Efforts to restart emergency diesel generators at the Fukushima Daiichi nuclear power plant were foiled early last year by the discovery of huge amounts of highly contaminated water flooding the basements of site buildings.
Over the following months, TEPCO developed and built temporary systems for water decontamination, which started up in June 2011. Initially the temporary water treatment system used a decontamination system from AREVA and was later supplemented by one, and then two, lines of a Toshiba SARRY decontamination system. Both systems incorporated adsorption towers with ion-exchange resins to grab and immobilise radionuclides present in the water.
The systems focused primarily on removing the relatively long-lived caesium radionuclides (Cs-134, Cs-137) which accounted for a major portion of the water’s activity, in absence of radioactive iodine (I-133), whose relatively short half-life of three months was progressively reducing the risk it posed.
Over the following months, the system was improved, and a return loop with desalination added so that treated water could be used to cool the reactors.
By the end of May 2012, the system had treated 177,000 m3 of water, and, by storing the treated water in huge tank farms on site, managed to lower the flooding levels in the buildings to below the water table.
But with nearly 160,000 m3 of highly-active water stored in tanks around the site, and the continuing use of some fresh water for the 500 m3 of daily cooling injection into reactor units 1, 2 and 3, the situation is clearly unsustainable.
So TEPCO has started to build a system to treat all of the contaminated water. Three aspects of the system’s designed scope are unprecedented outside of Fukushima.
First, its processing rate. Two decontamination trains (with a third installed but idle as a backup) aim to each process 250m3/day. For comparison, the EnergySolutions Advanced Liquid Processing System (ALPS) processing system designed for the shut-down UK Bradwell nuclear power plant aims to process about 500m3/year. Although the SARRY ion-exchange system currently in operation on site can process approximately double that much, it was designed specifically to keep up with reactor injection, according to Toshiba. The new system’s operating pressure is about the same as SARRY, it said. But the SARRY system only removes caesium isotopes.
Second, the new system will remove not one, but 62 radionuclides (see Table 1). These are not all of the radionuclides present—researchers have identified more than 1000—but are the ones judged to approach the closest to regulatory maximum levels for seawater, according to Japanese reactor regulations (section six, appendix 2). The nuclides present in the system fall into three types: fission products and transuranics, which come from the fission chain reaction inside the reactor, and corrosion products: particles corroded from originally-inert equipment that has become activated (such as Fe, Co, Mn). Alpha, beta and gamma radiation emitters are all included. One notable nuclide that will not be substantially removed by this system is tritium (beta-emitter, half-life 12.5 years), which TEPCO says is difficult to remove.
Third, TEPCO has set extremely stringent decontamination factors. Water treatment processes in general do not eradicate every last active molecule; they are designed to conform to discharge limits set by the national regulator. These vary by country and radionuclide. The maximum discharge level set by TEPCO for this project is not the maximum level set by law, but an amount 100 times lower. These guidelines bring radioisotope concentrations post-treatment close to, or below, the limits of detection. The low limits should help smooth the way for government approval to eventually release the water back to the sea, although TEPCO has not yet announced any plans to do so.
The project
TEPCO chose Toshiba as the prime contractor for the so-called ‘multi-nuclide removal system’. Although officially a subcontractor, EnergySolutions is credited by Toshiba as the creator of the system concept, and will work jointly with Toshiba to supply the system to TEPCO. Other vendors will supply components, including Fortum of Finland and subcontractors to supply ion-exchange resins. R&D for the project lasted six months, culminating in a pilot test on site that verified achievement of non-detect levels of 62 radionuclides.
Table 1 shows the results of this work, listing all of the high-risk nuclides by mass number and their decontamination levels experienced by both desalinated and concentrated salt water samples. However, TEPCO warns that because the decontamination data was collected at different times in different operational set-ups, it is difficult to draw conclusions based on the differences between concentrated salinated and desalinated water decontamination results.
An upland location on a former tennis court near an office building was chosen to locate the system, and civil construction work was underway in May. According to Toshiba, system elements will be installed for several months starting this month (July), after which cold and hot testing will begin. The system should begin operation toward the end of the year. According to a late 2011 roadmap, accumulated water processing will last into at least 2020.
The system
The system will be inserted into the current water treatment loop after oil separation and after caesium adsorption treatment, although it will also remove Cs-134 and Cs-137 (Figure 1).
Figure 1: Current water treatment system at Fukushima Daiichi (Source: TEPCO)
Most of the water will be taken post-desalination (a third, much smaller amount of pre-desalinated water will also be processed). Water to be treated includes both desalinated water and concentrated salt water, although Toshiba said that the two will be processed separately. Desalination is not required as pretreatment for decontamination; in fact, even treated water will remain saline, to match the salinity of the sea.
However, the competing ions present in seawater lower the efficiency of the ion-exchange resin by as much as three times, according to Fortum, which is supplying its CsTreat and SrTreat resins to remove caesium and strontium. It says that the resin’s maximum capacity is 150 TBq/kg, but specified that its maximum capacity, given seawater with Na 10 g/l, K 0.4 g/l, Mg 1.3 g/l, Ca 0.4 g/l and about 0.1 GBq/l of Cs, would be about 50 TBq/kg. TEPCO points out that the concentrated salt water is also expected to be more dense than the desalinated water, which would also tend to lower ion-exchange efficiency. Its own estimates of the amount of waste generated for the two water streams follow the same proportions: it is expecting that the salt water stream will fill 2.6 times as many radwaste containers as the desalinated water stream.
Extra precautions have had to be taken to guard against the corrosive effects of the flow of salty water (even though flows are expected to be at ambient, not elevated temperatures). According to Toshiba, tubing and flanges, which, if metal, would be at risk of corrosion from seawater, will be constructed instead from highly cross-linked polyethylene; in addition, some tanks will be lined.
The core of the treatment system is a modified version of the EnergySolutions ALPS, which consists of adsorption stage, in which ion exchange occurs, with extra pretreatment beforehand to remove precipitates and organics (Figure 2).
Figure 2: Schematic of the new Fukushima Daiichi water treatment system (Source: TEPCO)
The pretreatment stage consists of two different chemical processes: first, a ferric flocculation process that targets ruthenium isotopes; second, a co-precipitation process that removes calcium, magnesium and strontium as carbonates, says EnergySolutions. In both stages, a crossflow ultrafiltration step forces water through a porous ceramic pipe (hole diameter: 0.02 µm), trapping solids and organics whilst allowing water to pass through. Flocculates, precipitates and organics, which accumulate as sludge, are pumped into 1.9m tall by 1.6m diameter cylindrical radwaste storage containers.
In the adsorption stage, water flows through a series of adsorption towers. There are 16 in total, nearly three times as many as would be used in a typical installation, according to EnergySolutions. The large number of towers specified is due to the large amount of radionuclides to be removed; the system incorporates seven different ion-exchange capture mechanisms at a molecular level.
There are two types of adsorption tower in the system. Water first flows to 14 towers whose contaminated ion-exchange media are sluiced out (again to the same storage containers) and replaced. Sludge from desalinated water treatment is expected to fill half a storage container a day. Sludge from saltwater treatment is expected to fill 1.3 containers per day, according to TEPCO. Downstream, two larger towers made of fibreglass or a similar material mop up the small amount of low-level radioisotopes remaining. Because radionuclide concentrations are relatively low, their media are not worth sluicing out; instead, the entire tower is replaced once the resins are spent (on about a monthly basis, or 15 times a year, according to TEPCO).
Sampling lines will be installed at many points in the new system, but water testing will be carried out offline in a hot lab. There will be no online measurements, Toshiba said.
TEPCO said that the treated water would be stored initially in new purpose-built tanks. Before beginning the planning work to drain the water back into the sea, TEPCO has more urgent decontamination priorities, it said. These tasks include decontaminating leaked water, storing it, and preventing leaks of groundwater into the basements of Fukushima Daiichi station buildings.
Author Info:
Will Dalrymple is editor of Nuclear Engineering International. This article was first published in the July 2012 issue of NEI magazine.
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