The Severe Contaminated Water Situation at Fukushima Daiichi Nuclear Power Station Nuke Info Tokyo No. 156
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Storage tanks at Fukushima Daiichi, Photo by TEPCO
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Fukushima Daiichi Nuclear Power Station is currently facing severe problems with the leakage of water contaminated with radioactive substances. The reality on the site is that there is no quick-fix countermeasure for this contaminated water leakage and the situation appears to be deteriorating into confusion. This article is an attempt to take a straight look at the problems and uncover the fundamental issues involved.
The contaminated water leakage problem
On April 2, 2011, around 520 m3 of water containing radioactive substances leaked into the sea from an electrical cable pit in the area of the Unit 2 water intake. A coagulant was poured into the pit to stem the flow of water. This contaminated water had leaked from the reactor building into the turbine building. In a public announcement, TEPCO gave only the values of Iodine and Cesium, although the water would have naturally contained Strontium and various other nuclides. On May 11, 2011, 250 m3 of water also leaked from a pit in the area of the water intake of Unit 3, the pit being closed off to prevent further leakage.
At present, a coolant recycling system has been constructed and a large portion of the radioactive substances in the water used to cool the reactors is being removed before returning the cooling water to the reactors (there are problems with this, as will be noted below), but there has been a series of leaks of contaminated water from the coolant recycling system; leakages from pipes and from the joints within the contaminated water treatment equipment. To date, more than 60 of these incidents have occurred.
In April 2013, contaminated water was found to be leaking from underground water pools used to store the contaminated water (see NIT 154). TEPCO abandoned the use of the seven underground pools and decided to store the water in above-ground storage tanks. Because of this problem, it became impossible to gain the agreement of local fishing people to the “groundwater bypass,” which would have allowed groundwater to be pumped up before being contaminated and then released into the ocean. This failure to reach agreement left TEPCO with no option but to stop the oparation of groundwater bypass.
In later negotiations, just as agreement was being reached on the groundwater bypass, 300m3 of contaminated water was discovered to have leaked from one of the storage tanks (August 19) causing the negotiations to be aborted. 1,000 mSv/h of total beta and gamma radiation was measured at 50 cm above ground in the vicinity of this tank. Since TEPCO had vowed not to operate the bypass without the agreement of the fishing people, for the time being the only countermeasure remaining for the situation was to increase the number of storage tanks.
Further, on August 31, 1,800 mSv/h of total beta and gamma radiation was measured at 5 cm above ground among a different group of storage tanks, and leakage of contaminated water was discovered. The gamma radiation level was 1 mSv/h. This indicates the presence of nuclides such as Strontium, which emit only beta radiation.
As we can see from the above, the contaminated water leakage troubles can be classified into three types,
1. Leakage from the turbine buildings,
2. Leakage from the underground storage pools,
3. Leakage from the above-ground storage tanks.
Of these, the outflows of contaminated water from the turbine buildings and the contaminated water leaks from the above-ground storage tanks will be dealt with in detail below.
Outflow of contaminated water from the turbine buildings to the ocean
At the Fukushima nuclear power plant site, a large number of observation holes have been dug to check the contaminated water leakage situation, and the water pumped up is being tested for levels of radioactive substances. On June 19, TEPCO announced that high levels of Tritium and Strontium had been detected in the observation holes between Units 1 and 2. 460,000 to 500,000 Bq/l of Tritium had been detected in samples taken on May 24, 31 and June 7. 1,000 Bq/l of Strontium-90 were also detected in the sample taken on May 24, and in the sample collected on July 8, 630,000 Bq/l of Tritium was detected, the highest level up to that time. Levels higher than those detected in the observation holes between Units 1 and 2 were indicated in the observation holes on the ocean side of the turbine buildings. This appears to be related to contaminated water outflows to the ocean from fissures in the electrical cable pit that was blocked in April 2011. At this point, TEPCO closed off the discharge end (on the ocean side) of the pit. TEPCO also planned to close off the inlet (at the turbine building end), but the radiation level appears to have been too high to allow work to begin. The fact that high levels of radiation were detected in May suggests the possibility that deterioration has caused new fissures to appear over time and that the radioactive water has leaked out and contaminated the groundwater.
TEPCO has attempted to stem the outflow of contaminated water by chemical stabilization of the ground directly between Units 1 and 2 and the ocean, but this resulted in the groundwater level rising until it flooded above the level of the surface of the stabilized ground and simply drained into the ocean. TEPCO has thus taken the measure of pumping up the groundwater and returning it to the turbine building, and it appears that there is little option but to continue to do this for the time being. Looking at the developments thus far of the groundwater influx into the buildings and the outflow through the pits, it is hard to deny the unpleasant possibility that contaminated water is leaking out into groundwater through fissures in the buildings.
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Current state of the leakage of comtaminated water and contermeasures at Fukushima Daiichi
(Prepared by CNIC based on a report by the Ministry of Economy, Trade and Industry. Image ©2013 DigitalGlobe, ©2013 ZENRIN) |
Storage tank leaks
The tank that leaked was tank I-No.5 in the H4 tank group area. The tank has no level gage and it was only possible to calculate the leaked amount of 300 m3 by measuring the level of the remaining contaminated water from the top of the tank.
The tank is a cylindrical shape, 11 m in height and 12 m in diameter, capable of storing 1,000 m3 of water. These are flange type tanks bolted together in sections using a silicon gasket in the joints. The tank material is normal general construction SS400 rolled steel, not quench hardened, that is often used in bridges, ships, vehicles, and so on. The thickness of the steel is 6 mm for the lid, 12 mm at the side and 16 mm for the base. It is likely that this type of tank was chosen for speed of construction and cost. Differing press reports have stated that the tanks have a serviceable life of three to five years or perhaps only about two years, and that there were concerns about leakage from the joints from the outset.
According to TEPCO’s announcement concerning the analysis of the nuclides contained in the leaked water, Cesium-134 was 46,000 Bq/l, Cesium-137 was 100,000 Bq/l, Cobalt-60 was 1,200 Bq/l. Manganese-54 was 1,900 Bq/l, Antimony-125 was 71,000 Bq/l, and the total beta radiation was 80,000,000 Bq/l. The chlorine concentration was 5,200 ppm. No measured data has been released for Tritium content. Tritium cannot be removed by the multi-nuclide removal equipment (ALPS) and has become a problem since it is included in the water that has been targeted for ocean release.
TEPCO’s countermeasure has been to pump the contaminated water out of tank I-No.5 and transfer it to tank B-No.10 in the same area.
The tanks are surrounded by a 30 cm barrier and each tank group is fitted with drain valves to release rainwater through a rainwater collection box. Since having accumulated rainwater in the collection box would make it impossible to tell if leakage were occurring from the tanks, the drain valves are said to have been left open at all times. Perhaps because a large leak was not foreseen, the position of the valves was set as a measure for dealing with rainwater, not to prevent leakages. There is evidence that contaminated water flowed out through the drain valves, the surface of the valves themselves showing high radioactivity at 20 to 30 mSv/h. There is a rainwater outlet nearby and it is surmised that some of the contaminated water flowed into this channel and out into the ocean, but the amount is unknown. Part of the contaminated water has led to the pollution of groundwater.
TEPCO has opened observation holes in the area surrounding the leaky tank to test for possible pollution of groundwater. Groundwater sampled from an observation hole in the vicinity of the leaky tank on September 8 showed Cesium-134 at 2.5 Bq/l, Cesium-137 at 5.1 Bq/l, total beta radiation at 3,200 Bq/l, and Tritium at 4,200 Bq/l, indicating that groundwater has indeed been tainted with radioactivity.
TEPCO thus closed all the drain valves. The rainwater outlets have all been converted to underground drains and the barrier height has been increased. The top 20 to 40 cm of the polluted soil has also been removed. Tank patrols have been boosted by increasing the total number of personnel involved from 10 to 60 and increasing the number of patrols to six per day. The strengthened patrol regime may lead to the early discovery of problems, but it will also result in higher exposures for the personnel. The introduction of automatic monitoring of the dosage rate, the installation of surveillance cameras, and so on should be expedited.
It is also reported that the tank that leaked was one that had been disassembled and transferred to the H4 tank group area from H1 tank group area. The reason for the transfer was because the tank had tilted due to land subsidence and had therefore been disassembled and transferred to the current location. Two further tanks in the H4 area have also been relocated from other area. Leakages have not thus far been confirmed from the other two tanks, but TEPCO is taking the step of transferring the contaminated water to other tanks.
TEPCO had completed visual inspections of all 305 of the same type of tank by August 22 and had implemented measurements for radioactivity. The total number of storage tanks are approx. 930, containing welded tanks (as of August 23). Two tanks in the H3 area had shown high levels of radioactivity, but TEPCO reported, on August 23, that there had been no change in the water level inside the tanks. On August 31, a maximum of 1,800 mSv/h was measured from these two tanks. TEPCO therefore transferred the contaminated water from these two tanks to other tanks. At present, no obvious leakage has been observed, but cases of high dose rate measurements have increased. In the first place, due to their structural characteristics, it would be no surprise to find leakages from any of the flange type tanks, and further leakage problems are probably just a matter of time.
Two-and-a-half years have passed since the accident. If the installation of large and robust tanks had been planned at the outset, it is possible that this current state of affairs could have been avoided. Why was this not done? It is clear that this culture of leaving responses till the last minute is a problematic feature of the organizational system involved in the post-accident cleanup.
The fundamental groundwater problem at Fukushima Daiichi
The Fukushima Daiichi nuclear power plant has been dogged by groundwater problems from the beginning, long before the accident occurred. To suppress the buoyancy acting on the reactor buildings due to groundwater flowing toward the ocean from the inland side of the site, 23 drain holes had been dug around the reactor buildings and 850 m3 of water pumped out and released into the ocean each day. These drain holes became unusable after the earthquake and later explosions in the plant. It has proven impossible to restore them due to high radiation levels and they are still unusable today. According to an evaluation by TEPCO in August, 800 m3 of groundwater per day is flowing towards the ocean from the inland side of the buildings, of which around 400 m3 is flowing into the buildings and the remaining 400 m3 is draining into the ocean.
400 m3 of circulating coolant water is being pumped into the reactor buildings each day, and it is the excess of 400 m3 of groundwater flowing into the buildings that is being stored in tanks. The fact that the water influx and the circulated water have the same volume is to prevent contaminated water flowing out of the buildings through the influx routes (and so strictly the volume is not exactly the same; the circulating coolant water is of a slightly smaller volume). If the groundwater flowing into the buildings were reduced, the circulating coolant water would also have to be reduced. If not, the contaminated water would likely flow out of the buildings (and it has not been possible to identify where the influx routes are).
TEPCO now plans to install a water barrier around the buildings. Originally, this was a plan to construct a single linear water barrier on the ocean side of the buildings. This construction work began in April 2012 and is still continuing today. It is scheduled for completion in mid-2014. It has now been decided that the water barrier will adopt a frozen earth method. According to the materials submitted by the proposing construction company, Kajima Corporation, an example is that freezing tubes would be inserted into the ground at one-meter intervals, freezing the earth by circulating coolant at -40℃. Fourteen 400 kW refrigeration machines would be required to maintain the earth in a frozen state. The length of the barrier surrounding the buildings will be 1400 m and the construction will take between 18 months and two years to complete. Although Kajima Corporation has experience in the construction of tunnels, there is no precedent for maintaining earth in a frozen state over a long period and for long distances. The conceptual design is now being prepared and a feasibility study carried out. Construction will begin next year and is expected to cost 150 billion yen with running costs said to be in the region of several billion yen per year.
The frozen earth water barrier will carry a massive price tag for construction, running costs and maintenance, but it is as yet unclear for how long it is intended to be used. Will it be used until it is possible to air-cool the molten fuel debris, or until the removal of the molten debris is completed?
TEPCO claims that over the next ten years, the amount of contaminated water accumulating will be held down to around 800,000 m3 by the groundwater suppression measures (twice as much if the measures are not taken). However, the premises for this 800,000 m3 figure are that the groundwater bypass, the frozen earth barrier and so on are all implemented within the next few years and experience trouble-free operation thereafter. Since TEPCO holds to the policy of diluting the Tritium-contaminated water remaining after treatment in the multi-nuclide removal equipment to below the legally-permitted government standard and then releasing it into the ocean, it appears that the company believes the stored volume of water will reach a peak at some point, after which it will decrease.
Despite the adoption of unproven technologies, it seems that TEPCO firmly believes that everything will go according to plan. Surely this posture of refusing to consider unforeseen possibilities is making the contaminated water problem even more serious than it should be.
TEPCO and the government have brought about severe radioactive pollution that is causing untold suffering to the people of Fukushima and the adjacent prefectures, but appear to be unable to empathize with this suffering from the way they formulate policies that allow for the dilution of the radioactive contaminated water to below the standard for subsequent release into the ocean. Is it not true to say that the essence of this problem lies in their inability to understand the pain of local residents and their posture of the continual use of ad hoc measures without ever attempting to reach a fundamental resolution to the accident?
(Hideyuki BAN, Co-Director of CNIC)
