Clean-up programme

Although the release of radioactive materials from the damaged PCVs is now under control and the public ’s radiation exposure from additional releases has been significantly diminished (<1mSv/yr) [32], the explosion and radiation emission from the Fukushima accident results in a need to clean up the problematic wastes, a process that will take many years. This

task will need to include the treatment of a huge amount of accumulated water used for cooling the damaged SF pools and cleaning contaminated debris, surface soil, vegetation, structures surrounding the SF rods, as well as the sludge derived from processing the water potentially containing radioactive materials. The clean-up programme has been initiated. As required by the Japanese government, a mid — and long-term roadmap was drafted by TEPCO, the Agency for Natural Resources and Energy (ANRE) and NISA. This roadmap was finalized at the government and TEPCO ’s mid — and long-term countermeasures conference on December 21, 2011. The four basic policies addressing the mid — and long-term issues are the safety of local citizens and workers, which was given a top priority; the maintenance of transparent communications with local and national citi­zens; a continuously updated roadmap based on the on-site situation and the latest R&D results; and coordination efforts for TEPCO, ANRE, and NISA to achieve the clean-up goals [33].

In the Fukushima accident, around 56% of the total radioactive materials released into the atmosphere were noble gases [5]. The released gases chiefly consisted of xenon-131, which has a half-life of only 5.2 d, and most
of the gaseous emissions occurred in the early days of the accident. There­fore, the radioactive gases would have decayed, spread and been diluted to a very low radiation level. Gas pollutants are thus not assumed to be a threat to humans and the ecosystem and are not a focus of the clean-up programme. Merely as a preventive measure, PCV gas control systems were installed and are still operating at units 1, 2, and 3 to avoid any potential further emissions [32].

Treatment of the cooling water is clearly a bigger challenge than the gaseous waste processing in the clean-up programme. Firstly, the cooling water which was in direct contact with the damaged fuel rods was seriously contaminated by a variety of nuclides. Secondly, the injection of seawater into the damaged reactors not only promoted corrosion but also impeded the usual water treatment processes. The higher sodium concentration (ionic strength) of seawater reduces the specific adsorption of cesium when using zeolite to adsorb pollutants from water. As a result, more zeolite is required, and more spent zeolite waste will be produced. Before the com­mencement of operations of two water treatment systems in June 2011, a huge volume of contaminated water had been accumulated in surface storage tanks [21]. If water leakage had occurred into the RPV, the overall water treatment task would have been further burdened. In spite of these challenges, nearly 90% of the treatment system capacity was achieved by mid-August 2011 [21] . The Fukushima Daiichi nuclear power station now operates a large water treatment facility as shown in Fig. 24.6. As of July 3, 2012, the water treatment capacity had reached 540 m3/d, and the total volume of water treated to date had reached 196,091 m3. The treated water has been circulated into units 1, 2 and 3 for reactor building decontamina­tion at rates of 132 m3/d, 204m3/d, and 204 m3/d, respectively [34].

The treatment system (Fig. 24.6) comprises four parts [35]:

1 . an oil separator,

2. a Cs-adsorption system developed by the US company Kurion, which itself consists of three parts (a pre-treatment column packed with a surfactant modified zeolite aluminosilicate sorbent for removing remain­ing oil and Tc, four parallel columns of the sorbent herschelite for removing Cs, and a column packed with Ag-impregnated herschelite sorbent to remove I). These filters are porous zeolites that loosely bind metal ions and through a combination of adsorption and ion exchange trap Sr90, Cs134 and Cs137,

3. a system for removing the remnant Cs provided by the French company Areva which uses precipitation and coagulation,

4. on August 19, 2012, a second line called Sarry developed by Toshiba — Shaw, was added in parallel to the Kurion-Areva line. This line uses Cs adsorption by crystalline silico-titanates (CST).

Outline of water treatment facility system (highly concentrated accumulated water)

image274

24.6 Water treatment scheme used in the Fukushima Daiichi NPP to remove radioactive contaminants in the water [21]. Used with permission from Tokyo Electric Power Company (TEPCO).

The decontaminated water goes to tanks where it mixes with reagents such as nickel ferrocyanide and barium sulphate, along with polymers and sand. The dissolved radioactive metals form precipitates and colloids, which are trapped as a radioactive sludge, allowing the water to be desalinated by reverse osmosis and by evaporation, these desalination processes were added on June 24 and August 7, 2012, respectively, in a shielded ion-exchange module. The two processes reduce the concentration of cesium — the major element of concern from the reactors — in the water by up to a million times [35,36].

A problem still remaining in the water treatment task is the need for water storage. The American Nuclear Society (ANS) pointed out a contribution of 200-500 m3/day of contaminated water from groundwater in-leakage, but TEPCO is unable to release this water due to existing environmental policy [21] . Furthermore, a large volume of tritiated water, with a tritium concentration of 1,000 Bq/m3 . also needs storage, since the half-life of the tritium is about 12 years. Generally, the daily accumulation of water is about 200-700 m3 . and the need for water storage will eventually challenge the existing storage capacity, even with the new additions of >111,000m3 of tanks and 10,000 m3 of megafloat barges. Currently, the water storage operations, as well as the forecast conditions, are required to be submitted to the NISA weekly, providing oversight to the water processing [34].

After sealing the crack in the pit adjacent to the reactor 2 turbine hall and terminating the discharge of highly radioactive water, TEPCO took action to mitigate the level of radioactivity in the contaminated ocean. On April 16, 2011, TEPCO dumped about 10 bags of zeolite in the seawater area near the Fukushima Daiichi NPP, as shown in Fig. 24.7. Each bag con­tained 100 kg of ground zeolite and would be raised periodically to check the radiation level [37, 38].

The secondary solid waste (sludge, spent zeolite, and the used reverse osmosis membranes) generated from the water treatment operation are stored in the plant and labeled as radioactive solid waste. Treatment and immobilization of these secondary wastes is now an issue. One option being examined for the zeolite wastes is use of a mobile vitrification system using a form of in-can melting. The Areva process creates a sludge and the CST IE-911 inorganic Ti-based resin is also being used. No decision has yet been made on what to do with the Si and Ti spent resins but cementation and vitrification are being considered.

Currently, there is no clear plan for the treatment of such secondary waste, although the need for R&D in cementation and disposal techniques is mentioned in the mid and long-term roadmap published by TEPCO [33].

image275

24.7 Submerging zeolite into a water outlet from the Fukushima Daiichi NPP to reduce the radiation contamination to the ocean environment [39]. Used with permission from Tokyo Electric Power Company (TEPCO).

image276"Outside of debris storage

Подпись: Before removalПодпись: After removalDebris storage

24.8 Debris waste generated from the Fukushima Daiichi nuclear plant incident and its temporary storage [40]. Used with permission from Tokyo Electric Power Company (TEPCO).

In addition to the secondary solid waste, approximately 28,000 m3 of debris on the plant site from the disaster itself has already been collected by remote-controlled vehicles. These solid wastes were classified according to their material types as well as their level of radiation [32]. About 6,000 m3 of debris from the yard area around the nuclear plant is stored in -900 metal containers (with a volume of 4 or 8 m3 per container) and will be trans­ported for off-site treatment [21, 32] . Larger and less contaminated items are stored in bulk in a new solid waste building, pictured in Fig. 24.8. Before decontaminating the RAW, characterization and compositional analysis of the stored debris are required. The debris waste with the strongest radiation will be from the damaged reactors as well as in the damaged SF pools.

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