NUCLEAR FUEL ASSEMBLY

Abstract: The invention is concerned with the minimisation of radiological doses during manufacturing processes of fuel assemblies, by employing non-irradiated fuel as a shielding material against the radiological output of re-processed fuel materials. The non-irradiated fuel can be employed in peripheral fuel rods and/or in the end portions of fuel rods with beneficial effects.

Description: NUCLEAR FUEL ASSEMBLY This invention concerns improvements in and relating to nuclear fuel assemblies, particularly with regard to further minimising radiological doses to fuel manufacturing plant, fuel transport facilities, and nuclear power plant personnel. A number of tasks concerning fuel rods and assemblies such as manufacturing, handling, loading, and inspection m the nuclear power industry call for close proximity of personnel to the fuel rod or fuel assembly. This is particularly true of the body extremities such as hands and eyes. Close inspection, visually or by touch is typically required, for instance in inspecting the completed fuel rods or finished fuel assemblies following completion of manufacturing operations or transportation, or prior to insertion into the reactor core. Close inspection of this type reduces the distance between personnel and the radioactive source and as a consequence the degree of attenuation of the dose is reduced. Fuel for fuel rods for use in nuclear reactors falls into two basic categories, either virgin fuel from the primary extraction source which has been processed up to the required enrichment (so called non-irradiated uranium NIU) or alternatively reprocessed uranium (REPU) which has previously been m a reactor core and has subsequently been reprocessed prior to recycling it once more at the required reactivity equivalence . Both NIU and REPU fuel materials are suitable for use as fuel, however, reprocessed fuel differs from non- irradiated fuel in that it contains minor components including irradiation induced isotopes of various materials (including, but not limited to, uranium isotopes and their related "daughter" products) . These REPU components are significant from the radiological dose point of view, as they are of a higher spontaneous radioactivity than NIU of the corresponding grad . The present invention, amongst other aims, aims to provide fuel assemblies which enable REPL to be used alongside NIU and yet significantly minimise the consequential radiological dose to personnel. According to a first aspect of the invention we provide a nuclear fuel assembly comprising a plurality of fuel rods containing fuel wherein a proportion of the fuel in the fuel assembly is non-irradiated fuel and a proportion of the fuel in the fuel assembly is reprocessed fuel . In this way the non-irradiated uranium (NIU) provides a shielding effect to the radiological output of the reprocessed uranium (REPU) . According to a second aspect of the invention we provide the use of a . plurality of fuel rods containing non-irradiated fuel to shield against the radiological output of re-processed fuel provided in the fuel assembly. The first and / or second aspects of the invention may further provide the following possibilities. Optionally the NIU fuel maybe provided at or close to the periphery of the fuel assembly. The NIU fuel or a substantial part of it may be provided in the edge fuel rods or those fuel rods adjacent to the peripheral rods of the assembly. The REPU fuel may be provided internally within the fuel assembly. For instance the REPU fuel may be provided in non- peripheral fuel rods. Preferably the NIU is provided as a discrete portion of the fuel in a given fuel rod relative to REPU fuel. The discrete portion may be formed of fuel pellets . Preferably one or more of the fuel rods are provided with NIU fuel in one or both end portions of the fuel pellet stack. Preferably the rods are provided with NIU fuel only in such end portions . Preferably the NIU end portions represent between 1 and 15% and most preferably between 3 and 10%, of the length of fuel pellet stack in the fuel rod. A similar or dissimilar length NIU portion may be provided at the other end of the given fuel rod. The REPU may therefore constitute between 98 and 70% or between 94 and 80% respectively of the fuel in a given fuel rod. A shielding effect may be obtained with more than 40% of the fuel rods in an assembly being provided with one or both NIU end portions. Preferably the level is in excess of 70%, more preferably 80% and ideally in excess of 90%. Provision of all the fuel rods with such NIU end portions is highly desirable, but a partial screening effect can be obtained even where some lack the NIU end portion. Preferably a number of the peripheral fuel rods of the assembly are provided with NIU throughout all, or a substantial portion, of their length. Most preferably such peripheral fuel rods are provided with NIU fuel only. Most preferably all of the peripheral fuel rods are provided with NIU, but a screening effect can be obtained with more than 90%, more than 80%, more than 70% and even with 40% or more of the fuel rods being provided with NIU. The NIU may be mixed with REPU and still reduce the radiological dose or the NIU may be provided as a cylindrical coating or layer around a REPU core in the fuel rod or pellets. For simplicity sake the use of NIU alone is however preferred. In a particularly preferred embodiment of the invention all or substantially all of the peripheral fuel rods in a fuel assembly are provided with NIU fuel only and all or substantially all of the fuel rods have their end portion of fuel provided as NIU fuel. The non-peripheral fuel in such an assembly is preferably REPU or includes REPU. The fuel assembly may be of the types suitable for use in light water reactors (including boiling water reactors and pressurised water reactors) , advanced gas cooled reactors, WER or CANDU types, graphite moderated reactors (including RBMK or Magnox) , advanced thermal reactors and fast -reactor types (including radial blanket fuel). For example in an AGR fuel assembly between 9 and 18 and preferably all 18 of the peripheral fuel rods may be provided with NIU; for a CANDU fuel bundle between .9 and 18 and preferably all 18 of the peripheral fuel rods may once again be provided with NIU; for a new 43 pin CANDU fuel bundle between 10 and 21 and preferably all 21 peripheral fuel rods maybe NIU; for a WER-440 fuel assembly between 18 and 36 and preferably all 36 peripheral fuel rods are provided with NIU; for a BWR fuel assembly with for example, a 9 x 9 fuel rod array between 16 and 32 and preferably all 32 of the fuel rods may be provided with NIU. The fuel enrichment between individual rods and between portions of a given rod may vary. Peripheral rods may be provided with a lower fuel enrichment than internal fuel rods. The end portions of fuel rods may be provided at a lower enrichment than the middle portions of fuel rods. According to a third aspect of the invention we provide a nuclear fuel rod containing NIU fuel and REPU fuel . Preferably the NIU fuel is provided at one or both ends of the fuel rod. Most preferably the NIU fuel is provided in a discrete portion. The NIU maybe provided as a sub- stack of pellets adjacent to or in conjunction with REPU pellets. Preferably the NIU fuel portion comprises between 1 and 15 and most preferably between 3 and 10% of the total fuel stack within the fuel rod. A similar or different length portion may be provided at the alternate end of the fuel rod. According to a fourth aspect of the invention we provide the use of a nuclear fuel rod containing NIU fuel together with i) REPU fuel and/or with ii) one or more further fuel rods containing REPU fuel . Preferably the NlU-containing fuel rod is provided peripherally relative to the REPU fuel rod in use. According to a fifth aspect of the invention we provide a reactor core incorporating one or more fuel assemblies according to the- first aspect of the invention and / or one or more fuel rods according to the second aspect of the invention. According to a sixth aspect of the invention we provide a method of producing a nuclear fuel assembly comprising a plurality of fuel rods containing fuel, in which the fuel comprises both REPU and NIU fuel . Preferably the method provides a first fuel rod type in the assembly, the fuel rod having a NIU fuel portion at one or both ends of the fuel in the rod, the remaining fuel comprising REPU fuel. Alternatively or additionally a second fuel rod type maybe provided in the assembly, the second type fuel rod having NIU fuel throughout all or a substantial part of its length, at least some of the second fuel rod type being provided at or close to the periphery of the assembly with other fuel rods containing REPU. According to a seventh aspect of the invention we provide a method of fuelling or re-fuelling a nuclear reactor core comprising providing one or more fuel assemblies according to the first aspect of the invention and / or incorporating a fuel rod according to the second aspect of the invention and / or produced according to the method of the fifth aspect of the invention in the reactor core. According to a eighth aspect of the invention we provide electricity generated by a nuclear power station using any of the first to sixth aspects of the invention. Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which :- Figure 1 illustrates a plan view of a fuel assembly according to a first embodiment of the invention; Figure 2 illustrates a perspective view of a fuel assembly according to a second aspect of the invention; Figure 3 illustrates a perspective view of a fuel* assembly according to a third aspect of the invention; and Figure 4 illustrates embodiments of the invention employed in BWR, AGR, WER and CANDU fuel assembly types . Figure 1 shows in plan view the fuel rod array for a fuel assembly according to the invention. The central tube (3) is capable of retaining in-core neutron flux detection or similar instrumentation as required. The (diagonally hatched) tubes (5) correspond to guide thimbles designed to accommodate neutron absorbing reactor control rods upon insertion. Around the periphery of the assembly (crossed) fuel rods (7) are provided, with the remainder of the fuel assembly being made up of (plain) fuel rods (9) . The first type of fuel rod, (7) , is made up on non- irradiated (NIU) or so-called virgin fuel. This fuel represents a lower radiologj cally active material than the second type of fuel (9) which is made up of reprocessed fuel, REPU. REPU may of course include other components then uranium, for instance plutoniu . REPU fuel rods may be MOX fuel rods. By providing the NIU fuel in fuel rods around the perimeter of the assembly the fuel assembly is given a self- shielding capacity. The NIU rods (7) serve to attenuate the radiological activity of the REPU fuel rods (9). As a consequence radiological dose per unit of time given by the REPU fuel rods to an operator coming into proximity with the exterior of the fuel assembly is further reduced. In this first embodiment NIU fuel is provided in the fuel rods around the periphery of the assembly only. An alternative embodiment of the invention is shown in perspective in Figure 2. Here the fuel rod assembly is shown with a portion at the near front corner of the fuel assembly removed. Once again instrument tube (3) and control guide thimbles (5) are provided. In this embodiment the fuel rods are all of the same type but have distinct portions within them. At the end face a portion of the fuel rod pellet stack is provided from a number of discrete pellets (11) formed from NIU fuel. A similar portion may be provided at the other end of the fuel rods. The remainder of the length of the fuel rod is however formed from REPU fuel in pellet form. In this structure the NIU provides a shielding effect to the radiological dose emanating from the end faces of the fuel assembly. The present invention offers a significant benefit in situations where multiple fuel assemblies are employed in a reactor channel. By using fuel assemblies provided with NIU fuel in the extremity positions in the channel the screening effect reduces operator extremity dose during completion of total (multi-element) final assembly operations prior to reactor loading. Such a situation applies to AGR fuel stringers for instance. In a further refinement, the embodiment of Figure 3, the instrument and control rod guide thimbles are provided in the same locations as before. However, in this embodiment the peripheral rods (7) consist throughout their length of NIU fuel . In addition the non-peripheral fuel rods are provided with a NIU portion (11) at their ends, in a similar manner to Figure 2, with REPU fuel being provided in portions (13) covering most of their lengths. A fuel assembly according to this embodiment is thus shielded from top, bottom and all sides, thus reducing the radiological dose given by the REPU in all directions. Whilst the above examples demonstrate the use of the invention in PWR fuel rods and/or assemblies the invention is equally applicable in other nuclear fuel assembly types. Figure 4 illustrates the use of the invention in an AGR assembly (20) with guide tube (22) ; CANDU assembly (30) ,- WER or fast reactor radial blanket (40) and BWR assembly (50) . In each case, by providing some or all of the peripheral rods as NIU fuel the assembly is given a self -shielding capacity against the potential operator radiological dose arising from the REPU fuel (9) . In each case the assembly can be provided with end screening as exemplified in Figure 2, side screening as exemplified in Figure 1 or both side and end screening as exemplified in Figure 3. Whilst achieving the self -shielding effect discussed above the performance of the fuel assemblies remains unaltered as the equivalent reactivity of the NIU or REPU fuel is not necessarily varied. It is the distinction between the disposition of NIU and REPU material within the fuel rods and within the assembly which is used in achieving the effect of the invention, not the material enrichment. If desired the enrichment of fuel in any given rod or any given portion of a rod can be varied as required to give the desired reactivity / assembly peak power rating etc, with the fuel source NIU or REPU being selected to give shielding or not depending on the rods position within the assembly.

Claims: CLAIMS 1. A nuclear fuel assembly comprising a plurality of fuel rods containing fuel, wherein a proportion of the fuel in the fuel assembly is non- irradiated fuel and a proportion of the fuel in the fuel assembly is reprocessed fuel . 2. A fuel assembly according to claim 1 in which 40% of the peripheral fuel rods are provided with non- irradiated uranium. 3. A fuel assembly according to claim 1 or claim 2 in which more than 40% of the fuel rods in the assembly are provided with one or both end portions of non-irradiated uranium. 4. A fuel assembly according to any preceding claim in which peripheral fuel rods provided with non- irradiated uranium are provided with non- irradiated uranium fuel only. 5. The use of a plurality of fuel rods containing non- irradiated fuel to shield against the radiological output of reprocessed fuel provided in the fuel assembly. 6. The use of claim 5 in which a plurality of peripheral fuel rods containing non- irradiated fuel is employed. 7. The use of claim 5 or claim 6 in which a plurality of fuel rods are provided with non- irradiated fuel in one or both end portions of the fuel pellet stack. 8. The use of- claims 5, 6 or 7 in which 70% of the fuel rods are provided with non-irradiated fuel and / or 70% of the fuel rods are provided with one or both end portions of non- irradiated fuel . 9. The use of any of claims 5 to 8 in which all cr substantially all of the peripheral fuel rods in a fuel assembly are provided with non- irradiated fuel only and all or substantially all of the fuel rods have end portions of non- irradiated fuel. 10. A fuel assembly according to any of claims 1 to 4 or the use of claims 5 to 9 in which the fuel assembly is an AGR fuel assembly, CANDU assembly, WER fuel assembly or BWR fuel assembly . 11. A method of producing a nuclear fuel assembly comprising a plurality of fuel rods containing fuel, in which the fuel assembly is provided with a first fuel rod type, the fuel rod having a non- irradiated fuel portion at one or both ends of the fuel rod, the remaining fuel comprising re-processed fuel and / or a second fuel rod type, the second type fuel rod having non-irradiated fuel throughout all or a substantial part of its length, at least some of the second fuel rod type being provided at or near to the periphery of the assembly.

Inventors: MCCARTNEY JOHN, MCCARTNEY, JOHN
Applicants: BRITISH NUCLEAR FUELS PLC, MCCARTNEY JOHN, MCCARTNEY, JOHN
Application year: 1997
Main classification: G21C3
IPC codes: G21C3, G21C5