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

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Surface Grinding of Gravel for Waste Reduction and Reuse－ Decontamination Test of Gravel by General-Purpose Machine －100Reduction rate (%)

Condition of surface asperity

8060

Fig.1-9 Relationship between grinding
rate and reduction rate
In this test, we confirmed a reduction
rate of up to 66% relative to the surface
dose rate. We will research the correlation
bet ween the ground amount andreduction rate to determine the reduction
effect if the ground amount increases.

60 min40 min20 min

4020: Grinding time
005101520

Grinding rate (%)
Imaging graphics(Black part region indicates radioactive cesium)
BeforeDecontamination

After
Decontamination

Cross-sectional photographFig.1-8 Decontamination test by milling method
We tested gravel decontamination using a generalpurpose barrel finishing machine with 20 kg ofgravel, 20 ℓ of water, 150 rounds per min, and
20–60 min of grinding time per batch.

0102030 0102030(unit: mm)

Fig.1-10 Distribution state of radioactive cesium before and after testing
Using an IP, we confirmed that Cs gathers mainly in the black mica parts anddecreases in the ground parts of the gravel after decontamination (grindingtime is 20 min).

In houses and public facilities, gravel (pebbles, etc.) withradioactive cesium (Cs) is laid on. To reduce the air dose rate,
the gravel is stripped and led to waste. Gravel with low-levelcontamination is reused after high-pressure water washing to
remove fine-grained soil attached to its surface. Considering
the high price of gravel compared to that of crushed stone
and the need for waste reduction, an effective method of
decontaminating gravel with high-level contamination isexpected to be selected.
Surface grinding of gravel has the potential for producing astrong decontamination effect. Therefore, we tried to find an
effective procedure for decontaminating gravel by rubbing it
against itself using a milling method in which the grinding timeand amount of water added are varied.
We observed the condition of gravel during and after thetest and confirmed that the surface of the gravel was groundproperly (Fig.1-8). However, in this test, there were no groundparts on the concave regions of the gravel because the milling
was done with only gravel and water, without grinding material.
The test results were as follows. When the grinding timewas 20 min, the mean grinding rate was about 4%, and themean reduction rate of the surface dose rate was about 37%.

Furthermore, when the grinding time was 60 min, the meangrinding rate was about 10%, and the mean reduction rate of
the surface dose rate of the gravel was about 60% (up to 66%).
Both of these rates were low for these grinding times (Fig.1-9).
To confirm the cause of the low reduction rate, we observed thedistribution of Cs in the gravel before and after decontamination
using an imaging plate (IP). By comparing the observation ofa cross-section of gravel (granite) that was sliced in half andimaging graphics obtained with the IP (Fig.1-10), we confirmedthat the Cs was mostly attached to the black mica around thesurface of the gravel and penetrated slightly below the surface.
In this test, we used gravel that had been laid on heavily in
the garden and rooftop of Rifure-Tomioka. It took considerable
time to grind the gravel because it was slightly harder than
the gravel that was bought for a mock test. Furthermore, thereduction rate became lower than expected because the gravelcontained mica, which tended to absorb Cs. Various types ofgravel are laid on in houses. The color, shape, size, and hardnessof the gravel vary, so we must select reasonable conditions fordecontamination work. On the basis of the result of this test,
we will research decontamination methods and conditions forreusing gravel and reducing waste.

Reference
Japan Atomic Energy Agency, Decontamination Works in the Evacuation Zones concerning the Fukushima Daiichi Nuclear Power Plant Disaster【Part ofthe Decontamination Pilot Projects】Report, 2012, p.107 (in Japanese).
JAEA R&D Review 201315