JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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A Study on Complete Decomposition of Pyrrolidone
Precipitants by γ-Ray Irradiation
M. Nogami a) , Y. Sugiyama a) , T. Kawasaki a) , M. Harada a) , Y. Kawata b) ,
Y. Morita b) , T. Kikuchi c) and Y. Ikeda a)
a) Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology,
b) Division of Fuels and Materials Engineering, NSED, JAEA, c) Mitsubishi Materials Corporation
We have been developing a novel reprocessing system
for spent FBR fuels based on two precipitation processes 1) .
In this system, only U(VI) species are firstly precipitated in
nitric acid solutions dissolving spent fuels by using a
pyrrolidone derivative (NRP) with low hydrophobicity and
donicity which bring lower precipitation ability. Secondly
the residual U(VI) and Pu(IV, VI) are precipitated
simultaneously using another NRP with higher precipitation
ability. Use of a slight excess amount of the precipitants is
inevitable for complete precipitation of U and Pu species
due to the solubility of the precipitates. Therefore, the
residual precipitants included in the highly active waste
solution (HAW) generated after the second precipitation
treatment should be completely removed or decomposed for
safer disposal of the waste.
As one of candidate methods for the purpose mentioned
above, γ-ray irradiation to NRPs in HNO 3 of higher
concentration was examined in this study, because NRPs in
HNO 3 of 6 mol·dm -3 (= M) is found to be decomposed
easily with γ-ray irradiation compared with in HNO 3 of
lower concentration ranges 2) . This method has advantage
that no special equipments are necessary for the treatment,
because an increase in the concentrations of HNO 3 of HAW
is possible by inletting NOx gas or condensation.
A 9 M HNO 3 solution containing 0.1 M N-n-butyl-2-
pyrrolidone (NBP) which is one of the NRPs with lower
precipitation ability was put into a Pyrex glass tube.
Gamma-ray irradiation by the 60 Co source was performed at
11.8 kGy/h up to 210 kGy at room temperature under
ambient atmosphere. The irradiated sample solutions were
analyzed by 1 H and 13 C NMR (JEOL 400 MHz, solvent:
dimethyl sulfoxide-d 6). Pyridine was added as the internal
standard for determining the decomposition ratio of NBP.
The dependence of decomposition ratio of NBP on dose
Decomposition ratio / %
100
80
60
40
20
0
0 50 100 150 200 250
Dose / kGy
Fig. 1 Dependence of decomposition ratio of NBP on
dose in 9 M HNO3 by γ-ray irradiation.
JAEA-Review 2010-065
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is shown in Fig. 1. Approximately 70 % and 90 % of NBP
are found to be decomposed after the irradiation of 20 kGy
and 210 kGy, respectively. This suggests that the γ-ray
irradiation to NBP in HNO 3 of higher concentration is
effective for its decomposition.
1 H NMR spectra also showed a possibility of the
existence of acetic acid (ca. 2.1 ppm, CH 3, singlet) and
propionic acid (ca. 1.1 ppm, CH 3, triplet) in the irradiated
samples. The signal at 2.1 ppm was found to increase with
increasing dose. In the 13 C NMR spectra of the irradiated
samples, the signal which should be attributed to the carbon
atom of oxalic acid was detected at ca. 161 ppm. The
above results support our proposal for the degradation
mechanism of NBP in HNO 3 by γ-ray irradiation 2) .
Namely, the degradation of NBP starts from the cleavage of
the pyrrolidone ring by the addition of oxygen atom
originating from HNO 3, followed by the formation of chain
monoamides and C4 compounds by the continuous addition
of oxygen, finally leading to the generation of oxalic acid.
It is expected that acetic acid is produced from the side butyl
group through propionic acid. It was also revealed from
the NMR analyses that the abundance ratios of degradation
products which are not decomposed to oxalic acid or
propionic acid, i.e., the above-mentioned chain monoamides
and C4 compounds, are small.
These facts indicate that the complete decomposition of
NBP was not achieved under the present irradiation
condition. Further optimization of the condition for
decomposing organic acids such as acetic acid and oxalic
acid is necessary. The combination with heating is
expected to be effective for the practical use.
Acknowledgment
Present study is the result of “Development of Advanced
Reprocessing System Using High Selective and
Controllable Precipitants” entrusted to Tokyo Institute of
Technology by the Ministry of Education, Culture, Sports,
Science and Technology of Japan (MEXT). The authors
also greatly thank Dr. R. Yamagata, Department of
Advanced Radiation Technology, TARRI, JAEA, for his
help at the irradiation facility.
References
1) Y. Morita et al., J. Nucl. Sci. Technol. 46 (2009) 1129.
2) M. Nogami et al., JAEA Takasaki Ann. Rep. 2008 (2009)
25.