Fukushima Daiichi Nuclear Power Station Contaminated water management What is “slurry”? Why is it generated? How is it stored?
(English ver.) 2020-08-19
In March, 2011, an accident occurred due to the Great East Japan Earthquake at TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS). Our previous articles highlighted efforts toward decommissioning of the FDNPS as well as measures in dealing with contaminated water that contains radioactive materials. You may have some knowledge about the situation at the FDNPS from various news sources. However, we wonder if you are aware of efforts being made to ensure safety regarding the waste generated through contaminated water treatment process. This article focuses on waste called “slurry” that may not be commonly known, addressing questions such as: What is “slurry”? How is it stored? And how are risks to be reduced?
Two kinds of waste generated in the process of contaminated water treatment through ALPS
Fuel that melted and solidified inside the reactor (known as “fuel debris”) needs continuous cooling by water to suppress the heat it produces. This is the primary reason why contaminated water is generated. Furthermore, groundwater and rainwater flowing into the reactor buildings will come into contact with the radioactive materials inside the buildings, making additional contaminated water. Various countermeasures are being taken based on the three basic principles, namely 1.Preventing leakage of contaminated water, 2. Redirecting groundwater from contamination sources, 3. Removing contamination sources, in order to reduce the amount of contaminated water generated as well as to prevent it from flowing out of the buildings.
A variety of equipment is used for purification of contaminated water in order to reduce risks associated with radioactive materials. Contaminated water first goes through cesium absorption equipment (“Kurion” or “Sarry”), which removes cesium and strontium. It is then “desalinated” and treated for purification through the multi-nuclide removal equipment known as “ALPS” that removes 62 kinds of radioactive materials. Water treated by ALPS is referred to as “ALPS-treated water”,
ALPS-treated water still contains tritium that cannot be removed by ALPS.
Moreover, two kinds of waste products are generated in ALPS as well as in the process of contaminated water treatment prior to ALPS. One is a substance referred to as “slurry”, a viscous liquid mixture of water and fine particles created by injection of chemical agent into the equipment used before ALPS (shown as (1) in the illustration). The other is a “used absorbent” that is employed to trap radioactive materials (shown as (2) in the illustration).
These waste products are kept in a storage vessel made of polyethylene, which is referred to as a High Integrity Container (HIC).
HIC (covered with reinforcement)
HICs are currently kept in temporary storage facilities on-site at Fukushima Daiichi NPS, where they are contained in a large concrete box. The facilities have functions for safe storage not only to shield radiation the waste in HICs emits but also to release heat and hydrogen that it produces.
Efforts to further reduce risks associated with storage: stabilization treatment
There are several risks associated with the current method of HIC storage.
Major risks are: 1) Potential leak of water inherent in slurry kept in a HIC, and 2) Deterioration of an HIC caused by radiation.
With regard to 1) above, radiation emitted from waste products in an HIC will produce gases such as hydrogen. The pressure those gases put on the water could potentially cause leakage of water to the outside. With regard to 2) above, there is a risk of an HIC itself deteriorating as polyethylene is affected by radiation emitted by slurry.
These challenges facing HIC storage are being dealt with in a proper manner. Concerning 1), a smaller quantity of slurry is put into an HIC or supernatant water is skimmed from an HIC that is thought to be overfilled. Furthermore, proper measures have been put in place to deal with a potential leak so that leaked materials can be contained in a concrete box within the facilities. As a matter of course, periodical checks are conducted to make sure that there is no leak from HICs kept in the storage facilities. Concerning 2), radiation effects on an HIC are evaluated through experiments, whereby the useful life of an HIC is calculated to secure sufficient safety. HICs are under proper management within the prescribed useful life.
Although risks associated with the current status of storage are very low as explained above, plans for further enhancing the safety are underway. Specifically, by extracting slurry from an HIC and dehydrating it, the slurry changes from a liquid to a solid state for storage in the waste storage facilities. The process of extracting slurry from an HIC and dehydrating it is referred to as “stabilization treatment”. Changing to a solid state will lower the risk of leaking.
The process of dehydration is shown in the figure below. The slurry changes from a liquid to a solid state as shown in the pictures before and after dehydration. Dehydrated slurry will be stored in a metal container that is less vulnerable to radiation than an HIC. This will substantially reduce the risk of radiation effects on the container.
Process of dehydration, slurry before and after dehydration, Draft design of a container for storage of dehydrated slurry
Design and technological development of the equipment for slurry stabilization treatment is currently underway aiming to start operation in FY2022. The facilities will dehydrate about 600 HICs of slurry per year, and the number of HICs are expected to be reduced by about 3,500 over 6 years thereafter. Additionally, newly produced slurry will be continuously treated for stabilization as well, aiming to finish temporary storage of slurry by around FY2028.
Changes in the amount of slurry stored in HICs as stabilization treatment progresses
- HIC storage capacity is 4,192, out of which 3,500 were filled as of May, 2020.
- ALPS is currently processing:
- (i) Strontium removed water generated daily, and
- (ii) Strontium removed water generated in the past and stored*, which produces about 28 HICs of slurry per month with (i) and (ii) combined.
- Strontium removed water generated in the past and stored had been processed by August 8, 2020, and 10 HICs of slurry will be produced per month thereafter.
- ALPS will be upgraded to suppress slurry production, and 5 HICs of slurry will be produced per month thereafter.
- Dehydration of slurry will start in 2022, and slurry stored in HICs will be reduced by 50 HICs per month thereafter. It will take 6 years to process all the slurry stored in HICs, and then the storage capacity restriction will be resolved.
*Only cesium and strontium had been removed from contaminated water before ALPS was installed in March 2013. This strontium removed water is currently stored in tanks on-site at Fukushima Daiichi NPS. All of this had been treated by ALPS by August 8, 2020.
In the future, we will clarify details of the situation such as radiation effects on the boundary of the NPS (the boundary of the surrounding land), and construct necessary facilities according to plan toward the completion of dehydration treatment.
Division in charge
About the article
Nuclear Accident Response Office, Electricity and Gas Industry Department
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Research and Public Relations Office, Policy Planning and Coordination Division, Commissionerʼs Secretariat
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