*3.2.2. Colamination of UMo with Zry‐4*

Both Zry‐4/U7Mo/Zry‐4 stacked plates TIG welded (sandwich) were hot colaminated using 150 mm rolls. The Stanat rolling mill had entrance guides to quickly position plates that were heated at a temperature of 675°C in an air atmosphere. To minimize the oxide layer growth the heating time between passes was set up at minimum values that could guarantee tension relieving and recrystallization of UMo. Totally welded interfaces ND near final thickness were reached after eight passes. Mean reduction step was 17.5% and mean heating time was 7.5 min; all the colamination process takes a total time of one hour. The cooling after the last lamination step must guarantee the retention of the metastable γ‐U7Mo phase.

## *3.2.3. Final procedures*

After the hot colamination, the plates were straightened in a multi roller machine and the oxide layer must be removed. Wet silicon carbide sand papers were used for this last purpose. The final thickness was reduced in 0.1 mm with the oxide layer removal and surface polishing.

Ultrasonic testing was performed to control 100% welding, and X‐Ray radiography was used for checking meat dimensions, density homogeneity, dog boning, meat folding, etc. Cutting to obtain final dimensions was done with a guillotine. Identification of the miniplates was the last step of the fabrication procedure (**Figure 18**).

**Figure 18.** γ‐U7Mo/Zry4 finished mini‐plate of 100 x 25 x 1 mm<sup>3</sup> size. Meat and cladding thickness are 0.50 and 0.25 mm, respectively.

#### **3.3. Monolithic miniplates characteristics and irradiation**

The final characteristics of miniplates MZ25 and MZ50 are summarized in **Table 1** showing plate and cladding thickness, the dimensions and density of the monolithic γ‐U7Mo meat and the total uranium with its meat uranium bulk and surface density.


Gamma Uranium Molybdenum Alloy: Its Hydride and Performance http://dx.doi.org/10.5772/63652 105


**Table 1.** MZ25 and MZ50 characteristics. Miniplates size is 25 x 100 mm2 .

Miniplates MZ25 and MZ50 were irradiated 90 effective full power days (EFPD) during the RERTR‐7A test experiment in the ATR. Local burn‐ups of <sup>235</sup>U were between 28.5 and 53.3% for MZ25 and between 25.2 and 48.3% for MZ50. Average total burn‐ups were 37.5 and 33.1% for MZ25 and MZ50, respectively. The estimated beginning of life (BOL) thermal conditions, operating parameters, peak surface heat flux, power density generated at the beginning and end of irradiation, minimum and maximum clad surface temperatures, fission density, internal and external temperatures, heat flux and total swelling were previously shown [43, 59–63].

#### **3.4. Post‐irradiation examination**

*3.2.2. Colamination of UMo with Zry‐4*

*3.2.3. Final procedures*

104 Nuclear Material Performance

mm, respectively.

Both Zry‐4/U7Mo/Zry‐4 stacked plates TIG welded (sandwich) were hot colaminated using 150 mm rolls. The Stanat rolling mill had entrance guides to quickly position plates that were heated at a temperature of 675°C in an air atmosphere. To minimize the oxide layer growth the heating time between passes was set up at minimum values that could guarantee tension relieving and recrystallization of UMo. Totally welded interfaces ND near final thickness were reached after eight passes. Mean reduction step was 17.5% and mean heating time was 7.5 min; all the colamination process takes a total time of one hour. The cooling after the last lamination

After the hot colamination, the plates were straightened in a multi roller machine and the oxide layer must be removed. Wet silicon carbide sand papers were used for this last purpose. The final thickness was reduced in 0.1 mm with the oxide layer removal and surface polishing.

Ultrasonic testing was performed to control 100% welding, and X‐Ray radiography was used for checking meat dimensions, density homogeneity, dog boning, meat folding, etc. Cutting to obtain final dimensions was done with a guillotine. Identification of the miniplates was the

The final characteristics of miniplates MZ25 and MZ50 are summarized in **Table 1** showing plate and cladding thickness, the dimensions and density of the monolithic γ‐U7Mo meat and

**Miniplate Units MZ25 MZ50**

size. Meat and cladding thickness are 0.50 and 0.25

step must guarantee the retention of the metastable γ‐U7Mo phase.

last step of the fabrication procedure (**Figure 18**).

**Figure 18.** γ‐U7Mo/Zry4 finished mini‐plate of 100 x 25 x 1 mm<sup>3</sup>

**3.3. Monolithic miniplates characteristics and irradiation**

the total uranium with its meat uranium bulk and surface density.

U enrichment % 235U 19.86 U composition % w/w U 92.91 ± 0.09 Mo composition % w/w Mo 7.04 ± 0.07

The post irradiation examination (PIE) was performed at the Hot Fuel Examination Facility of the Material and Fuel Complex (INL) [58, 59]. Plate swellings were 3.6% for MZ25 and slightly higher than 4% for MZ50 miniplate. Meat swellings of 15 and 12% were measured for MZ25 and MZ50, in accordance with fission densities. The swelling is low and uniform and consistent with other monolithic UMo plates with aluminium cladding irradiated in previous experi‐ ments (RERTR‐6 and RERTR‐7) with similar burn‐ups.

An eddy current probe (HELMUT FISCHER model Delta Scope MP30) modified for hot cell was used for measuring the oxide thickness. Zero calibration was done using nonirradiated Zry‐4. The average oxide layer thickness after irradiation was 2.6 and 3.2 +/− 0.5 μm for miniplates MZ25 and MZ50, respectively.

**Figure 19** shows a montage of metallographic images of MZ25 miniplate cut transversally and polished. The arrow indicates the direction of the ATR core center. At the nearer end, the neutron flux is higher and burn‐up reached 53.3%. The burn‐up gradient across the width of the plate was 1.85. **Figure 20** shows a higher magnification optical image of the UMo/Zry‐4 irradiated interface with fuel and clad remaining adherent and with no evidence of fission gas bubbles.

**Figure 19.** Optical metallographic montage of a polished transversal cut of MZ25 fuel plate.

**Figure 20.** Metallographic cross‐section of the interaction zone showing fuel (bottom) and clad (top) of MZ25 irradiat‐ ed miniplate.

**Figure 21** shows another high magnification optical image montage of the width end of plate MZ50 fuel/cladding interface plate that faces the ATR core centerline; a zone with the highest fission density rate and highest temperature with a 48.3% final burn‐up. The width of the interaction layer between γ‐U7Mo fuel and Zry‐4 cladding is extremely thin and can hardly be seen. No fission gas bubbles were visible in the fuel, and the bonding between fuel and cladding is intact. The swelling is uniform.

**Figure 21.** Metallographic cross‐section of a hot zone in MZ50 miniplate.

PIE reports in both miniplates states that the fuel/clad bonding looks excellent.
