atw 2017-06

atw Vol. 62 (2017) | Issue 6 ı June
DECOMMISSIONING AND WASTE MANAGEMENT 404
| | Fig. 2.
Percentage differences between measured and computed nuclide compositions for Obrigheim NPP (measured at Ispra).
Four samples were analyzed in both
laboratories for cross-checking.
The proposed method was applied
to a total of 728 nuclide composition
or nuclide ratio data for 17 samples
measured at Ispra to identify those
data that deviate significantly from
other data and therefore became
the candidates for detailed analysis.
Various detailed analysis appropriate
to identify the root causes of the
significant deviation from other data
were applied and described below.
3.1 Burnup for GEROBRPWR-9
Figure 2 shows the percentage differences
between the calculated and
measured nuclide composition data for
the samples from Obrigheim NPP measured
at the Ispra laboratory. Relatively
high percentage differences between
the calculated and measured data were
observed for one sample, which
was found to be the GEROBRPWR-9
sample. According to Barbero et al.
[24], the burnup for the GEROBRPWR-9
sample measured at Karlsruhe using
the Nd-148 method (22,700 MWd/
MTU) was abnormally high compared
to the burnup measured at Ispra using
the Nd-148, non-destructive Cs-137,
and destructive Cs-137 methods
(17,130, 16,970, and 17,490 MWd/
MTU, respectively). Because the
burnup in the OECD/NEA SFCOMPO
database was based on the measurement
at Karlsruhe, the code calculation
was performed again using the burnup
measured at Ispra with the Nd-148
method (17,130 MWd/MTU). Figure 3
shows the percentage differences
after the burnup correction for the
GEROBRPWR-9 sample. It can be
seen that the deviations of the
GEROBRPWR-9 sample from other
samples were properly corrected.
Figure 3 also identifies those
nuclide composition or nuclide ratio
data that need to be analyzed in more
detail. The Pu-241/Pu-239 ratio
shows relatively high percentage
differences. The percentage differences
of Cs-137/U-238 ratio are
divided into two groups. A large
uncertainty and a large deviation in
percentage differences are observed
for Am-241 and Am-242, respectively.
Detailed analysis on each of the
identified nuclide composition or
nuclide ratio data are described in the
following sections.
3.2 Cooling time of plutonium
isotopes
As indicated in Fig. 3, the Pu-241/
Pu-239 ratio was found to have
higher percentage differences among
samples than other nuclide ratios.
Barbero et al. [24], the original source
of the measurement data, reported
the nuclide ratios of uranium and
plutonium with the dates of the
measurements. The actual cooling time
of the samples can be calculated from
the date of discharge (August 16, 1974)
and the date of measurement (e.g.,
April 12, 1978 for GEROBRPWR-3).
However, the cooling times of the
data were specified as zero in the
OECD/NEA SFCOMPO database,
which means that the data were
adjusted to the time of discharge. The
code calculations were performed
assuming that the cooling times of
the samples were zero. Because the
half-life of Pu-241 (14.325 years) is
comparable with the cooling time of
the samples, a non-negligible amount
of Pu-241 decayed out; therefore, the
calculated Pu-241/Pu-239 ratios were
higher than the measured ones.
| | Fig. 3.
Percentage differences between measured and computed nuclide compositions for Obrigheim NPP (measured at Ispra) after burnup
correction for GEROBRPWR-9 sample.
3.3 Possible errors during
Cs-137/U-238 ratio calculation
from measured data
As indicated in Fig. 3, the Cs-137/
U-238 ratios were found to fall into
two groups. One group consists of 5
samples with very small percentage
differences, and the other group consists
of 12 samples with percentage
differences of up to −35 %. The existence
of two distinct groups motivated
a detailed analysis considering possible
systematic errors in the data.
Figure 4 shows the amounts of
Cs-137 buildup and the remaining
U-238 in the spent nuclear fuel per
metric ton of final uranium. As a
burnup monitor, the amount of Cs-137
buildup shows a linear relationship
with the burnup. Although the
amount of U-238 remaining shows a
Decommissioning and Waste Management
Validation of Spent Nuclear Fuel Nuclide Composition Data Using Percentage Differences and Detailed Analysis ı Man Cheol Kim