Seminar: In situ activity determination in decommissioned nuclear facilities

If concrete structures in nuclear power plants are activated or contamination are
introduced, is a crucial question in nuclear dismantling. For answering this question,
conventional drill cores are taken and examined afterwards in a laboratory analysis.
A fiber optic radiation sensor (FORS), which is based on a GAGG scintillation
crystal coupled to a polymer optical fiber and a photomultiplier tube, was developed
in this thesis as a more efficient alternative sampling method. It is designed for
measurements in thin on-site boreholes in nuclear power plants. A specific activity
of 0.1 Bq g−1 of the nuclide 137Cs was statistically significantly detected in laboratory
experiments in a concrete phantom within a measurement time of 4 min. Therefore,
analyses were applied based on the “Guide to the Expression of Uncertainty in
Measurement” (GUM) and the Kolmogorov-Smirnov test.
A second method of examination enables not only the detection of a contamination
but also the quantitative determination of its specific activity. This was possible to a
lower limit of 0.1 Bq g−1 for the nuclide 137Cs. For this reason, an analysis parameter
was determined based on the two-dimensional Kolmogorov-Smirnov test, which is
proportional to the specific activity. Therefore, the measurment to be investigated is
compared to combinations of measurements with different specific activities.
In two field tests in the nuclear power plant Unterweser, which is in the process
of dismantling, the activity of localized contaminations and acitived structures was
determined. The activity per area of an activated metal shield was estimated in
the walls of the reactor and spent fuel pool. In the investigation of the calotte, the
determination of a specific activity of 60Co was possible. The value of it exceeded
0.1 Bq g−1 only in a very localized area. If the spatial expansion is estimated correctly,
the results of the FORS analyses are in good accordance with drill core analyses at the
same sample places.