11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 CW-124920-CONF-003 UNRESTRICTED FABRICATION OF SIMULATED INTERMEDIATE-BURNUP ACR FUEL IN THE RFFL D. Woods, G. Cota-Sanchez and I. Dimayuga Atomic Energy of Canada Limited, Chalk River, Ontario, Canada ABSTRACT - Due to the differences in reactor design between the Advanced CANDU Reactor (ACR®) and a standard CANDU® reactor, a program of reactor physics validation measurements is being conducted in the ZED-2 reactor to support the use of the reactor physics computer code tool set for application in ACR. These measurements include a series of experiments using MOX fuel that simulated intermediate-burnup ACR fuel. The Recycle Fuel Fabrication Laboratories (RFFL) at the Chalk River Laboratories, a facility designed to produce experimental quantities of MOX fuel for reactor physics and irradiation tests [1], conducted a fabrication campaign to manufacture this MOX fuel. The objective of the RFFL fabrication campaign was to produce 41 MOX fuel bundles with the ACR geometry, which is a modified 43-element CANFLEX® design [2]. The ACR fuel bundle consists of 42 11.5 mm diameter elements in the outer rings and a 20 mm diameter centre element. Forty of these bundles were assembly welded and one was a demountable bundle that allows special elements to be installed and removed for fine structure experiments. Based on a study done to determine the composition of the simulated intermediate-burnup ACR fuel, the MOX fuel bundles contained different fuel compositions (i.e., different Pu and Dy contents and different 235U enrichments) for each ring of elements. The fabrication process used, from the starting fuel powders to the finished elements and bundles, will be presented, including qualification results and fabrication data. 1. Introduction The RFFL is a facility specially designed for the fabrication of alpha-active fuels [1]. A campaign to fabricate MOX fuel to simulate mid-burnup ACR fuel for the lattice physics testing in the ZED- 2 reactor was completed recently. The objective of the RFFL fabrication campaign was to produce 41 MOX fuel bundles with the ACR geometry, consisting of forty two 11.5-mm diameter elements in the outer three rings (i.e., 21 elements in the outer ring, 14 elements in the intermediate ring, and 7 elements in the inner ring) and one 20-mm diameter centre element [2]. The composition of the 11.5 mm diameter elements is given in Table 1. The 20 mm diameter centre elements were not fabricated in the RFFL and are not discussed further in this paper. Forty MOX fuel bundles were assembly welded and one demountable bundle was fabricated to allow special demountable elements to be installed and removed for fine structure experiments. 11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 Five special processes (processes that cannot be verified during the fabrication campaign) were qualified for use during fabrication: low enriched uranium powder blending, MOX powder blending, sintering of MOX fuel pellets, fuel element welding, and bundle assembly welding. The results of these qualifications are presented in the appropriate sections below. Table 1. Composition of ACR MOX Fuel Elements for ZED 2 Element Type Number of Elements Fuel Composition Welded Bundles Demountable Bundle Total Inner 280 7 287 1.805 %235U/U + 0.259% (Pu+Am)/U + 0.108 %Dy/U Intermediate 560 14 574 1.583 %235U /U + 0.187% (Pu+Am)/U + 0.102 %Dy/U Outer 840 21 861 1.258 %235U /U + 0.303% (Pu+Am)/U + 0.149 %Dy/U TOTAL 1722 2. Fabrication process description Figure 1 depicts the sequence of manufacturing and inspection steps carried out in the RFFL during the fabrication process of MOX fuel bundles. The first step in the fabrication process involved the blending of the starting low Enriched uranium (LEU) powder by down-blending the 4.95% Enriched Uranium (EU) with Natural Uranium (NU) and a specified amount of Dy2O3. After blending, the U isotopic and Dy contents were determined by chemical analyses. The next step consisted of the milling the starting PuO2 powder to ensure an optimal particle size distribution. Then, the MOX fuel powder was blended using a two-step process to ensure a homogeneous distribution of Pu in the MOX fuel. The MOX powder was pre-pressed using an isostatic press, to convert it into compacts, which were, in turn, fed into a low-speed blade mill to be granulated. After this step, zinc stearate was added as lubricant to the resulting free-flowing granules. The total granulated powder was then final pressed into green pellets using a single- cavity hydraulic press. The geometric density of the green pellets was measured frequently to control the pressing parameters. The green pellets were sintered in a reducing atmosphere. After sintering, several analyses were performed, including O/M ratio, geometric density, determination of Pu rich areas and microstructure. The next step of the production process consisted of centreless grinding of the sintered pellets to the specified diameter and surface finish. The final diameter was measured and the pellet density was determined by the immersion density method. 11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 After visual inspection, ground pellets were loaded into empty Zircaloy sheaths (the sheaths had previously had one endcap resistance welded in place). The second endcap was welded to the loaded sheath using a Gas Tungsten Arc Welding (GTAW also known as Tungsten Inert Gas (TIG) welding) system. The sealed elements were then helium leak-tested, and scanned for surface alpha contamination. Bundle assembly was performed in the RFFL by GTAW welding the end cap spigots to the endplate. A bundle assembly jig was used to locate the elements and align the element ends with the endplate. The elements were arranged such that all of the GTAW-welded endcaps were located at one end of the bundle. The assembly welds were visually inspected and the finished bundles were weighed, dimensionally inspected and helium leak-tested. Starting EU Powder Starting Dy Powder Starting NU Powder Starting Pu Powder Slightly Enriched U Powder PuO2 Milling Blending U Isotopic Dy Content Mastermixing Final Blending Pu Content Pre-Pressing Granulating Final Pressing Green Geometric Density Geometric Density Sintering O/M Ratio Pu Rich Areas Microstructure Grinding, Washing and Drying Immersion Density Pellet Diameter Pellet Loading into Sheaths Visual Inspection Pellet Stack Length Pellet Stack Weight Element Welding Visual Inspection He Leak Test Element Length Element Weight Bundle Assembling Bundle Welding Visual Inspection He Leak Test Bundle Length Bundle Weight PCD Figure 1. MOX Fuel Fabrication Process 11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 3. Qualification of special processes The following special processes were qualified for the fabrication of MOX fuel bundles: Low Enriched Uranium (LEU) Powder Blending, MOX Powder Blending, Sintering of MOX Fuel Pellets, Welding of Fuel Elements and Welding of MOX Fuel Bundles. 3.1 Low enriched uranium (LEU) powder blending Pre-calculated weights of NU and EU (4.95% 235U) were blended in a Lancaster K-Lab mixer. Four 0.5-g samples taken randomly were analysed for Dy content by High Performance Liquid Chromatography (HPLC), and U content and isotopics by Thermal Ionization Mass Spectroscopy (TIMS). Table 2 and 3 show the results of Dy content and U isotopics, respectively. Table 2. Dysprosium Content of the Qualification Test Dysprosium Content Dysprosium Content (wt% Dy/UO2) (wt% Dy/U) Average = 9.35E-02 0.108 Std Dev = 9.7E-4 9.57E-4 Table 3. Isotopic Analysis of the Qualification Test U-234 U-235 U-236 U-238 (wt%) (wt%) (wt%) (wt%) Average 0.0152 1.8090 0.0007 98.1750 Std Dev 0.00010 0.0028 0.00009 0.00283 As the acceptance criteria for qualification required, the average values of the Dy and 235U contents were within 99-101% of the target value while the standard deviations were 0.001 and 0.0028 for Dy and 235U content, respectively. 3.2 MOX powder blending Pre-calculated weights of LEU powder and PuO2 were blended in a 2 L high-speed mastermixer and a 20 L turbula mixer. Four powder samples were analyzed for Pu content by Thermal Ionization Mass Spectroscopy (TIMS). Table 4 shows the qualification results in terms of Pu/MOX and Pu+Am/U. 11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 Table 4. Pu Content by TIMS and (Pu+Am)/U Ratio Pu/MOX (Pu+Am)/U wt% wt% Average = 0.216 0.258 Std Dev = 0.003 0.004 3.3 Sintering of MOX fuel pellets The densification of MOX fuel pellets depends on the operating parameters of the sintering process, as well as the initial density (green density) of the pellets, which in turn depends on the operating parameters of the final-press. The optimal parameters for the final-pressing and sintering processes of MOX fuel pellets were determined prior to this qualification process. In this test, 48 pellets were pressed varying the ram pressure of the final-press. These pellets were later sintered in a reducing atmosphere. The processes of final pressing and sintering were later evaluated using the density results of both the green and sintered pellets. Figure 2 depicts the results of both green and sintered densities as function of the ram pressure of the final-press. 11th International Conference on CANDU Fuel Sheraton Fallsview Hotel and Conference Centre Niagara Falls, Ontario, Canada, 2010 October 17-20 Figure 2.