Development of recycling processes for clean rejected MOX fuel pellets

Development of recycling processes for clean rejected MOX fuel pellets

Nuclear Engineering and Design 270 (2014) 227–237 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.els...

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Nuclear Engineering and Design 270 (2014) 227–237

Contents lists available at ScienceDirect

Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes

Development of recycling processes for clean rejected MOX fuel pellets P.M. Khot, G. Singh, B.K. Shelke, B. Surendra, M.K. Yadav, A.K. Mishra, Mohd. Afzal ∗ , J.P. Panakkal Advanced Fuel Fabrication Facility, Bhabha Atomic Research Centre, Tarapur Complex 401502, Maharashtra, India

h i g h l i g h t s • • • • •

Dry and wet (MWDD) methods were developed for 100% recycling of CRO (0.4–44% PuO2 ). Dry method showed higher productivity and comparable powder/product characteristics. MWDD batches demonstrated improved powder/product characteristics to that of virgin. Second/multiple recycling is possible with MWDD with better powder/product characteristics. MWDD batches prepared by little milling showed better macroscopic homogeneity to that of virgin.

a r t i c l e

i n f o

Article history: Received 30 May 2013 Received in revised form 16 December 2013 Accepted 17 December 2013

a b s t r a c t The dry and wet recycling processes have been developed for 100% recycling of Clean Reject Oxide (CRO) generated during the fabrication of MOX fuel, as CRO contains significant amount of plutonium. Plutonium being strategic material need to be circumvented from its proliferation issues related to its storage for long period. It was difficult to recycle CRO containing higher Pu content even with multiple oxidation and reduction steps. The mechanical recycling comprising of jaw crushing and sieving has been coupled with thermal pulverization for recycling CRO with higher Pu content in dry recycling technique. In wet recycling, MicroWave Direct Denitration (MWDD) technique has been developed for 100% recycling of CRO. The powder prepared by dry and wet (MWDD) recycling techniques was characterized by XRD and BET techniques and their effects on the pellets were evaluated. (U,21%Pu)O2 pellets fabricated from virgin powder and MWDD were characterized using optical microscopy and ␣-autoradiography and the results obtained were compared. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Advanced Fuel Fabrication Facility (AFFF), BARC, Tarapur has manufactured MOX fuel for experimental irradiation in Indian Boiling Water Reactors (BWRs), Pressurized Heavy Water Reactors (PHWRs) and Fast Breeder Test Reactor (FBTR). Presently, MOX fuel for Prototype Fast Breeder Reactor (PFBR) is being fabricated through powder metallurgical route involving cold compaction and sintering. The Annular MOX fuel pellets of two compositions i.e. 21% and 28% PuO2 are being fabricated for the first core of PFBR. During fabrication of (U,Pu)O2 MOX fuel, a small percentage (10–15%) of pellets gets rejected at various stages. These process rejects comprises of green rejects formed during the cold pressing and rejected sintered pellets. Dust produced during grinding of oversize sintered pellets also contributes to process rejects. The sintered rejects are

∗ Corresponding author. Tel.: +91 2525244165; fax: +91 2525244913. E-mail address: [email protected] (Mohd. Afzal). 0029-5493/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.12.060

broadly divided into two categories; clean reject oxide (CRO) and dirty reject oxide (DRO) depending on the chemical and physical characteristics of the pellets. The CRO consist of sintered MOX pellets rejected either due to low density, physical defects such as cracks, chips, blisters etc. or not meeting the physical specifications as mentioned in Table 1. Various techniques have been developed for recycling of the rejected sintered pellets. The wet recycling route tried initially for UO2 rejects involved dissolution in boiling HNO3 , liquid–liquid extraction, precipitation and conversion to UO2 (IAEA, 1995, 1999). This multi-stage process is costly, useful especially for recycling highly enriched fuel. Due to the large number of operations and associated liquid waste generated in wet recycling processes, new methods like oxalate recycle route, carbonate route, direct thermal denitration route have been explored (Vijayan et al., 1989; Lerch and Norman, 1984). These processes offered promise initially, but were not proved on industrial scale of operation. Some other problems associated with wet recycling methods are sensitivity to the characteristics of the product to process parameters, criticality, low

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productivity, additional packaging and transport. The dry routes of recycling therefore gained significance due to many advantages over the wet routes. In literature, studies have been reported on dry recycling of UO2 scrap material (Larson et al., 2002). In this process, good quality UO2 powder from the scrap material containing UO2 was prepared by low temperature oxidation (350–400 ◦ C) followed by reduction at higher temperature (800 ◦ C). The dry process for recycling of sintered (U,Pu)O2 MOX in MIMAS (Micronization and Master blend) process involved pre-crushing followed by micronization in ball mill for incorporation of scrap powder in primary blend of process (Vandergheynst et al., 2004). 40% of the rejected MOX pellets from MIMAS process was recycled by the above mentioned recycling process. A co-conversion process for (U,50%Pu)O2 using direct microwave heating technique has also been developed (Koizumi et al., 1983). It has a number of advantages such as good powder characteristics, good homogeneity of (U,Pu)O2 , simplicity of the process and low liquid waste generation. However, no systematic work has been reported for developing dry as well as wet method which recycles 100% of (U,Pu)O2 MOX CRO containing 0.4–44% of PuO2 . The dry and wet recycling (MWDD) technique has been developed at Advanced Fuel Fabrication Facility, BARC, Tarapur for 100% recycling of MOX CRO. The CRO contains significant amount of plutonium which has to be recovered and recycled back into the production line. The long term storage of CRO leads to fissile content degradation and increase men-rem to operators due to build of Am241 . The issues such as proliferation and fissile content degradation related to its storage for long period has led the development of processes for 100% recycling. Due to the demand of recycling of CRO in shorter time with better quality of the product, trials with microwave denitration technique has been attempted. After a number of trials, fabrication procedures and parameters were established. This paper deals with development of procedures for fabrication of MOX fuel by dry as well as MWDD recycling technique using 100% CRO containing 0.4% to 44% of PuO2 . During this study, the change in powder characteristics and its effect on process parameters by dry and MWDD techniques with respect to virgin powder were evaluated. The (U,Pu)O2 MOX fuel fabricated by recycling techniques was characterized by optical microscopy and alpha autoradiography to verify the microstructure and distribution of fissile content which is an important criterion in deciding its inpile behavior (Lee et al., 2001; Leyva et al., 2002; IAEA, 2003a). The results of (U,Pu)O2 MOX pellets fabricated from virgin powder were compared with pellets fabricated by dry and wet recycling routes. 2. Fabrication The flowsheet for fabrication of MOX fuel along with dry and wet recycling routes for recycling of CRO and the quality control checks at different process steps is shown in Fig. 1. UO2 and PuO2 powder were first weighed and then milled together in attritor to get the required composition and homogeneity. The milled material was pre-compacted and the pre-compacts were granulated in the size range of 400–2000 ␮m. These granules were used in final Table 1 Physical and chemical specification of PFBR MOX fuel. Outer diameter of pellet Inner diameter of pellet Length of pellet Linear mass of pellet PuO2 enrichment (nominal) O/M ratio Equivalent hydrogen content Total concentration of impurities

5.55 ± 0.05 mm 1.8 ± 0.2 mm 7.0 mm (nominal) 2.25 ± 0.15 g/cm 21 ± 1% and 28 ± 1% 1.96–2.00 <3 ppm <5000 ppm

Table 2 Details of microwave oven parameters. S. No.

Parameters

Parameter explanation

1. 2. 3. 4. 5. 6.

Frequency () Power (P) Applicator/cavity Applicator volume Incident power Magnetron

2450 ± 50 MHz 3 kW Multimode stainless steel ∼60 l Adjustable in the span of 0–100% Water cooled

compaction to make the green pellets. Final compaction of MOX annular pellets was carried out in a rotary press with a core rod in bottom and top plunger so as to fabricate annular pellets (Yadav et al., 2009; Shrotriya et al., 2009). The sintering of green pellets was carried out in a batch type resistance heating furnace under reducing atmosphere (mixed N2 –7% H2 gas) at 1600 ◦ C for 4–6 h. The oversize pellets were ground to acceptable size by a dry centerless grinder. The physical and chemical specifications of sintered pellets for PFBR are given in Table 1. The inspected pellets were loaded into clad tubes for encapsulation. 3. Experimental 3.1. Apparatuses used 1. Microwave oven: An indigenously developed, glove box adapted, stainless steel microwave oven was used for the study is shown in Fig. 2. All the components, control panel and magnetron were remotely located from the cavity to eliminate/minimize the active maintenance jobs inside glove box. Provisions were made for gas/air/vapor inlet and exhaust, thermocouple insertion up to load/charge and illumination of the cavity. The multimode applicator consisted of a fan/mode stirrer to homogenize the microwaves inside the cavity and hence the heating. The instrumental parameters are given in Table 2. 2. Denitration process vessel: Three types of quartz process vessels (1 l each) namely dish shaped, wide neck and narrow neck beakers were taken for MicroWave Direct Denitration experiments. 3. Jaw crusher: The feed material was passed through the norebound hopper before entering the crushing chamber. The size reduction takes place in the wedge shaped area between the fixed crushing arm and the one moved by an eccentric drive shaft. The elliptical motion crushed the pellets which then fell under gravity into a removable collector. The sintered MOX pellets with a large feed size of 6 mm were crushed down to <0.5 mm in a single step. 4. Micrometer and weighing balance: The green and sintered densities were measured by geometric method as per ASTM standard C 833-95a (ASTM, 2011a) using micrometer and weighing balance. 5. TGA (Thermogravimetric analyzer): Oxygen to metal (O/M, where M = U + Pu) ratio of the sintered pellet was measured thermogravimetrically using a Thermal Analyzer. The accuracy of measurement in weight was within ±1 ␮g. The O/M ratio of the sample was obtained from the weight change on heating at 800 ◦ C in flowing Ar–8%H2 atmosphere having oxygen potential approximately −100 kcal/mol, at which the dioxide is stoichiometric (ASTM, 2011b). 6. BET: The specific surface area of powder was measured using the Brunauer Emmett Teller (BET) method with helium as adsorbate gas. 7. XRD: The phase analysis was performed using X-ray diffractometry. The X-ray diffraction pattern of the sample was obtained by Cu-K␣ radiation with curved graphite monochromator. The accuracy of the equipment for X-ray intensity is ±5%.

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229

Fig. 1. Flowsheet for fabrication of MOX fuel with dry and wet recycling route.

8. Optical microscope: Microstructure was characterized by an optical microscope.

3.2. Development of dry methods for 100% recycling of MOX CRO The dry methods were developed for 100% recycling of CRO containing various percentage of PuO2 . 3.2.1. CRO containing low % PuO2 (<3%) The CRO containing low % PuO2 (<3%) was recycled for making (U,21%Pu)O2 MOX fuel using route I shown in Fig. 1. It was recycled

completely by thermal pulverization technique involving only oxidation and reduction. In this technique, CRO was oxidized at 700 ◦ C for 4 h in air atmosphere. This oxidized CRO was then reduced at 700 ◦ C for 4 h in presence of N2 –7% H2 atmosphere to eliminate the excess oxygen present and reduced CRO was used for fabrication of MOX fuel pellets. A batch size of 5 kg was prepared using reduced CRO powder along with required quantity of PuO2 , 1% (wt) polyethylene glycol (PEG) as binder, 0.8% (wt) oleic acid (OA) as lubricant followed by milling in an attritor for 40 min. The milled CRO was further processed as per flowsheet shown in Fig. 1. The experimental trials were carried out to find out the suitable size of the tooling sets

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shown in Fig. 1. The CRO was initially pulverized mechanically with the help of attritor. The mechanically pulverized CRO was subjected to oxidation and reduction as described earlier. The reduced CRO was stabilized in air at 70 ◦ C for 24 h. The stabilized CRO powder was used for fabrication of MOX fuel pellets. The multiple crushing, oxidation and reduction treatments were required to achieve powder properties comparable to that of the virgin powder. Different batches were prepared using (a) virgin 56% (wt) Nat. UO2 and 44% (wt) PuO2 powder, (b) 100% stabilized CRO, (c) 90% stabilized CRO and 10% (wt) oxidized CRO powder, (d) 95% (wt) stabilized CRO and 5% (wt) oxidized CRO powder, and (e) 85% (wt) stabilized CRO and 15% (wt) oxidized CRO powder in M3 O8 form along with organic binder and lubricant. The batches were further processed as per flowsheet shown in Fig. 1. The sintering of the batches was done at 1550 ± 25 ◦ C for 6 h with an intermediate soak at 800 ◦ C for 2 h in N2 –7% H2 atmosphere in a resistance heating furnace.

Fig. 2. A 3 kW, indigenously developed and glove box adapted microwave heating system.

and finally compacted with 6.3 mm diameter to get pellets of specified size. The sintering was carried out in reducing atmosphere at 1600 ◦ C for 6 h. CRO having low Pu content could be easily disintegrated into powder form after thermal pulverization due to lower plutonium content. During oxidation of CRO, phase changes from UO2 to U3 O8 led to concomitant volume change of about 30%. The stresses developed due to large volumetric change and temperature effectively break up the rejected pellets into finely divided powder without crushing and grinding. The sinterability and compressibility depend on the O/M of the starting powders. The reduction of oxidized CRO was carried out to get O/M ratio of the starting powder in the range of 1.96–1.99 so as to achieve specified O/M of MOX pellet. 3.2.2. CRO containing high % PuO2 The CRO containing high % PuO2 was recycled for MOX fuel fabrication using route II as shown in Fig. 1. The route II is further divided into route II-A (21% PuO2 ) and II-B (44% PuO2 ). Direct thermal pulverization of CRO was unable to disintegrate rejected pellets into powder even at higher oxygen potential due to its higher plutonium content. The stresses generated from the phase change of UO2 –U3 O8 were less than CRO containing low % PuO2 (<3%). It was observed that only a few pellets of CRO transformed into powder that too partially. 3.2.2.1. CRO containing 21% PuO2 . In order to study the effect of % CRO addition on green and sintered densities, batches of about 500 g were fabricated with varying content of CRO. A batch of MOX powder of 5 kg was prepared from virgin powder as well as from 100% CRO powder and further processed as per flowsheet shown in Fig. 1. The mechanical pulverization technique was coupled with thermal pulverization method for 100% recycling of 21% PuO2 CRO in route II-A. The mechanical pulverization was carried out using a jaw crusher. The modification in the feeding hopper of the jaw crusher was carried out to minimize dust generated during mechanical pulverization of MOX pellets. The fine MOX CRO powder obtained after jaw crushing provided more reaction site for oxidation of UO2 . The crushed CRO was further oxidized and reduced in the same way, as described in Section 3.2.1. 3.2.2.2. CRO containing 44% PuO2 . CRO containing 44% PuO2 was recycled for (U,44%Pu)O2 MOX fuel fabrication using route II-B as

3.3. Development of wet method for 100% recycling of MOX CRO A flow sheet was developed for 100% recycling MOX CRO containing 21% PuO2 using MicroWave Direct Denitration (MWDD) technique as shown in Fig. 1 (Route III). The non-pulverized CRO in batches (∼500 g each) was dissolved in 16 M HNO3 in the narrow neck quartz denitration vessel employing 0.6–0.8 kW microwave incident power to boil the solution smoothly. After dissolution, mixed uranyl nitrate and plutonium nitrate solution was heated to concentrate the solution by evaporating excess H2 O and HNO3 to get crystals of nitrates. Heating further at incident microwave power of 1–1.2 kW, water of crystalisation of nitrates was removed to get anhydrous nitrates. The anhydrous nitrates were decomposed to respective oxides on further heating above 300 ◦ C. Temperature was measured by inserting a thermocouple up to the charge. The direct microwave co-conversion process took approximately 2 h to get completed. Various process steps could be seen through an observation window provided on door of the microwave oven and an illumination source. The spongy cake formed after denitration was heated until it became incandescent. The fumes and vapors (HNO3 /NOx ) generated during processing were routed through a condenser followed by an alkali scrubber to the exhaust line. The condensed HNO3 was collected (although of lower molarity) and reused for dissolution of fresh batches along with concentrated HNO3. The denitrated and calcined powder was removed from the process vessel with a spatula as the product formed was spongy and fragile even with gentle touch. The chemical reactions taking place at various process steps and their respective temperature are as follows (Bao and Song, 1998; Kato et al., 2004, 2005; JAEA, 2006; Greiling and Lieser, 1984; Koizumi et al., 1983): (a) Dissolution (115–120 ◦ C) The CRO pellets were dissolved in concentrated HNO3 to get their soluble nitrates without addition of any catalyst like HF or dissolution aid (Eq. (1)). UO2 + PuO2 + 7HNO3 + 8H2 O → UO2 (NO3 )2 ·6H2 O + Pu(NO3 )4 ·5H2 O + HNO2

(1)

(b) Concentration (115–120 ◦ C) The mixed uranyl nitrate and plutonium nitrate solution was heated to concentrate the solution to get crystals of nitrates as shown in Eq. (2).

P.M. Khot et al. / Nuclear Engineering and Design 270 (2014) 227–237 Table 3 Microwave power and temperature profile during denitration/calcination.

231

4.2. Dry recycling

S. No.

Reaction/process

Incident power (W)

Time/duration (min)

Temperature (◦ C)

1. 2. 3. 4. 5.

Dissolution Concentration Dehydration Denitration Calcination

600–800 600–800 1000–1200 1000–1200 1000–1200

30–40 20–30 20–30 30–40 30–40

115–120 115–120 120–300 300–700 ∼700

4.2.1. CRO containing low % PuO2 (<3%) The green density of the batches fabricated from recycled CRO was found higher than that of virgin batches may be attributable to higher apparent and tap density of MOX powder and granules obtained from CRO batches (Table 5). However, the sintered densities of CRO batches were found to be nearly same as that of virgin batches. The yield was marginally higher in CRO batches compared to that of virgin batches.

Pu(NO3 )4 ·5H2 O + UO2 (NO3 )2 ·6H2 O + H2 O(Excess) + HNO3 (Excess) → PU(NO3 )4 ·5H2 O + UO2 (NO3 )2 ·6H2 O (2) (c) Dehydration (120–300 ◦ C) In this process, water of crystalisation of nitrates were removed to get anhydrous nitrates as shown below.

4.2.2. CRO containing 21% PuO2 The effect of addition of % CRO to virgin on green and sintered densities of annular MOX pellets is shown in Fig. 3. The addition of CRO up to 50% showed significant improvement in green density from 56 to 65% TD due to increased apparent and tap density of the MOX powder. However, the addition of CRO above 50% showed

⎫ ⎬

Pu(NO3 )4 · 5H2 O + UO2 (NO3 )2 · 6H2 O → Pu(NO3 )4 · 3H2 O + UO2 (NO3 )2 .3H2 O + 5 H2 O ⎪ Pu(NO3 )4 · 3H2 O → PuO2 (OH)NO3 + HNO3 + 2H2 O + 2NO2

⎪ ⎭

UO2 (NO3 )2 · 3H2 O → UO2 (OH)NO3 + HNO3 + 2H2 O (d) Denitration (above 300 ◦ C) The mixed anhydrous nitrates were decomposed to respective oxides on further heating above 300 ◦ C results in the denitrated product of composition mixed ␤-UO3 , PuO2 (Eq. (4)). UO2 (OH)NO3 → ˇ-UO3 + HNO3 PuO2 (OH)NO3 → PuO2 + HNO3 + 0.5O2



(4)

(e) Calcination and reduction The product was further calcined at ∼700 ◦ C in air in the same process vessel for 20–30 min (Eq. (5)). UO3 + PuO2 → UO2 + PuO2 + 0.5O2

(5)

The microwave incident power was adjusted to get required temperature profile during each process steps manually as shown in Table 3.The temperature profile was maintained at each stage to achieve desired product as per reactions (1)–(5). The reflected microwave power during the experiments was ∼0 W. The powder obtained after microwave calcination was further oxidized and reduced as per route III shown in Fig. 1. Two batches namely MWD-I and II were prepared from 100% microwave denitrated powder along with 0.8% PEG and 0.6% O.A. In MWD-I, CRO was recycled by MWDD technique following route III. The CRO generated by dry recycling method was in the form of sintered MOX pellets rejected due to physical defects. In MWD-II, CRO generated from dry recycling method (i.e. CRO from route II-A) was recycled 100% using MWDD technique following route-III.

(3)

least improvement in the green density of MOX pellets. Sintered density decreased initially with addition of CRO up to 20%. The sintered density showed an increasing trend up to 50% CRO and thereafter it again decreased. The increase in sintered density with addition of CRO in the range of 40–50% might be due to the fact that the effect of increase in green density on sintered density was higher than that of the opposite effect of lowering of shrinkage attributable to CRO addition. On the other hand, the decrease in sintered density with CRO addition above 50% may be attributed to significant reduction in sinterability of powder due to its lower surface area and larger particle size (Table 4). It was revealed from Fig. 3 that using 100% CRO, the sintered densities are in acceptable range (>90% TD). Subsequently 100% CRO recycling was attempted by dry recycling methods. The green density of the batches fabricated from recycled CRO was higher than that of virgin batches. On the other hand, the sintered densities of CRO batches were found to be nearly same as that of virgin batches. The higher green densities of CRO batches compared to virgin batches may responsible for lowering shrinkage in

4. Results and discussion 4.1. BET surface area analysis BET surface area analysis of starting materials used for batch preparation is given in Table 4. The oxidized reduced MOX CRO powder showed lower specific surface area and higher particle size as compared to virgin MOX powder. The microwave denitrated CRO showed slightly improved particle size and surface area as compared to virgin powder. On the other hand, second recycled microwave denitrated CRO powder showed slightly higher surface area as compared to first recycled microwave denitrated CRO powder.

Fig. 3. Effect of % CRO addition on green and sintered density of annular MOX pellets.

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Table 4 BET analysis of starting (U–21% Pu)O2 MOX powder obtained via different routes for batch preparation. Recycling method

Recycling route

Starting material for batch preparation

Particle size (␮m)

Specific surface area (m2 /g)

Virgin Dry Wet (MWD-I) Wet (MWD-II)

– II-A III II-A and then III

Virgin MOX powder Oxidized and reduced CRO powder First recycled MWDD powder Second recycled MWDD powder

2–3 5–10 1–2 1–2

2.63 0.69 2.71 3.29

Fig. 4. XRD pattern of stabilized and non-stabilized MOX CRO powder.

100% CRO batches. The rate of sintering and shrinkage of the powder metallurgy compacts is also influenced by initial powder properties such as particle size and specific surface of the powder used (Matsui et al., 2007; ASM, 2007). The shrinkage of 100% CRO batches (∼13%) was lower compared to virgin batches (∼18%) may be due to lower surface area (0.69 m2 /g) and higher particle size (5–10 ␮m) compared to virgin powder which in turn affected the sinterability of the powder. So tooling of lower size was required for CRO batches to achieve sintered to size pellet because of less diametrical shrinkage in green pellets fabricated. The yield was marginally higher in CRO batches compared to that of virgin batches. 4.2.3. CRO containing 44% PuO2 The CRO having 44% PuO2 could not be easily disintegrated in the powder form since, the degree of thermal pulverization (UO2 –U3 O8 conversion) was lesser compared to that of CRO containing 0.4–21% PuO2 . Therefore, the dry recycling of 44% PuO2 MOX CRO requires crushing, multiple oxidation and reduction treatments for achieving powder properties comparable to that of virgin powder. However, during powder conditioning treatments and storage, there was a change in O/M ratio of MOX powder. A stabilization treatment of reduced CRO carried out at in air 70 ◦ C for 24 h solved the problem of the further oxidation of MOX powder which led to the change in O/M ratio. In order to understand the passivation mechanism of stabilized powder, XRD analysis of stabilized and non-stabilized powder samples was carried out and is shown in Fig. 4. From the XRD pattern it has been observed that the stabilized sample showed presence of additional U3 O8 phase in comparison with non-stabilized sample. The probability of formation of U3 O8 on the surface of the powder particle was more than that of the interior portion as stabilization treatment was carried out at lower temperature. For complete oxidation of the powder particles, air had to permeate through the dense particles which would be less probable phenomena at lower temperature (70 ◦ C). The U3 O8 layer formed around the stabilized powder particles may further inhibit oxidation during storage or powder conditioning treatments.

The oxidized CRO addition in dry recycling of 0.4–21% PuO2 MOX CRO (Route I, II-A) was not required since it was possible to achieve the sintered density in the acceptable range (>90% TD) without addition of oxidized CRO. During dry recycling of 44% PuO2 MOX CRO batches without oxidized scrap (i.e.100% stabilized CRO batches) showed sintered density near to lower side of the acceptable range (∼90% TD). In order to further improve sinterability of the conditioned MOX powder, oxidized scrap addition was done in 44% PuO2 MOX CRO recycling. As shown in Table 5, addition of oxidized CRO up to 10% (wt) in M3 O8 form showed an improvement in sintering rate as compared to that of batches fabricated without oxidized scrap. The U3 O8 present in oxidized scrap will be reduced to UO2.00 in N2 -7% H2 atmosphere even at 700 ◦ C if it is in powder form. In this study, however, the green pellets having densities in the range of 62–65% T.D. were used. Since the rate of reduction of U3 O8 was surface controlled, the reduction of U3 O8 in to UO2 could not be easily completed as the gas had to permeate through the dense mass of pellets. This suggested that U3 O8 had not been fully reduced to UO2 at the early stage of sintering but, might have been reduced to UO2+x . Lay and Carter (Lay and Carter, 1969) have evaluated the role of O/U ratio on sintering of uranium dioxide and reported that the diffusion coefficient of uranium at the initial stages of sintering of uranium dioxide was dependent on the O/U ratio. They have reported that the diffusion coefficient of uranium in uranium dioxide having an O/U ratio of 2.02 is 108 times greater than that having an O/U ratio of 2.00. In fact, the diffusion coefficient of uranium increased in proportion to x2 . This may be the probable reason for improvement in sintering rate of (U,Pu)O2 MOX using oxidized CRO up to 10%. According to Schwartz et al. (1979) addition of U3 O8 in UO2 resulted in decreased closed porosity due to the catalytic effect of the excess oxygen supplied by U3 O8 , on the sintering kinetics of uranium dioxide. On the other hand, addition of U3 O8 resulted in increased open porosity due to large decrease of volume accompanied by the conversion of U3 O8 in to UO2 . This might be the reason for increase in sintered density of the batches fabricated with oxidized scrap. It was observed that the addition of 15% oxidized CRO (in M3 O8 form) showed decrease in sintered density (Table 5). The addition of oxidized CRO increases open porosity as a result of volume change. It also reduces the closed porosity attributable to catalytic effect of excess oxygen. The reduced sintered density with the addition of 15% oxidized CRO might be due to increase in open porosity was higher than that of decrease in closed porosity. The green and sintered density of CRO batches were higher than that of batches fabricated from the virgin powder under similar compaction and sintering parameters.

4.3. Wet recycling of 21%PuO2 MOX CRO The effect of shape of the process vessel on powder characteristics was studied. In order to select a process vessel for denitration/calcination, three types of quartz vessels were investigated namely dish, wide neck and long narrow neck beakers and corresponding observations and related parameters are recorded in Table 6. The denitration experiments carried out in dish shaped and wide neck beakers showed a product which was sticky, hard and difficult to discharge from the process vessels. A long narrow neck

Table 5 Physical characteristics of powder and pellets obtained via different recycling method. Method

21% PuO2

44% PuO2

Batch details

Virgin powder (21% Pu)

Virgin powder (Nat. UO2 and PuO2 powder) 100% CRO (21% Pu)

21% PuO2 Dry recycling 100% CRO (low % Pu)

44% PuO2

Wet recycling

21% PuO2

100% stabilized CRO 95% stabilized CRO + 5% oxidized CRO in M3 O8 form 90% stabilized CRO + 10% oxidized CRO in M3 O8 form 85% stabilized CRO + 15% oxidized CRO in M3 O8 form MWD-I First recycling of 100% MWDD CRO

MWD-II Second recycling of 100% MWDD CRO

Powder characteristics

Tool size (ø) mm

Powder lot

Apparent density (g/cm3 )

Tap density (g/cm3 )

Milled Powder (40 min) Granules (120 MPa, 10#) –

2.83

3.35

3.82

4.1





3.33

3.84

4.166

4.23

2.74–2.81

3.32–3.4

Pellet characteristics (280 MPa)

% Yield

Green density (% TD)

Sintered density (% TD)

6.7

54.47–56.39

93.08–96.34

89–90

6.7

53.64–56.40

90.92–93.08

89–91

6.3

66.59–67.85

92.78–96.43

91–92

6.3

65.97–67.54

91.36–95.53

90–92

Milled Powder (40 min) Granules (120 MPa, 10#) Milled Powder (40 min) Granules (120 MPa, 10#) – –

3.85

4.15

– –

– –

6.3 6.3

66.60–67.85 65.97–67.54

89.47–90.45 90.65–93.45

91–93 91–92







6.3

65.94–66.91

92.69–96.70

92–94







6.3

65.47–66.50

88.47–90.55

89–90

Milled powder (20 min) Granules (120 MPa, 10#) Milled powder (20 min) Granules (120 MPa, 10#)

2.9–3.0

3.5–3.7

6.3

61.81–64.70

92.56–94.62

91–93

4.0649

4.57

2.92–3.2

3.6–3.75

6.3

63.45–67.23

93.8–97.42

92–95

4.2

4.62

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Virgin

Composition

233

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Table 6 Effect of shape of process vessel on denitrated/calcined powder characteristics. S. No.

Shape of quartz process vessel

Dimension (mm)

Observation

De-nitrated/calcined powder characteristics

1.

Dish

2.

Wide neck beaker

Insignificant effervescence Slight effervescence

Sticky and hard, very difficult to remove from vessel Slightly sticky and hard mass

3.

Long beaker with narrow neck

Height = 60 Diameter = 180 Height = 140 Diameter = 120 Height = 280 Diameter = 75

Vigorous effervescence

Enormous gas pockets, spongy and fragile. Not sticky and easy to remove from process vessel

beaker was chosen for MicroWave Direct Denitration (MWDD). This was due to the fact that long narrow neck beakers offered small evolution area for outgoing gases/vapors so effervescence was maximum. Since solidification occurred suddenly while the evolution of the off gases was still continuing, the product formed consisted of enormous gas pockets and porosity. The physical characteristics of the powder and pellets fabricated via MWDD are presented in Table 5. The powder characteristics of microwave denitrated powder as compared to virgin powder were found to be improved with regard to particle size, surface area, apparent and tap density. It was observed that the batches fabricated from microwave denitrated CRO showed better sintered density as well as yield, possibly due to the improved powder characteristics as discussed earlier (Table 4). The fuel pellets fabricated from second recycled CRO (MWD-II) met the specifications with better sintered density and yield as compared to pellets fabricated from first recycled CRO (MWD-I). The higher sintered density in second recycled CRO batches was attributable to higher surface area of starting powder (3.29 m2 /g) compared to first recycled CRO (2.71 m2 /g) powder (Table 4). The 100% second/multiple recycling of CRO with dry recycling technique was not feasible after following route II-A due to poor powder characteristics. Subsequently second/multiple recycling of CRO was attempted with MWDD technique following route III. The second/multiple recycling with MWDD technique (route III) does not need dry recycling process steps such as jaw crushing, multiple oxidation and reduction etc. prior to MWDD processing. However oxidation and reduction are required after MWDD processing for powder conditioning to obtain desired O/M of the starting materials. It was observed that second/multiple recycled batches with MWDD showed improved powder/product characteristics. The second/multiple recycling of 100% CRO which was difficult task in dry recycling method, was easily carried out with MWDD technique. During microwave heating, volumetric heat generation caused rapid heating and reversed thermal gradient in the hot material (Sutton, 1989; Thostenson and Chow, 1999; Jhao and Chen, 2008). The higher thermal stresses developed due to rapid heating and reversal of thermal gradient in microwave heating as compared to conventional heating might be responsible for higher surface area (2.71 m2 /g) and smaller particle size (1–2 ␮m) of the powder obtained via microwave denitration route. This might be the reason for the improved sinterability of powder obtained via microwave denitration route as compared to that of powder obtained via dry recycling route explained earlier. The higher sintered density of microwave denitrated batches reduced handling defects and thus improved the yield compared to that of the virgin batches. 4.4. Characterization 4.4.1. O/M The O/M ratio of the starting powders after conditioning treatments and the sintered pellet were measured thermogravimetrically using gas equilibration technique. O/M ratio of starting powders used for batch preparation and MOX pellets fabricated

Fig. 5. XRD pattern of (U–21% Pu)O2 MOX pellet (virgin batch).

from different recycling route are recorded in Table 7. The O/M ratio of pellets prepared via recycling route I, II and III were slightly higher than that of MOX pellets fabricated from virgin powder due to higher O/M of starting powder. The interstitial oxygen introduced into the fluorite lattice during multiple powder treatments such as crushing, oxidation etc. which might have resulted in higher O/M of starting powder used for recycling batches. The O/M ratio of MOX pellets fabricated from powder obtained via route II-A and III was slightly higher than that of recycling route I might be due to additional processing steps such as crushing, multiple oxidations, denitration, etc. The pellets fabricated from all recycling routes met the O/M specification. 4.4.2. XRD The XRD analysis was carried out to identify the phases existing in the stabilized and non-stabilized MOX powder. Fig. 4 shows XRD pattern of stabilized and non-stabilized MOX CRO powder. The stabilized sample showed additional U3 O8 phase in comparison with that of the non-stabilized sample. Fig. 5 shows the XRD pattern of (U,21%Pu)O2 pellet sintered in N2 –7% H2 . The XRD pattern of (U,21% Pu)O2 pellets revealed the presence of single phase fcc solid solution. 4.4.3. Microstructure The sintered pellet was mounted in Araldite cement and ground successively by emery papers. The final polishing was done using diamond paste. The pellet was then removed from the mount by dissolving the cement in acetone. Thermal etching of polished pellet was performed by holding it in reducing atmosphere (N2 –7% H2 ) at 1600 ◦ C for 1 h for ceramographic inspection (Charollais et al., 2001). The microstructure of MOX pellets prepared by different recycling routes was analyzed. Figs. 6 and 7 show the

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Table 7 O/M ratio of starting powder and (U–21% Pu)O2 pellets (sintered in reducing atmosphere). Recycling Method

Recycling route

Starting material

Powder conditioning treatments

O/M starting powder

O/M sintered pellet

Dry Dry

I II-A

100% CRO (<3% PuO2 ) 100% CRO (21% PuO2 )

1.98 ± 0.02 1.99 ± 0.02

1.97 ± 0.02 1.98 ± 0.02

Wet (MWDD)

III

100% CRO (21% PuO2 )

1.99 ± 0.02

1.98 ± 0.02





Virgin powder (21% PuO2 )

Oxidation, reduction Crushing, oxidation, reduction Microwave dissolution, denitration, calcination, oxidation and reduction –

1.97 ± 0.02

1.96 ± 0.02

Fig. 8. ␣-autoradiograph of (U–21% Pu)O2 MOX pellet (virgin batch).

Fig. 6. Microstructure of (U–21% Pu)O2 MOX fuel pellet (virgin batch).

microstructures of UO2 –21%PuO2 fuel pellets fabricated from virgin and microwave denitrated powder, respectively. In the microstructure U–Pu grains are visualized. The microstructure of UO2 –21%PuO2 pellet fabricated from virgin and microwave denitrated powder and sintered in reducing atmosphere was

Fig. 7. Microstructure of (U–21% Pu)O2 MOX fuel pellet (microwave denitrated batch).

nearly uniform throughout the pellet. The grain size was determined by intercept method. The average grain size of pellets fabricated from virgin powder was 15–20 ␮m. On the other hand, grain size of the pellets fabricated from microwave denitrated batches was 25–30 ␮m. As grain growth is a diffusion controlled phenomena, it depends upon defect concentration such as oxygen interstitials or metal vacancies and sintering temperature. As the temperature employed in this study was the same, the concentration of the defects would be the controlling factor for diffusion controlled process such as grain growth. The higher grain size in the fuel pellets fabricated by microwave denitrated powder might be due to higher non stoichiometric defects generated which was indicated by higher O/M ratio of its starting powder used for batch preparation (Table 7). 4.4.4. ˛-autoradiography The macroscopic homogeneity was evaluated by means of alpha-autoradiography. The alpha autoradiography is a well established technique and widely used to check plutonium distribution in Pu based fuels (Kegley, 1972; Kamath, 2003). The presence of Pu agglomerates in the pellet affects the fuel performance as they act as hot spots. It enhances fission gas release resulting in increase of fuel pin internal pressure (IAEA, 2003b). So it was necessary that the size of Pu agglomerate should be as low as possible and its distribution should be uniform throughout the matrix. The ␣-autoradiograph of the pellets fabricated from the virgin batch was compared with pellets fabricated from microwave denitrated batches. Figs. 8 and 9 show ␣-autoradiographs of UO2 –21%PuO2 pellet obtained from virgin and MWDD batches, respectively. It was observed from Figs. 8 and 9 that the distribution

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Fig. 9. ␣-autoradiograph of (U–21% Pu)O2 MOX pellet (microwave batch).

Table 8 Comparison of dry and wet (MWDD) route for MOX fuel fabrication.

Powder characteristics Productivity Second/Multiple recycling

Dust raising and Pu loss Liquid waste

Dry

Wet (MWDD)

Comparable to that of virgin powder. High Difficult due to requirement of multiple oxidation, reduction and crushing More due to requirement of grinding and crushing Nil

Comparable or even better than that of virgin powder Low Comparatively easier due to less process steps

Less due to absence of grinding and crushing Nil or negligible as condensed nitric acid was reusable

of Pu rich islands (dark spots) in uranium matrix was uniform and did not show any agglomerate greater than 30 ␮m. The MOX pellets fabricated from MWDD batches showed better distribution of plutonium in uranium matrix than that of virgin batches. As plutonium and uranium nitrate were mixed in solution state, microwave denitrated batches showed better macroscopic homogeneity with little milling. The smaller particle size of microwave denitrated powder in comparison with that of virgin powder also reduces diffusion distance required to achieve desired homogeneity. 4.5. Comparison of dry and wet (MWDD) recycling techniques The advantages and disadvantages of the dry and wet methods for recycling are recorded in Table 8. The method for CRO recycling has to be chosen by the fuel fabricator considering various factors such as powder characteristics, productivity and time of processing. 5. Conclusion In this study, it was demonstrated that high density (U,Pu)O2 MOX pellets can be fabricated by dry as well as wet (MWDD) techniques using 100% CRO as a starting material. The parameters for dry recycling of MOX CRO containing PuO2 in the range 0.4–44% were optimized. The parameters of wet (MWDD) recycling route of 21% PuO2 MOX CRO were also optimized. From the detailed study of these recycling processes following conclusions have been drawn:

(a) 100% CRO containing 0.4% (wt) to 44% (wt) of PuO2 has been recycled successfully by dry recycling techniques. The production yield of MOX fuel pellets fabricated from 100% CRO has been improved slightly as compared to the pellets fabricated from virgin powders. The higher productivity and powder characteristics comparable to the virgin powder made dry recycling technique highly efficient for recycling CRO at higher rate. (b) The characteristics of the powder obtained by MWDD technique were better than the virgin powder. The wet recycling methods are not preferred as these involve many process steps and associated generation of liquid waste. The MWDD technique approaches the concept of zero discharge since the condensed HNO3 (although of lower molarity) can again be used for dissolution of fresh CRO batches along with concentrated HNO3 . The main drawback of MWDD technique was lower productivity due to criticality considerations in solution processing. (c) The microstructure of MOX pellets from MWDD batches was nearly uniform with a grain size in the range of 25–30 ␮m. The grain size of MOX pellets from virgin batches was 15–20 ␮m. The macroscopic homogeneity of MWDD batches was better than that of the virgin batches even though MWDD batches were prepared with little attritor milling. (d) The second/multiple recycling of 100% CRO which was difficult to recycle by dry recycling method was easily carried out with MWDD technique with improved powder/product characteristics.

Acknowledgements The authors are sincerely thankful to all colleagues of OFS (Oxide Fuel Section) and QCS (Quality Control Section) of AFFF for the encouragement and useful suggestions during the course of this work. The authors would like to acknowledge the cooperation extended by Radiometallurgy Division (RMD) of BARC for carrying out surface area and XRD analysis. The authors are grateful to Dr. K.B. Khan, RMD for his keen support during the course of this work.

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