherent expense in holding tolerances of a structure where multiple plates are involved and tolerances must be simultaneously held. SUMMARY OF THE INVENTION It is an object of the invention to defer a portion of the investment in a fuel storage rack for some years until increased capacity of the rack is required. It is further an object to obtain the ability to store fuel of an enrichment greater than that which has been forecast. It is a further object to reduce the cost of a flux trap type storage rack. A nuclear fuel storage apparatus for use in a water filled pool is fabricated of a material such as stainless steel in a form of an egg crate structure having vertically extending openings. Adjacent openings have a common wall between them which extends throughout the height of the active length of the fuel to be stored. Fuel may be stored in this basic structure in a checkerboard pattern with high enrichment fuel or in all openings when the fuel is of low effective enrichment. A plurality of inserts of a material such as stainless steel are adapted to fit within these openings. The inserts have two plates, one parallel to each of two adjacent sides of an opening, and the plates extend throughout a length generally equal to or greater than the active length of the fuel to be stored. The plates are stored in a similar location in each opening so that a water gap and, therefore, a flux trap is formed between adjacent fuel storage locations. These inserts may be added at a later time and fuel of a higher enrichment may be stored in each opening. When it is desired to store fuel of still greater enrichment, poison plates may be added to the water gap formed by the installed insert plates, or substituted for the insert plates. Alternately or in addition thereto fuel may be installed in high neutron absorption poison boxes which surround the fuel assembly to store fuel or still greater enrichment. It is normally expected that the inserts must be removed at this time because of physical problems of storing the same size fuel assembly with a surrounding box. Stainless steel boxes installed in this manner would function to produce an effective flux trap. The stainless steel inserts and the poison plates are each not required until the capacity of the basic egg crate structure is approached. Purchase of these items can, therefore, be deferred for many years. Should the fuel to be stored be of higher enrichment than initially forecast, the deferred decision on the poison plates makes it possible to obtain increased poison in the plates to satisfy the newly discovered requirement. Even if the storage rack were to be initially supplied with all the inserts in place in accordance with the flux trap principle, construction costs may be reduced. While basic tolerances must still be maintained on the original egg crate structure, the inserts are formed with their own tolerances, which while related to the basic structure tolerances need not be simultaneously maintained.