The process consists, basically, in the simultaneous adding of the gaseous ammonia (or ammonium hydroxide solution) and the acid solution containing the actinides, keeping the pH, temperature and agitation, constant along all the process.
The precipitant agent used in our case was the ammonia gas and not the ammonium hydroxide because it is very important for us to reduce the liquid wastes as much as possible when the work is carried out inside glove boxes during the fabrication of mixed fuel using uranium – plutonium. Figure 1. shows the different steps of the process.
Fig. 1. A generalized flow sheet for conversion of mixed solutions of uranium into sintered pellets.
Feed solution composition and acidity adjustment
Coprecipitation and filtering
Calcination - Reduction
Milling
Pressing into pellets
Sintering NH3(g)
NITRIC SOLUTION OF : UO2(NO3)2
or
UO2(NO3)2 + Pu(NO3)4 or
UO2(NO3)2 + Pu(NO3)4 + PuO2(NO3)2 or
UO2(NO3)2 + Gd(NO3)3
The simplified chemical reactions involved in each step are shown in Table I.
Table I.
2.1. Precipitation reactor
The precipitation reactor consists of a double-wall Pyrex glass tube through which water at constant temperature circulates; thus, the selected work temperature of the solution in the inner container remains constant along all the process. Furthermore, the reactor has a vertical stirrer with a propeller powered by an electrical motor. A glass electrode connected to a pH-meter allows to testing the solution pH constantly. A peristaltic pump with a constant flow
Adjustment of oxidation state (for Pu+4 precipitation only)
PuO2+2 + H2O2 + 2 H+ → Pu+4 + 2H2O + O2 ↑ (1) Precipitation reactions
NH3 + H2O → NH4+ + OH- (2) H+ + OH - → H2O (acid neutralization) (3) 2 UO2+2 + 6 OH- → U2O7= + 3H2O (4) U2O7= + 2 NH4+ → U2O7(NH4)2↓ Ammonium diuranate (ADU) (5) Pu+4 + 4 OH- → Pu(OH)4↓ Plutonium hidroxide (6) 2 PuO2+2 + 6 OH- → Pu2O7(NH4)2↓ Ammonium diplutonate (ADPu) (7) Gd+3 + 3 OH- → Gd(OH)3↓ Gadolinium hidroxide (8) Thermal Decomposition
400º C
U2O7(NH4)2 → 2UO3 + 2NH3 + H2O (9)
400º C to 650º C
3UO3 → U3O8 + ½ O2 (10)
400º C to 650º C
Pu(OH)4 → PuO2 + 2 H2O (11)
400º C to 650º C
2 Gd(OH)3 → Gd2O3 + 3 H2O (12) Reduction
650º C
U3O8+ 2 H2 → 3 UO2 + 2 H2O (13) Sintering
~ 1700º C
UO2+x (green pellet) + x H2 →UO2,00 (sintered pellet) + x H2O (14)
NH3
pumps the solution into the reactor. The ammonia gas supply is controlled using a manual flow regulation valve.
Both, the ammonia and the acid solution of actinides enter simultaneously through a lid on the top of the reactor. The end of either dosage pipe is placed very close to the stirrer propeller.
FIG. 2. Schematic drawing of the precipitation reactor.
3. EXPERIMENTAL
3.1. Assays with uranium only
The co precipitation method adjustment and the powder thermal treatments were carried out in a first step, with uranium only and outside glove boxes.
3.1.1. Actinides solution preparation
For the first assays and the determination of work parameters three uranyl-nitrate solutions with different uranium concentration in 1M free nitric acidity were prepared. This acid concentration assures the non-polymerization of Pu+4 when the assay is carried out with solutions containing plutonium, and is low enough to avoid a high spending of ammonia due to the acid neutralization with formation of a high quantity of ammonium nitrate salt. The selected actinide concentration was 300 g/l in order to increase the yield in each precipitation batch without affecting the powder quality.
3.1.2. Precipitation pH selection
From experiments carried out in other countries and from the subsequent bibliography, we concluded that the best precipitation pH is between 4 and 6 [1]. This pH range assured a simultaneous and quantitative precipitation of both elements and avoided the selective precipitation of one of them as it could occur in the direct method with variable pH. Besides, the precipitate obtained under those conditions presented an easy filtration process.
1 : pH-meter.
2 : Thermostatic bath.
3 : Stirrer.
4 : Glass electrode.
5 : Ammonia flow regulation valve.
6 : Peristaltic pump for actinides solution.
7 : Ammonia flow meter.
8 : NH3excess and air outlet.
9 : Propeller.
10 : Actinides solution vessel.
11 : Drain.
12 : Lid.
13 : Double glass wall.
14 : Discharge valve.
15 : NH3inlet .
16 : Actinides solution inlet.
9
3.1.3. Precipitant agent selection
The selected precipitation agent was the gaseous ammonia according to the reasons explained in Section 2.
3.1.4. Precipitation temperature choice
Previously, three constant working temperatures were tested to heat the actinide solution:
20ºC, 40ºC and 60ºC.The ADU (ammonium diuranate) precipitate, formed at 60ºC and proceeding from a solution with 300 g/l of uranium, was easily filterable with an average particle size of ~12 microns. On the other hand, the precipitate formed at 40ºC showed smaller particle sizes (~6 microns) and a longer filtration time, and the 20ºC one was gelatinous and very difficult to filtrate. That is why the selected temperature was 60ºC.
3.1.5. Precipitation
Every experiment was performed using 1000 ml of actinide solution that were pumped into the reactor at a rate of ~ 17 ml/min. The ammonia gas flowing was manually controlled using a regulation valve in order to maintain the pH shown in the pH-meter within the range 4 – 6.
3.1.6. Filtration
The slurry of yellow ADU precipitate remained in the reactor one more hour at the same temperature, pH and agitation speed to allow the particles to grow. Afterwards, the reactor was drained and the mother water was separated by filtration using a vacuum pump and a Büchner funnel provided with the adequate filter paper. The retained precipitate is then washed with a diluted ammonium hydroxide solution, and later, dried 24 hrs in an oven at 120ºC in air. The analysis of the mother water showed an uranium content lower than 3 ppm, which means, a quantitative precipitation.
3.1.7. Decomposition-reduction. Thermal treatment.
In a horizontal electric furnace, the ADU precipitate was heated according to a controlled heating program. In the following diagram, the times to reach each temperature and the corresponding atmosphere are shown.
75 min. 60 min. 35 min. 60 min. ~ 15 hrs.
20ºC 400 ºC 400ºC 650 ºC 650 ºC 20ºC ADU air N2 N2 N2/H2 (8%) N2 UO2+x (decomposition) (reduction)
Where N2means nitrogen atmosphere, and N2/H2 (8%) means mixed atmosphere of nitrogen-hydrogen (8% nitrogen-hydrogen).
UO2+x indicates that the oxide composition is not stechiometric ( O/U ratio greater than 2.00).
3.1.8. Milling
A portion of the UO2+x powder obtained from the thermal treatment was milled in a cylindrical horizontal mill provided with stainless steel balls during 4 hours. This process was necessary to increase the bulk density of the powder, which facilitates the press die loading, and increases subsequently, the density of “green” pellets and sintered pellets.
The processes described below were carried out on both kinds of powders: milled and non - milled ones.
3.1.9. Pressing
Several pressures were assayed using an automatic press. Two methods of lubrication were tested: internal lubrication, by adding zinc diestearate into the powder (~0.4%) and mixing homogeneously, and external lubrication, applying vegetal oil on the die wall.
3.1.10. Sintering
Two thermal cycles in argon–hydrogen (8%) atmosphere, were studied depending on the used lubrication method. The mentioned cycles are as is indicated:
Cycle 1 (used to eliminate the internal lubricant):
200ºC/h 60 min. 325ºC/h 120 min. -420ºC/h 20ºC 400ºC 400ºC 1650ºC 1650ºC 20ºC
Cycle 2 (without an elimination “plateau”, for pellets pressed with external lubrication):
300ºC/h 160ºC/h 120 min. - 420 ºC/h 20ºC 900 ºC 1650ºC 1650ºC 20ºC 3.1.11. Characterization of the obtained products
The following table shows the product features:
Table II
Measured parameter Non-milled powders 4-hour milled powders
Specific surface area (m2 /g) 3.32 4.09
O/U ratio 2.06 2.08
Average particle diameter
(microns) 8.93 3.10
Tap density (g/cc) 2.61 3.33
In table III, the average densities of 10 pellets are shown, depending on the way of conditioning and pressing pressure.
As we can see in Table III, the pellets proceeding from milled powders, pressed using external lubrication and sintered according the cycle 2 (Section 3.1.10.) showed sintered densities equal or higher than 95% T.D. (theoretical density) being this one the lowest density admitted for the pellets required to be used in power reactors such as Atucha or Embalse (Candu fuel) existing in Argentina at present time (year 2003).
Lubricant elimination
Table III
NON-MILLED POWDER 4-HOUR MILLED POWDER
▼
Internal Lubrication External Lubrication Internal Lubrication External Lubrication PressingPressure 500 MPa* 600 MPa 500 MPa 600 MPa 500 MPa 600 MPa 500 MPa 600 MPa Green density
(g/cc) 5.94 6.09 6.54 6.71 6.56 6.21 6.53 6.74
Cycle used 1 1 1 2 1 2 1 1 1 2 1 2
Sintering
density (g/cc) 10.0 10.2 9.9 10.3 10.0 10.3 10.3 10.3 10.4 10.6 10.4 10.6
%T.D.
(Theoretical density)
91.2 93.1 90.3 94.0 91.2 94.0 94.0 94.0 94.9 96.7 94.9 96.7
* : MPa = mega-pascal pressure unit = 10-2 Ton/cm2.
3.2. Glove box assays using a mixed uranium-plutonium (IV) solution
With the results and knowledge acquired with uranium, the “reverse strike” co precipitation technique was carried out inside glove boxes with a mixed solution of uranium-plutonium.
The plutonium was stabilized as Pu (IV) using hydrogen peroxide (H2O2) as it is shown in the reaction (1) in Table I.
Due to the smaller working place inside the glove box, the precipitation reactor used was smaller than the one used outside with uranium only, and the volume of the mixed solution was 500 ml only.
The solution characteristics and work conditions are summarized as follows:
Feeding solution Work conditions and results Volume………...…..500 ml Temperature……….60ºC Acid concentration………...0.9 M (nitric acid) pH………4 – 6 Uranium concentration………...228.10 g/l Used solution volume…..474 ml Plutonium concentration………...57.05 g/l Precipitant reactive …… NH3 (g) U+Pu concentration………..285.15 g/l Liquid wastes volume.….1000 ml Filtration time ……...~ 10 min.
The resultant mother water was analyzed and no presence of U or Pu was detected, then, the precipitation was entirely quantitative.
3.2.1. Thermal treatments
The obtained mixed precipitate was calcined under the conditions detailed in Section 3.1.7.
The result was a pyrophoric mixed oxide. That is why the calcination – reduction cycle had to be modified in order to reduce the specific surface area of the powder and, therefore, avoid its re oxidation in air at laboratory temperature. Three thermal cycles (A, B and C) were studied.
In one of them the reduction temperature was increased and in all of them the reduction
“plateau” time was also increased.
Table IV shows the characteristics of the powders obtained under those conditions.
Table IV
As we can see, cycle C is the treatment that gave the lowest specific surface area and the highest tap density. These two features are the most convenient to the next treatments of powders.
3.2.2. Pressing
Due to the low tap density of the powders, it was necessary to select pressures lower than the ones used in the experience with uranium in order to obtain pellets with the required dimensions. The extremely low density of the powders obtained with cycle A, made its pressing very difficult, so, the results were not repetitive. In all cases, external lubrication with vegetal oil was used. The next table shows some pressures applied and the resulting
“green” densities obtained, depending on the cycle used.
Table V Pressing pressure (MPa) Cycle B
(Green density –g/cc-)
The sintering of pellets was carried out using the Cycle 2 (Section 3.1.10.) because this one gave the best results in the assays with uranium only. The sintered pellet density was measured by weighing each one, first, in air, and after that, in water (Archimedes´ principle).
Some results are shown in table VI, where the pressing pressure applied and the thermal cycle used, are indicated (T.D. = 11.06 g/cc for 20% of Pu).
Table VI Pressing pressure (MPa) Cycle B
(Sintering density –g/cc-)
A solubility assay was done using one of the sintered pellets. The pellet was attacked with 7 M nitric acid in a reflux bath during 6 hours without fluorhidric acid (FH) adding. At the end of three hours, the pellet was entirely dissolved. Nevertheless, after 6 hours, the solution was cooled and filtered through the finest pore filter paper. The filtered solution and the content on the paper were analyzed, and the following result was found:
Mass of Pu found in solution
Solubility % = --- 100 = 99.7 % (Mass of Pu found in solution + mass of Pu found on the paper)
It is interesting to compare this result to the solubility of pellets proceeding from mechanical blending of UO2+ PuO2 powders ( with a Pu content of 0.55% w/w in the pellet) used in a previous fabrication for irradiation assays [2]. In this case the results obtained for the solubility in the same conditions and furthermore with FH adding, were only ~ 97 % .
3.2.5. Ceramographic characterization
Some pellets were cut, polished and attacked using conventional ceramographic techniques to study their microstructures. The porosity distribution observed in these pellets was inhomogeneous but not in the case of the grain size distribution, which turned out to be homogeneous, with an average grain size of 11 microns. Individual PuO2 particles were not observed in the alpha-auto radiographies.
3.3. Glove box assays with mixed solution of uranium and (Pu+4 + PuO2+2)
In this experiment with mixed solution of U and Pu, the plutonium was not conditioned to be only as Pu+4 using hydrogen peroxide, as it was in the previous one. Thus, both species, Pu+4 and PuO2+2
were present (~60% and ~40% respectively), as well as the UO2+2
, all of them proceeding from the attack with nitric acid of scraps resultant from previous fabrications. The aim was to study the final quality of the products and their features. Almost all the other work conditions and processes were preserved. The dried precipitate obtained under these conditions, was divided into two portions (A and B). Portion A was calcined using the cycle C described in Section 3.2.1. taking into account the reasons already mentioned for the assay with Pu+4 . Portion B was calcined with the same cycle, but using a temperature of 800ºC during the reduction step in order to decrease the specific surface area even more.
Table VII shows the obtained results.
Table VII
Both powders showed a homogeneous appearance and a very stable behavior regarding the oxidation when the samples were exposed to the air.
3.3.1. Pressing
The higher tap density of these powders allowed to apply a pressure greater than the ones already used in powders of Pu+4 (250 MPa ).
3.3.2. Sintering
The green pellets were sintered using Cycle 2 (Section 3.1.10.). The results are shown in the next table.
Table VIII Reduction temperature
(ºC)
Green density (g/cc)
Sintering density
(g/cc) % Theoretical density
700 (Portion A) 5.38 9.84 88.9
800 (Portion B) 5.67 9.83 88.9
The T.D. of sintered pellets were much lower than the ones observed in the assays with Pu+4. The ceramographic analysis showed a greater amount and volume of pores that agrees with the observed T.D. decrease in the pellets. The assay of solubility with nitric acid without FH adding showed a similar result to the one of previous step with Pu+4 (greater than 99.7%).
3.4. Co precipitation assays with mixed uranium-gadolinium nitric solutions outside glove boxes
The “reverse strike” co precipitation method was also used to obtain mixed oxides of U-Gd for pellets fabrication. The supposed and simplified chemical reactions involved in different steps of the process were already mentioned in Table I. The precipitation reactor, the calcination furnace and the mill, are the same as those used in the experiments with uranium only.
3.4.1. Preparation of the uranium-gadolinium solution
Five solutions of 1000 ml each containing 0, 2, 4, 6 and 8% of gadolinium regarding the total present metal, were prepared. The total metal content (U+Gd) in each solution was ~300 g per liter. As always, the medium was 1 M nitric acid.
TAP density Specific surface area (g/cc) (m2/g)
Portion A (700 ºC) 2.62 6.70 Portion B (800 ºC) 2.68 3.60
3.4.2. Precipitation pH selection
The selected working pH was 9, which proceeded from previous experiments carried out in order to obtain a quantitative and simultaneous precipitation for both elements. The other process parameters were the same as those that were used with uranium. The following table summarizes the characteristics of the obtained precipitate for each Gd concentration.
Table IX
% of Gd (U + Gd = 100% w/w)
Average particle diameter (Microns)
Specific surface area (m2/g)
0 0,37 8,8
2 1,00 12,2
4 2,60 10,8
6 11.5 9.4 8 12,5 8,8 All the precipitates were analyzed by X-ray diffractometry. The diffractograms showed peaks
that had no correspondence with those observed for ADU, Gd(OH)3 or another known compound of U or Gd that could be formed under these physicochemical conditions. For this reason, the existence of a new mixed phase is presumed (see Fig. 3):
FIG.3. Diffractograms of obtained precipitates with different gadolinium content.
3.4.3. Thermal treatment of the precipitates
Previous calcination assays of the 0% Gd powder (uranium only, precipitated at pH 9) with the cycle mentioned in Section 3.1.7. showed that the resulting powder had a specific surface area about 5 m2/g . Considering that the same powder obtained at pH 4-6 had a specific surface area of 4.09 m2/g, some modifications of that cycle were performed in order to decrease that parameter and consequently to get a greater stability in air. All the obtained precipitates of U-Gd were calcined under the cycle shown below.
75 min. 60 min. 35 min. 120 min. 60 min. 15 hours
Precipitate at 20ºC 400ºC 400ºC 650ºC 650ºC 650ºC oxides at 20ºC Air N2 N2 N2 N2 / H2 N2
PERFORMED MODIFICATION
intensity (a.u.) new phase
2 theta (deg.)
3.4.4. Milling, pressing and sintering
The powders thus obtained, were milled during four hours in a stainless-steel ball mill in order to increase their bulk densities. The results obtained after the milling are shown in the next table. Data of milled UO2 powder precipitated at pH 4-6 are also given in order to compare.
Table X
% Gd (metal) Average diameter (microns)
(pH 4-6 precipitation) 3,10 4,09 2,08 3,33
Several pressing pressures were studied, resulting the 240 MPa the selected one because it gave the highest “green” density in each case. Afterwards, the green pellets were sintered according to Cycle 2 (Section 3.1.10.). The final obtained features can be summarized in the following table.
Table XI
Tested oxide Green density (1) (g/cc)
(1) : Average 10 pellets measured in each case.
(2) : Average 2 pellets measured in each case.
(*) : The theoretical density for each gadolinium content, can be calculated upon the data issued by Fukushima [3].
The analysis by X-ray diffraction of both mixed powders of U–Gd oxides and their corresponding sintered pellets, showed the existence of a single phase that crystallized in the face centered cubic system, f.c.c., whose lattice parameters changed according to the Gd concentration [4]. These parameters were smaller than the ones observed in the UO2 lattice (5.471 Angström). This lattice shrinking was due, apparently, to the gadolinium incorporation into the UO2 lattice [3,5]. The qualitative analysis carried out by EDS (Energy Dispersive Spectroscopy) and WDS (Wavelength Dispersive Spectroscopy) techniques on sintered pellets, showed a homogeneous distribution of gadolinium inside the uranium matrix.