the facility. One beam line for NRTA and three beam lines for NRCA/PGA are
placed as shown in Fig. 2.1. The sample size for NRTA is assumed to be 10–30 cm
in diameter and 1–2 cm in thickness. In comparison, the sample size for NRCA/
PGA is smaller; the diameter is 1–2 cm, and the thickness is 1–2 cm. A collimator is
placed between the NRCA/PGA sample and the γ-ray detector to reduce the
background γ-rays from the sample. Because optimal sample thickness for NRTA
strongly depends on the amount of impurities or matrix material, the quantity of the
interfering nuclei in debris has to be measured roughly by NRCA/PGA preceding
NRTA measurements [12].
The statistical uncertainties of NMs quantified by NRTA were estimated
[12]. The size of a MF sample is assumed to be 1 cm in thickness and 30 cm in
diameter. The weight of the sample becomes about 4 kg: it consisted of nuclear fuel
(64 vol.%),
nat Fe (8 vol.%),
nat B (8 vol.%), and 20 vol.% of vacancy. The composition of the nuclear fuel was taken from Ando and Takano [13] [a fuel of 40 GWd/t
burn-up in a boiling water reactor (BWR)]. The measurement was assumed to be
carried out for 40 min, in which 20 min was for sample and 20 min for background.
Table 2.2 shows the estimated statistical uncertainties of quantified Pu and U
isotopes in the sample. The achieved statistical uncertainties are less than 1 %.
With the measurement cycle given here, about 0.15 ton of debris can be handled
in a day; this enables us to measure 30 tons of debris in a year (200 working-days
are assumed). This amount can be increased with the number of NRTA beam lines.
2.3 Development of a γ-Ray Spectrometer for NRCA/PGA
The γ-ray background from debris is expected to be strong. The strongest radioactive isotope in a MF of the TMI-2 accident was
137 Cs, which ranged from 10
6 to
3 Â 10
8 Bq/g [14]. The energies of the prominent γ rays from nuclei listed in
Table 2.1 is larger than the 661 keV γ-rays from
137 Cs, except for
10 B. Accordingly,
most of the measurements of the NRCA/PGA will not have interference with the
γ-rays from
137 Cs. On the other hand, the Compton edge of the 661 keV γ-rays
surely overlaps with the 478-keV γ-ray peak originating the
10 B(n, αγ)
7 Li reaction.
Table 2.2 Estimated statistical uncertainty of quantities of U and Pu isotopes in a sample
Nucleus
Concentration in a fuel (kg/tHM)
Statistic error (%)
238
Pu
0.19
0.85
239
Pu
5.25
0.074
240
Pu
2.13
0.051
241
Pu
1.23
0.23
242
Pu
0.48
0.069
235
U
14.6
0.049
238
U
928
0.010
The measurements are assumed to be carried out for 40 min with a 10
12 n/s neutron source
16
M. Koizumi et al.
placed as shown in Fig. 2.1. The sample size for NRTA is assumed to be 10–30 cm
in diameter and 1–2 cm in thickness. In comparison, the sample size for NRCA/
PGA is smaller; the diameter is 1–2 cm, and the thickness is 1–2 cm. A collimator is
placed between the NRCA/PGA sample and the γ-ray detector to reduce the
background γ-rays from the sample. Because optimal sample thickness for NRTA
strongly depends on the amount of impurities or matrix material, the quantity of the
interfering nuclei in debris has to be measured roughly by NRCA/PGA preceding
NRTA measurements [12].
The statistical uncertainties of NMs quantified by NRTA were estimated
[12]. The size of a MF sample is assumed to be 1 cm in thickness and 30 cm in
diameter. The weight of the sample becomes about 4 kg: it consisted of nuclear fuel
(64 vol.%),
nat Fe (8 vol.%),
nat B (8 vol.%), and 20 vol.% of vacancy. The composition of the nuclear fuel was taken from Ando and Takano [13] [a fuel of 40 GWd/t
burn-up in a boiling water reactor (BWR)]. The measurement was assumed to be
carried out for 40 min, in which 20 min was for sample and 20 min for background.
Table 2.2 shows the estimated statistical uncertainties of quantified Pu and U
isotopes in the sample. The achieved statistical uncertainties are less than 1 %.
With the measurement cycle given here, about 0.15 ton of debris can be handled
in a day; this enables us to measure 30 tons of debris in a year (200 working-days
are assumed). This amount can be increased with the number of NRTA beam lines.
2.3 Development of a γ-Ray Spectrometer for NRCA/PGA
The γ-ray background from debris is expected to be strong. The strongest radioactive isotope in a MF of the TMI-2 accident was
137 Cs, which ranged from 10
6 to
3 Â 10
8 Bq/g [14]. The energies of the prominent γ rays from nuclei listed in
Table 2.1 is larger than the 661 keV γ-rays from
137 Cs, except for
10 B. Accordingly,
most of the measurements of the NRCA/PGA will not have interference with the
γ-rays from
137 Cs. On the other hand, the Compton edge of the 661 keV γ-rays
surely overlaps with the 478-keV γ-ray peak originating the
10 B(n, αγ)
7 Li reaction.
Table 2.2 Estimated statistical uncertainty of quantities of U and Pu isotopes in a sample
Nucleus
Concentration in a fuel (kg/tHM)
Statistic error (%)
238
Pu
0.19
0.85
239
Pu
5.25
0.074
240
Pu
2.13
0.051
241
Pu
1.23
0.23
242
Pu
0.48
0.069
235
U
14.6
0.049
238
U
928
0.010
The measurements are assumed to be carried out for 40 min with a 10
12 n/s neutron source
16
M. Koizumi et al.