Dissolution of synthetic uranium dibutyl phosphate deposits in oxidizing and reducing chemical formulations

Journal of Hazardous Materials 254–255 (2013) 263–269

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Dissolution of synthetic uranium dibutyl phosphate deposits in oxidizing and reducing chemical formulations A.L. Rufus, V.S. Sathyaseelan, S.V. Narasimhan, S. Velmurugan ∗ Water and Steam Chemistry Division, Bhabha Atomic Research Centre Facilities, Kalpakkam 603102, TN, India

h i g h l i g h t s

g r a p h i c a l

• Combination of oxidation and reduc-

SEM of the U-DBP coated stainless steel coupon before and after exposure to chemical formulation containing acid permanganate at 80 ◦ C.

tion processes efficiently dissolves U-DBP deposits. • NP and NAC formulations are compatible with SS-304. • Dissolved uranium and added chemicals are effectively removed via ion exchangers.

a r t i c l e

i n f o

Article history: Received 24 August 2012 Received in revised form 14 March 2013 Accepted 16 March 2013 Available online 29 March 2013 Keywords: Radiation hazard Uranium Dibutyl phosphate X-ray florescence Ion exchange.

a b s t r a c t

a b s t r a c t Permanganate and nitrilotriacetic acid (NTA) based dilute chemical formulations were evaluated for the dissolution of uranium dibutyl phosphate (U-DBP), a compound that deposits over the surfaces of nuclear reprocessing plants and waste storage tanks. A combination of an acidic, oxidizing treatment (nitric acid with permanganate) followed by reducing treatment (NTA based formulation) efficiently dissolved the U-DBP deposits. The dissolution isotherm of U-DBP in its as precipitated form followed a logarithmic fit. The same chemical treatment was also effective in dissolving U-DBP coated on the surface of 304stainless steel, while resulting in minimal corrosion of the stainless steel substrate material. Investigation of uranium recovery from the resulting decontamination solutions by ion exchange with a bed of mixed anion and cation resins showed quantitative removal of uranium. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nuclear reprocessing facilities use tributyl phosphate (TBP) in the solvent extraction process to recover uranium and plutonium from used fuels [1,2]. During operation of the reprocessing plant or on extended storage of the reprocessing wastes, TBP

∗ Corresponding author. Tel.: +91 44 27480203; fax: +91 44 27480097. E-mail addresses: [email protected], [email protected] (S. Velmurugan). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.03.050

hydrolyzes to form dibutyl phosphate (DBP) and monobutyl (MBP) phosphate. These phosphates then react with uranium in solution to form uranium monobutyl phosphate (U-MBP) and uranium dibutyl phosphate (U-DBP). Solubility of U-DBP is low in aqueous media and so it precipitates as a sticky coating on contacting surfaces [3,4]. Precipitation of uranium in any form will lead to nuclear criticality safety issues. Besides, the coating can also act as a host for various radioactive nuclides and fission products present in the reprocessing waste resulting in radiation exposure hazard to the maintenance and waste disposal personnel.

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Following are the reactions that result in the precipitation of uranium. H2 O/H+

[CH3 (CH2 )3 O]3 − P = O −→ [CH3 (CH2 )3 O]2 − P(OH) = O + CH3 (CH2 )3 OH

(1)

DBP

TBP

[CH3 (CH2 )3 O]2 − P(OH) = O → ([CH3 (CH2 )3 O]2 − PO = O)− + H+ DBP

(2)

MBP

− UO2+ 2 + 2([CH3 (CH2 )3 O]2 − PO = O) DBP−

− UO2+ 2 + 2([CH3 (CH2 )3 O]2 − PO = O)

dil HNO3 (∼0.1 M)

−→

UO2 ([CH3 (CH2 )3 O]2 − PO = O)2 ↓

(3)

U-DBP(Yellow precipitate) Conc.HNO3

−→

[UO2 ([CH3 (CH2 )3 O]2 − PO = O)2 (NO3 )− ]H+ ↓

(4)

Yellow precipitate

Apart from the above reactions, uranyl solution (U6+ ) undergoes photo-reduction to U4+ , which will react with DBP to form green crystalline compound of the composition U[(C4 H9 )2 PO4 ]4 [5]. In order to mitigate the criticality safety issues and radiation exposure hazard, it is necessary to remove such deposits from the surfaces by chemical dissolution. With this objective, U-DBP was prepared synthetically and its dissolution in chemical formulations was investigated. From the literature, it was seen that the concentration of nitric acid in the storage tank and reprocessing wastes would be around 0.1 M [3]. Hence, during the synthesis of U-DBP in the laboratory, the nitric acid concentration was maintained at 0.1 M. It is reported that under low acidic conditions (up to 0.2 M of HNO3 ), uranium precipitates predominantly as U-DBP of the composition given in the equation-3. At higher concentrations (≥1.0 M of HNO3 ) molecular substitutions occur to form the compound given in the equation-4. In the present study, since the concentration of HNO3 was maintained at 0.1 M, the synthesized U-DBP can be presumed to be of single phase [6]. Chemicals that have been employed for decontamination of reprocessing plants include mineral acids, alkalies, sodium carbonate and tartaric acid. Eurochemic Reprocessing Plant at Belgium was decontaminated using dilute and concentrated nitric acid, which sometimes contained reducing additives. In this case, initial rinsing with nitric acid was followed by treating with various chemical formulations viz., sodium hydroxide, mixture containing 0.1 M potassium permanganate and 5–8 M nitric acid, 0.1 M oxalic acid, 0.5 M hydrazine nitrate, 10 wt.% sodium tartarate and 10 wt.% sodium citrate solutions [7]. At Savannah River Site and other DOE sites, decontamination was carried out by a two-step process consisting of an oxidation step using potassium permanganate or nitric acid, followed by reduction step using hydroxylamine sulphate or hydroxylamine nitrate [8]. In general, it is preferable to employ a decontamination chemical formulation similar to the chemicals used during the normal operation. This will avoid drastic changes in the chemistry conditions. Nitric acid is one of the chemicals used in the nuclear reprocessing plants. Hence, the formulation employed in the present study to dissolve the U-DBP deposits was based on nitric acid. Besides, DBP being organic in nature, use of strong oxidizing agents will destabilize the U-DBP lattice and thereby enable its easy dissolution. Thus, a formulation containing nitric acid and potassium permanganate, a strong oxidizing agent, was evaluated for the dissolution of U-DBP. The usual structural material of reprocessing plants is stainless steel, which generally has a passive oxide film containing iron, nickel and chromium over its surface. Thus, it will be convenient to have a single process to dissolve this contaminated passive oxide layer along with the precipitated U-DBP deposits. It is well known that a combination of oxidation and reduction treatment will dissolve chromium containing oxides effectively [9–13]. Hence, apart from oxidizing treatment with permanganate, a reducing treatment will help in achieving good decontamination factor. In the

present study, a chemical formulation based on nitrilotriacetic acid (NTA) containing ascorbic acid and citric acid was employed for the reducing treatment. The process employed and the chemical formulations used should result in the effective dissolution of the contaminants with no or minimum attack on the structural materials. Hence, dilute chemicals of millimolar concentrations were used instead of molar concentrations, which are being normally employed [7,8]. After the cleaning process, quick and complete removal of the dissolved metal ions and restoring the plant back to its normal operating conditions is desirable. Hence, ion exchange studies on the removal of dissolved metal ions and the added chemicals have been carried out and the results are presented in this paper.

2. Methods and materials All the chemicals used were of AR/GR grade. The ion exchange resins used were Tulsion-T46, a strong acid cation exchange resin and Tulsion-A33, a strong base anion exchange resin from Thermax India Limited. The resins were regenerated with 5% HCl and NaOH respectively prior to their use.

2.1. Chemical formulations Table 1 gives the composition of the chemical formulations used in our study. NP formulation is an oxidizing formulation normally used for the dissolution of chromium containing oxides from the surfaces of chromium based alloys such as stainless steel. NAC formulation is normally used for the dissolution of ferrites deposited over carbon steel surfaces [12]. This formulation essentially consists of an acid, a reducing agent and a chelating agent. Citric acid and ascorbic acid are used as the source of H+ and reducing agent respectively. NTA is the chelating agent that will complex with the dissolved metal ions and retain them in solution and thereby preventing reprecipitation. However, depending upon the conditions, the three constituents can act as acids and/or chelants, as all are carboxylic acids.

Table 1 Composition of the chemical formulations used for the dissolution of U-DBP. Formulation Abbreviation

Composition

Nature of the formulation

NP

5 mM nitric acid + 2.5 mM potassium permanganate 1.4 mM NTA + 1.7 mM ascorbic acid + 1.4 mM citric acid NP followed by NAC

Oxidizing

NAC

NP + NAC

Reducing

Oxidation followed by reduction

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2.2. Analytical 2.2.1. Determination of uranium by X-ray fluorescence (XRF) PW4025 MiniPal energy dispersive X-ray spectrometer with 9 Watt air-cooled Rh X-ray tube and a semiconductor detector was used for the determination of uranium. An aliquot (200 ␮l) of the sample was dried under IR lamp over poly propylene film. The spectrum of the dried film was recorded to carry out both qualitative and quantitative measurements. The concentrations of the samples were determined from a calibration graph obtained with known standards. There was no interference from other ions present in the solution. 2.2.2. Determination of uranium by UV–vis spectrophotometry In some experiments, uranium was also measured spectrophotometrically as a yellow colored thiocyanate complex formed by its reaction with ammonium thiocyanate in alcohol medium [14]. There were many absorption peaks for the complex viz., 294, 304, 310, 375, 420 and 460 nm and the calibration curves in the concentration range of 0–50 ppm was found to be linear at all these wavelengths. In the present study, 304 nm was selected because of high sensitivity. In the presence of potassium permanganate, citric acid was added to decompose permanganate prior to the determination of uranium. Similar treatment was given to the standards during calibration. 2.2.3. Determination of permanganate Absorption spectrum of permanganate showed peaks at 312, 526, 536 and 546 nm and the calibration in the concentration range of 0–50 ppm was found to be linear at all these wavelengths. However, the measurements were carried out at 536 nm because of high sensitivity. Presence of uranium did not interfere in the analysis. In the presence of MnO2 particles, the samples were filtered through 0.2 ␮m filter prior to the analysis. 2.2.4. Surface analytical techniques Scanning Electron Microscope (SEM) from Philips, model No. XL-30, with EDX attachment was used for the surface analysis of U-DBP coated stainless steel coupons. 3. Experimental 3.1.1. Oxidation of DBP In a round bottomed flask, 250 ml of double distilled water of resistivity 7 M was heated to 80 ◦ C. When the temperature was steady, 98.8 mg of potassium permanganate (corresponding to 2.5 mM) and 78 ␮l of 15.8 M nitric acid (corresponding to 5 mM) were added to the flask followed by required amount of DBP. The mixture was stirred continuously. At desired intervals, 4 ml of the solution was sampled, cooled and filtered through 0.2 ␮m filter. The filtrate was diluted 10 times and the absorbance was measured at 536 nm against water as blank. The above experiment was repeated by varying the concentration of KMnO4 (1.5 mM and 2.5 mM) and maintaining the DBP concentration constant at 4 mM. 3.1.2. Preparation of U-DBP and its coating over stainless steel coupons

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at 80 ◦ C for 1 hour. Based on the concentration of HNO3 employed for the synthesis, the precipitate should be predominantly consisting of a single phase. Even if other minor phases are present, the dissolution behavior may not be affected owing to the use of permanganate based strong oxidizing formulation for their dissolution. U-DBP was coated on the stainless steel (SS-304) surface by immersing the metal coupons during the precipitation process described above. Two sets of coupons, one set polished to diamond grit (particle size–1 ␮m) and the other polished to 220 grit (particle size–68 ␮m), were immersed. The coupons were left in the solution for 10 h, removed, washed and dried in an oven at 80 ◦ C for 3 h. The thickness of the coating calculated was in the range 4–6 ␮m. Thickness of the coating in ␮m =

w × 104 A

(5)

where w = weight gain in g;  = density of (U-DBP) in g cm−3 ; A = surface area of the coupon in cm2 ; 104 = multiplication factor to convert cm to ␮m. 3.1.3. Dissolution experiments All the experiments were carried out in a table-top set-up consisting of a round bottomed flask with provisions for heating, stirring and purging inert gas. For powder dissolution, 250 ml of the desired formulation and 100 mg of the synthesized U-DBP powder were taken in the flask. The contents were maintained at desired temperature under inert condition by purging argon gas. In the case of dissolution of U-DBP coated over metal surfaces, the coated coupons were directly immersed in a flask containing the desired formulations at the required temperature. The extent of dissolution was monitored by determining the concentration of uranium released to the solution at regular intervals. 3.1.4. Material compatibility experiments Corrosion of SS-304 in the NP formulation was studied by exposing welded and control specimens for 24 h. The solution was neutralized with citric acid to decompose the unreacted permanganate and also to dissolve the MnO2 precipitate formed. The metal coupons were washed, dried, weighed and subsequently exposed to NAC formulation for another 24 h. At the end of the experiment, the coupons were removed, washed, dried, weighed and subjected to microscopic examination. 3.1.5. Ion exchange experiments The contents of the flask at the end of the dissolution experiments were passed through a mixed bed ion exchange resin column consisting of 12 ml of cation exchanger and 24 ml of anion exchanger. The effluent was collected in one lot. Concentration of uranium, organic content (chemical oxygen demand – COD), pH and conductivity were measured before and after the ion exchange treatment. The amount of ions picked up on the ion exchange column was obtained from the difference between the inlet and outlet concentrations. 4. Results and discussion 4.1. Oxidation of DBP by NP formulation

U-DBP was prepared by solution precipitation by adding a suspension of 0.9 g of DBP in 0.1 M of HNO3 to 50 ml of 11.9 g/L solution of uranium in 0.1 M HNO3 . The precipitate was digested for about an hour at 60–70 ◦ C. The solution was tested for complete precipitation by adding 0.2 g of DBP. The precipitate was filtered and dried

U-DBP is an organic compound of uranium. As permanganate based formulations are good in oxidizing organic compounds, NP formulation was evaluated for dissolving the U-DBP deposits. Efficiency of NP formulation in oxidizing pure DBP was also studied.

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Fig. 1. (a) Decrease in the concentration of permanganate during the oxidation of DBP by NP formulation (b) Rate of decomposition of permanganate at 80 ◦ C as a function of initial concentration of DBP.

Fig. 1a gives the concentration of permanganate in solution as a function of time during the oxidation of DBP by NP formulation at 80 ◦ C. The rate of decomposition was monitored by measuring the concentration of permanganate in the solution and it was found that the concentration decreased linearly as a function of time. The rate of decomposition was also found to be linearly dependent on the initial concentration of DBP (Fig. 1b). Oxidation of DBP was also carried out by varying the concentration of permanganate (1.5 and 2.5 mM) while maintaining the concentration of DBP constant (4 mM). The rate of the reaction in terms of decomposition of permanganate was found to be 0.45 and 0.77 ppm/min, respectively. The reactions involved in the oxidation of DBP are:(6)2KMnO4 + 6HNO3 → 2KNO3 + 2Mn(NO3 )2 + (O)

3H2 O + 5(O)(7)(C4 H9 O)2 PO − OH−→oxidation products According to the order of the reaction calculated (1.2 with respect to permanganate and 1 with respect to DBP), the number of nascent oxygen involved in the reaction will be 3. Thus, DBP will not undergo complete oxidation to form water, oxides of carbon and phosphorous. Hence, only partial oxidation will be involved. 4.2. Dissolution of U-DBP Fig. 2 gives the build-up of uranium in solution during the dissolution of as precipitated form of U-DBP in the NP formulation at 80 ◦ C. The curve was found to follow a logarithmic fit, y = b ln(x − a),

Fig. 2. Build-up of uranium in solution during the dissolution of U-DBP in NP reagent at 80 ◦ C.

where the rate of dissolution was quite high in the initial stages. On calculation, the dissolution achieved at the end of 6 h was around 20%. The concentration of uranium in solution showed an increasing trend toward the end of the experiment. This implies that if the experiment had been continued for longer duration, more powder would have dissolved as long as sufficient formulation was available. This was verified by a longer duration experiment carried out for 22 h, which resulted in 40% dissolution. Dissolution of U-DBP was also carried out at 65 ◦ C to understand the influence of temperature. Besides, a comparative study was carried out with other formulations viz., NAC formulation (wholly a reducing treatment) and a two-step process involving a combination of oxidizing treatment for 3 h followed by reducing treatment for another 3 h. Fig. 3a gives the build-up of uranium in solution as a function of time and Fig. 3b gives the percentage dissolution obtained at the end of 6 h. 4.2.1. Process of dissolution It is seen from the uranium build-up curve (Fig. 3a) that the rate of dissolution is high in the initial stages for both oxidizing and reducing formulations. Based on this observation, the process of dissolution can be viewed as given in Fig. 4. In the oxidizing formulation, as the reaction between permanganate and U-DBP progressed, the organic entity (>DBP) is oxidized and the surface of the particles were enriched in uranium. As a result, the rate of dissolution decreased. After 3 h, there was practically no dissolution. Similar behavior was seen in the case of reducing formulation. As the reaction progressed, the surface was enriched in DBP and the rate of dissolution decreased. At the end of 6 h, less than 20% dissolution only could be achieved in both the cases. For effective dissolution, a multistep process involving alternate oxidation and reduction step was essential. During the oxidation step, apart from uranium enrichment on the surface, precipitation of insoluble MnO2 also takes place. In the initial stages of dissolution, there will be little MnO2 in the system. Hence, the formulation will have an easy access to the U-DBP surface and the dissolution will proceed at an appreciable rate without any hindrance. At later stages, appreciable quantity of MnO2 will be formed that will act as a barrier between the formulation and the U-DBP surface. This will result in reduced rate of dissolution. In a multi-step process, oxidation and reduction steps will be carried out alternatively. When the rate of dissolution of U-DBP saturates in the oxidation step owing to uranium enrichment on the surface and/or MnO2 precipitation, reduction step will be carried

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Fig. 3. a Build-up of uranium in solution during the dissolution of U-DBP and (b) Efficiency of various formulations in dissolving U-DBP.

out. The reduction step can be either the addition of NAC or the addition of citric acid followed by NAC. During this stage, the precipitated MnO2 will be dissolved and the dissolution of U-DBP will be resumed. MnO2 + 2H+ → Mn2+ + H2 O + (O)

(8)

Mn2+ + Citric acid → Mn(II)citrate + 2H+

(9)

Advantage of the multi-step process is obvious in Fig. 3b, which compares the efficiency of various formulations in dissolving UDBP. Under identical conditions, less than 20% dissolution was achieved in the single step processes, while 30% dissolution was achieved in the case of a multi-step process.

the surfaces, U-DBP was coated on stainless steel coupons and used in the dissolution experiments. NP formulation was used for the dissolution of coated U-DBP. The progress of dissolution was monitored by measuring the concentration of permanganate, which decreased continuously during the entire 6 h of exposure. The concentration decreased from an initial value of 2.5–1.8 mM. At the end of 6 h, the solution was filtered to remove precipitated MnO2 and uranium was measured in the filtrate and residue using XRF. The coupons were also subjected to XRF and SEM-EDX analysis before and after the exposure. On analysis, it was found that the filtrate accounted for 70% of uranium, while the remaining was physically dislodged due to solution agitation and was present along with MnO2 in the residue. It is possible that the less adherence of U-DBP coating is due to the

4.3. Dissolution of U-DBP coated over stainless steel coupons U-DBP deposits can be formed either during the operation or on long standing of the reprocessing waste, with the deposits being formed on the surfaces of the reprocessing facility. The dissolution of U-DBP in the precipitated form will be quite different from that of U-DBP deposited on the metal surfaces. In the case of loose samples, dissolution will be quite easy as the surface area of the particles exposed to formulation will be very high. However, it will not be so in the case of U-DBP deposited on surfaces. In some cases, the underlying base metal will also play a role on the rate of dissolution. Hence, in order to mimic the dissolution of deposits from

Oxidising formulation

U-DBP Particle U enriched and DBP depleted surface + MnO2 precipitate

U-DBP Parcle Reducing formulation

U-DBP Parcle U depleted and DBP enriched surface

Fig. 4. Schematic for the process of dissolution.

Fig. 5. XRF spectrum of U-DBP coated stainless steel coupons (a) before and (b) after the exposure to NP reagent at 80 ◦ C (c) XRF of the residue.

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Fig. 6. SEM of the U-DBP coated stainless steel coupon (a) before and (b) after the exposure to NP formulation at 80 ◦ C.

Fig. 7. Microscopic examination of the coupons exposed to NP formulation (a) Control before exposure (b) Control after exposure (c) Weld metal before exposure (d) Weld metal after exposure.

less rigorous conditions such as low temperature and low pressure under which it is formed in the laboratory. Similar conditions prevail in the reprocessing plants. It is reported in literature that the basic physico-chemical processes leading to contamination in reprocessing plants are mainly adsorption, ion exchange and precipitation [6]. Appreciable quantities of deposits formed under such conditions can be brought into solution by physically dislodging them using water flushing techniques. Fig. 5 gives the XRF spectrum of the U-DBP coated specimen before and after exposure to NP formulation. There was a prominent peak for uranium at 13.6 KeV due to U-DBP coating on the surface before exposure to the formulation. After exposure, the peak corresponding to uranium was absent. Fine tuning quantification by XRF showed 6.5 ␮g/cm2 of uranium retained on the surface after cleaning against an initial value of 7 × 106 ␮g/cm2 of uranium. Fig. 6 shows the surface morphology of the U-DBP coated stainless steel surface before and after the exposure to NP formulation. Table 2 gives the corresponding EDX results quantifying the elements present on the surface.

The Au peak during EDX analysis is due to the gold coating given to the specimen prior to SEM-EDX analysis while Al and K are due to the spurious/background signals. Hence, these three peaks were not considered while calculating the atom percent. The peaks for Fe, Ni and Cr are due to the base metal SS 304. Before exposure to the formulation, the peaks due to the base metal are very weak while

Table 2 EDX analysis before and after exposure to NP formulation. Before exposure

After exposure

Element

Atom %

Element

Atom %

Al K Cr Fe Au Ni U

– – a BDL 12.2 – BDL 87.8

Cr Fe Ni Au U

17.9 70.9 11.2 – 0.0

a

BDL—below detection limit.

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Table 3 Weight loss measurements on the coupons exposed to a complete cycle of oxidation and reduction treatment. Specimen

Surface area (cm2 )

Wt. loss after oxidation step (mg)

Wt. loss after reduction step (mg)

Total wt. loss (mg)

Total wt. loss (mg/cm2 )

Control Welded specimen#1 Welded specimen#2

2.4 2.4 6.8

0.01 0.0 0.09

0.03 0.05 0.11

0.04 0.05 0.20

0.02 0.02 0.03

Table 4 Parameters measured before and after the ion exchange purification. Parameters

pH Conductivity in ␮S/cm Uranium in ppm Organics in ppm

5. Conclusions

Oxidation step alone

Complete cycle

Before

After

Before

After

2.74 1184 49.2 Not analyzed

5.7 1.2 BDL 0.4

2.52 1574 71.2 Not analyzed

5.7 1.1 BDL BDL

the peak due to uranium is strong indicating the presence of UDBP on the surface. After exposure, the uranium peak disappears and strong peaks due to base metal are observed indicating the effective dissolution of U-DBP coating. 4.4. Compatibility of materials in the formulations During the dissolution of U-DBP deposits from metal surfaces, it is necessary to keep the base metal corrosion to a minimum. Welded joints are highly prone to corrosion. Hence, welded specimens of SS-304 along with control coupons were exposed to a complete cycle of dissolution process, consisting of the oxidation and reduction steps. Table 3 gives the weight loss measurements carried out on the control and the welded specimens. The observed weight losses for both control and welded specimens were similar, which shows there is no enhanced corrosion attack by the decontamination process on the base metal, including welded regions. Fig. 7a–d shows results of the microscopic examination of control and welded specimens. It can be seen that both the specimens undergo similar form of mild attack that involves dissolution of some surface inclusions. 4.5. Ion exchange purification of the dissolved uranium and the added chemicals Removal of the dissolved metal ions and the added chemicals over a column of mixed bed ion exchange resins is usually practiced after a decontamination campaign, in order to purify the system and restore it to its normal operating conditions. Ion exchange treatment is also widely employed in waste management for compacting and immobilizing the undesired chemical species such as fuel reprocessing wastes. Hence, it was necessary to understand the ion exchange removal of dissolved species and the constituents of the added chemicals. In one set of the experiment, the ion exchange removal over mixed bed was carried out immediately after the oxidation step, while in the second case it was carried out after the complete cycle consisting of an oxidation step and a reduction step. Table 4 gives various parameters of the influent and effluent of the ion exchange columns. The observations show that the dissolved uranium from UDBP and the added chemicals can be efficiently removed over ion exchange resins.

Dissolution studies carried out with U-DBP in dilute solutions of chemical formulations showed the following results: 1. The kinetics of oxidation of DBP by potassium permanganate was found to be linear. The order of the reaction with respect to permanganate was found to be 1.2, while it was 1 with respect to DBP. 2. Dissolution isotherm of as-precipitated form of U-DBP followed a logarithmic fit, y = b ln(x − a). Besides, the rate of dissolution was higher at higher temperature. 3. Higher percentage dissolution of U-DBP was obtained, when it was subjected to a multi-step process consisting of an oxidation step followed by a reduction step. U-DBP deposited over SS-304 surface could also be dissolved efficiently. 4. The chemical formulations used in the dissolution process were compatible with structural material (SS-304) of reprocessing plants and waste storage tanks. 5. Dissolved uranium and the added chemicals were efficiently removed over mixed bed ion exchange resin columns enabling easy handling by waste disposal plants. References [1] Spent fuel reprocessing options, IAEA technical report, IAEA TECDOC-1587, 2008. [2] M.F. Simpson and J.D. Law, Nuclear fuel reprocessing, Idaho National Laboratory technical report, INL/EXT-10-17753, 2010. [3] R.A. Pierce and M.C. Thompson, Dibutyl phosphoric acid solubility in high-acid, uranium–bearing solutions at SRS (U), Westinghouse Savannah River Company, technical report WSRC-TR-98-00281, 1998. [4] B.A. Powell, J.D. Navratil, M.C. Thompson, Compounds of hexavalent uranium and dibutyl phosphate in nitric acid systems, Solvent Extr. Ion Exch. 21 (3) (2003) 347–368. [5] R.M. Wagner, The hydrolysis products of tributyl phosphate and their effect on the tributyl phosphate process for uranium recovery, AEC Research and Development Report, HW-19959, 1952. [6] R.A. Pierce, M.C. Thompson and R.J. Ray, Solubility limits of dibutyl phosphoric acid in uranium-nitric acid solutions, Westinghouse Savannah River Company, Technical Report WSRC-MS-99-00321, 1999. [7] Decontamination of operational nuclear power plants, IAEA Technical Report, IAEA-TECDOC-248, 1981. [8] D.G. Harlow, R.E. Felt, S. Agnew, G.S. Barney, J.M. McKibben, R. Garber, M. Lewis, DOE Technical Report on Hydroxylamine Nitrate. DOE/EH-0555, 1998. [9] J. Semmler, Recent advances in Canadian decontamination technologies, Int. Conf. Nucl. Plant Chem., NPC-2010, Canada, 2010. [10] H. Khedim, R. Podor, C. Rapin,w, M. Vilasi, Redox – control solubility of chromium oxide in soda-silicate melts, J. Am. Ceram. Soc. 91 (11) (2008) 3571–3579. [11] K. Kim, H. Lee, M. Choi, D. Kang, S. Inoue, Establishment of an optimal decontamination process by the newly designed semi-pilot equipment, Nucl. Eng. Des. 229 (1) (2004) 91–100. [12] A.L. Rufus, V.S. Sathyaseelan, S. Velmurugan, S.V. Narasimhan, NTA based formulation for the chemical decontamination of nuclear power plants, Nucl. Energy 43 (1) (2004) 47–53. [13] V.S. Sathyaseelan, A.L. Rufus, H. Subramanian, B. Anupkumar, W. Shiny, S.V. Narasimhan, S. Velmurugan, High temperature dissolution of oxides in complexing media, J. Nucl. Mater. 419 (1–3) (2011) 39–45. [14] M.M. Tillu, D.V. Bhatnagar, T.K.S. Murthy, Colorimetric estimation of uranium with ammonium thiocyanate and its application to determination of uranium in minerals particularly monazite concentrates, Proceedings Math. Sci. 42 (1) (1955) 28–35.