Abstract:
Safety concerns at the Hanford Reservation require a way to measure the
moisture content of material contained in the high level radioactive waste tanks. The
freezing of material next to a probe was proposed by scientists at Pacific Northwest
Laboratories (PNL) as one approach to solve this problem. It was theorized that due
to water's high latent heat of fusion, the local temperature profile of the waste during
the freezing period should vary significantly with moisture content.
Experiments were conducted at PNL to test the Phase Change Calibration
Probe's ability to measure moisture in a simulating material. The initial results looked
promising in the probe's ability to distinguish between various moisture contents.
Computer modeling was performed at Oregon State University (OSU) to validate the
results of the PNL experiments. After several models were tested it was determined
that the computer results could not match the experimental results. Several questions
were raised from the modeling about the true cause of the PNL results.
To explore the questions raised by the modeling, controlled experiments were
also undertaken at OSU with various mixtures of water and waste simulant. Using a
facility designed to handle the requirements of the testing program, data was collected
and analyzed for several waste mixtures, ranging from pure simulant to pure water
samples.
The results of the experiments showed that the analysis technique used by PNL
can be confounded by changes in the coolant temperature between tests. We also
showed that the overall trend of the tests appeared to be consistent with the
hypothesis, however the resultant effect from freezing was not large enough to easily
discriminate between certain moisture levels. Of significant concern was the observed
moisture threshold level, below which no notable thermal effect was measured. Since
the region of greatest interest and concern lies below this point, it was concluded that
the probe would probably not perform in the region of greatest interest for the waste
tanks.
