A Jaw crusher (M/s RETSCH, Germany) was used for mechanical crushing of the sintered pellets into coarse powder. It consists of a no-rebound hopper for feeding the sintered pellets. It has a fixed crushing arm and other movable by an eccentric drive shaft. The elliptical motion of the second arm crushes the pellets (9 - 11 mm) into the powder <1 mm. A vacuum based dust collection system collects the radioactive dust formed in crushing operation. The other details of the jaw crusher can be found elsewhere [43] .

The Attritor mill [43] consists of a vertical cylindrical stainless steel vessel,

central shaft and horizontal impellers made-up of hardened tool steel. The optimized size hardened chrome steel balls of diameter 6 mm were used as grinding media. The material was charged at 100 rpm into the attritor. The milling in the attritor was carried out for 40 min. at 200 rpm. The discharging of the material was carried out at 300 rpm.

Indigenously developed microwave processing system of maximum microwave output power of 6 kW was used. The output power of the system can be continuously varied between 10% - 100%. The aqueous processing was carried out in a multimode applicator of volume nearly 60 liters. The microwave transmission up to the applicator is achieved through standard WR340 wave-guides from remote microwave generator. The system is equipped with real time temperature and power measurement during the processing. The other technical details of this system are described elsewhere [27] .

The flow sheet for recycling of ThO₂ or (Th, U)O₂ fuel rejects by integrated method is shown in Figure 1. The sintered pellets were crushed using a jaw crusher to coarse powder. The coarse powder in case of ThO₂ was directly attritor milled and in case of (Th, U)O₂, it was subjected to air oxidation at 700˚C for 4 h in conventional resistance furnace and subsequently for attritor milling. The attritor milling (40 min., 200 rpm) further resulted in micronisation of the coarse powder. The attritor milled powder was sieved through ASTM standard sieves.

The sieved powder (in batches) was taken for aqueous processing using microwave. It was dissolved in 16 M HNO₃ EMPARTA^® (M/s Merck, Germany) in open quartz cylindrical vessels (height 280 mm and dia.75 mm) in microwave processing system. Several batches of 500 g of sieved powder in 1000 ml of

HNO₃ were dissolved and denitrated. The typical temperature profile during microwave dissolution and denitration of a batch is shown in Figure 2. Figure 2 also depicts the various stages of the process up to metal oxide conversion. The product obtained is low density, porous and dry cake of oxide or mixed oxide. The dry cake was removed from the process vessel using spatula in the form of clinkers. The clinkers were attritor milled and subjected to air calcination at 700˚C, 2 h for ThO₂. The calcination and reduction (700˚C, 4h) for (ThO₂, U₃O₈) was carried out.

1) Dissolution and Concentration

The attritor milled and sieved powder was dissolved in HNO₃ at 115˚C - 120˚C to form their soluble nitrates using catalyst HF as shown in Equation (1-2) [31] [32] [33] [51] [52] . The dissolved solution was further heated to concentrate and solidify by removal of excess H₂O and HNO₃. This resulted in formation of hydrated crystals of respective nitrates.

T h O 2 + 4 H N O 3 + 5 H 2 O → T h ( N O 3 ) 4 ⋅ 5 H 2 O + 2 H 2 O (1)

U O 2 + 3 H N O 3 + 5 H 2 O → U O 2 ( N O 3 ) 2 ⋅ 6 H 2 O + H N O 2 (2)

2) Dehydration and Denitration

In the process of dehydration, water of crystallisation of nitrates was removed

to get nitrates with reduced water of crystallisation above 120˚C. The partially hydrated nitrates were decomposed to respective oxides or mixed oxide on further heating as described by Equation (3)-(6). The denitration reaction for thorium nitrate pentahydrate is shown by Equation (3) [53] [54] . The nitrogen dioxide (NO₂) combines with available H₂O and forms HNO₃. After partial dehydration, the uranyl nitrate trihydrate undergoes oxidative denitration to form UO₃ through formation of uranyl hydroxy nitrate compound as shown by Equation (4-6) [25] [55] [56] .

T h ( N O 3 ) 4 ⋅ 5 H 2 O → T h O 2 + 4 N O 2 + 2 O 2 + 5 H 2 O (3)

U O 2 ( N O 3 ) 2 ⋅ 6 H 2 O → U O 2 ( N O 3 ) 2 ⋅ 3 H 2 O + 3 H 2 O (4)

U O 2 ( N O 3 ) 2 ⋅ 3 H 2 O → U O 2 ( O H ) ( N O 3 ) + H N O 3 + 2 H 2 O (5)

U O 2 ( O H ) N O 3 → U O 3 + 0.5 H 2 O + N O 2 + 0.25 O 2 (6)

3) Calcination and Reduction

The mixed oxide product obtained after denitration was porous and was further calcined in air converting UO₃ into U₃O₈ in the same process vessel as represented by Equation (7) [57] . Another, calcination (700˚C, 2 h) was carried out after attritor milling for removal of residual nitrates, H₂O, carboneous impurities (if any) and quantitative conversion of UO₃ into U₃O₈. The calcined material was reduced in flowing (N₂ + 7% H₂) atmosphere at 700˚C, 4 h and the reduction reaction for (ThO₂, U₃O₈) is shown by Equation (8) [58] [59] . The reduction step for calcined ThO₂ was not carried out.

3 U O 3 + T h O 2 → U 3 O 8 + T h O 2 + 0.5 O 2 (7)

T h O 2 + U 3 O 8 → T h O 2 + 3 U O 2 + O 2 (8)

Mechanical micronisation using jaw crusher and attritor mill produces powder from the sintered pellets. The particle size and the surface area data after different stages of the integrated process (Figure 1) are recorded in Table 3. The sieve analysis was carried out for powders after jaw crushing, air oxidation and attritor milling of sintered rejects using ASTM standard sieves. The values in the parenthesis show (Table 3) the wt. fraction of powder in the corresponding particle size range during sieve analysis. The laser particle size analyser Cilas 1190 LD (M/s Cilas, France) was used for microwave denitrated and attritor milled powders and values recorded are average particle size (Table 3). The powder obtained after jaw crushing operation (Table 3) was compared with the sinterable grade feed powder (Table 1). The specific surface area and particle size were found to be poor, which may not be sintered to the densities required for an oxide fuel typically > 90% of TD. Further improvement in the powder physical properties was carried out by attritor milling. Though, the powder so obtained also showed better surface area and smaller particle size as compared to the precursors (Table 3). However, it cannot be recommended as sinterable grade (Table 1). The extended micronisation using attritor mill may be useful however, this leads to prolonged processing cycle. More importantly, the issues of concern are the impurity pick-up due to wear of attrition media and the associated radioactive dust hazard in the extended micronisation. The dust raised frequently chokes the HEPA filters calling their frequent replacement. High energy ball or attritor milling is useful however, is energy intensive. Thus, mechanically micronized powder (after attritor milling, 40 min) so obtained is not suitable as fuel feed material. Else, mechanically micronized powder can be mixed with fresh feed powders for fuel fabrication. Mishra et al. [44] reports the addition of maximum 15 wt% of the (Th, 30%U)O₂ MOX sintered rejects recycled via mechanical micronisation process (crushed and attritor milled) for fabrication of fuel.

The conventional oxidative or thermo-mechanical pulverization techniques have been employed in recycling of defective UO₂ fuel pellets for long [18] - [24] . A large (~36%) volumetric increase resulting from oxidation of face centered cubic (FCC) UO₂ to orthorhombic U₃O₈ leads to generation of internal stresses and hence pulverization of sintered compacts. The UO₂ to U₃O₈ transformation has been thoroughly studied and proceeds via formation of intermediates U₃O₇/U₄O₉ [30] .

The oxidation of ThO₂ owing single oxidation state (+4) is not feasible like UO₂ [29] . However, presence of UO₂ in the ThO₂ matrix has caused its oxidation thus giving (Th, U)O_2+x. Figure 3 shows the increase in the% oxidation in various (Th, U)O₂ MOX in the range of UO₂ from 0 to 30 wt%. The% oxidation was calculated using the method followed by G. Singh et al. [23] based on the O:M ratio of the initial and final oxidized material. It represents the percentage with respect to the theoretical ultimately achievable oxidation for a given composition. The O:M of ThO₂ is non-variable upon oxidation and taken as 2.00.

It is evident from the Figure 3 that as UO₂ content increases in the MOX, the % oxidation increases during air oxidation (after jaw crushing). The content of UO₂, thus steers the oxidation behavior of (Th, U)O₂ matrix. The rate of oxidation of (Th, U)O₂ further depends upon several factors alike oxidation UO₂ and (U, Pu)O₂ [23] [30] . The surface area, particle size, grain size, density, temperature and oxygen partial pressure are key parameters governing the oxidation kinetics. The physical form of the MOX plays a crucial role in the oxidation process. The powder form gives more oxidation than solid pellets [23] . Thus, oxidation step was introduced after jaw crushing in the integrated process (Figure 1) for (Th, U)O₂ recycling. However, oxidation process was not carried

out for recycling of ThO₂ sintered pellets in the integrated process (Figure 1). It is also concluded from Table 3 that the air oxidation of jaw crushed (Th, 14% U)O₂ powder improves the specific surface area and reduces the particle size. The particle size and specific surface area of jaw crushed (Th, 30%U)O₂ were 0.9 - 1 mm (>0.9 wt. fraction) and <1 m²∙g⁻¹ respectively and after air oxidation were 600 - 700 µm (>0.8 wt. fraction) and 1.8 m²∙g⁻¹ respectively. This was attributed to oxidation of (Th, U)O₂ to (Th, U)O_2+x due to presence of uranium. Higher the UO₂ content in (Th, U)O₂, more is the % oxidation (Figure 3). This in turn improves the powder properties (surface area and particle size) and hence the sinterability. The oxidation of UO₂ to U₃O₈ decreases the density thereby increasing the surface area and reducing the particle size [30] [60] . The sinterability can further be increased by following multiple oxidation-reduction cycles [60] .

The applicability of microwave dissolution and denitration is based upon the microwave coupling behavior of the precursors and compounds that are formed in the course of the process. H₂O and HNO₃ are strong couplers at microwave frequency 2450 MHz [61] [62] . UO₂ and U₃O₈ are excellent microwave absorbers [63] [64] . The UO₃ and ThO₂ are week absorbers [25] [39] [65] . However, the presence of other absorbing materials raises their temperature. As the temperature increases, UO₃ transforms to U₃O₈ which is strong absorbing material. A small amount of U₃O₈ is sufficient for transforming the entire mass of UO₃ into U₃O₈ or else a pinch of U₃O₈ is intentionally added as microwave absorbing seed [66] . The temperature raises the microwave dissipation factor and thereby the absorption increases along with the temperature [67] [68] . The microwave denitration of thorium nitrate is feasible [36] [37] [38] . The mechanical micronisation and/or air oxidation increase the surface area and thus enhance the dissolution of ThO₂ tremendously. Thus, micronized and/or air oxidized powder was used for dissolution-denitration (Figure 1).

The effect of various parameters influencing the powder properties of denitrated product was thoroughly studied. Based on the multi-parametric study, the optimized conditions were selected to deliver sinterable powder. The influence of the various parameters on the cake bulk density (CBD), powder bulk density (PBD) and BET surface area is depicted in Figure 4.

1) Effect of method of heating

The aqueous processing involving dissolution-denitration was carried out in microwave and conventional hotplate. It was found that the specific surface area in the microwave denitrated product was higher than the hot plate processed (Figure 4(a)). The cake and powder bulk densities were found to be lower in microwave denitrated ThO₂. This was attributed to the direct material and bulk heating associated to microwave contrary to conventional. The solidification (Figure 2) occurs quickly in case of microwave throughout the bulk. Thus, the evolution of gases and vapours during solidification and subsequent steps are many more times larger which forms the cake to be porous. The shape of the

denitration process vessel plays a key role in enhancing the cake porosity [22] . Long narrow neck vessel (used in the present work) provides small area for outgoing gases and vapours (nitrogen oxides called NOx, HNO₃, H₂O etc.). However, the heat conducts from the surface of the material in the hot plate. The solidification starts from the surface and take longer to reach up to the core of the material. Thus, there is no quick evolution of gaseous products during solidification. Therefore, the product formed in the in hot plate denitration possess high bulk density (thus, low porosity) and low surface area. The increase in the surface area in microwave denitration is due to de-agglomeration of particles because of quick evolution of gases and vapours, which prevents the nucleation and the particle size growth. These outcomes are in line with the findings by Chandramouli et al. [39] where they found very low specific surface area of the ThO₂ powder synthesized via hot plate denitration. However, the surface area was higher when the denitration was performed using microwave. Thus, the microwave denitration was embedded in the integrated process (Figure 1) to improve the powder physical properties.

2) Effect of HF addition

To investigate the effect of completely dissolved ThO₂ on the microwave denitrated powder, the microwave dissolution of the attritor milled powder was performed in HNO₃ in the presence and absence of HF. The HF (0.025 M) in HNO₃ was added to increase the ThO₂ dissolution and. Figure 4(b) reveals that the dissolved ThO₂ improves the specific surface area of the microwave denitrated powder to a greater extent. The bulk densities of cake and powder were found to be lower in case of the microwave denitrated ThO₂ from completely dissolved solution. Without fluoride the solution contained undissolved and sintered ThO₂ particles which subsequent to the denitration remain in the dense form. These undissolved and dense particles are responsible for poor powder properties. Thus, complete dissolution is ascertained before concentration and solidification (Figure 2). Therefore, in all MOX batch recycling experiments using integrated method (Figure 1), HF was added for dissolution.

However, it was seen that the denitrated product without HF has slightly better surface area (2.1 m²∙g⁻¹) (Figure 4(b)) than the starting micronized ThO₂ (1.6 m²∙g⁻¹) (Table 3). This may be attributed to the presence of very small crystallites of ThO₂ in micronized ThO₂ powder. The ThO₂ powder with small crystallites (<56 Å) undergoes complete dissolution without fluoride [51] [54] . More the fraction of such small crystallites in the micronized powder, higher will be the extent of dissolution and subsequently the better powder properties. However, no attempt has been made to see the presence of such crystallites in the present work. The future work may be pursued in this direction.

3) Effect of PVA addition

Further improvement in the characteristics of the microwave denitrated powder was achieved by adding PVA to the completely dissolved ThO₂ (using HF). The combustion synthesis of ThO₂ from its aqueous nitrate solution using PVA has already been reported [39] [40] [41] [42] . There occurred a vigorous reaction between PVA as organic fuel and HNO₃ as oxidant. The reaction is violent because of the exothermicity of the denitration and oxidation of the PVA. The product of denitration is highly porous because of the evolution of excessive gaseous products (CO₂, NOx etc.). PVA provides a surface for the denitration while preventing aggregation and sintering of particles, thereby helping the formation and stabilisation of the small crystallites of the powders. Thus, powder obtained has high surface area (>7 m²∙g⁻¹) as shown in Figure 4(c) and Table 3. The ratio of the organic fuel and oxidant are crucial in deciding the crystallite size and hence the powder properties. The aqueous solution of PVA (8% - 10%) was mixed with aqueous thorium nitrate solution obtained after dissolution of attritor milled ThO₂ in 1:2 volume ratio in a batch of denitration after dissolution (Figure 2). The specific surface area and the bulk cake and powder densities of the ThO₂ so obtained are shown in Figure 4(c). The large surface area of the powder obtained is in line the findings by researchers [39] [40] [41] [42] . The further studies are directed towards investigation of the effect of concentration of thorium and/or uranium, nitric acid molarity, ratio of fuel to oxidant etc. and are not in the scope of present work.

4) Effect of UO₂ in ThO₂

A profound effect on the surface area and bulk density of the denitrated product was found due to presence of UO₂ in ThO₂ as shown in Figure 4(d). This is because the denitrated product contains UO₃. The density of UO₃ is lower (6.69 - 8.62 g∙cm⁻³) as compared to ThO₂ (10 gcm⁻³) depending upon the allotrope formed [57] . It was reported that the microwave denitration of uranyl nitrate produces β-UO₃ [55] [56] . The studies have shown that for denitration of uranyl nitrate, the crystal structure of the oxide product depend upon temperature and heating rate with desired porous product formed at faster heating rates [69] . The calcination of UO₃ further gives voluminous U₃O₈ having density 8.326 - 8.395 g∙cm⁻³ [54] . The surface area and bulk density of cake and powder in the microwave denitrated (Th, 14%U)O₂ are shown in Figure 4(d). Thus, Figure 4(d) revealed that the bulk density decreases and surface area increases as UO₂ content increases in the (Th, U)O₂.