In January of this year, TEPCO conducted an investigation in the inside of the Primary Containment Vessel (PCV), of Unit 2 at Fukushima Daiichi. They had been assuming that the water in the PCV was around OP9500, but were unable to locate the water level inside of the containment vessel and ultimately guessed that the top of the water must be just out of their ability to investigate.
In August, workers began preparatory work at Reactor 1 to investigate the inside of the primary containment vessel (PCV). This week, workers at Fukushima Daiichi began the four-day investigation at Unit 1 with an endoscope camera.
During investigation, photos will be taken by a camera, data collected on radiation dose, ambient temperature, water temperature and water level will be collected and sampling of accumulated water will be conducted. New PCV ambient temperature thermometers and water gauges will also be installed to improve monitoring.
The goal of the work is to determine the current conditions in the water, and the amount of contaminated water in the vessel, but after the first day of investigation, workers were unable to find the water level in the reactor, and some damage was found.
For Units 1 to 3, it is assumed that nuclear fuel and core internals have melted and resolidified (fuel debris) and have escaped in varying amounts from the reactor pressure vessel (RPV) and are thought to currently be located primary containment vessel (PCV). The nuclear fuel can now only be removed while under a pool of water to ensure the temperature and radiation levels are controlled.
As hydrogen concentration and Kr-85 radioactivity density have been changing since April, it is assumed that the hydrogen generated in the early stage of the accident and Kr-85 which remain in the upper part of the suppression chamber are intermittently released into the dry well.
First, the inability to locate the water obviously means that the water being pumped into the containment is escaping, either through a large hole in containment, or a series of smaller holes.
Secondly, if there is no accumulated water, than there is far less shielding and decay heat removal available than TEPCO has accounted for, which would also infer the melted fuel in the containment vessel may be exposed.
The radiation levels inside of the reactor are one of the key indicators used to determine the location and formation of melted nuclear fuel. If the nuclear fuel was exposed, the radiation fields produced by the melted nuclear fuel would’ve produced radiation fields so strong they would be detectable outside of the reactor building, and no radiation levels that high have been confirmed by the utility.
These new findings by TEPCO may suggest that the water is not adequately cooling any melted fuel which may be located in the containment vessel, or may also infer that the melted fuel has relocated to another location and the containment may be working as a shield to block the radiation emitted.
When looking at raw data provided by TEPCO, one can see that there has been no major flux in temps or radiation levels in a while, which may mean that this condition has existed for some time. The key question is how long has the majority of the coolant water pumped into the reactor been escaping and providing little if any shielding to the melted nuclear fuel.
In short, we have been playing the ‘where is the corium’ game since the first week of the disaster, and the recent finding (no water found in containment vessel) is increasing evidence, when verified by the temperature and radiation levels as given by TEPCO, that the fuel may not still be located where they thought it was.