SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai
Reducing effect of mixing on the dissolution kinetics of UO2 pellets in nitric acid
N. Desigan1, Nirav P Bhatt2, N. K. Pandey1*, U. Kamachi Mudali1, R. Natarajan3 and J. B. Joshi4
1
Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam
2
Dept. of Chemical Engineering, Indian Institute of Technology, Madras
3
Former Director, Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam
4
Institute of Chemical Technology, Matunga, Mumbai and HBNI, Mumbai
desi@igcar.gov.in, niravbhatt@iitm.ac.in, nkpandey@igcar.gov.in, kamachi@igcar.gov.in, jbjoshi@gmail.com
1. Introduction:
Understanding the dissolution behaviour of UO2 is important for designing a dissolution system for spent nuclear fuel
processing. Our study establishes the retarding effect of mixing on UO 2 dissolution in nitric acid. This was justified by
ascertaining the role of nitrous acid in the reaction. Since UO2 undergoes oxidative dissolution in nitric acid, the nitrous acid
generated insitu [1], accelerates the dissolution.
But on stirring the reaction mixture, the nitrous acid formed at the
pellet/solution interphase [2] diffuses into the bulk of the solution and its availability for the reaction and thereby the dissolution
rate decreases.
2. Material and Methods:
The natural UO2 pellets used are from Nuclear Fuel Complex (NFC), Hyderabad which are the typical driver fuel for the
Indian PHWRs. All the reagents used are AR grade from reputed suppliers in India. The reactor is a cylindrical glass
vessel with provisions for mixing (using four flat bladed impeller), reagent addition, sampling, heating with temperature
control etc. In all experiments, 4 pellets were dissolved in about 285ml of 8M nitric acid at 80C as a function of various
mixing speed from 100 to 1500 RPM and unstirred condition. At regular intervals of time, samples were taken and
analyzed for [U(VI)], [HNO 3] and [HNO2]. Uranium was analysed spectrophotometrically using pyridine azo resorcinol as the
chromogenic reagent [3]. Nitrous acid was estimated by ion chromatography [4] and spectrophotometry as well employing
sulfanilamide and N-1-naphthylethylenediamine as coloring reagents [5]. Free acidity was determined by titrimetry using a
standard alkali with prior complexation of metal ions using a suitable ligand [6].
3. Significant Results and Discussion
It was observed that as the agitation rate of the reaction mixture was increased from unstirred to 600 RPM, the rate of
dissolution reaction gradually decreased. That is the time taken for the total dissolution of the pellets increased with
increasing mixing rate. Beyond 600 RPM there was no further noticeable change in the dissolution rate indicating that
diffusion control was almost eliminated beyond that. The uranium concentration profile (fig. 1) obtained for all RPM was
same and was typical for an autocatalytic reaction [4]. As uranium dissolves oxidatively in nitric acid, presence of any
oxidizing agent accelerates the reaction.
Nitrous acid is one of the dissolution product generated in -situ during the
dissolution of UO 2 in nitric acid [1]. As the any solid-liquid reaction takes place only at the interphase [2], presence of
nitrous acid at the interphase catalyzes the reaction. Hence as the reaction mixture is agitated, the nitrous acid produced at
the interphase is dispersed into the bulk of the solution. This dispersion of nitrous acid is directly proportional to the extent
of agitation. Hence at higher level of mixing, the nitrous acid availability at the interphase is reduced and thereby the ra te
of dissolution of UO 2. To confirm this some dissolution runs were carried out under unstirred condition and it was found
*
Corresponding author: nkpandey@igcar.gov.in
1
SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai
that the rate was maximum during that. Similarly the nitrous acid concentration profile given in figure 2 also confirms the
above explanation.
% dissolution
100
% Dissolution
80
1500 RPM
60
1200 RPM
900 RPM
40
600 RPM
300 RPM
20
0 RPM
0
0
200
400
600
800
1000
Time (mins)
Figure 1: Percentage dissolution of UO2 as a function of mixing rate
0.4
[HNO2] vs Time
0.35
[HNO2], mmol/L
0.3
0.25
0.2
0 RPM
300 RPM
600 RPM
900 RPM
1200 RPM
1500 rpm
0.15
0.1
0.05
0
0
200
400
600
800
1000
Time (mins)
Figure 2: Nitrous acid concentration profile as a function of time during UO 2 dissolution in nitric acid
4. Conclusions:
UO2 dissolves in nitric acid oxidatively and hence the nitrous acid generated in-situ during the reaction catalyzes it. But when
the reaction mixture of agitated, it was found to impart a negative effect on the dissolution rate. This was explained on the basis
of reduction of nitrous acid concentration at the reaction location as it was dispersed into the bulk at increased agitation.
References (Times new roman font size 10)
[1] Tetsuo Fukasawa et al. , Nucl. Tech. 94, 1991, 108-113
[2] L. K. Doraiswamy, M. M. Sharma, “Heterogeneous reactions: Analysis, Examples and Reactor design: Volume 2”, 1984, John Wiley & Sons
[3] Levenspiel, O. “Chemical Reaction Engineering”, 1972, second edition, John Wiley and Sons, New York
[4] N. Desigan et al., “Determination of the composition of NOX gases during the reprocessing of spent nuclear fuel”, CHEMENT-2014, 06-07 March 2014,
Kalpakkam, India
[5] Florence T. M., Yvonne. Farrar, “Spectrophotometric Determination of Uranium with 4-(2-Pyridylazo) resorcinol”, Anal. Chem., 1963, 35 (11), 1613–1616
[6] Schneider, R. A., Harmon, K. M., (1961) “Analytical Technical Manual”, HW-53368
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