Post-accident Cleanup Process at TEPCO Fukushima Daiichi Nuclear Power Station (April to June 2013) Nuke Info Tokyo No. 156
Situation at the Plant
Figure 1; Releases of radioactivity from Units 1 to 3 of Fukushima Daiichi Nuclear Power Station
From materials prepared and submitted by the secretariat to the Government and TEPCO’s Mid-to-Long Term Countermeasure Meeting and Secretariat of the Council for the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station. (However, this was for assessing leakage from exhaust outlets and gaps in building covers and does not include leakage to groundwater, etc.)
Many of the instruments in the measuring systems at the Fukushima Daiichi Nuclear Power Station (FDNPS) suffered breakdowns due to the accident. There is doubt about the accuracy of the values being currently measured, but taking these values as a premise it is possible to estimate from the containment temperature and releases of Xenon-135 gas that the reactors are stable. Even now, however, radioactive materials are continually being released and the situation does not allow for much optimism (Figure 1).
Post-accident Cleanup Process
1. Molten Nuclear Fuel
Decommissioning work is progressing on the basis of the “Mid-and-Long-Term Roadmap towards the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station Units 1-4” drawn up by the government and TEPCO on December 21, 2011 (amended on July 30, 2012 and June 27, 2013. Hereafter known as the “Roadmap”).
The Roadmap indicates that the plan for removing the molten nuclear fuel will be implemented in approximately the following nine stages.
1. Decontamination of reactor buildings,
2. Inspection of the lower part of the containment,
3. Waterproofing of the lower part of the containment and the reactor building,
4. Partial filling of the containment with water,
5. Inspection and sampling of the interior of the containment,
6. Repair of the upper part of the containment,
7. Filling of the containment and reactor pressure vessel with water,
8. Opening of the upper lid of the reactor pressure vessel and inspection of the interior of the reactor,
9. Removal of the molten nuclear fuels.
At present, inspections are being implemented for all units to confirm the status of each of them in the moves toward the decontamination of the buildings and to determine the locations of leaks in the containments.
2. Spent Nuclear Fuel
The Roadmap states that the spent nuclear fuel removal plan will be implemented approximately according to the following stages.
1. Removal of rubble and wreckage,
2. Construction of covers and cranes,
3. Design and manufacture of transport containers and canisters,
4. Securing of capacity in the common pool and remodelling of equipment,
5. Removal, storage and management, and transport away from site of the spent nuclear fuel in the cooling pools.
By December 2012, Stage 1 had been completed at Unit 4. At present the construction for Stage 2 is in progress and Stage 5 is scheduled to begin in November 2013. Stage 1 is progressing at Unit 3, and the measures for reducing radiation are due to be implemented on the operating floor, the fifth floor of the reactor building, before long. In order to carry out the work for Stage 1 in Unit 1 in mid-2013, initiation of work to demolish the current reactor building cover has been scheduled, but there is concern over increased releases of radioactive substances after the demolition. For Unit 2, high radiation has made it impossible to confirm the situation in the interior of the building.
Several problems exist in the eventual removal of the spent nuclear fuel. For example, concerns over the possibility of corrosion by seawater and damage due to wreckage, the possibility of accidents due to falling fuel during removal, and damaged fuel that has been in the spent fuel pool for some time, even since before the accident.
3. The Problem of Contaminated Water
|Figure 2; Diagram of current water regime at Fukushima Daiichi Nuclear Power Station
TEPCO Fukushima Daiichi Nuclear Power Station, June 27, 2013, Mid-and-Long-Term Roadmap towards the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station Units 1-4, figure on p.40 partially amended
At FDNPS, in order to prevent the plant buildings rising due to seepage of groundwater and rising of the water table, roughly 850 m3/day of water was being pumped out of the subdrains (pump wells) of Units 1 to 4 and allowed to flow into the ocean.
Following the accident, the subdrains became unusable and the water table has risen. According to an estimate by TEPCO, approximately 800 m3/day are flowing into the buildings. All 400 m3 of groundwater flowing into the reactor buildings, if TEPCO estimates are correct, is being treated as contaminated water (see Figure 2). Part of the remaining 400 m3 is coming into contact with the highly radioactive contaminated water that has accumulated in the many trenches (underground conduits for pipes and electrical cables) onsite and is currently flowing out into the ocean. There is also the possibility that this highly radioactive contaminated water is leaking into the surrounding soil through the bottom of the trenches.
The Roadmap shows the following as countermeasures for groundwater problems.
1. On the upper inland side of the plant, a “groundwater bypass” has been constructed to pump up groundwater and release it into the sea in order to lower the water table and reduce the amount of water flowing into the reactor buildings. (The construction is complete, but the consent of local people has not been obtained and the bypass is therefore not in operation.)
2. Restoration of the subdrains for management of the water table (scheduled to be operational in mid-2014).
3. Construction of an inland frozen earth water barrier (an ice barrier created by burying refrigeration pipes at set intervals around Units 1 to 4 – scheduled to begin operation in the first half of FY2015).
4. Preventing the draining of water from the buildings of Units 1 to 4 through the (roughly 880) holes, etc. in the outer walls of the buildings.
5. Reduction of soluble content of contaminated water using the Advanced Liquid Processing System (ALPS) to separate other nuclides from the “treated water” still containing Tritium* (scheduled to be fully operational in mid-2013),
6. There are plans to increase the number of tanks and so on, and an ocean-side water barrier is scheduled to be constructed by September 2014 as a countermeasure against groundwater outflow into the ocean.
The Roadmap also states that holding capacity for contaminated water will be approximately 800,000 m3 in January 2021, following which a state of equilibrium will be reached. The premises for this, however, are that water from the ground water bypass and water pumped out from the subdrains is released into the ocean and that all the other measures are successful. In addition, TEPCO also considers that release of the Tritium-containing water arising from 5. into the ocean is one possible option.
Please also see the article on p.1 for more information on the many contaminated water leakage problems that are occurring in the meantime.
(Hajime MATSUKUBO, CNIC)
* Releases weak beta-particles (max. 18.6, average 5.7keV) and no gamma radiation. Often said to have little impact on the human body, but caution is necessary. If Tritium-containing water taken into the human body replaces hydrogen atoms that make up DNA, there is a risk of beta-particles damaging surrounding genes. Genetic disorders, neonatal deaths and infant leukemia have been reported in Canada.