Patent Number: 06233302&
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One conventional type of nuclear reactor in which the MOX fuel can be utilized is the pressurized water reactor (PWR). This type of reactor typically combusts a uranium oxide (UO.sub.2) fuel to produce steam. These NSSS (nuclear steam supply systems) traditionally include two-loop arrangement with two steam generators, two hot legs, and four cold legs each with a reactor coolant pump. One specific example of a PWR in which the embodiments of the invention can be implemented is ABB Combustion Engineering's System 80.RTM. reactor which loads 241 fuel assemblies. Each assembly, as seen in FIG. 1, is mechanically identical to the others and consists of a 16.times.16 fuel rod array 20 with five large structural guide tubes 21 that each occupy 2.times.2 fuel lattice locations. The four outer guide tubes are for control element assembly (CEA) fingers, while the center guide tube is used for in-core instrumentation. The in-core instruments are bottom-entry, and therefore do not interfere with the upper internal design for CEA guidance. Each fuel assembly contains 236 fuel rods 22. As seen in FIG. 2, the CEA's have either 4 or 12 element arrangements. The 12 element CEA has the unique characteristic of inserting into five adjacent fuel assemblies, as shown in FIG. 3. This characteristic is made possible by the unique upper guide structure design. of the reactor internals, which provide continuous guidance for each individual CEA element into the fuel assembly guide tube. This upper guide structure, shown in FIG. 4, is a rugged, all-welded structure, and protects each individual CEA element from flow forces and dynamic loads. In this UO.sub.2 core design, burnable absorber pins which contain erbia (Er.sub.2 O.sub.3) admixed with enriched UO.sub.2 are used in the fuel assemblies. These burnable fuel rods are located in predetermined locations to provide reactivity hold down and control power peaking. Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. FIGS. 5A to 14A show ten different 16.times.16 fuel assembly designs containing MOX fuel which were developed for use in the equilibrium cycle core designs of the above mentioned type of nuclear reactor. According to the invention, it is possible to select a number (e.g. 241) of one or more of the these fuel assembly designs and to compile the core in a manner which will enable a particular set of combustion characteristics, such as produced by using only conventional uranium fuel, to be replicated. Burnable absorber rods containing erbia are incorporated into these MOX fuel assembly designs to provide reactivity hold down and control power peaking. These are fuel-bearing burnable absorbers, but do not contain MOX in accordance with the above mentioned ground rules/restraints which have been imposed. Instead, the burnable absorber rods employed in these MOX assemblies are, in the disclosed embodiments, an admixture of erbia and enriched UO.sub.2, and are functionally identical to the burnable absorber pins described earlier in the discussion of the traditional UO.sub.2 core design. The fuel assembly designs in FIGS. 5 to 14 are differentiated by the number of MOX fuel rods and the number of urania-erbia (UO.sub.2 --Er.sub.2 O.sub.3) rods within each assembly as well as by the specific arrangement of these rods. In FIGS. 5 to 14, "M" represents a MOX fuel rod and "E" represents an urania-erbia fuel rod. The number of urania-erbia rods in the fuel assembly designs in the arrangements shown in FIGS. 5 to 14 ranges from 24 to 88. Within each fuel assembly design, the locations of the burnable absorber (urania-erbia) rods and the MOX fuel rods are fixed. Both the UO.sub.2 enrichment in the urania-erbia rods and the plutonium enrichment (wt % of Pu-239) in the MOX fuel rods can be varied during the core design process. Typically, there are 5 to 8 different plutonium enrichments in the MOX fuel rods within any given fuel assembly. For the urania-erbia rods, the UO.sub.2 enrichment is the same in all of the rods within a particular fuel assembly. Each of the fuel assembly designs in FIGS. 5A to 14A were developed on an octant basis and are octant-symmetric. Each of FIGS. 5B to 14B and 5C to 14C depict, for an assembly octant, the specific Pu-239 enrichment of each MOX fuel rod and the resulting normalized intra-assembly power distribution. Since the enrichment of the burnable absorber rods is fixed within any one of these fuel assembly designs, the respective octant maps in FIGS. 5B to 14B and 5C to 14C identify them within each assembly with the letter "E". Actually, two such octant maps are depicted for each. assembly design, representing data for a low enrichment case in FIGS. 5B to 14B and a high enrichment case in FIGS. 5C to 14C, respectively. Between these two cases, each fuel pin's enrichment differs by exactly 1.0 wt. % For the MOX pins, the Pu-239 enrichment is as shown. For the erbia pins, a fixed UO.sub.2 enrichment of 4.0 wt. % is selected for the low enrichment case and a fixed UO.sub.2 enrichment of 5.0 wt. % is selected for the high enrichment case. Each MOX assembly is designed to provide optimal performance over this range of enrichments represented by the low enrichment case and the high enrichment case. Detailed neutronics, generated for both cases, indicates that the neutronics behavior is characterized as a function of fuel enrichment. This design approach makes it possible to consider the effects of varying assembly enrichments during an equilibrium cycle core design phase without the need of re-generating any additional assembly data. By using different fuel rod enrichments within each MOX fuel assembly as described herein and as shown in the corresponding figures, it is possible to both optimize the intra-assembly power peaking, which enhances the performance of the fuel assemblies during operation, and to maximize the throughput of weapons-grade plutonium in each core. The burner absorber rod characteristics for the MOX assembly designs are also arranged to optimize the intra-assembly power peaking and have the secondary benefit of enhancing the throughput of weapons-grade plutonium in each core. FIG. 5A shows the MOX fuel assembly design of a first embodiment of the instant invention having a 16.times.16 fuel rod array including 24 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 5B shows a low enrichment octant map and FIG. 5C shows a high enrichment octant map of this embodiment. Each of these maps depicts one octant of the 236 rod arrangement shown in FIG. 5A. As will be noted, in the case of the low enrichment, while most of the MOX rods have a Pu-239 enrichment of 4.8wt %, a number of the rods, which are in proximity of the guide tubes 21, have lower values which are as low as 3.3wt %. The corresponding MOX rods according to the high enrichment schedule are, as mentioned above, 1% richer. FIG. 6A shows the MOX fuel assembly design according to a second embodiment of the instant invention and which has a 16.times.16 fuel rod array including 32 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 6B shows a low enrichment octant map and FIG. 6C shows a high enrichment octant map of this embodiment. FIG. 7A shows the MOX fuel assembly design of a third embodiment of the instant invention having a 16.times.16 fuel rod array including. 40 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 7B shows a low enrichment octant map and FIG. 7C shows a high enrichment octant map of this embodiment. FIG. 8A shows the MOX fuel assembly design of a fourth embodiment of the instant invention having a 16.times.16 fuel rod array including 48 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 8B shows a low enrichment octant map and FIG. 8C shows a high enrichment octant map of this embodiment. FIG. 9A shows the MOX fuel assembly design of a fifth embodiment of the instant invention having a 16.times.16 fuel rod array including 56 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 9B shows a low enrichment octant map and FIG. 9C shows a high enrichment octant map of this embodiment. FIG. 10A shows the MOX fuel assembly design of a sixth embodiment of the instant invention having a 16.times.16 fuel rod array including 60 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 10B shows a low enrichment octant map and FIG. 10C shows a high enrichment octant map of this embodiment. FIG. 11A shows the MOX fuel assembly design of a seventh embodiment of the instant invention having a 16.times.16 fuel rod array including 64 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 11B shows a low enrichment octant map and FIG. 11C shows a high enrichment octant map of this embodiment. FIG. 12A shows the MOX fuel assembly design of an eighth embodiment of the instant invention having a 16.times.16 fuel rod array including 72 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 12B shows a low enrichment octant map and FIG. 12C shows a high enrichment octant map of this embodiment. FIG. 13A shows the MOX fuel assembly design of a ninth embodiment of the instant invention having a 16.times.16 fuel rod array including 80 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 13B shows a low enrichment octant map and FIG. 13C shows a high enrichment octant map of this embodiment. FIG. 14A shows the MOX fuel assembly design of a tenth embodiment of the instant invention having a 16.times.16 fuel rod array including 88 erbium (UO.sub.2 --Er.sub.2 O.sub.3) rods. FIG. 14B shows a low enrichment octant map and FIG. 14C shows a high enrichment octant map of this embodiment. In accordance with the invention, equilibrium cycle core designs using MOX fuel can be developed using a subset consisting of any combination (e.g. up to three) of the ten fuel assembly designs shown in FIGS. 5A to 14A. FIGS. 15 and 16 show examples of two different equilibrium cycle core loading patterns having a feed batch size of 81 fuel assemblies (i.e. 81 new fuel assemblies). FIG. 17 shows an equilibrium cycle core loading pattern having a feed batch size of 88 fuel assemblies. In FIGS. 15 to 17, "X" represents a fresh assembly, "Y" represents a once-burned assembly and "Z" represents a twice-burned assembly, "O" represents an assembly sub-type with 24 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods, "3" represents an assembly sub-type with 48 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods, "4" represents an assembly sub-type with 56 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods, "5" represents an assembly sub-type with 60 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods and "7" represents an assembly sub-type with 72 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods. FIG. 15 shows a feed batch having 25 assemblies with 24 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods and 56 assemblies-with 56 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods. The average enrichment of MOX fuel rods in this feed batch is 5.16 wt % Pu-239. FIG. 16 shows a second feed batch arrangement having 17 assemblies with 24 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods, 36 assemblies with 56 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods and 28 assemblies with 72 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods. The average enrichment of MOX fuel rods in this feed batch is 5.01 wt % Pu-239. FIG. 17 shows a third feed batch arrangement having 64 assemblies with 48 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods, 12 assemblies with 56 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods and 12 assemblies with 60 UO.sub.2 --Er.sub.2 O.sub.3 fuel rods. The average enrichment of MOX fuel rods in this feed batch is 4.67 wt % Pu-239. A summary of some important design parameters for the equilibrium cycle for a MOX core design and a typical UO.sub.2 core design is shown in Table 1 set forth at the end of this disclosure. These equilibrium cycle core designs using MOX fuel were evaluated to assess their performance characteristics relative to a typical UO.sub.2 equilibrium cycle core design. As will be appreciated, the invention enabled the MOX core to perform in a manner which closely corresponds to the power level--average coolant temperature series of parameters produced with a UO.sub.2 core. Table 2, which is also set forth at the end of this disclosure, shows a comparison of some important core performance characteristics for the 88 feed batch assembly MOX core design shown in FIG. 17 and a typical 18-month cycle UO.sub.2 core design. The core average burn-up for the MOX-based 18-month cycle core design (17,000 MWd/MTHM) is consistent with that for a similar UO.sub.2 -based cycle (17,500 MWd/MTHM). The maximum fuel rod burn-up is within the licensed limit of 60,000 MWD/MT. The discharge burn-up (46,000 MWd/MTHM) is consistent with discharge burn-ups (45,000 MWd/MTHM) for comparable UO.sub.2 -based fuel cycles. The hot full power (HFP) all-rods-out (ARO) BOC critical boron concentration (CBC) for the MOX-based core. design is 1990 ppm, compared to 1250 ppm for a UO.sub.2 -based core design. Although larger than the value for a typical UO.sub.2 core, the HFP BOC CBC for the MOX core is less than the maximum allowable value of 2000 ppm necessary to remain within the existing analysis envelope for existing plants. The power distributions for the MOX-based core design are similar to those for a comparable UO.sub.2 -based core design. The maximum expected peaking factors for the MOX core are slightly higher than those for the UO.sub.2 core, but within the allowable limits (less than or equal to 1.72 for Fr, 2.00 for Fz, for HFP ARO conditions) necessary to remain within the existing analysis envelope. The MOX-based equilibrium cycle core designs developed in the instant invention achieve a throughput of approximately 1.5 MT (Metric Tons) of weapons-grade plutonium per 18 month cycle. As a result, the disposal of 50 MT of weapons-grade plutonium could be accomplished in three System 80.RTM. reactors in approximately 17 years of plant operation. This includes the transition from a conventional, low enrichment UO.sub.2 core to a MOX core. Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. For example, as will be self-evident from the above disclosure, if a fuel qualification for MOX fuel wherein the use of erbia in the MOX rods was permitted, a substantial increase in the throughput of plutonium would be enabled with an attendant reduction in the time needed to dispose of any given quantity of weapons grade plutonium. TABLE 1 Core Design Parameters UO.sub.2 Core MOX Core Power Level 43876 Mwth 3876 Mwth Nominal Cycle Length 18 Months 18 Months .about.460 EFPD 463.5 EFPD 17,500 GWd/MTHM 17,000 GWd/MTHM Fuel Assemblies 241 241 Fuel Assembly Configuration 16 .times. 16 16 .times. 16 Fuel Rod Locations/Assembly 236 236 Active Core Height (inches) 150 150 Fuel Loading (MTHM) 102.3 102.3 Fuel Type Enriched U-235 Enriched WG Pu.sub.2 O.sub.3 in Tails UO.sub.2, Burnable Absorber Type Er.sub.2 O.sub.3 in Enriched UO.sub.2 Er.sub.2 O.sub.3 in Enriched UO.sub.2 Fuel Management 3-batch, mixed central zone 3-batch, mixed central zone Erbia Loading (integral) &lt;2.5 wt % (integral) &lt;2.5 wt % Feed Batch Size Assemblies 72-104 81-88 Feed Fuel Enrichment &lt;4.5 wt % U-235 .about.4.5-5.0 wt % Pu-239.sup.(1) Soluble Burnable Absorber Natural B.sub.10 Natural B.sub.10 Control Element Assemblies Standard Configuration 76 Full-Length, Full Strength 76 Full-Length, Full Strength 13 Part-Length, Part Strength 13 Part-Length, Part Strength or Enhanced Configuration 89 Full-Length, Full Strength Average Heat Generation Rate 5.45 KW/FT 5.45 KW/FT Average Coolant Temperature 585 .degree. F. 585 .degree. F. .sup.(1) Average for MOX pins only. TABLE 2 Core Performance Characteristics System 80 .RTM. Equilibrium Cycle Core Design UO.sub.2 Core MOX Core BOC EOC BOC EOC Burnup Data, MWd/MTHM Core Average 13,700 31,200 17,000 34,000 Maximum Fuel Rod -- 51,600 -- 57,400 Discharge Batch Average -- 45,000 -- 46,400 Critical Boron Data, PPM HFP, ARO 1250 1 1990 90 Inverse Boron Worth, PPM/% .DELTA..rho. Hot Full Power -130 -107 -227 -169 Maximum Peaking Factors Fr (HFP, ARO) 1.51 1.64 Fq (HFP, ARO) 1.83 1.96 Moderator Temperature Coefficent (MTC), 10.sup.-4 .DELTA..rho./.degree. F. Hot Zero Power +0.17 -- -1.62 -- Hot Full Power -0.72 -2.89 -1.80 -3.91 Standard CEA Standard CEA Enhanced CEA Configuration Configuration Configuration CEA Worths, % .DELTA..rho..sup.(1) Total Net Total Net Total Net BOC, HZP 12.6 10.0 11.0 8.7 11.9 9.9 EOC, HZP 15.1 11.3 13.3 10.2 14.4 11.3 EOC, Cold (68.degree. F.) 11.1 7.5 10.2 6.7 11.1 7.7 .sup.(1) The CEA worths are raw values, with no biases or uncertainties, for comparison purposes only.