**3. Monolithic γ‐UMo miniplates with cladding of Zircaloy‐4**

UMo and aluminum have interlayer incompatibility under irradiation and different thermo‐ mechanical properties that complicates a monolithic plate fabrication. These conditions can be avoided if the aluminum is replaced by a more friendly material with UMo such as Zircaloy‐ 4, used in fuel tubes in power nuclear reactors [49, 50]. UMo has low plastic deformation compared with Zry‐4; the latter has a coefficient of thermal expansion (6.10‐6 °C‐1) less than half of the UMo coefficient [55]. Probably for this reason never before hot rolling colamination was applied to the system UMo/Zry, although UMo alloys have been co‐extruded with Zircaloy‐2 in plate and rod shapes [40, 56]. Hot rolling colamination needs several deformation steps while coextrusion is performed at higher temperatures in only one deformation step. The interaction zone growth kinetics between UMo and Zry is well known [40] and more recently it was shown by calculation that there exists a low interaction between Zr and UMo alloys [57]. Finally, a fabrication technique was developed [51], and two fuel miniplates were produced, irradiated and the post irradiation examination (PIE) was performed [43, 54, 58, 59].

### **3.1. Colamination parameters**

Colamination temperature was chosen around 650°C where γ‐UxMo phase is stable and Zry‐ 4 is in α phase (<850°C) minimizing oxide layer growth in an air atmosphere. A uranium 7% (w/w) molybdenum alloy was chosen to maximize uranium content. Total welding in the sandwich hot colamination can only be achieved through sequential deformation steps. Differential contraction of UMo and Zry‐4 in the cooling between colamination steps was avoided by quick reentrance to the heating furnace. It is illustrative to repeat a detailed description of this process mentioned in reference [43]: "The process of furnace extraction, lamination and placement again in the furnace was carried out as quickly as possible mini‐ mizing contact with cold surfaces. Thickness measurements were replaced by a very fast measurement of the plate length with a ruler from which the reduction pass was calculated. If the reintroduction in the furnace is not quick enough, repeated "crick" sounds are heard, indicating the breaking of the partial welds that were formed. After several steps, the contact surfaces are totally welded. The involved difference in contraction after final cooling are of the order of 1% and the stresses involved are absorbed by plastic deformation, as it is clear from past successful extrusion experiences. Very different sounds are heard when dropping good and bad laminated miniplates in a table: an'applause' sound stands for not totally welded surfaces, while a metallic sound is heard with a 100% surface welded miniplates. Welding between meat and frame was checked by destructive metallographic techniques."

#### **3.2. Fabrication of miniplates**

After several testing developments steps of the hot co‐lamination processes (**Figure 15**), two monolithic LEU γ‐U7Mo fuel miniplates with Zry‐4 cladding were fabricated in CNEA for irradiation in the Advanced Testing Reactor (ATR) at Idaho National Laboratory (INL). Miniplates final size was 100 x 25 mm with a total thickness of 1 mm; nominal meat thicknesses were 0.25 (MZ25) and 0.50 (MZ50) with 0.36 and 0.25 mm of cladding thickness, respectively.

**Figure 15.** Monolithic γ‐U7Mo/Zry‐4 miniplates used along the different development stages. The three at the right extreme are in an intermediate step of surface oxide removal.

#### *3.2.1. Alloy melting and sandwich preparation*

**3. Monolithic γ‐UMo miniplates with cladding of Zircaloy‐4**

UMo and aluminum have interlayer incompatibility under irradiation and different thermo‐ mechanical properties that complicates a monolithic plate fabrication. These conditions can be avoided if the aluminum is replaced by a more friendly material with UMo such as Zircaloy‐ 4, used in fuel tubes in power nuclear reactors [49, 50]. UMo has low plastic deformation compared with Zry‐4; the latter has a coefficient of thermal expansion (6.10‐6 °C‐1) less than half of the UMo coefficient [55]. Probably for this reason never before hot rolling colamination was applied to the system UMo/Zry, although UMo alloys have been co‐extruded with Zircaloy‐2 in plate and rod shapes [40, 56]. Hot rolling colamination needs several deformation steps while coextrusion is performed at higher temperatures in only one deformation step. The interaction zone growth kinetics between UMo and Zry is well known [40] and more recently it was shown by calculation that there exists a low interaction between Zr and UMo alloys [57]. Finally, a fabrication technique was developed [51], and two fuel miniplates were produced,

irradiated and the post irradiation examination (PIE) was performed [43, 54, 58, 59].

between meat and frame was checked by destructive metallographic techniques."

After several testing developments steps of the hot co‐lamination processes (**Figure 15**), two monolithic LEU γ‐U7Mo fuel miniplates with Zry‐4 cladding were fabricated in CNEA for irradiation in the Advanced Testing Reactor (ATR) at Idaho National Laboratory (INL). Miniplates final size was 100 x 25 mm with a total thickness of 1 mm; nominal meat thicknesses were 0.25 (MZ25) and 0.50 (MZ50) with 0.36 and 0.25 mm of cladding thickness, respectively.

Colamination temperature was chosen around 650°C where γ‐UxMo phase is stable and Zry‐ 4 is in α phase (<850°C) minimizing oxide layer growth in an air atmosphere. A uranium 7% (w/w) molybdenum alloy was chosen to maximize uranium content. Total welding in the sandwich hot colamination can only be achieved through sequential deformation steps. Differential contraction of UMo and Zry‐4 in the cooling between colamination steps was avoided by quick reentrance to the heating furnace. It is illustrative to repeat a detailed description of this process mentioned in reference [43]: "The process of furnace extraction, lamination and placement again in the furnace was carried out as quickly as possible mini‐ mizing contact with cold surfaces. Thickness measurements were replaced by a very fast measurement of the plate length with a ruler from which the reduction pass was calculated. If the reintroduction in the furnace is not quick enough, repeated "crick" sounds are heard, indicating the breaking of the partial welds that were formed. After several steps, the contact surfaces are totally welded. The involved difference in contraction after final cooling are of the order of 1% and the stresses involved are absorbed by plastic deformation, as it is clear from past successful extrusion experiences. Very different sounds are heard when dropping good and bad laminated miniplates in a table: an'applause' sound stands for not totally welded surfaces, while a metallic sound is heard with a 100% surface welded miniplates. Welding

**3.1. Colamination parameters**

102 Nuclear Material Performance

**3.2. Fabrication of miniplates**

The LEU uranium molybdenum alloy for the MZ25 and MZ50 miniplates was melted in the same way as described for powder production in section 2.1. using a graphite vertical mold to cast a 75 x 100 mm2 plate of 2 mm thickness. Sandwich preparation was performed using the lids and frame technique thoroughly described in a previous work [43]; overall thickness of both packs was 4 mm (**Figures 16** and **17**)

**Figure 16.** Finished frames, lids and coupons of MZ25 and MZ50 ready to be stacked and welded to conform the two sandwiches.

**Figure 17.** TIG welded sandwich of monolithic U7Mo meat and Zry‐4 clad ready for hot co‐lamination.
