**4. Discussion**

#### **4.1. HMD γ‐U7Mo powder performance**

Hydriding in the presence of stresses can be easily corroborated using two annealed probes of γ‐U7Mo, one of which has been indented after the heat treatment with a Vickers hardness tip. The beginning of hydriding can be noticed only at the indented probe after slow heating of both samples up to 300°C in a hydrogen atmosphere and slow cooling. Another way of obtaining stressed samples is by melting alloy buttons in an arc furnace with a refrigerated copper crucible. Also a stressed sample of γ‐UMo can probably be cathodically charged filling with hydrogen the traps preparing the alloy for massive hydriding in an hydrogen atmosphere.

Microscopic fractures are produced during the hydriding of γ‐U7Mo without producing a nanosized powder as it happens with the hydriding of pure uranium. The rate of hydrogen incorporation during massive hydriding of γ‐U7Mo is greater than a square root dependence with temperature because of fragmentation of the material in the phase transformation. Hydrogen incorporation increases with pressure with no variation in the absorption temper‐ ature range. UMo samples with higher purity take less time for total hydride formation [64].

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

ed miniplate.

106 Nuclear Material Performance

**4. Discussion**

cladding is intact. The swelling is uniform.

**4.1. HMD γ‐U7Mo powder performance**

**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.

Hydriding in the presence of stresses can be easily corroborated using two annealed probes of γ‐U7Mo, one of which has been indented after the heat treatment with a Vickers hardness

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

**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 The vibrating chamber allowed many of the thermal treatments at temperatures above 500°C avoiding powder sintering. The vibration was connected during hydriding, dehydriding and passivation for moving the interior material, enhancing diffusion and increasing cooling rates for quenching the metastable gamma phase. Vibration was also used for testing different coverage techniques with surrogate and UMo particles.

The oxidation kinetics of γ‐UMo alloy is lower than in pure uranium. The hydride can spontaneously burn at room temperature in air. The burning of the hydride is reduced after heating in vacuum at 325°C with the elimination of hydrogen in traps. In all experiments the temperature control was performed with a thermocouple immersed in the powder, allowing immediate control of hydriding (exothermic), dehydriding (endothermic) and passivation (exothermic) processes. Direct pressure control was achieved by maintaining the chamber closed, when possible; evolution of pressure can be directly correlated with hydrogen absorp‐ tion and desorption. Powders of γ‐U7Mo in contact with air diminish their density from one year to another evidencing surface oxidation and hitherto should be kept in inert atmosphere containers.

#### **4.2. Performance of monolythic γ‐UMo fuel with nonaluminum cladding**

Special precautions must be taken to allocate UMo monolithic coupons in the frame with very small tolerances to avoid blistering formation during the lamination process. As usual, surface contaminations of any kind—in the assembling of the fuel coupon with the lids and frame, in the welding of the sandwich, in the cleaning of the casting molds, in the maintenance of fresh reactives, etc.—must be avoided. Colamination of monolythic UMo with Zry‐4 or stainless steel used as cladding materials is performed at temperatures higher than 650°C where the gamma UMo high temperature phase is stable. No special precautions are needed to avoid decomposition at working temperatures and TTT diagrams are used only in the evaluation of final cooling velocities to retain the γ‐UMo phase.

The different coefficients of thermal expansion of U7Mo and Zry‐4 needed a special procedure of not allowing the cooling at intermediate steps of hot Colamination. When finally cooling to room temperature with 100% welded interfaces, the appearing stresses produced by differ‐ ential contraction coefficients will be absorbed by elastic or plastic deformation of Zry‐4; they probably disappear during final straightening and polishing or even during irradiation. No special treatment is needed and this is not an issue. In the case of stainless steel claddings, there is no great mismatch between thermal coefficients, and no problems are presented during hot colamination with monolithic UMo.

Nonaluminum monolithic fuel plates need to be straightened after hot lamination. More power machinery than with aluminum dispersed fuel plates is needed. The oxide layer of the Zry‐4 cladding after hot colamination could only be removed by mechanical means. Since sand blasting must be done in an inert atmosphere, wet sand papers were used semimanually. In the case of stainless steel claddings the polishing can be assisted with chemical means. A more industrialized polishing can be performed using a scanning abrasive water blasting that can follow small surface deformations maintaining uniform cladding thickness. Also abrasive powders assisted by brushes can be used.

Nonaluminum claddings in UMo monolithic fuels can be smaller than 150 μm [9]. MZ50 miniplate had a cladding thickness of 250 μm and a total plate thickness of 1 mm. This cladding reduction thickness can compensate lower thermal conductivity, compared with aluminum alloys, in heat extraction. The reduction of cladding and fundamentally plate thickness can help in new designs introducing a more satisfactory adjustment of neutron moderation ratios.

The growth kinetics of oxide layer in Zry‐4 and stainless steel claddings during irradiation is much lower than in aluminum claddings. The lids and frame process of monolithic γ‐UMo with nonaluminum cladding using the lids and frame hot colamination fabrication process can be scaled up to full size plates.
