S-P2-4
Radioactive Waste Disposal by Nuclear Transmutation
核変換による放射性廃棄物処理について
Takanori Kitada, Atsuki Umemura and Kohei Takahashi
北田孝典, 梅村篤輝，高橋公平
Osaka University, Graduate School of Engineering, Division of Sustainable Energy and Environmental Engineering
Yamada-oka 2-1, Suita, Osaka 565-0871, Japan
大阪大学工学研究科 環境・エネルギー工学専攻 〒565-0871 大阪府吹田市山田丘2-1
Radioactive waste can be reduced by transmuting radioactive nuclides into stable nuclides through nuclear
reactions. As target nuclides, 90Sr, 99Tc, 129I, and 137Cs are selected and the difference of transmuting
efficiency among several neutron spectrum fields is investigated in this study. Numerical results showthat neutron spectrum of thermal reactor is the best to reduce the radioactivity of 99Tc and 129I, and D-T
neutron spectrum is the best to reduce the radioactivity of 90Sr and 137Cs by larger transmutation of thetarget nuclide through (n,2n) reaction than that of the neutron spectra of fission reactors.

1. Introduction
Radioactive waste can be reduced through theuse of nuclear reactions such as capture, fission,and (n,2n) etc. by changing element and/or atomic
mass. This transmutation efficiency of the target
nuclide depends on its cross section which has aremarkable dependency on the irradiated neutronenergy and also different value among nuclides.
Therefore the recommended neutron spectrum toannihilate the radioactive waste is supposed to bedifferent for different target nuclides. In thismanuscript, the cause of the efficiency differenceby changing neutron spectrum fields and the target
nuclides is shown with numerical results.
The target nuclides in this survey are 90Sr, 99Tc,
129
I and 137Cs: these are major radioactive waste(fission product:FP) from fission reactors and also
to be annihilated, and the neutron spectrum fields
used in this survey are from two fission reactors andone fusion reactor: Pressurized Water Reactor(PWR), Fast Breeder Reactor (FBR), and D-T
neutron (DT).
2. Neutronic Parameter
Neutronic parameters to affect the transmutation
efficiency are cross section (XS) of each nuclide
and neutron spectrum.
In addition to these,burnup chain which shows the transmuting relation
among nuclides is also to be considered to discuss
the results. In this section, those data are briefly
described for the sake of easy understanding.
2.1 Cross Section
The major reaction types to be considered in thetransmutation are capture and (n,2n) for FP. Fig. 1shows XS in barn unit as a function of incident
neutron energy for the nuclides: 90Sr, 99Tc, 129I and137

Cs. It is noted the XS of (n,2n) reaction type isalmost the same value and shape for all nuclides.

137Cs(n,2n)
90Sr(n,2n)
137Cs(n,γ)

90Sr(n,γ)Fig.1 (a) Cross sections of 90Sr and 137Cs

99Tc(n,γ)129I(n,2n)
99Tc(n,2n)
129I(n,γ)Fig.1 (b) Cross sections of 99Tc and 129I

2.2 Neutron Spectrum
Neutron spectrum is also important to consider
the transmutation because of the remarkable energy
dependence of XS. The neutron spectra used in thissurvey are shown in Fig. 2. The amount of thetransmutation is evaluated as the proportional value

99

to the product of XS and neutron flux.
1.2

Neutron flux (arb. unit)

1.0

FBR
PWR

0.8

DT
0.6

0.4

0.2

0.0
1E-5

1E-3

1E-1

1E+1

1E+3

1E+5

1E+7

Energy (eV)Fig.2 Neutron spectrum in PWR, FBR and DT

2.3 Burnup Chain
Radioactive nuclide transmutes as the results ofnuclear reaction and also radioactive decay. Therelation among the nuclides is called burnup chain.
Fig. 3 shows a piece of burnup chain related to the90
Sr. Exact burnup chain includes the relation(n,p), (n,3n), alpha-decay etc. and all the relation isconsidered in the following results.

Tc. Although total radioactivity increase at theend of irradiation (1000day) because of theproduction ofshort half-life nuclides, theradioactivity becomes smaller than the initial value
at the end of cooling (4500day) except for the caseof 129I in PWR. The increment in PWR is caused by
the accumulation of 134Cs. However, the half-life of134
Cs is 2.1y and this is remarkably shorter than thetarget nuclide. In addition, by considering thesmaller mass of 129I after irradiation, theradioactivity decreases rapidly compared to thenon-irradiated case. It should be mentioned that toirradiate 137Cs and 90Sr in FBR and PWR ismeaningless, and DT usually shows goodperformance to reduce the radioactivity. These
differences mainly come from the facts that DT is
likely to cause (n,2n) reaction compared to FBRand PWR, and the XS of (n,2n) reaction at14(MeV) is higher than that of capture reaction
especially for 137Cs and 90Sr, and almost all thetransmuted nuclides are stable or have shorter
half-life compared to the target nuclides.
Table 1 Summary of the transmutation efficiency
Mass Ratio

89Sr

88Sr

β(50d)88Y

89Y

β(107d)90Srβ(29y)90Yβ(64h)

91Srβ(9.6h)(n,γ)(n,2n)

137Cs30y91Y

β(58d)

Fig. 3 Burnup chain related to 90Sr

3. Calculation and Results
In this study, calculations were performed byMVP-II[1] and ORIGEN2[2]. MVP-II is used toevaluate effective XS to be used in ORIGEN2
burnup (irradiation and cooling) calculation. The
amount of neutron flux is set to 5×1015[#/cm2sec]
during irradiation period of 1000 days, and cooling
period after irradiation is set to 3500 days.
Non-irradiation (only cooling) case is also
performed to be clear the transmutation efficiency
by the irradiation.
Results are summarized in Table I with half-livesof each nuclide, where the mass of target nuclide
and total radioactivity of all transmuted nuclides arenormalized by initial value.
4. Discussions
Table I shows that mass of the target nuclides
decrease for all cases and the transmutation
efficiency remarkably changes by changing thespectrum fields. However, the spectrum field isdifferent to get the best efficiency: DT is the bestfor 90Sr and 137Cs, and PWR is the best for 129I and90

Sr29y129I
1.6e+7y99

Tc
2.1e+5yTotal Radioactivity Ratio

01000

20004500

01000

20004500

FBR

1.00

0.93

0.88

0.75

1.00

1.04

0.87

0.74

PWR

1.00

0.93

0.87

0.74

1.00

1.04

0.87

0.74

DT

1.00

0.45

0.42

0.36

1.00

3.60

0.42

0.36

cooling_only

1.00

0.94

0.88

0.75

1.00

0.94

0.88

0.75

FBR

1.00

0.93

0.87

0.74

1.00

1.01

0.87

0.74

PWR

1.00

0.93

0.88

0.74

1.00

0.99

0.88

0.74

DT

1.00

0.52

0.49

0.41

1.00

2.94

0.49

0.41

cooling_only

1.00

0.94

0.88

0.75

1.00

0.94

0.880.75

FBR

1.00

0.84

0.84

0.84

1.00

1.28E+6

0.94

0.85

PWR

1.00

0.10

0.10

0.10

1.00

2.81E+6 1.67E+3 1.70E+2DT

1.00

0.52

0.52

0.52

1.00

4.57E+6

0.53

0.52

cooling_only

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

FBR

1.00

0.75

0.75

0.75

1.00

2.43E+4

0.75

0.75

PWR

1.00

0.03

0.03

0.03

1.00

1.40E+4

0.03

0.03

DT

1.00

0.54

0.54

0.54

1.00

4.83E+2

0.56

0.56

cooling_only

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.005. Conclusions
To annihilate the radioactive waste byneutron irradiation, the difference of neutron
spectrum causes remarkable difference intransmutation efficiency even in the sameneutron fluence. DT is found to be the best for
137Cs and 90Sr, and PWR is the best for 99Tc and129I by considering the shorter half-life of 134Cs
than that of 129I.
References[1] Y. Nagaya, K. Okumura, T. Mori et al, JAERI 1348,(2005).
[2] A. G. Croff, ORNL/TM-7175, July 1980.