An Extreme Disposition Method

For Low Level Radioactive Wastes

Using Supercritical Water

　　　 Wataru Sugiyama*, 　 Tomoyuki Koizumi*, 　 Akira Nishikawa*, 　　　　　 Yuuji Sugita*, 　 Ki Chul Park#, 　 Hiroshi

Tomiyasu#

*Chubu Electric Power Co., Inc.

#Shinshu University

Introduction

A large amount of radioactive wastes have been accumulated in nuclear power plants. These are mostly fire retardant materials and at this moment stored in the 200L dram cans with mortar after the following process.

Combustibles(Paper, Wood etc.)

Plastics(Fire retardant sheet

etc.)

Incombustibles

(Metal etc.)

Incineration MeltCompression

200L dram cans with mortar

Figure The disposition process of low level radioactive wastes

The present process has problems as follows:

○ No significant decrease in total amounts of wastes

○ Plastics, if involved in combustible wastes, may produce hazardous gases

The objective of the present study is to establish an extreme disposition method to minimize the wastes as close as to zero with zero addition during disposition.

A disposition using supercritical water could be an only method for this purpose.

Introduction

The effective disposition method for low level radioactive wastes has not been established yet

Objective of the present study

　 Supercritical Water Oxidation method using oxygen as an oxidant in supercritical water are generally known and widely used. However, complete decomposition is not possible for stable materials such as aromatic compounds by this method.

Recently, we have developed a new method using RuO2 as a catalyst in supercritical water†. With this catalyst, the complete decomposition of fire retardant materials became possible without any other addition.

The present study is to use this RuO2 catalyst for the disposition of low-level wastes to achieve an extreme disposition method.

† ： ref.

1. W. Sugiyama, K. C. Park, H. Tomiyasu, et. al., Super Green 2002,., Suwon, Korea, (2002)

2. K. C. Park, H. Tomiyasu;, ChemComm. (2003) 694

Residuals resulted from supercritical water oxidation treatment for p-di-chlorobenzene using equivalent (right) and twice equivalent (left) H2O2 as an oxidant under the following condition: 450 and 30 min. of reaction ℃time.

Solid residuals by the supercritical water oxidation treatment for p-di-chlorobenzene using equivalent (left) and twice equivalent (right) H2

O2 under the following condition: 450℃ and 30 min. of reaction time. p-di-chlorobenzene is used for the simulation of PCB.

Solid residual after the treatment by supercritical water oxidation (right) for polyvinylchloride powder (below).

Solid residual by our new method (left)

What is supercritical water?

• Critical Point • 22 MPa and 374℃• This is the Critical Point of Water

• Supercritical Water• Above the critical point (22 MPa and 374 ) in ℃

the phase diagram of water, water is no longer liquid, but not gas either.

Supercritical conditionCritical pointRoom temperature High temperature

L

Co

L

L

L

LL Co

L

L

LL

tetrahedronoctahedron

Vapor Vapor Liquid Liquid Two phase One phase

A Characteristic of Supercritical Fluids A Characteristic of Supercritical Fluids

● 　 Lower viscosity, Higher diffusive(gaslike)● 　 Higher thermal conductivity(liquidlike)● 　 Lower dielectric constant, Larger ion product

Supercritical fluids can simultaneously control with slight variation in density.

(from liquidlike to gaslike)

1H NMR spectra of water measured in the the range of 25-400 at 30MPa. ℃

【 1H, 17O-NMR Chemical shift of water vs. Temp.】

17O chemical shifts of water and the extent of hydrogen bonding as a function of temperature

at 25 and 30MPa.

Increasing temperature

Decreasing hydrogenbond

Highfield shift

Fig. 6 Proton spin-lattice relaxation times (T1) of water as a function of temperature.

Structure of 95% DStructure of 95% D22OO

Structure of 95% COStructure of 95% CO33CDCD22ODOD

Structure of 95% COStructure of 95% CO33ODOD

D

O

H O

D D

CD

DD

CD D

O

H C

DD

DCD D

O

D

CD

DD

O

HC

DD

D

O

D

Fig. 5 Structure of water, ethanol and methanol (95%deuterations)

Spin-Lattice relaxation time T1

at temperatures from 25 to 400℃

•T1 is controlled

•　 in low temperature 　 (below 200℃) • by the magnetic moments of adjacent atoms• because of slow molecular motion •　 (e.g. 1H gives larger magnetic influence than 2D)

• •　 in high temperature • under sub or supercritical conditions• by the rate of intra-molecular rotation

Model compounds of coal

Ref. Hayatsu, R., Scott, R. G. Nature, 1975, 257, 378.

O

HN

O 1,3-Diphenylpropane

Benzyl ether

N-Phenylbenzylamine

Phenyl ether

Bridged aromatics

N

ONH

ON

Benzene

Naphthalene

Phenanthrene

Dibenzofuran

BenzonaphtofuranPyridine

Quinoline

Carbazole

S

S

Dibenzothiophene

Benzo[b]thiophene

Condensed aromatics and heterocycles

O

HN

CH3

NH2OHCH3

CH3

CH3

OH O

1,3-Diphenylpropane

390℃, 3 h

SCW

390℃, 3 h

SCW

390℃, 3 h

SCW

Benzyl ether

N-Phenylbenzylamine

Toluene Ethylbenzene

+

+

+

+

+

Benzyl alcohol Benzaldehyde

Aniline

Aromatic rings are highly stable in SCW

Decomposition of bridged aromatics by SCW

Aromatics

Lower hydrocarbons with higher H/C ratioCatalysts

Polymers

CC

H

H

H

n

Hydrogenation ( H donor ： H2O )

CO2

PolymersAromatics plasticsCoal Biomass

A nearly complete gasification of aromatics and polymers was achieved by stoichiometrically insufficient amounts of RuO2 in SCW to provide CH4, CO2 and H2 as major products.

Sample : 150mg

RuO2 : 30mg

Water : 3mL

Experimental procedure

HASTELLOY batchwise reactor

Reaction

Time : 5,30,60 and 180min.

Temp. : 673,723 and 773 K

Cooling at room temp.Water

,

CHCl3

Open

Decantation

Evaporate CHCl3

Solid residue

Organic residue

Weigh

Weigh

Rinse

Filter RuO2

and solid residue

Experimental procedure

Figure Experiment equipment

On-line gas chromatography apparatus

Gas chromatographs ： Shimadzu, TCD-GC8APT, FID-GC8APFAnalysis conditions

Hydrocarbons ： Porapak Q, Col. Temp. 60 ℃, He carrier

H2 ： Molecular sieve 5A, Col. Temp. 50 ℃, Ar carrier

CO2 ： Silica Gel, Col. Temp. 60 ℃, He carrier

Table 2 Experimental results on RuO2-catalyzed gasification of naphthalene in SCW

Org.Atomic ratio Molar ratio

C-conv.(%)

Product distribution (%) Molar ratio

H/C O/C [Org]/[RuO2] CH4 CO2 H2 [O]CO2/[O]RuO2 [H]Gas/[H]Org

0.80 0 5.12 96.7 48.8 42.7 8.4 23.1 2.90

Carbon conversions of organic compounds (feed) to gaseous products were calculated according to the following equation; C-conv. (%) = 100×[C] in gaseous products/[C] in feed, where [C] represents the moles of carbon.

Org.Atomic ratio Molar ratio

C-conv.(%)

Product distribution (%) Molar ratio

H/C O/C [Org]/[RuO2] CH4 CO2 H2 [O]CO2/[O]RuO2 [H]Gas/[H]Org

1.00 0 6.32 100.7 53.7 39.4 6.9 21.5 2.47CC

H

H

H

Table 3 Experimental results on RuO2-catalyzed gasification of polystyrene in SCW

Carbon conversions of organic compounds (feed) to gaseous products were calculated according to the following equation; C-conv. (%) = 100×[C] in gaseous products/[C] in feed, where [C] represents the moles of carbon. Molar quantities of polymers are the apparent values calculated by assuming monomeric units to be molecules.

Table 4 Summary of the experimental results on RuO2-catalyzed gasification of organic compounds in SCW

Org.Atomic ratio Molar ratio a

C-conv.(%) b

Product distribution (%) d Molar ratio

H/C O/C [Org]/[RuO2] CH4 CO2 H2 [O]CO2/[O]RuO2 [H]Gas/[H]Org g

0.80 0 5.12 96.7 48.8 42.7 8.4 23.1 2.90

0.75 0 3.94 87.9 c 52.7 40.6 6.7 18.1 2.86

0.83 0.08 3.87 99.9 45.8 48.8 5.4 23.9 (22.0) e 2.46

0.67 0.08 3.92 101.7 51.0 43.6 5.5 22.0 (20.1) e 3.46

PE 2.00 0 23.5 100.6 66.6 28.0 5.3 14.0 1.47

PP 2.00 0 15.7 99.9 66.5 26.9 6.5 13.5 1.49

PS 1.00 0 6.32 100.7 53.7 39.4 6.9 21.5 2.47

PET 0.80 0.40 3.44 97.2 37.3 51.0 11.5 19.3 (12.6) e 2.44

Cellulose 0.80 0.83 5.12 97.0 34.2 42.7 14.6 14.0 (4.2) e 1.18

a Molar quantities of polymers are the apparent values calculated by assuming monomeric units to be molecules. b Carbon conversions of organic compounds (feed) to gaseous products were calculated according to the following equation; C-conv. (%) = 100×[C] in gaseous products/[C] in feed, where [C] represents the moles of carbon. c The lower conversion is ascribed to the adsorption of CO2 by the resulting NH3; the wt.% conversion based on its feed

and recovery was 98.6 wt.%. d C2H6 and C3H8 were detected as minor products, though the proportions (< 0.2%) are

not listed here. e The values in parenthases were caluclated according to ([O]CO2− [O]Org)/[O]RuO2. g Molar ratios of hydrogen atoms in gaseous products ([H]Gas) to those in the organic compounds converted ([H]Org). In carbazole, [H]Org was calculated using the wt.% conversion.

O

O

NH

PE = polyethylene, PP = polypropylene, PS = polystyrene, PET = poly(ethylene terephthalate)

Results

Figure 　 Decomposition of laminating sheet

Reaction time : 180min.

Reaction temperature : 723K

Figure 　 Decomposition of fire retardant tape

Results

Results

Figure 　 Decomposition of anion exchange resin

Results

Figure 　 Decomposition of rubber gloves

Results

The decomposition calculated using a formula as follows.

×100 (w%)a －

ba

a : mass before experiment (mass of sample)

b : mass after experiment (mass of decomposed sample)

Results

Samples BasisDecomposition*

(w%)

Laminating sheet polyethylene 98

Cover sheet (ALARA sheet) polyethylene 98

Attention rope with tiger striping

polyethylene 99

Suit for controlled area (zipper)

nylon 99

Fire retardant sheetpolypropylen

e98

Fire retardant tapepolypropylen

e98

Anion exchange resin polystyrene 94

Rubber glovesnatural rubber

79

Table Decomposition

* : Reaction time : 180min. Reaction temperature : 723K

The samples, which are used in nuclear power plants, are commercially available ones from CHIYODA TECHNOL CORPORATION. Anion exchange resin was DOWEX 1-X8.

Results

Five typical samples are chosen to determine the best condition

Samples Basis

Laminating sheet polyethylene

Fire retardant sheet polypropylene

Fire retardant tape polypropylene

Anion exchange resin

polystyrene

Rubber gloves natural rubber

0

25

50

75

100

0 1 2 3

Time（hr）

Dec

ompo

sitio

n（w

%）

Figure Dependence of temperature and time for laminating sheet

Results

▲ : at 673K◆ : at 723K■ : no catalyst at 723K

0

25

50

75

100

0 1 2 3

Time（hr）

Dec

ompo

sitio

n（w

%）

Figure Dependence of temperature and time for fire retardant sheet

Results

▲ : at 673K◆ : at 723K■ : no catalyst at 723K

0

25

50

75

100

0 1 2 3

Time（hr）

Dec

ompo

sitio

n（w

%）

Figure Dependence of temperature and time for fire retardant tape

Results

▲ : at 673K◆ : at 723K■ : no catalyst at 723K

0

25

50

75

100

0 1 2 3

Time（hr）

Dec

ompo

sitio

n（w

%）

Figure Dependence of temperature and time for anion exchange resin

Results

▲ : at 673K◆ : at 723K■ : no catalyst at 723K

0

25

50

75

100

0 1 2 3

Time（hr）

Dec

ompo

sitio

n（w

%）

Figure Dependence of temperature and time for rubber gloves

Results

▲ : at 673K◆ : at 723K● : at 773K■ : no catalyst at 723K

Discussion and Conclusion

1. Decompositions and gasification of fire retardant plastics were performed nearly 100 by use of RuO2 as a catalyst in supercritical water, but a little residuals remained for anion exchange resin and natural rubber

Gases produced during the decomposition of all wastes were CH4, CO2 and H2 and no hazardous gas such as CO was not observe

Discussion and Conclusion

2. The catalytic effects by RuO2 are dependent on temperature and reaction time, but independent of time after 30 minutes

Decomposition reactions are controlled by the catalyst rather than thermal decompositions

Discussion and Conclusion

Temp. : 450 ℃

Time : 30min.

3. The best condition for the present catalytic reaction is as follows

4. Only rubber gloves showed lower decomposition ratio

The reason is expected that the gloves contain C=C bonds originated from natural rubber and that these double bonds might prohibit the decomposition

Conclusion

• The present RuO2 catalytic disposition method in supercritical water enables nearly 100% decomposition for low-level wastes except natural rubber.

• Radioactive metals such as Fe, Co Ni were recovered as oxide precipitations.

• Nothing except the catalyst was added during the disposition..

• Ruthenium can be recovered easily to be used recycled.• In conclusion, this disposition might be close to the

extreme method, that is, to make wastes zero with zero addition.

Acknowledgement This study started at the beginning in Titech by the support of Future Program of

the Japan Society for the Promotion of Science. The author (HT) expresses his

thanks to the following persons:

Core members of Future Programs in JSPS:

Prof. Yoshio Yoshizawa

Prof. Yasuhiko Fujii

Co-workers:

Prof. Yasuhisa Ikeda

Dr. Masayuki Hara

Dr. Tomoo Yamamura, Dr. Yun-Yul Park, Dr. Seong-Yun Kim,

Dr. Zsolt Fazekas, Dr. Norioko Asanuma, Dr. Takehiko Tsukahara,

Dr. Varga Tamas, Dr. Yuichiro Asano, Dr. Koh Hatakeyama, Dr.Koji Mizuguchi,

Prof. Gilvert Gordon (Volwiler Distinguish Professor of Miami University)

Prof. Kunihiko Mizumachi (Emeritus Professor of Rikkyou University)