Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 38 (2005)

48(LNS Experiment: #2518, #2530)

Distribution of 59Fe, 60Co and 85Sr as Low-level Radioactive
Wastes Included in Nonﬂammable Organic Materials
in the Decomposition by Supercritical Water with RuO2
T. Yamamura1, I. Satoh1, W. Sugiyama2, M. Takahashi1,Y. Shiokawa1, H. Tomiyasu3, and T. Ohtsuki4
123

Institute for Materials Research, Tohoku University, Sendai 980-8577
Electric Power Research and Development Center, Chubu Electric Power Co., Inc., Nagoya 459-8522

Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Nagano 380-8553
4Laboratory for Nuclear Science, Tohoku University, Sendai 982-0826

We investigated the distribution behavior of iron, cobalt and strontium attached to nonﬂammableorganic materials, in solid, liquid and gas phases during the decomposition of these materials insupercritical water mixed with RuO2. The distributions of these elements were determined by using
their radioisotopes as simulated low-level radioactive wastes in order to ease the detection of trace
amounts of elements even in solid and gas phases. The obtained results indicate that all of the elements
investigated in this study (iron, cobalt, and strontium) can be recovered successfully by thissupercritical-water process using RuO2. Consequently, this process is suggested as a predominantcandidate for the treatment of nonﬂammable organic materials in low-level radioactive wastes (LLW).

§1. Introduction
Low-level radioactive wastes (LLW) generated at nuclear power plants can be classiﬁed into three
categories in Japan: the wastes with (i) very low-level, (ii) relatively low-level and (iii) relatively highlevel radioactivity. The conventional method for dealing with these wastes includes compression orincineration before solidiﬁcation with mortar or concrete, and storing then in 200 L drum containers [1].
Even excluding the above-mentioned category (iii), more than half a million drum containers ﬁlled withLLW were being stored at domestic nuclear power plants by the end of FY 2001. One of the reasons
why such a huge volume has accumulated is that the main components of the LLW are bulky
nonﬂammable organic materials. No practical method for compressing or decomposing these wastes has
been established yet and therefore volume reduction is not achieved satisfactorily.
A method using supercritical water with RuO2 as a catalyst [2] was recently developed for achieving
the complete gasiﬁcation of nonﬂammable organic compounds, including even such compounds as stablearomatic structures. One of the most important applications of this method would be the decompositionof nonﬂammable organic materials contaminated with low-level radioactivity, in order to reduce theirvolume. We have reported on an investigation using this method to decompose nonﬂammable organicmaterials used in nuclear power plants [1]. For practical use, what is required is the removal of trace

49
quantities of radioactive elements attached to these plastics from gas and liquid phases and theirconcentration in the solid phase.
Radioisotopes contained in the LLW from nuclear power plants can be classiﬁed into two types byorigin. One type is the radioactivated materials in nuclear power plants, e.g. iron-59 and cobalt-60 fromthe stainless steel (SUS) used as a major construction material, and the other is the ﬁssion products ofuranium, such as strontium-90, which originate in the nuclear fuel, due to leakage.
In this study, the distribution of the above nuclides into solid, liquid and gas phases wereinvestigated by applying the supercritical-water reaction with RuO2 for decomposing the nonﬂammableorganic materials. The objective of this study is to elucidate the optimum conditions for theconcentration of the radionuclides into the solid phase. In order to detect trace quantities of elementseven in the gas and solid samples collected after supercritical-water reaction, radiometric analysis wasapplied.

§2. Experimental
Laminate sheeting, a popular product used in domestic nuclear power plants, is commerciallyavailable from Chiyoda Technol Corp., Japan. Reagents and radioactive reagents were used aspurchased. Strontium-85, the tracer of strontium, was prepared by bremsstrahlung irradiation of SrCO3
at the condition of 50 MeV, 100-120 μA, 8 hours with the electron linear accelerator (LINAC) at
Laboratory of Nuclear Science, Tohoku University. Radioactive iron and cobalt hydroxides, as simulated
radioactive scales of SUS, were prepared as follows already reported [3].
2.1 Supercritical-water reaction conditions
The supercritical batch-wise reactor made of hastelloy C-22 with a nominal inside volume of 10.8
cm was supplied by Taiatsu Techno Corp., Japan. A certain amount of radioactive compound and non3

radioactive carrier, 150 mg of laminate sheeting, 30 mg of RuO2 and 3 mL of water were loaded into theabove reactor. By using a temperature controller, the reaction condition of 450ºC-43 MPa was kept for
30 min.
After the reactor was cooled to room temperature, the volume of the gas produced by thesupercritical-water reaction was measured using a gas pressure meter and the gas was collected in a
100 mL gas vial; then everything (liquid and solid phases) remaining inside the reactor was recoveredand placed in a 10 mL polypropylene test tube. The inner surface of the reactor was rinsed with water
three times (total 6 mL) and the washings were collected and added to the contents of the reactor. The
supernatant liquid was separated into another polypropylene test tube by centrifuging these contents.
2.2 Determination of distribution factor
The radioactivity of 59Fe, 60Co, and 85Sr were determined from the areas of their peaks at 1099.3,1173.2, and 514.0 keV, respectively, by using the γ-ray spectrometer (GEM-28185-P, ORTEC Inc.,
USA). The distribution of their radioactivity in solid, liquid and gas phases was determined as given by

50

Dphase ＝Aphase
AT(1),

where Dphase designates the distribution ratio, AT and Aphase are the total radioactivity of each phase after
the supercritical-water reaction and the radioactivity of each phase respectively, and the subscription
phase is "sol", "liq" or "gas", referring to the solid phase, the liquid phase and the gas phase, respectively.

§3. Results and discussion
As reported previously [1], the laminate sheeting initially added is completely decomposed without
any solid residues by the supercritical-water reaction using RuO2. Also in this study, only the clear andcolorless supernatant solutions and inorganic precipitates are recovered in the all runs. The gases
produced during the decomposition of the laminate sheeting may consist of CH4, CO2 and H2, according
to our previous study [2]. Experimental conditions, the volume of the gas produced and the distribution
in three phases in experiments on 59Fe, 60Co, and 85Sr are summarized in Table 1.
3.1 Distribution of 59Fe and 60Co
There is a tendency for the iron to precipitate almost quantitatively and only a trace amount istransferred to the liquid when a small amount of carrier is added (Table 1). The gas produced during
the decomposition of the laminate sheeting contains no activity of 59Fe in any of the runs. This result
indicates that both the decomposition of nonflammable plastics and the recovery of iron in the solid
phase are successful. In contrast to the results for radioactive iron, about a quarter of 60Co distributes
Table 1. Distribution of Fe, Co, and Sr in three phases after reaction in supercritical-water of
nonﬂammable plastics [3]ElementFeCarriercompoundFe(OH)3

Amount ofcarrier/ mg

Tracer

SrCO3

102Dliq2)

102Dgas2)990

97.5±1.5

2.5±0.0

0.0±0.0

1.3

0.60×105979

99.9±1.6

0.1±0.0

0.0±0.0

2.94×105970

100±1.4

0.0±0.0

0.0±0.0

3.90×105941

100±1.4

0.0±0.0

0.0±0.0

8.33×1049103

75.4±1.0

24.6±0.2

0.0±0.0

9.02×104990

76.7±0.823.3±0.20.0±0.04)

8.27×10492787.0±0.9

13.0±0.1

0.0±0.0trace

8.21×102965

15.1±0.484.9±1.20.0±0.0

6.52×103960

32.3±0.567.7±1.0

0.0±0.0

2.97×104944

78.0±0.3

22.0±0.1

0.0±0.01)
5913 1)

Fe4)

23 4)
230

Sr102Dsol2)

3.12×105

2.3
Co(OH)3Amountof gas
phase/ mL3)

0.13 1)130 1)Co

Activity of Amounttracerof liquid
addedphase/ Bq/ mL

55060

Co85Sr1) Amount of Fe(OH)3 calculated on the basis of the amount of Fe(NO3)3·9H2O used to prepare Fe(OH)3.
2) Value indicates measurement error [8].
3) Value calculated from gas pressure after reaction.
4) Amount of Co(OH)3 calculated on the basis of the amount of CoCl2·6H2O used to prepare Co(OH)3.

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Table 2. Distribution of Co and Sr in three phases when 200 mg of non-radioactive ironhydroxide1) was added for coprecipitation [3]ElementCo

Sr

CarriercompoundCoCl2・6H2O

Amount ofcarrier/ mgnot added
5

SrCO3

0.5

SrCO3

5Sr(NO3)25

Tracer60

Co85SrActivity of Amounttracerof liquid
addedphase/ Bq/ mL

Amountof gas
phase/ mL3)

102Dsol2)

102Dliq2)

102Dgas2)

2.19×105927

98.5±0.9

1.5±0.0

0.0±0.0

2.18×105927

99.4±1.0

0.6±0.0

0.0±0.0

2.66×103920

7.3±0.1

92.7±1.0

0.0±0.0

4.55×104921

91.7±0.3

8.3±0.10 0.0±0.0

5.82×1091697.2±0.6

2.8±0.1

30.0±0.01) 200 mg of Fe(OH)3 calculated on the basis of the 500 mg of FeCl3・6H2O used to prepare Fe(OH)3.
2) Error indicates measurement error [8].
3) Value calculated from gas pressure after reaction.

to the liquid phase. In order to shift the distribution to the solid phase, 200 mg of non-radioactive ironhydroxide was added to the reactor and the supercritical-water reaction was examined (Table 2).
Cobalt is quantitatively transferred to the solid phase, as seen only in the case of iron (Table 1). This
dramatic improvement could be explained by the coprecipitation of the cobalt with the iron. The gas
produced during the decomposition of laminate sheeting contains no activity of 60Co in any run.
3.2 Distribution of 85Sr
Table 1 indicates that 85Sr mainly stays in the liquid phase, and only a small amount is transferredto the solid phase, even when SrCO3 carrier is added. The gas produced during the reaction contained
no 85Sr in any run. To recover radioactive strontium as a solid, two additional examinations wereperformed: (1) adding the non-radioactive iron hydroxide to the reactor before the reaction and (2)adding a precipitation reagent to liquid obtained after the reaction. In the case of examination (1), 200
mg of the non-radioactive Fe(OH)3 was added to the reactor before the reaction (Table 2), and thestrontium quantitatively transferred to the solid phase as seen only in the case of iron (Table 1). This
dramatic improvement in the recovery of strontium in the solid phase could be explained by thecoprecipitation of the strontium with the iron. In the case of examination (2), Na2CO3 or NaHCO3
aqueous solution was added to radioactive Sr(NO3)2 aqueous solution as follows: A certain amount of
non-radioactive Sr(NO3)2 as the carrier and 85SrCO3 dissolved in a 1 M HNO3 aqueous solution, which isthe simulated solution obtained after the supercritical-water reaction, was mixed into 3 mL of water ina polypropylene test tube. By adding 1 mL of 0.4 M Na2CO3 or NaHCO3 aqueous solution to the abovementioned solution, a white suspension was obtained in all runs. After centrifuging, the supernatantliquid phase was transferred to another polypropylene test tube with a polyethylene dropper. The
determined distribution of 85Sr in this examination (Table 3) indicates that Na2CO3 and NaHCO3 areexcellent precipitate reagents for strontium. Thus, strontium dissolved in the liquid obtained after thesupercritical-water reaction could be precipitated with Na2CO3 and NaHCO3 aqueous solution.

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Table 3. Distribution of Sr in solid and liquid phases after adding precipitation reagent tosimulated solution obtained after reaction by supercritical-water mixed with RuO2 [3]Element

Sr

Precipitate
reagent

CarriercompoundAmount ofcarrier/ mg

TracerActivity oftracer
added/ Bq

102Dsol1)

102Dliq1)

Na2CO3 2)

0.5

5.03×10399.1±0.7

0.9±0.0

Na2CO35

5.13×10399.2±0.7

0.8±0.0

4.87×10399.1±0.7

0.9±0.0

4.81×10399.1±0.5

0.9±0.02)

NaHCO3 2)
NaHCO3 2)

Sr(NO3)2

850.55Sr1) Error indicates measurement error [8].
2) Adding 1 mL of Na2CO3 or NaHCO3 aqueous solution.

3.3 Precipitation mechanism in the case of Fe and Co
Under the conditions with small amounts (0.13 and 1.3 mg) of the non-radioactive iron hydroxide
as the carrier, iron is not recovered in the solid completely and still remains in the liquid (Table 1). One
possible explanation of these results is that the iron(III) could be simultaneously reduced to iron(II)during the reductive decomposition of the nonﬂammable organic materials. To obtain information on
the oxidation state of the dissolved iron under these conditions, 2,2' -bipyridyl, a colorimetry reagent for
Fe2+, were added to the liquid obtained after the supercritical-water reaction with 0.13 and 1.3 mg of
the non-radioactive iron hydroxide. The liquid turned pink and the absorption spectrum in the visible
region showed an absorption band at 520 nm, which is attributed to Fe2+ [4]. In the case of cobalt, thecolor of the liquid obtained after reaction was initially pale pink and changed to golden yellow when
2,2' -bipyridyl was added. Also non-radioactive condition, we prepared a cobalt(II) aqueous solution
using CoCl2·6H2O. The color of this solution was initially pale pink and changed to golden yellow when
2,2' -bipyridyl was added. These results indicated the production of Fe2+ in the liquid by the reactions
with 0.13 and 1.3 mg of the non-radioactive iron hydroxide and also Co2+ in the liquid by supercriticalwater reaction. These results show that iron(II) and cobalt(II) generated in the liquid after reaction.
Iron and cobalt added into the reactor before the reaction was Fe(OH)3 and Co(OH)3, respectively.
Therefore, iron(III) and cobalt(III) were respectively reduced to iron(II) and cobalt(II), and this meansthat a reductive mechanism is operating in the reaction using supercritical water mixed with RuO2. It
is worthwhile to note that the reduction of Co(III) to Co(II) requires very reductive conditions, asindicated by the standard electrode potential of -1.92 V vs. SHE [5] in comparison with that of Fe(III) toFe(II), which is -0.771 V vs. SHE [5]. Once the divalent metal ion is formed, the cobalt(II) ion shouldhave greater solubility than the iron(II) ion on the basis of the solubility product Ksp obtained at theambient temperature for Fe(OH)2 and Co(OH)2 8 ×10 － 16 [6] and 1.6 × 10 － 15 [7], respectively.
Furthermore, all of the solids obtained after the supercritical-water reaction of iron showed
ferromagnetism when checked with a magnet. Thus, a non-radioactive sample was prepared by theprocedure wherein 200 mg of non-radioactive Fe(OH)3, 30 mg of RuO2, 150 mg of laminate sheeting and53
3 mL of water reacted under standard conditions. The X-ray powder diffraction (XRD) was performed
at RINT 2000 (Rigaku Corp., Japan) using CuK α1. The XRD pattern of the obtained solid agreed very
well with Fe3O4. Since the corresponding radioactive cobalt samples also exhibited ferromagnetic
behavior, the non-radioactive sample was prepared by the same way of iron samples. the XRD patternof the obtained solid agreed well with CoO. A detailed analysis of the solid obtained after the reaction
using supercritical water mixed with RuO2 is now underway.
With reference to the gasification mechanism proposed in ref. [2] for the decomposition of
nonflammable organics using supercritical water with RuO 2 catalyst, the atmosphere of thesupercritical-water reaction is suggested to be reductive. The above analysis of the solid phase obtainedin the reaction also supports this reductive mechanism, where a portion of iron(III) and all of cobalt(III)are reduced to iron(II) and cobalt(II), respectively.

§4. Summary
The present investigation indicates that during the decomposition of laminate sheeting, theproduced gas contains no radioactivity in any run. The distribution of Fe, Co, and Sr was determinedby radiometric analysis after laminate sheeting was decomposed using supercritical water mixed with
RuO2. Iron, cobalt and strontium were recovered as solids after the present supercritical-water reaction
by the addition of an excessive quantity of iron hydroxide before the reaction. In the case of strontium,almost all radioactivities were found in the liquid phase. However, by adding precipitate reagents tothe liquid phase obtained in the reaction, strontium can be recovered in the solid phase. This methodshould be a very effective way to decompose the nonﬂammable organic materials of LLW generated by
nuclear power plants.

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W. Sugiyama et al. : J. Nucl. Sci. Technol. 42 (2005) 256.

[2]

K.-C. Park et al. : Chem. Commun. (2003) 694.

[3]

W. Sugiyama et al. : J. Supercritical Fluid (2005) (Available online at doi:10.1016/ j.supﬂu.2005.02.
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[4]

M.H. Ford-Smith et al. : J. Am. Chem. Soc. 83 (1961) 1830.

[5]

A.J. Bard et al. : “Standard Potentials in Aqueous Solutions” , ed. ; IUPAC (Marcel Dekker): New York(1985).

[6]

D.L. Leussing et al. : J. Am. Chem. Soc. 75 (75) 2476.

[7]

A.O. Gübeli et al. : Helv. Chim. Acta 16 (1970) 1229.

[8]

G. Gilmore: “Practical Gamma-Ray Spectrometry” , ed. ; John Wiley & Sons: Chichester, 1995.