Sensitivity Analysis of Actinide Decay Heat Focused on Mixed Oxide Fuel
Naoto HAGURA, Tadashi YOSHIDA
Musashi Institute of Technology, Tamazutsumi 1-28-1, Setagaya-ku, Tokyo 158-8557, Japan
E-mail: g0567014@sc.musashi-tech.ac.jp

Nuclear power plants in the world accumulate a vast amount of spent fuels. In Japan, the spent fuelis reprocessed and is not disposed directly. Therefore utilization of this reprocessed fuel, i.e. MixedOxide (MOX) Fuel, will eventually expand. Under this background, hereafter, the MOX fuel will playa role of greater importance. It is, then, essential to understand the characteristics of the spent MOX
fuel. In this work, the uncertainty of the fission-product and actinide decay heats was studied
introducing the uncertainty of the nuclear data and the prediction accuracy of isotopic generation. At
100 year after discharge, for example, 7% error of the decay heat will be introduced by 10% uncertaintyof Pu-241 generation calculation through Am-241 production.

1.

Introduction
There are 440 nuclear power plants in the world. They hold a vast amount of spent fuels. In Japan,the spent fuel is reprocessed and is not disposed directly. Therefore utilization of this reprocessed fuel,i.e. Mixed Oxide (MOX) Fuel, will eventually expand.
Under this background, hereafter, the MOX fuel will play a role of greater importance. It is, then,essential to understand the characteristics of the spent MOX fuel. Above all, the radioactivity anddecay power of spent MOX fuel are very crucial in storage or disposal terms. In this work, theuncertainty of the fission-product and actinide decay heats was studied introducing the uncertainty of thenuclear data and the prediction accuracy of isotopic generation.
2.

Procedure of Analyses
In this study, we made use of SWAT code system1 developed by Japan Atomic Energy ResearchInstitute (JAERI) and ORIGEN2 code2 developed by Oak Ridge National Laboratory (ORNL). Burnup
calculation is performed by SWAT code, and decay heat calculation is carried out by ORIGEN2 code.
In SWAT, typical PWR pin cell model is selected as calculation model. Component of MOX fuel is
specified as PWR33G-MOX3 (enrichment of 0.22% depleted uranium and 5.3% Pu). Calculationconditions are as follows: Thermal power is 37.9MW/t, normal discharged burnup is 30GWd/t, andlibrary is JENDL3.3. Isotopic inventory calculated by SWAT is translated into the input of ORIGEN2code.

3. Calculation Results
3.1 Spent fuel characteristics in perspective
For keeping the decay heat and radioactivity of spent fuel in perspective, Fig.1 shows the results ofstandard calculation of this study. On these figures, red line means MOX fuel, while blue line means
5% enriched uranium (EU) fuel. We should pay attention to the Actinide component. The difference
that exceeds one decade is seen between MOX fuel and EU fuel in the actinide component. As a result,
total decay heat (ACT and FP) has been raised. In each cooling time, the nuclides that mainly show

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contribution are as follows: Cm-242 (~1yr), Pu-238 (~100yr), Am-241 (~500yr), Pu-240 (~104yr),
Pu-239 (~105yr), and nuclides of neptunium series (>105). Characteristics of radioactivity also indicate
similarity between MOX fuel and EU fuel (Fig.1 (b)).
1.E+081.E+04

MOX fuel (Red line)
5%EU fuel (Blue line)

1.E+0330GWd/t

1.E+02
1.E+01
1.E+00
1.E-01

Total(MOX)
ACT(MOX)FP(MOX)
Total(EU)ACT(EU)FP(EU)

1.E-02
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Radioactivity, Ci/tonneIHM

Decay Heat, W/tonneIHM

1.E+05

MOX fuel (Red line)
5%EU fuel (Blue line)

1.E+07
1.E+0630GWd/t

1.E+05
1.E+04
1.E+03
1.E+02
1.E+01

Total(MOX)
ACT(MOX)FP(MOX)
Total(EU)ACT(EU)FP(EU)

1.E+00
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Time after Discharge, yr

Time after Discharge, yr(a) Absolute of Decay heat of spent fuel(b) Absolute of Radioactivity of spent fuelFig.1 Characteristics of spent fuel at 30 GWd/t
Figure 2 shows each decay heat by deference of discharged burnup per unit amount of power
generation4. The representation of decay heat per unit amount of power generation is appropriate forthe viewpoint of the economy. On MOX fuel the curve per unit amount of power generation of thisfigure leads the advantage of higher burnup approach clarify.
1.E+07

10GWd/t

Decay Heat

1.E+0650GWd/t [W/GWe･yr]

1.E+05
1.E+04
1.E+03
1.E+0230GWd/t

1.E+01[W/tonneIHM]

1.E+00
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Time after Discharge, yr

Fig.2 Decay heat per gigawatt (electric) year versus time after discharge
3.2 Impact of uncertainty of prediction accuracy of isotopic generation
Figure 3 indicates an example5 of discrepancy of C/E ratio for Pu vector and MAs. These figuresmeans that Pu isotopes have a few percent disagreement and that miner actinide isotopes have tens ofpercent disagreement. Therefore, evaluation of decay heat containing their uncertainty is required. On
Pu-238, -239, -240, -241, Am-241, Cm-242, the ratio to the standard calculation result is shown about
decay heat when the combustion calculation result contains the uncertainty from 2 to 20 percent asfollows (Fig.4 (a) - (d)).
These figures show the variations of decay-heat behavior with respect to the intentional charge in thedischarge amount of each isotope, for example 239Pu in Fig.4 (a). The peak in Fig.4 (c), is not coming
directly from Pu-241 itself but through Am-241. Pu-241 decays into Am-241 with the half-life of 14.35years. It is Pu-241 that dominates the amount of Am-241 long after discharge. Am-241 is an
important nuclide for decay heat for an important period for storing spent fuels from ten-odd years tothousands of years. According to this figure, at 100 year after discharge, 7% error of the decay heat will
be introduced by 10% uncertainty of Pu-241 generation calculation through Am-241 production.

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2.0

Calculated/Experimental

DU-1
DM-1
DM-2
DM-3
DM-4

1.5

DU-1
DM-1
DM-2
DM-3
DM-4

Calculated/Experimental

2.0

1.5

1.0

1.0

0.5

0.5

0.0

0.0
Pu238

Pu239

Pu240

Pu241

Pu242

Np237

Am241

Am242m

Am243

Cm242

Cm244

Cm245

(a) Pu vector(b) MAs
Fig.3 C/E ratio for Pu-isotopes and minor actinides520%15%⊿DH/DH⊿DH/DH

15%

20%

2%
5%
10%15%20%

10%

2%
5%
10%
15%
20%

10%

5%

5%

0%
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+060%
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Time after Discharge, yr

Time after Discharge, yr(a) Pu-239(b) Pu-240

20%

15%

⊿DH/DH⊿DH/DH

15%

20%

2%
5%
10%
15%
20%

10%

2%
5%
10%
15%
20%

10%

5%

5%

0%
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+060%
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Time after Discharge, yr

Time after Discharge, yr(d) Am-241(c) Pu-241
Fig.4 Uncertainty of the FP and actinide decay heats introduced by uncertainty of the prediction accuracy of
isotopic generation of each nuclide
3.3 Impact of uncertainty of one-group cross section
On the spent fuel storage term (several 100 years from 10 years), Am-241 is dominant nuclide.
Importance of the amount of Pu-241 has already been described. In a reactor in operation, generation ofPu-241 is remarkable. Evaluation value6 of uncertainty of Pu-240 capture cross sections is shown in
Figure 5. In this figure, capture cross section of Pu-240 has 20% from 10% uncertainty in whole energy
range. Then the influence on decay heat when capture cross sections of Pu-240 and Pu-239 werechanged from 10 to 50 percent was examined.

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Fig.5 Evaluation value of uncertainty of Pu-240 capture cross sections620%

20%

15%

15%

10%

10%

5%

5%

⊿DH/DH⊿DH/DHFigure 6 shows the peak caused by Am-241 around 500 year cooling. Fig.6 (a) indicates that decay
heat increases by 3% when the capture cross section of Pu-239 is increased by 10%. Fig.6 (b) showsthat decay heat increases by 5% when the capture cross section of Pu-240 is increased by 20%. The dipof about 104 year results from reduction of Pu-239 and Pu-240, respectively.

0%-5%
-10%
-15%
-20%
1.E-01

Pu-239 cap 10%up
Pu-239 cap 20%up-10%

Pu-239 cap 30%up
Pu-239 cap 40%up-15%

Pu-239 cap 50%up

1.E+00

1.E+010%-5%

1.E+02

1.E+03

1.E+04

Time after Discharge, yr

1.E+05

1.E+06-20%
1.E-01

Pu-240 cap 10%up
Pu-240 cap 20%up
Pu-240 cap 30%up
Pu-240 cap 40%up
Pu-240 cap 50%up

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06Time after Discharge, yr(a) Pu-239(b) Pu-240
Fig.6 Uncertainty of the total decay heats introduced by uncertainty of the one-group capture cross section
4.

Summary and Future Plan
It is important to evaluate the characteristics of spent MOX fuel, especially decay heat, appropriately.
In this study, sensitivity analyses of prediction accuracy of isotopic generation and uncertainty of
one-group cross section were performed. Between 100 and 1,000 years after discharge of the spent
MOX fuel, the total decay heat is dominated by Am-241. We should pay attention to the generation of
Pu-241 which decays into Am-241. The amount of Pu-241 has a close relation to the capture crosssection of Pu-239 and -240. The uncertainty of the burnup calculation is discussed. At 100 year after
discharge, for example, 7% error of the decay heat will be introduced by 10% uncertainty of Pu-241
generation calculation through Am-241 production.
We will make sensitivity analyses of capture cross section of notable isotopes on each energy group.
Reference
1. K. Suyama, T. Kiyosumi and H. Mochizuki, JAERI-Data/Code 2000-027, JAERI (2000).
2. A.G.croff, ORNL/TM-7175, Oak Ridge National Laboratory (1980).
3. Y. Ando, H. Takano, JAERI-Research 99-004, JAERI (1999).
4. Zhiwen Xu, Mujid S. Kazimi, and Michael J. Driscoll, Nucl. sci. Eng., 151, 261(2005).
5. Y. Ando, K. Yoshioka, I. Mitsuhashi, K. Sakurada and S. Sakurai, Proc. of the 7-th International
Conference on Nuclear Criticality Safety, Tokai, Japan, Oct.20-24, 2003, pp494-499 (2003).
6. G. Aliberti, et. al., Proc. of the Int. Workshop on Nucl. Data Needs For Gen. IV Nucl. Energy Systems,Antwerp, Belgium, Apr.5-7, 2005 (2005).

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