Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-16 Nuclear Material Accountancy for Molten Core Material byγ-ray Measurement－ Development of Nuclear Material Measurement Technology using Passive γ Spectrometry of Molten Core Material －Debris removal from reactor
4.0E‐03

Debris stored in canister

Segmented gamma scanning
Quantifying low-volatile FP mass154

(with correction of photon-shielding)

Eu/Pu weight ratio

Mass, average density for shielding correction

Burnup estimation

Approx. curve

3.0E‐03
2.5E‐03

Core ave. 1.93E-03

2.0E‐031.5E‐03

3 cycle1 cycle1.0E‐03

0.0E+00

cycle ave.
4.6%Eu0１0(kgU, kgPu, kgUfis, kgPufis)

Fig.1-35 Data flow of γ spectrometryfor U, Pu quantification
U and Pu are quantified by coupling
passive γ spectrometry for radioactivity
quantification of FP with FP/U, Pu
correlation by inventory calculation, andphoton transport calculation.

Total leakage/source photon

1Completion of fuel quantification

２0３0

４0５0

Burnup (GWd/t)

144

Quantifying fuel by low-volatile FP/fuel ratio

4 cycle2 cycle3.0%Eu5.0E‐04

Eu/fuel, Ce/fuelfrom inventory calculations
154

Fig.1-36 Correlation between 154Eu
and Pu quantity (1F2)
154
Eu: Pu ratio increases monotonicallywith respect to burnup.

Eu/Pu(1F2)

154

3.5E‐03¹⁰⁶Rh（2.71MeV）¹⁴⁴Ce/¹⁴⁴P（r2.19 MeV）¹⁵⁴Eu（1.27 MeV）¹³⁷C（¹⁵⁴Eu（1.59 MeV）
s 662
keV）

0.1

¹⁵⁴Eu
（248 keV

0.01

0.001）¹⁵⁴Eu（123 keV²⁴¹Am（）59.5 keV）¹⁵⁵Eu（45.3 keV

0Even after the accident at the TEPCO’s Fukushima Daiichi
NPS (1F), it is required to provide assurance to the nationaland international communities that uranium (U) and plutonium(Pu) in the molten fuels have not been diverted to militaryuse. We have been conducting a feasibility study of U andPu quantification measurement by passive γ spectrometry ofmolten core material (fuel debris) from outside of a canister
after the removal of fuel debris.
Although U and Pu themselves emit γ-rays, it is very
difficult to measure them directly because their energies aremuch lower than the energies of fission products (FPs). We
focused on the high-energy γ-emitting and low-volatile FPs
that coexist with U and Pu in the fuel melting process during
a severe accident. Cerium (Ce) and europium (Eu) are wellknown as low-volatile FPs, and 144Ce and 154Eu emit highenergy (>1 MeV) γ-rays. Experience in quantifying U and Pu
by measuring γ-rays from 144Ce in fuel debris was gained in
the TMI-2 accident.
Fig.1-35 shows the assumed data flow of passive γ
spectrometry of FPs for U and Pu quantification. Because thisis an indirect methodology for quantifying U and Pu using
FP/U and the Pu ratio by an inventory calculation at the timeof the 1F accident, the sensitivity to various parameters that
affect the quantities of FPs, U, and Pu was analyzed. Fig.1-36,510）

15

Fig.1-37 Sensitivity of leakage
photon spectrum to debris
sizeHigh-energy γ-rays leak moreeasily than low-energy γ-rays.
Self-shielding correction isessential to obtain the source
photon intensity because ofstrong attenuation in large
pieces of fuel debris.

20Debris diameter (cm)as an example, shows a good correlation between the ratioof 154Eu to Pu and burnup, regardless of the neutron flux andwater void fraction, because the variance is offset.
In γ-ray measurement of fuel debris, it is necessary toanalyze the sensitivities to parameters such as the selfattenuation in the fuel debris and the geometry of the fueldebris and canister. A fundamental analysis was performed
using a simple spherical model with a variety of parameters,such as the debris size, composition, porosity, density,homogeneity, or heterogeneity. It is assumed that low-energy
γ-rays (<500 keV) are sensitive to the size and composition of
the debris, and high-energy γ-rays (>1 MeV) are less sensitive.
Fig.1-37 shows the sensitivity of the energy-dependent leakage
photon ratio to the debris size. For 154Eu, strong attenuation
was observed, up to about 20% and 50% with fuel debris
2.5 cm and 10 cm in diameter, respectively. Correction of selfshielding is essential. The use of the correlation between thedifference in the photopeak flux and the energy of 154Eu has
been analyzed as an option.
We will continue the feasibility study of this methodology
by numerical analysis with the canister model and conceptual
system design, and improve the precision of nuclear material
quantification.

Reference
Sagara, H., Tomikawa, H. et al., Feasibility Study of Passive Gamma Spectrometry of Molten Core Material from Fukushima Dai-Ichi Nuclear Power
Station Unit 1, 2 and 3 Cores for Special Nuclear Material Accountancy －Low-Volatile FP & Special Nuclear Material Inventory Analysis and Fundamental
Characteristics of Gamma-ray from Fuel Debris －, Journal of Nuclear Science and Technology, vol.51, issue 1, doi: 10.1080/00223131.2014.852994.
JAEA R&D Review 201327