Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 233-235, 2005

Behavior of Technetium in Supercritical Water: Distribution to Liquid, Solid Phases andonto Metal Surfaces
T. Yamamura,*, a I. Satoh,a N. Okuyama,a Y. Shiokawa,a T. Sekine,b W. Sugiyama,c K.-C. Park,dand H. Tomiyasud
aInstitute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan
Department of Chemistry, Faculty of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
cElectric Power Research and Development Center, Chubu Electric Power Co., Inc., Kitasekiyama, Odaka-cho,Midori-ku, Nagoya 459-8522, JapandDepartment of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato,Nagano 380-8553, Japanb

Received: July 31, 2005; In Final Form: September 30, 2005
The distributions of technetium among solid, liquid and gas phases in supercritical water reactions were investigated for the decomposition process of nonflammable plastic materials contaminated with technetium. In addition
to supercritical water (SCW) methods with or without H2O2 which have been investigated extensively for decomposition method for the plastics, SCW with RuO2, developed by us, characteristic of reductive reaction was investigated for technetium distributions. The distributions of technetium were determined by using 95mTc tracer with orwithout its carrier 99Tc. The gases produced during the decomposition of the organic material contain no radioactivity under all conditions in this study. Also, technetium shows a tendency to transfer to solid phase but not toliquid phase in SCW with RuO2. These results indicate that this process is suggested as a predominant candidatefor the treatment of nonflammable organic materials in low-level radioactive wastes (LLW).

1. Introduction

2. Experimental

Above 647 K and 22 MPa, water is a supercritical fluid,which possesses unique solvating and transport properties
compared to liquids or gases. Supercritical water (SCW),
which shows liquid-like density and gas-like diffusivity, has
the ability not only to dissolve materials soluble in liquid water
but also to promote particular reaction. Using the supercriticalwater, gasification of organic materials using RuO2 as a catalyst
has been developed.1 We applied this gasification method tothe decomposition of bulky non-flammable organic materials
generated in nuclear power plants, classified as low-level radioactive wastes (LLW). 2, 3 Radioactive iron, cobalt, cesium,iodine, strontium attached to the organic materials were foundto be recovered in the solid phase with or without precipitation
reagents and showed no transfer to gas phase.
Appreciable yield of 6.2% in the thermal neutron fission
leads to the formation of technetium-99 with long-lived halflife (t1/2 = 2.1 × 105 y). The LLW also contains the technetium,
which is known to have oxidation states from 0 to VII andshows variety of chemical properties including sublimation ofTc2O7 above 584 K.4 For the decomposition of LLW by theSCW method, the distribution behavior of technetium is to beclarified. In this study, distribution of technetium after supercritical water reaction among solid, liquid and gas phases weredetermined. The distribution behavior was discussed concerning with differences (i) between SCW reaction with rutheniumoxide (RuO2) and with hydrogen peroxide (H2O2), (ii) with orwithout its carrier and (iii) between technetium and alkaline
metal. In addition, adsorption of technetium on surface of various metal materials, which are candidates for reactor materials,
during the SCW reactions were discussed.

Technetium-95m was produced by 93Nb(α,2n) 95mTc reaction
at Cyclotron and Radioisotope Center of Tohoku University
and purified by sublimation, followed by dissolution in waterto prepare HTcO 4. NH499TcO 4, possessed by Faculty of Science, Tohoku University, was used as a carrier. Ruthenium(IV)oxide (purity: > 99.9%) and granular polyethylene (mediumdensity) were purchased from Kishida Chemical Co., Japan andAldrich Chemical Company, Inc., U.S.A., respectively, andused without further treatment.
A batchwise reactor with 10 mL capacity made of Hastelloy
C-22 was used. A small portion of an aqueous solution including about 1 mg of technetium, three pieces of metal plates(SUS304, Hastelloy C22, Inconel 625, 5 × 10 × 1 mm size, polished with #2000), water and either of oxidant (H2O2) or reductant (RuO2 and 150 mg of granular polyethylene) were loaded
into the reactor and the supercritical reaction under the condition of “723 K-43 MPa-30 min.” was carried out. After cooled
off to room temperature, distribution coefficient was determined by measurement of γ-ray spectra of three phases separated (solid, liquid and gas) and metal pieces. Cesium solution
with a tracer of 137Cs was used for a reference of the behavior.
The radioactivity of 95mTc was determined from the areasof peak at 204.11 keV by using a γ-ray spectrometer (GEM28185-P, ORTEC Inc., USA). The distribution of their radioactivity in solid, liquid and gas phases was determined as given
by*Corresponding author. E-mail: yamamura@imr.tohoku.ac.jp.
FAX: +81-22-215-2121.

Dphase =Aphase,AT(1)where Dphase designates the distribution ratio, AT and Aphase areradioactivity of sample before the supercritical-water reaction
and 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. The radioac-© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

234 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Yamamura

tivity of each radioisotope in a sample was extrapolated to thetime point at which the radioactivity, loaded into the supercritical-water reactor, was measured before the supercritical-water
reaction. The typical time periods required for the γ-ray measurements were 30 minutes for the solid phase, 3 hours for theliquid phase and 48 hours for the gas phase.
3. Results and Discussion
3.1. Preliminary experiment. In a preliminary experiment
for examining reaction of technetium with supercritical water,
only around 10% of technetium was found in a recovery (liquidphase and solid phase) when SCW-RuO2 reaction including
technetium was performed (Figure 1, 1st reaction). Technetiumwas not detected in recovered gas phase at all. By measuring γrays from inside of the hastelloy reactor, appreciable quantity oftechnetium was found in the reactor, in spite that inside walls
were rinsed many times with water after SCW-RuO2 reaction.
In order to remove adsorbed technetium, SCW-H2O2 reaction
without technetium was examined and more than 80% of technetium to loaded quantity was recovered (Figure 1, 2nd reaction). According to these results, SCW-H2O2 reaction effectively
removes technetium adsorbed inside the hastelloy reactor,
whereas SCW-RuO 2 reaction has not show such ability.
Actually, SCW-RuO2 reaction was examined again in 3rd reaction, but only less than 1% of technetium was recovered, whichis in clear contrast to further SCW-H2O2 reaction done in 4–7
reactions which remove technetium significantly. Since in the
5th reaction by SCW-H2O2 yield only less than 2% of technetium, we decided that 5 times of SCW-H2O2 reaction should bedone for finishing a supercritical water experiment with technetium.
3.2. Distribution of technetium with or without carrier

Figure 1. Activity of Tc-95 loaded and found in recovered samples
obtained by supercritical water with RuO2 and polypropylene (No. 1and 3) or with H2O2 (No. 2 and 4-7).

and/or precipitating agent. Amount of technetium found in
the solid, liquid and gas phases recovered after supercriticalwater reaction (i.e. not including the recovery of washings by
SCW with H2O2 reaction to remove adsorbed technetium insideof the reactor) are indicated in ratio D in Table 1. Both in H2O2and RuO2 methods, technetium does not transfer to gas phasein spite of its low boiling point but limited to solid phase. This
distribution of technetium was not affected by addition of its
carrier of 99Tc but shifted to solid phase by addition of Fe(OH)3.
Summarizing the results, technetium can be recovered in solid
phase without transfer to gas phase.
3.3. Distribution of technetium onto metal surfaces. It is
worthwhile to note that technetium shows a large tendency to
be adsorbed onto metal surfaces and the adsorbed technetium
should be removed by repeating washing by H 2O2 method forthe next reaction. In order to elucidate the adsorption on metal
surfaces, supercritical water reactions were carried out withthree types of metal materials (Table 2). Moreover, the results
for technetium was compared with those for cesium, which is amember of alkaline metal ion whose character is simple ionic.
Technetium was found to adsorbed onto various metal materials. Washing by SCW with H 2O2 for 5 times is required toremove the adsorbed technetium from the reactor.
In RuO2 method technetium may be reduced to colloidal or
polymeric TcO 2 or TcO(OH) 2 which has a great affinity tometal surface.5, 6 The adsorbed TcO2 can be oxidized to TcO4 −by H2O2 and dissolved in solution. Effect of supercritical waterwith RuO2 on technetium chemistry requires further investigation.
4. Conclusion
Distribution of technetium after supercritical water reaction
was investigated. Technetium does not transfer to gas phase inspite of its low b.p. but distributes to solid phase in SCW andSCW with RuO 2 . Furthermore, technetium was found toadsorbed on to various metal materials. Washing by SCW withH 2O2 for 5 times is required to remove adsorbed technetiumfrom reactor. The result of adsorption of technetium by SCWwith RuO2 and the dissolution by SCW with H 2O2 suggested
that the reductive atmosphere of SCW with RuO2 may result inthe formation of colloidal or polymeric TcO 2 or TcO(OH) 2
which have a great affinity to metal surface. The supercriticalwater process can be used for decomposition of non-flammableplastics of LLW with limiting technetium to solid and onto
metal surfaces.
Acknowledgements. We would like to thank Mr. M. Takahashi of Experimental Facility for Alpha-Emitters of IMR,Tohoku University, for his kind cooperation. This work was
performed at the International Research Center for Nuclear
Materials Science, Institute for Materials Research, Tohoku
University.

TABLE 1: Distribution of Technetium to Three Phases After Supercritical-Water Reactions
Reaction
RuO2 method

H2O2 method‡

Carrier
Tc-99 /mg Fe(OH)3 /mg

102 Dsol‡†

102 Dliq‡†

102 Dgas‡------

23.75 (97.11)

0.71 (2.89)

0.00

0.5

---

22.86 (97.26)

0.64 (2.74)

0.00

0.5

13

17.30 (99.30)

0.12 (0.70)

0.00------

86.54 (99.28)

0.63 (0.72)

0.00

0.5

---

41.98 (97.91)

0.90 (2.09)

0.00

Values of Dphase were determined according to eq 1. †Values in parentheses are calculated by Dphase /(Dsol + D liq + Dgas)

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 235

Behavior of Technetium in Supercritical Water

TABLE 2: Distribution of Tc and Cs to Each Site After Supercritical-Water Reactions of Nonflammable Plastics ¶No reaction‡
Contents

Phases recovered
after reaction

Metal specimens

Washings withSCW-H2O2 reaction

Cs-137 Tc-95m

Cs-137

RuO2 method
Cs-137 +Tc-95m +Tc-95m
CsNO3Tc-99

H2O2 method
Tc-95m +Cs-137 Tc-95m
Tc-99

Solid phase

---†

---†

5.8

15.63

Liquid phase

83.7

85.5

64.5

80.29

Gas phase

---†

---†

00

SUS304000

Hastelloy

000

Inconel

000

1.4

1.38

0

0.9

0.71

1st reaction

3.1

4

6.9

4.17

22.6

24.12

12.1

51.8

28.55

2nd reaction

1.3

1.6

5.9

0.63

4.8

1.79

2.4

3.8

5.58

3rd reaction

---

---

2.9

0.17

2.3

0.52

1.1

1.7

1.11

4th reaction

---

---

1.6

0.12

3.8

0.41

1.3

1.3

0.26

1.4

0.38

0.5

0.8

0.51101.01

80.3

97.99

99.5

84.6

79.18

5th reaction
Total amount recovered------

0.8

88.1

91.1

88.4

33.9

65.75

1.9

17.8

16.75

0.5

0.76

0080.2

1.5

23.62

000

1.9

1.95

0

1.2

1.47

7.7

0.93

0

3.8

0.62

¶

Distributions are indicated as percentile to the amount initially loaded. ‡Solutions and specimens were left in the reactor for two hours without
supercritical water reaction. †No solid and gas phases owing to no reaction.

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