**6. Production of MTR nuclear fuel**

40 Radioisotopes – Applications in Physical Sciences

Fig. 9. Crucible arrangement of before melting to produce the intermetallic. U3Si2 product and its x-ray diffractogram results compared to CERCA product and JPDF 47-1070 for pure

U3Si2.

The reference industrial process to produce plate-type fuel involves roll-milling together the fissile core, or fuel meat (a blend of an uranium compound and aluminum powders), and the cladding (aluminum alloy plates). This process can draw on considerable feedback from experience, since nearly all research reactors use this type of fuel. The process has seen large-scale implementation with NUKEM, in Germany, UKAEA, in the United Kingdom, CERCA, in France, and Babcock, in the United States.

Fig. 10. Fabrication process of silicide fuel elements.

In general, the MTR type fuel element fabrication process using silicide (U3Si2) can be divided into the following main steps: hydrolysis of UF6 through its reaction with water; production of uranium tetrafluoride (UF4); production of metallic uranium; U3Si2 powder production from uranium metal; production of fuel cores from U3Si2 and aluminum powders; production of fuel plates with U3Si2-Al dispersion; assembling of fuel elements; recovery of uranium; effluent treatment; quality control.

The simplified block diagram of the fabrication process for silicide fuel elements is shown in Figure 10. The manufacturing process of the fuel begins with the UF6 processing. The UF6 is enriched to 19,75 wt% 235U, a enrichment level that categorize the fuel as LEU (low enriched uranium). Bellow the main stages of manufacture of such fuel are discussed.

According to the calculation for the masses of U3Si2 and aluminum powders, the charges for pressing are weighed separately. The weighing is carried out in glass bottles specially designed for installation in a homogenizer. Once the powders are weighed, the charge is mixed inside a glove box with inert atmosphere. This blending ensures that the specified amount of 235U is homogeneously distributed throughout the briquette to be pressed. The weighing operation is performed carefully and, after homogenization, the cautious handling

The homogenization operation is performed using a special homogenizer with a capacity for simultaneous mixing of eight charges. The duration of homogenization is 120 minutes under rotation of 36 rpm and angle of 45o. To prepare the briquettes, the homogenized charges are pressed at room temperature using a hydraulic press with capacity for 700 tons, which is placed in a glove box. The pressing pressure is adjusted to get the desired thickness, keeping the residual porosity from 5 to 7% by volume. The bottle containing the homogenized charge is transferred from the glove box used for homogenization to the glove box used for pressing. Within this glove box, the charge of a briquette is emptied into the die cavity with the inferior puncture initially raised. The powder is placed in layers with the aid of a special smoother to prevent segregation and to minimize the variation of the thickness of the briquette, lowering the punch inferior gradually until all the charge is loaded, when the punch is fully lowered to its position during the pressing. Then, the superior punch is inserted and pressure is applied and maintained for 15 seconds. The entire array is then opened to eject the briquette and the punch superior, which is manually removed. The thickness of the briquette is defined based on final specifications valid for the fuel meat. This thickness is theoretically calculated and then

Immediately prior to the transfer of the briquettes to be used in the manufacture of fuel plates, they are vacuum degassed at 2 x 10-3 torr in a retort. The temperature is 250 oC kept for 1 hour. After remaining inside the degassing retort for the time and temperature specified, the briquette is removed for cooling, keeping the vacuum system working until the room temperature is reached. Thus, the briquettes that will compose the cores of the fuel plates are used in the new phase of processing, or assembling the sets for rolling. Figure 11

of the charge is critical to avoid segregation.

adjusted through manufacturing tests.

illustrates the process for preparing the briquettes and the set.

Fig. 11. Process for briquettes preparation and degassing.

#### **6.1 Fuel cores production from U3Si2 and aluminum powders**

The U3Si2 ingot produced in the previous step is transferred to a glove box with inert atmosphere of argon, since the U3Si2 is pyrophoric. Inside the glove box, the ingot is subjected to a preliminary grinding, resulting in granules less than 4 mm in size with the smallest fraction of fines (< 44 μm) possible. This operation is performed with the aid of a manual crusher. After doing the preliminary grinding, the material is placed directly on a set of sieves, and then sieved by hand. The sieve set comprises a coarse sieve with 4 mm opening, a fine sieve with 150 μm opening and a background compartment. The granules with a diameter greater than 4 mm are crushed again. The granules with size between 4 mm and 150 μm are collected for final grinding and particles smaller than 150 μm are collected separately for particle size classification.

The U3Si2 obtained after the preliminary grinding is manually milled again. The material collected during the preliminary grinding (between 4 mm and 150 μm) is processed in this step. The grind is done carefully, with intermediate sieving, to classify the powder in the range from 150 to 44 μm. The specification allows 20 wt% fines fraction (below 44 μm) as maximum. The fraction above the specification (150 μm) is sent back to the final grinding system. The fraction inside the specified range (between 150 μm and 44 μm) is collected and stored. The fraction of fines (< 44 μm) is collected and stored separately. The glove box contains a vibrating screening machine, which performs the separation of three size fractions of silicide powder, above 150 μm, between 150 and 44 μm and bellow 44 μm. The batch U3Si2 powder composition is adjusted to have maximum fines content in the level of 20 wt%, as specified.

The next process step is the fabrication of the fuel cores, which will form the core of the fuel plates, or fuel meats. The core of the fuel plate contains U3Si2 as the fissile material. This core is fabricated by means of powder metallurgy techniques and is normally called briquette or fuel compact. Initially, the mass and composition of the briquette are calculated based on the analyzed values of total uranium and isotope enrichment of the U3Si2 powder. The criterion for calculating the briquette mass is the amount of the isotope 235U specified for the fuel and the dimensions of the briquette. Based on the calculated mass of the briquette, the silicide Al powders mass are determined separately and mixed together to ensure that the specified 235U amount is uniformly distributed. These charges are cold pressed to form the fuel compacts, and the briquettes are measured and weighed. The final dimensions of the fuel meat in the finished fuel plate are set by specification and the volume of the briquette is calculated from these data by their values of thickness, width and length. The thickness of the briquette is obtained by multiplying the specified thickness of the fuel meat by the deformation dimension resulted after rolling operation, assuming zero enlargement. The core content of voids depends only on the volume fraction of fuel powder content. To optimize the final geometry of the rolled core, the briquette has rounded corners, and the volume of the corners is included in the calculation of volume.

The difference between the volume of the briquette, obtained as described above, and the volume of the fuel powder, as determined by the division between the mass of the powder and its density, determines the amount of aluminum powder to be added to the mass of the briquette. As the theoretical density of the system cannot be achieved during the compaction of the briquette, the volume of aluminum is reduced by the amount of pores that remain after pressing. The total mass of the briquette is given by the calculated mass of fissile material powder added to the calculated mass of aluminum powder.

The U3Si2 ingot produced in the previous step is transferred to a glove box with inert atmosphere of argon, since the U3Si2 is pyrophoric. Inside the glove box, the ingot is subjected to a preliminary grinding, resulting in granules less than 4 mm in size with the smallest fraction of fines (< 44 μm) possible. This operation is performed with the aid of a manual crusher. After doing the preliminary grinding, the material is placed directly on a set of sieves, and then sieved by hand. The sieve set comprises a coarse sieve with 4 mm opening, a fine sieve with 150 μm opening and a background compartment. The granules with a diameter greater than 4 mm are crushed again. The granules with size between 4 mm and 150 μm are collected for final grinding and particles smaller than 150 μm are collected

The U3Si2 obtained after the preliminary grinding is manually milled again. The material collected during the preliminary grinding (between 4 mm and 150 μm) is processed in this step. The grind is done carefully, with intermediate sieving, to classify the powder in the range from 150 to 44 μm. The specification allows 20 wt% fines fraction (below 44 μm) as maximum. The fraction above the specification (150 μm) is sent back to the final grinding system. The fraction inside the specified range (between 150 μm and 44 μm) is collected and stored. The fraction of fines (< 44 μm) is collected and stored separately. The glove box contains a vibrating screening machine, which performs the separation of three size fractions of silicide powder, above 150 μm, between 150 and 44 μm and bellow 44 μm. The batch U3Si2 powder composition is adjusted to have maximum fines content in the level of

The next process step is the fabrication of the fuel cores, which will form the core of the fuel plates, or fuel meats. The core of the fuel plate contains U3Si2 as the fissile material. This core is fabricated by means of powder metallurgy techniques and is normally called briquette or fuel compact. Initially, the mass and composition of the briquette are calculated based on the analyzed values of total uranium and isotope enrichment of the U3Si2 powder. The criterion for calculating the briquette mass is the amount of the isotope 235U specified for the fuel and the dimensions of the briquette. Based on the calculated mass of the briquette, the silicide Al powders mass are determined separately and mixed together to ensure that the specified 235U amount is uniformly distributed. These charges are cold pressed to form the fuel compacts, and the briquettes are measured and weighed. The final dimensions of the fuel meat in the finished fuel plate are set by specification and the volume of the briquette is calculated from these data by their values of thickness, width and length. The thickness of the briquette is obtained by multiplying the specified thickness of the fuel meat by the deformation dimension resulted after rolling operation, assuming zero enlargement. The core content of voids depends only on the volume fraction of fuel powder content. To optimize the final geometry of the rolled core, the briquette has rounded corners, and the

The difference between the volume of the briquette, obtained as described above, and the volume of the fuel powder, as determined by the division between the mass of the powder and its density, determines the amount of aluminum powder to be added to the mass of the briquette. As the theoretical density of the system cannot be achieved during the compaction of the briquette, the volume of aluminum is reduced by the amount of pores that remain after pressing. The total mass of the briquette is given by the calculated mass of fissile

**6.1 Fuel cores production from U3Si2 and aluminum powders** 

volume of the corners is included in the calculation of volume.

material powder added to the calculated mass of aluminum powder.

separately for particle size classification.

20 wt%, as specified.

According to the calculation for the masses of U3Si2 and aluminum powders, the charges for pressing are weighed separately. The weighing is carried out in glass bottles specially designed for installation in a homogenizer. Once the powders are weighed, the charge is mixed inside a glove box with inert atmosphere. This blending ensures that the specified amount of 235U is homogeneously distributed throughout the briquette to be pressed. The weighing operation is performed carefully and, after homogenization, the cautious handling of the charge is critical to avoid segregation.

The homogenization operation is performed using a special homogenizer with a capacity for simultaneous mixing of eight charges. The duration of homogenization is 120 minutes under rotation of 36 rpm and angle of 45o. To prepare the briquettes, the homogenized charges are pressed at room temperature using a hydraulic press with capacity for 700 tons, which is placed in a glove box. The pressing pressure is adjusted to get the desired thickness, keeping the residual porosity from 5 to 7% by volume. The bottle containing the homogenized charge is transferred from the glove box used for homogenization to the glove box used for pressing. Within this glove box, the charge of a briquette is emptied into the die cavity with the inferior puncture initially raised. The powder is placed in layers with the aid of a special smoother to prevent segregation and to minimize the variation of the thickness of the briquette, lowering the punch inferior gradually until all the charge is loaded, when the punch is fully lowered to its position during the pressing. Then, the superior punch is inserted and pressure is applied and maintained for 15 seconds. The entire array is then opened to eject the briquette and the punch superior, which is manually removed. The thickness of the briquette is defined based on final specifications valid for the fuel meat. This thickness is theoretically calculated and then adjusted through manufacturing tests.

Immediately prior to the transfer of the briquettes to be used in the manufacture of fuel plates, they are vacuum degassed at 2 x 10-3 torr in a retort. The temperature is 250 oC kept for 1 hour. After remaining inside the degassing retort for the time and temperature specified, the briquette is removed for cooling, keeping the vacuum system working until the room temperature is reached. Thus, the briquettes that will compose the cores of the fuel plates are used in the new phase of processing, or assembling the sets for rolling. Figure 11 illustrates the process for preparing the briquettes and the set.

Fig. 11. Process for briquettes preparation and degassing.

removal from the furnace, the fuel plates are visually inspected for observation and recording of bubbles (47; 48). Fuel plates that present bubbles are registered as reject and

The cold rolling operation is performed in the same rolling mill used in the hot rolling. In this operation the specified thickness is achieved with precision. The total cold reduction is approximately 10% in thickness and is applied in one or two passes. During cold rolling, the length of the fuel meat is checked, ensuring the fulfillment of the specification for the

After cold rolling, the fuel plates are pre-cut for facilitate handling during the subsequent fabrication operations, as flattening, radiography and final cut. The fuel plates obtained in cold rolling have their surfaces still undulating, requiring a flattening operation. This operation is performed using a roll-flattener, which is basically consisted with a group of flattener cylinders controlled by a position adjustment system to keep the cylinders in a flat

The next step is the final cut of the fuel plate to reach the specified dimensions. This cut is made using a guillotine cutter machine and is oriented by x-ray radiography. This radiography is obtained by using an industrial system set, where the fuel meat can be perfectly positioned inside the fuel plate and, then, the plate receives line tracing to guide the final cut. Next, the fuel plates are degreased in acetone and pickled in a solution of NaOH 10wt% for 1 minute at 60 oC. Then, they are washed in water for 1 minute, neutralized in cold 40wt% HNO3 for 1 minute, rinsed again in running demineralized water for 5 minutes (spray), washed by immersion in hot demineralized water and dried manually with the aid of hot air blast. Figure 13 shows a drawing of the fuel plate, illustrating its fuel meat. Figure 14 shows the sequence of operations performed to

forwarded for chemically recover of uranium.

minimum core length and for the thickness of the fuel plate.

position. Only one pass is enough to flatten the fuel plates.

Fig. 13. Illustration of the process for preparing the assemblies and rolling

manufacture the fuel plates.

## **6.2 Production of fuel plates with U3Si2 – Al dispersion**

The technology of fuel plates manufacture adopts assembling and rolling of a set composed by the fuel meat (briquette), a frame plate and two cladding plates. In this way, after the rolling operation, it is fabricated a fuel plate containing inside the fuel meat totally isolated from the environment, which is done through the perfect metallurgical bonding between the core and frame with the claddings. The frame and cladding plates are made from commercial aluminum Al 6061 alloy (48).

In order to prepare the rolling assemblies, the frame plate is heated in a furnace at 440 °C. The cold briquette is then assembled inside the frame plate. Once cooled the frame, the briquette should be perfectly housed and fixed in the frame cavity by mechanical interference. The other cladding plates are placed above and below the frame plate with the core, completing then the assembling to be rolled. This assembly set is then fixed in a rotating welding bench and welded at its edges. The welding is TIG type protected with argon. A continuous welding bead is done on the four corners of the assembly, leaving the ends free in order to allow air to be exhausted in the first rolling pass. Figure 12 illustrates the procedure of preparing the assemblies for rolling.

The welded assemblies are properly identified and inserted in a furnace for 60 minutes at a temperature of 440 °C. The hot rolling is performed in several passes following a wellestablished rolling schedule. The rolling schedule defines thickness reduction per pass in order to control the end defects and the final dimensions of the fuel meat. The rolling schedule is determined by theoretical calculations and empirical data from manufacturing tests and must guarantee the metallurgical bonding and the control and reproducibility of the fuel meat deformation. The rolling mill usually has an accuracy of 0.025 mm and is equipped with rolling cylinders coated with a chrome layer. It is important the perfect lubrication of the rolling cylinders. Between each pass, the assemblies are reheated for 15 minutes. After the final hot-rolling pass, the fuel plates are identified again in the same position of the initial identification in a region outside the fuel meat, using mechanical marker.

After hot rolling, a blister test is performed to test the metallurgical quality of bonding between meat-frame-claddings. The hot rolled plates are heated at 440 °C for 1 hour. After

The technology of fuel plates manufacture adopts assembling and rolling of a set composed by the fuel meat (briquette), a frame plate and two cladding plates. In this way, after the rolling operation, it is fabricated a fuel plate containing inside the fuel meat totally isolated from the environment, which is done through the perfect metallurgical bonding between the core and frame with the claddings. The frame and cladding plates are made from

In order to prepare the rolling assemblies, the frame plate is heated in a furnace at 440 °C. The cold briquette is then assembled inside the frame plate. Once cooled the frame, the briquette should be perfectly housed and fixed in the frame cavity by mechanical interference. The other cladding plates are placed above and below the frame plate with the core, completing then the assembling to be rolled. This assembly set is then fixed in a rotating welding bench and welded at its edges. The welding is TIG type protected with argon. A continuous welding bead is done on the four corners of the assembly, leaving the ends free in order to allow air to be exhausted in the first rolling pass. Figure 12 illustrates

**6.2 Production of fuel plates with U3Si2 – Al dispersion** 

the procedure of preparing the assemblies for rolling.

Fig. 12. Diagram illustrating the assembling of the set core-frame-claddings.

The welded assemblies are properly identified and inserted in a furnace for 60 minutes at a temperature of 440 °C. The hot rolling is performed in several passes following a wellestablished rolling schedule. The rolling schedule defines thickness reduction per pass in order to control the end defects and the final dimensions of the fuel meat. The rolling schedule is determined by theoretical calculations and empirical data from manufacturing tests and must guarantee the metallurgical bonding and the control and reproducibility of the fuel meat deformation. The rolling mill usually has an accuracy of 0.025 mm and is equipped with rolling cylinders coated with a chrome layer. It is important the perfect lubrication of the rolling cylinders. Between each pass, the assemblies are reheated for 15 minutes. After the final hot-rolling pass, the fuel plates are identified again in the same position of the initial identification in a region outside the fuel meat, using mechanical

After hot rolling, a blister test is performed to test the metallurgical quality of bonding between meat-frame-claddings. The hot rolled plates are heated at 440 °C for 1 hour. After

commercial aluminum Al 6061 alloy (48).

marker.

removal from the furnace, the fuel plates are visually inspected for observation and recording of bubbles (47; 48). Fuel plates that present bubbles are registered as reject and forwarded for chemically recover of uranium.

The cold rolling operation is performed in the same rolling mill used in the hot rolling. In this operation the specified thickness is achieved with precision. The total cold reduction is approximately 10% in thickness and is applied in one or two passes. During cold rolling, the length of the fuel meat is checked, ensuring the fulfillment of the specification for the minimum core length and for the thickness of the fuel plate.

After cold rolling, the fuel plates are pre-cut for facilitate handling during the subsequent fabrication operations, as flattening, radiography and final cut. The fuel plates obtained in cold rolling have their surfaces still undulating, requiring a flattening operation. This operation is performed using a roll-flattener, which is basically consisted with a group of flattener cylinders controlled by a position adjustment system to keep the cylinders in a flat position. Only one pass is enough to flatten the fuel plates.

The next step is the final cut of the fuel plate to reach the specified dimensions. This cut is made using a guillotine cutter machine and is oriented by x-ray radiography. This radiography is obtained by using an industrial system set, where the fuel meat can be perfectly positioned inside the fuel plate and, then, the plate receives line tracing to guide the final cut. Next, the fuel plates are degreased in acetone and pickled in a solution of NaOH 10wt% for 1 minute at 60 oC. Then, they are washed in water for 1 minute, neutralized in cold 40wt% HNO3 for 1 minute, rinsed again in running demineralized water for 5 minutes (spray), washed by immersion in hot demineralized water and dried manually with the aid of hot air blast. Figure 13 shows a drawing of the fuel plate, illustrating its fuel meat. Figure 14 shows the sequence of operations performed to manufacture the fuel plates.

Fig. 13. Illustration of the process for preparing the assemblies and rolling

In IPEN, two types of fuel elements are manufactured. The standard fuel element consists of 18 fuel plates, 2 side plates (right and left), a nozzle, a handling pin and 8 screws. The control fuel element is composed of 12 fuel plates, two side plates (right and left), two guide plates, a nozzle, a dashpot and 12 screws. The dimensional characteristics of the fuel elements are specified. All structural components of the fuel element are manufactured

The process begins with the assembling of fuel plates to form a case that is the structural body of the fuel element. The plates are fixed to the side plates (left and right) by mechanical clamping. Subsequently, the nozzle is fixed. For the standard fuel element, the handling pin is fixed on the side opposite to the nozzle. In the case of the control fuel element, the dashpot is fixed on the side opposite to the nozzle. After cleaning and inspection, the fuel element is packed and stored until transportation to the reactor. Figure 15 illustrates the

After fixing the fuel plates in the side plates to form the main case, the next component to be installed is the nozzle. The nozzle is used to fix the fuel elements in the reactor core. It is fixed by screws at the lower end of the main case. The nozzle is aligned with the case of fuel element through an adjustment operation by using precision measuring instruments. The holes in the nozzle that are used to fix the side plates are already machined. The holes to hold the external fuel plates at the nozzle are machined with the nozzle already fixed in the side plates, with the aid of a milling machine. The screws used are made with aluminum and are already qualified and properly cleaned before use. The final tightening is done after a previous dimensional characterization, once verified the alignment of the nozzle in the main case. If alignment does not meet the specification, it is adjusted. In the case of the control fuel element, the procedure for fixing the nozzle and dashpot is the same as

The handling pin is used to handle the standard fuel element inside the reactor pool. It is installed at the upper end of the main case, which contains two holes where the handling pin is fixed by clinching. In this operation, the ends of the handling pin, which have cavities, are deformed by pressure with the aid of a drilling machine. In the case of the control fuel element, this pin is replaced by the dashpot, which is aimed at damping the control or security bars that operates within this type of fuel elements. Figure 16 illustrates the

**6.3 Assembling of fuel elements** 

according to designs that are part of the specifications.

steps for the fuel elements assembling process.

Fig. 15. The process of assembling the fuel elements.

standard fuel element and its components.

described above.

Fig. 14. The schematic illustration of the finished fuel plate (after rolling).

The finished fuel plates are characterized dimensionally, measuring in its length, width and thickness. Fuel plates that do not meet the dimensional specifications are rejected and sent for uranium recovery.

After the final cut, two new radiographs are obtained. The first one aims at checking the position of the fuel meat inside the fuel plate, as well as to verify its dimension, length and width. The second radiography aims to check the uranium distribution homogeneity in the fuel meat and also its integrity, as well as the possible presence of "white spots" and fissile particles outside the fuel meat zone.

To check the reproducibility and stability of the manufacturing process of fuel plates, the residual porosity of the fuel meat of all fuel plates produced are determined using the Archimedes principle.

Every 24 fuel plates produced, one fuel plate is separated to characterize the end defects in the fuel meat, which are basically the cladding thickness reduction in the area of the "dog-boning", inspection of the "diffuse zone" (end of the fuel meat) for studying the "fish tail" defect and to do the final geometry inspection of the fuel meat. This analysis is performed destructively to allow metallographic image analysis. In the case of fuel plate production routine, the quality analysis samples is randomly made (1:20) over all produced plates to check possible defects that do not meet specifications. In case, the sample is rejected then a second fuel plate is randomly taken from the batch and is destroyed to be examined. If this second sampled plate also proves defective then the entire batch is rejected. This metallographic analysis is performed using standard metallographic techniques and specific equipment for this purpose. All fuel plates rejected are forwarded for uranium chemical recovery.

The metallurgical bonding quality of the assembled plates set, after rolling, is checked by means of bending tests. This test is performed at two occasions, after pre-cutting and after the final cut. This test is performed in the leftover material from the cutting operations. The material is extensively bent in an angle of 180° and in reverse. In case of bonding failure, which is easily detected by visual inspection, the fuel plate is rejected and sent to uranium chemical recovery.

#### **6.3 Assembling of fuel elements**

46 Radioisotopes – Applications in Physical Sciences

Fig. 14. The schematic illustration of the finished fuel plate (after rolling).

All fuel plates rejected are forwarded for uranium chemical recovery.

for uranium recovery.

Archimedes principle.

chemical recovery.

particles outside the fuel meat zone.

The finished fuel plates are characterized dimensionally, measuring in its length, width and thickness. Fuel plates that do not meet the dimensional specifications are rejected and sent

After the final cut, two new radiographs are obtained. The first one aims at checking the position of the fuel meat inside the fuel plate, as well as to verify its dimension, length and width. The second radiography aims to check the uranium distribution homogeneity in the fuel meat and also its integrity, as well as the possible presence of "white spots" and fissile

To check the reproducibility and stability of the manufacturing process of fuel plates, the residual porosity of the fuel meat of all fuel plates produced are determined using the

Every 24 fuel plates produced, one fuel plate is separated to characterize the end defects in the fuel meat, which are basically the cladding thickness reduction in the area of the "dog-boning", inspection of the "diffuse zone" (end of the fuel meat) for studying the "fish tail" defect and to do the final geometry inspection of the fuel meat. This analysis is performed destructively to allow metallographic image analysis. In the case of fuel plate production routine, the quality analysis samples is randomly made (1:20) over all produced plates to check possible defects that do not meet specifications. In case, the sample is rejected then a second fuel plate is randomly taken from the batch and is destroyed to be examined. If this second sampled plate also proves defective then the entire batch is rejected. This metallographic analysis is performed using standard metallographic techniques and specific equipment for this purpose.

The metallurgical bonding quality of the assembled plates set, after rolling, is checked by means of bending tests. This test is performed at two occasions, after pre-cutting and after the final cut. This test is performed in the leftover material from the cutting operations. The material is extensively bent in an angle of 180° and in reverse. In case of bonding failure, which is easily detected by visual inspection, the fuel plate is rejected and sent to uranium In IPEN, two types of fuel elements are manufactured. The standard fuel element consists of 18 fuel plates, 2 side plates (right and left), a nozzle, a handling pin and 8 screws. The control fuel element is composed of 12 fuel plates, two side plates (right and left), two guide plates, a nozzle, a dashpot and 12 screws. The dimensional characteristics of the fuel elements are specified. All structural components of the fuel element are manufactured according to designs that are part of the specifications.

The process begins with the assembling of fuel plates to form a case that is the structural body of the fuel element. The plates are fixed to the side plates (left and right) by mechanical clamping. Subsequently, the nozzle is fixed. For the standard fuel element, the handling pin is fixed on the side opposite to the nozzle. In the case of the control fuel element, the dashpot is fixed on the side opposite to the nozzle. After cleaning and inspection, the fuel element is packed and stored until transportation to the reactor. Figure 15 illustrates the steps for the fuel elements assembling process.

Fig. 15. The process of assembling the fuel elements.

After fixing the fuel plates in the side plates to form the main case, the next component to be installed is the nozzle. The nozzle is used to fix the fuel elements in the reactor core. It is fixed by screws at the lower end of the main case. The nozzle is aligned with the case of fuel element through an adjustment operation by using precision measuring instruments. The holes in the nozzle that are used to fix the side plates are already machined. The holes to hold the external fuel plates at the nozzle are machined with the nozzle already fixed in the side plates, with the aid of a milling machine. The screws used are made with aluminum and are already qualified and properly cleaned before use. The final tightening is done after a previous dimensional characterization, once verified the alignment of the nozzle in the main case. If alignment does not meet the specification, it is adjusted. In the case of the control fuel element, the procedure for fixing the nozzle and dashpot is the same as described above.

The handling pin is used to handle the standard fuel element inside the reactor pool. It is installed at the upper end of the main case, which contains two holes where the handling pin is fixed by clinching. In this operation, the ends of the handling pin, which have cavities, are deformed by pressure with the aid of a drilling machine. In the case of the control fuel element, this pin is replaced by the dashpot, which is aimed at damping the control or security bars that operates within this type of fuel elements. Figure 16 illustrates the standard fuel element and its components.

The aggregate amount of scrap recycled via chemical recovery may reach 10% or more of finished fuel material weight. Chemical recovery is naturally more costly than direct recycle of metallic scrap to remelt. These considerations justify various expedients to by-pass chemical recovery by recycling metallic scrap. However, particular emphasis is given to the recovery of all residues solids and liquids because of the higher intrinsic value of the enriched material. As an example, the IPEN process to produce U3Si2 involves metallic uranium as an intermediate product, through magnesiothermic reduction which produces slags containing uranium. The recovery process consists on slag lixivium of calcined by-products from metallic uranium reduction. The results from researching this process confirmed that this method could be integrated in treatment and recovery routines of uranium. The chemical route avoids dealing with metallic uranium since this material is unstable, pyroforic and extremely reactive. On the other hand, U3O8 is a stable oxide with low chemical reactivity, and it justifies the slags calcination of metallic uranium reduction by-products. This calcination occurs under oxidizing atmosphere and transforms the metallic uranium into U3O8. Some experiments have been carried out using diferente nitric molar concentrations, acid excess contents and temperature control of the lixivium process. The nitric lixivium main chemical reaction for calcined metallic uranium slags is represented by the equation:

U3O8 (s) + 8 HNO3 (I) → 3 UO2(NO3)2 (I) + 2 N02(g) + 4H2O(l) (26)

• Granulometric control: sieving and segmentation of calcined slag in the range of 100-

As results, the full lixivium took 9 hours; the fluoride concentration in lixivium was 0,002g/L. Lixivium made at lower temperatures and lower nitric concentrations reduced both the magnesium and calcium fluorides solubility and the corrosion effect caused bifluoride ions was not prominent. This ensured a stable and secure lixivium from the operational point of view. The nitric dissolution of metallic uranium slags produced uranyl nitrate solution, which has been reused as a feed-in compound for uranium purification system made by solvent extraction method, using diluted n-tributhylphospate. The purified uranium product was then precipitated as ammonium diuranate (ADU) at 60°C, by injecting ammonium gas diluted with air. Aiming at returning the recovered product to the fuel fabrication cycle with nuclear quality level, the purified ADU was converted into uranium tetrafluoride (UF4) by U3O8 route. The final yield in U content was 94%, proving the viability

Thanks are due to IPEN for providing generously the technology of Nuclear Fuel Center, fully exemplified in this chapter, providing so nuclear know-how to a more peaceful, safer and healthy world. We are especially thankful to our colleagues who provided lots of information shown here, mainly Mr. Davilson Gomes da Silva who made many of the

• Temperature and time: calcination of metallic uranium slag at 600°C during 3h;

• Concentration: lixivium adjustment of HNO3 at 1 molar; HNO3 excess (120%);

The adopted process has the following parameters:

• Agitation: 300 rpm, turbine stem type (45° inclination).

of IPEN´s slag recovering from uranium magnesiothermic reduction.

• Lixivium temperature: 40 - 50°C;

200 mesh;

**8. Acknowledgements** 

illustrations to qualify better this text.

Fig. 16. Schematic illustration of the fuel element produced at IPEN.

Once qualified, the fuel element is washed in a bath of ethyl alcohol and dried manually with the aid of a jet of hot air. After this cleaning, a visual inspection is conducted, especially inside the cooling channels (the channels between the fuel plates), trying to detect possible obstructions caused by chips or foreign material. After washing and inspection, the fuel element is transferred to the reactor.
