TEPCO recently inspected the unit 1 torus room for a second time as we reported here. Workers involved in the inspection work made the comment that the torus tube and the proximity to the probe may play a role in the new radiation readings. The levels being found within the torus indicate that work in this region of the building could prove quite difficult and may require specialized robotic equipment.
February 2013 Unit 1 torus inspection, probe (blue line) compared with torus and water proximity.
We have reviewed multiple handouts issued by TEPCO concerning the inspection of the torus areas at 2 locations along the west side of the reactor building. The first being in June 2012 and the second in Februrary 2013. Each was conducted at different locations about 16900mm apart and each with varying patterns of radiation levels at those torus area locations.
Our impression is that at both locations the radiation levels are extremely high for the conditions there. At the first location, the radiation levels grew higher underwater toward the bottom of the torus room.
The second data set (Feb 2013 inspection) showed radiation levels that rose then lowered as approaching the water surface to the bottom. We have performed radiation dose calculations on the second set of data to investigate the possibility of a nearby radiation source point.
All attempts to determine a radiation source level, based on distance and the shielding properties of the known amounts of water & steel in the torus resulted in extremely high estimated source readings. Our rough estimate for the source is about 100 million R/hr, about 1,000,000 sieverts. This estimate was concurred by an outside lab.
We think some volume of corium could be located within the torus down comer pipe at the pipe cap area, having come from containment.There may also be melted fuel debris trapped in the sparger (the smaller tubes inside the torus) area. The corium or fuel debris may have located to the area within the torus at the second inspection location. The water in the torus is very turbid and very likely contains high levels of Cs137.
An alternative possibility is that the water within the torus tube has a high level of dissolved cesium. This could be tested for by moving a probe at a set distance from the torus tube then tracing around the circumference of the tube at that distance. If the readings stayed constant it would be more likely to be a water based source rather than a fuel debris collection. Additionally repeating the same steps at another nearby location of the torus tube that does not line up with a downcomer pipe could help confirm or eliminate the potential for a dissolved water source. TEPCO claims the torus tube and downcomers to be full of water. The water level found in the bottom of containment supports this idea. Yet there is obviously some form of leak from the lower containment or the torus structure. This allows enough water out that containment can not be filled with water but the cooling water injection and leak points keep the lower structures from draining down further.
The previous inspection in June found extremely high levels of radiation below the water closer to the torus room floor. These readings were an ever increasing rate as the probe went down closest to the torus tube as with the more recent inspection. Then the rates drop in the water until the probe approaches the torus room floor where the rate jumps considerably. Debris at the June inspection location and the differing localized damage at this location hint that there could have been a different event here and different possible fuel debris deposition.
These findings indicate that the torus and torus room locations will present a major challenge to TEPCO for both further inspection and for any decommissioning work. These problems will have to be resolved through creative means or new technologies.
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