FABRICATION OF SIMULATED INTERMEDIATE-BURNUP ACR FUEL IN THE RFFL

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CW-124920-CONF-003 UNRESTRICTED

FABRICATION OF SIMULATED INTERMEDIATE-BURNUP ACR FUEL IN THE RFFL

D. Woods, G. Cota-Sanchezand 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.

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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.

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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 Dy Powder

Starting EU Powder Starting NU Powder

Slightly Enriched U Powder Blending

Mastermixing

Final Blending

Sintering Granulating Pre-Pressing

Grinding, Washing and Drying

Pellet Loading into Sheaths Final Pressing

Element Welding

Bundle Welding Bundle Assembling

PuO2 Milling Starting Pu Powder

U Isotopic Dy Content

Pu Content

Geometric Density O/M Ratio Pu Rich Areas Microstructure

Immersion Density Pellet Diameter

Visual Inspection Pellet Stack Length Pellet Stack Weight Visual Inspection

He Leak Test Element Length Element Weight

Visual Inspection He Leak Test Bundle Length Bundle Weight

PCD Green Geometric

Density

Figure 1. MOX Fuel Fabrication Process

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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

(wt% Dy/UO2)

Dysprosium Content (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

(wt%)

U-235 (wt%)

U-236 (wt%)

U-238 (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.

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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.

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Figure 2. ACR-P7 Experimental Test

Based on the results from the scoping tests, a qualification test was performed using the optimal operating parameters. For this qualification test, ten sintered pellets per tray (there are 80 pellets total on a tray) were selected diagonally across the tray for inspection by immersion density.

Figure 3 depicts a typical density distribution of the sintered pellets along the diagonal sample, for each sintering furnace tray.

As the figure shows, a relatively even pellet density distribution was obtained during sintering.

Overall the results showed that the average sintered density 10.45 g/cm3 and the standard deviation of 0.06 g/cm3 meets the density specifications.

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Figure 3. Sintered Densities of the Qualification Batch. .

Ceramography shows that a random and uniform grain size distribution was obtained. In turn, auto-radiography indicates that a random and uniform distribution of Pu-rich areas was produced in the MOX Powder Blending process.

3.4 Endcap GTAW process (fuel element welding)

Fuel element welding was performed in a GTAW welder which included an Intellitig 4

Programmable Precision DC GTAW Controller by Miller Electric. Helium leak detection was performed using a Varian Helium Leak Detector. Scoping tests were performed to determine the optimum welding parameters.

Based on the results of the scoping tests, ten elements were welded using the optimal welding parameters. Overall, the results showed that the average element diameter at the weld was 11.59 mm with a standard deviation of 0.032 mm. All welds passed visual inspection; all welds passed freely through the ring gauge; and all welds passed the helium leak test. Figure 4 depicts a typical metallographic image of an endcap weld, showing a full penetration weld with no overheating, no wall thinning, and free of pores and fractures.

Figure 4. Metallography of a Qualification Endcap Weld

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3.5 Endcap to endplate GTAW process (MOX fuel bundle welding)

To qualify the GTAW process, 10 dummy element welds were made by each welder using a standard manual GTAW welder and Zr filler wire. Standard torque tests were performed using a standard torque wrench and the torque results were evaluated according to the specifications.

Table 4 shows that all GTAW welds met the strength requirements.

Table 5. Torque Test Results for Bundle Assembly Welds Welder Range of Torque Strengths

Measured, Nm Average Torque Strength  Standard Deviation, Nm

Welder 1 12.5 – 20.0 16.2  2.58

Welder 2 10.0 – 22.0 18.2  3.25

In addition, three MOX fuel bundles were welded and visually inspected at the different stages of the welding and shipping process. All welds met the acceptance criteria.

4. MOX fuel manufacturing

MOX fuel bundles were produced according to the fabrication process depicted in Figure 1. The following sections describe the different manufacturing steps of the MOX fuel production

process, along with a summary of the fabrication data.

4.1 Low enrichment uranium (LEU) blending

Low enriched uranium blends were produced by down-blending pre-calculated weights of Enriched Uranium with Natural Uranium and Dy2O3 powders in a Lancaster K-Lab mixer.

Powder samples were taken randomly every four blended batches and analyzed for Dy content by High Performance Liquid Chromatography (HPLC) and for U and isotopic contents by

Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Table 6 shows the results of Dy content and Table 7 shows the results of uranium isotopic content for the different element types produced during the fuel manufacturing campaign.

Table 6. Dy Content for the Various ACR Element Types

Dy/UO2 Dy/U

Element Type wt% wt%

Inner Elements Average = 0.0896 0.1018

Std. Dev. = 0.0027 0.0030

Intermediate Elements

Average = 0.0854 0.0992

Std. Dev. = 0.0011 0.0039

Outer Elements Average = 0.1213 0.1378

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Std. Dev. = 0.0016 0.0018

Table 7. U Isotopic Content for the Various ACR Element Types

234U 235U 236U 238U

Element Type wt% wt% wt% wt%

Inner Elements

Average

= 0.016 1.82 <0.01 98.16

Std Dev

= 0.001 0.01 0.01

Intermediate

Elements Average

= 0.014 1.60 <0.01 98.39

Std Dev

= 0.001 0.01 0.01

Outer Elements Average

= 0.010 1.27 <0.01 98.72

Std Dev

= 0.001 0.01 0.01

4.2 MOX powder blending (PuO2 milling, mastermix and final blending)

The starting PuO2 powder was milled in a Vibratory Mill to ensure an optimal particle size

distribution. This milled powder was then blended to MOX fuel powder using a two-step process to ensure a homogeneous distribution of Pu in the MOX fuel.

First, a mastermix was prepared by blending the total amount of PuO2 required for each powder batch in the process line with the blended LEU powder (Section 4.1). The mastermix was

blended using a mixing vessel equipped with high-speed knife blades. Second, the mastermix was blended down with additional LEU to the specified final blend concentration of the production batch using a low-intensity mixer (Turbula blender).

Thermal Ionization Mass Spectroscopy (TIMS) analysis was performed on composite samples prepared using MOX powder taken from three consecutive batches.

4.3 MOX pellet pressing (pre-pressing, granulating and final pressing)

After final blending, the MOX powder was pre-pressed using an isostatic press to convert the MOX powder into compacts. The MOX powder compacts were then granulated in a low-speed blade mill equipped with a mesh screen. The resulting free-flowing granules were blended with zinc stearate in a rolling mill. The total granulated powder was then final pressed into green

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pellets using a single-cavity hydraulic press. The geometric density of the green pellets was measured frequently to control the pressing parameters.

4.4 MOX pellet sintering and grinding

The green pellets were sintered in a sintering furnace using a reducing atmosphere composed of 10% hydrogen and 90% nitrogen. After sintering, several analyses were performed, including O/M ratio, geometric and sintered density (Table 8).

Table 8. Typical Results for MOX Sintered Pellets

Pellet Diameter Pellet Height Geometric Density Sintered Density O/M Ratio Average Std Dev Average Std Dev Average Std Dev Average Std Dev Average Std Dev

(mm) (mm) (mm) (mm) (g/cm3) (g/cm3) (g/cm3) (g/cm3)

10.475 0.013 13.11 0.76 10.46 0.10 10.46 0.06 2.002 0.002

The next step in the production process consisted of centreless grinding the sintered pellets to the specified diameter and surface finish. The pellets were washed with water and dried in air. After grinding, the pellet diameters of a sample of 45 pellets were measured. In addition, the sintered pellet density was determined by the immersion density method. Pu-rich areas of sintered pellets were also measured to evaluate the capability of the manufacturing process to produce a

homogenous MOX solid solution. Overall, standard ceramography and alpha autoradiography analyses were performed every six batches of sintered pellets to determine Pu-rich area and grain size distribution. Table 9 shows Pu-rich areas and grain size distribution obtained in the

production process of MOX fuel.

Table 9. Average Size of Pu-Rich Areas and Grains in MOX Sintered Pellets

Grain Size Pu-Rich Area Average Std Dev Average Std Dev

(µm) (µm) (m) (m)

8.40 1.05 1.70 1.20

Figure 5 depicts a typical ceramographic image of an etched MOX pellet. A random and uniform grain size distribution can be observed with an average grain size of 8 m and standard deviation of 0.5 m. Figure 6 shows a non-etched ceramographic image and Figure 7 shows a typical auto- radiograph of a MOX pellet. Alpha auto-radiography showed that a random and uniform

distribution of Pu-rich areas was normally produced with the MOX powder blending process.

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Figure 5. Grain Size Distribution of an Etched MOX Pellet from Batch ACR-P78. Magnification:

200x

Figure 6. Non-Etched Ceramographic Image of a MOX Pellet from Batch ACR-P18.

Magnification: 100x

Figure 7. Alpha Auto-Radiography of a MOX Pellet from Batch ACR-P12. Magnification: 25x

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4.5 MOX pellet loading and element welding

After visual inspection, ground pellets were formed into stacks and loaded into empty Zircaloy sheaths which had one endcap previously resistance welded in place.

The second endcap was welded to the loaded sheath using a Gas Tungsten Arc Welding (GTAW) process. After welding, the element length was measured on a sample of 38 finished elements.

The finished elements were required to pass a helium leak-test and were scanned for surface alpha contamination. Table 10 shows average values for the MOX fuel elements produced.

Table 10. Welded MOX Elements

Stack Length Element Length Loaded Element Wt Stack Wt. No. of Pellets Average Std dev Average Std dev Average Std dev Average Std dev Average Std dev

(mm) (mm) (mm) (mm) (g) (g) (g) (g)

481.0 0.145 491.89 0.07 471.4 2.045 421.1 2.025 42 2.7

4.6 MOX bundle assembly welding

Bundle assembly was performed in the RFFL by GTAW welding the endcap spigots to the

endplate. A bundle assembly jig was used to locate and align the element ends with the endplate.

The elements were arranged such that all of the GTAW-welded endcaps were located on one end of the bundle. The loaded bundle jig was placed in a tubular chamber flooded with argon to minimize oxidation of the elements during welding. All bundles were welded with a small GTAW weld with filler wire on each side of the endplate where it lay over the flat area on the element end cap. The assembly welds were visually inspected and the finished bundles were weighed,

dimensionally inspected and helium leak-tested. Table 11 shows average values for the MOX fuel bundles produced.

Table 11. MOX Fuel Bundle Assemblies

Bundle Length Average = 495.545 mm

Standard Deviation = 0.074

Bundle Fuel Weight Average = 17689.03 g

Standard Deviation = 17.85

Total Bundle Weight Average = 20694.19 g

Standard Deviation = 17.45 Pitch Circle

Diameter

Inner Elements Average = 33.988 mm

Standard Deviation = 0.120 Intermediate

Elements

Average = 58.569 mm

Standard Deviation = 0.548

Outer Elements Average = 86.649 mm

Standard Deviation = 0.230

Dy Weight Inner Elements Average = 0.375 g

Standard Deviation = 0.008

Intermediate Average = 0.359 g

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Elements Standard Deviation = 0.005

Outer Elements Average = 0.507 g

Standard Deviation = 0.003 (Pu+Am) Weight Inner Elements Average = 0.914 g

Standard Deviation = 0.026 Intermediate

Elements

Average = 0.660 g

Standard Deviation = 0.009

Outer Elements Average = 1.072 g

Standard Deviation = 0.005

U Weight Inner Elements Average = 369.069 g

Standard Deviation = 2.301 Intermediate

Elements

Average = 370.518 g

Standard Deviation = 1.225

Outer Elements Average = 369.782 g

Standard Deviation = 1.800

5. Conclusions

Five special processes were successfully 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. Forty-one MOX bundles with ACR geometry were then fabricated in the RFFL for use in reactor physics code validation measurements to be conducted in the ZED- 2 reactor. 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.

6. References

[1] F.C. Dimayuga, “Development in CANDU MOX Fuel Fabrication”, Proc. 8th Int. Conf.

on CANDU Fuel, Honey Harbour, Ontario (2003 September 21-24).

[2] P. Reid, M. Gacesa and M. Tayal. “The ACR-1000 Fuel Bundle”, Proc. 10th Int. Conf.

on CANDU Fuel, Ottawa, Ontario (2008 October 5-8).

7. Acknowledgement

A special thank you to all of the RFFL staff for their valuable contributions throughout the campaign.

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