Regarding the full-scale removal of "nuclear fuel debris," which is considered the most difficult task in decommissioning the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant, a national specialized agency has decided to pour filling material into the reactor and solidify the debris together. We proposed the use of a new method to extract the waste.
At the Fukushima Daiichi Nuclear Power Plant, an accident 13 years ago caused a meltdown in which nuclear fuel melted away in Units 1 to 3, and the amount of ``nuclear fuel debris'' that was a mixture of nuclear fuel and surrounding structures is estimated to be approximately 880 tons. I am.
However, the trial extraction scheduled for Unit 2 has been repeatedly postponed due to lack of progress in installing equipment, and the full-scale extraction scheduled to begin at Unit 3 has not yet been determined, and there is no prospect that it will be able to begin. yeah.
On the 8th, the Japan Nuclear Damage Compensation and Decommissioning Support Agency, a national specialized agency that provides technical advice to TEPCO, held a press conference and announced new recommendations regarding removal methods.
The proposal focuses on
the ``aerial method,'' which works mainly in the air without filling the reactor or containment vessel with water, and ``filling and solidification,'' which involves
pouring a filler such as cement material, hardening it together with debris, and removing it. '' should be considered in some areas.
The disadvantage of the ``partial air method'' is that there is a high risk of exposure because there is no water to block the radiation, but when combined with ``filling and solidification,'' it has the advantage of preventing exposure to radiation and the spread of radioactive materials.
On the other hand, the ``flooding method,'' which involves covering the reactor building with a huge structure and filling the entire area with water, was also considered, but although it has the advantage of blocking radiation with water, it takes a long time to prepare. For these reasons, it was determined that it was difficult to make a choice at this point.
Mr. Toyoshi Fuketa, former chairman of the Nuclear Regulation Authority and head of the review committee set up by the Agency, said, ``The filling and solidification method still requires research and development,'' with the plan to complete decommissioning in 40 years at the most. "It's still at a stage, but considering the roadmap for the entire decommissioning process, we can't afford to remain idle forever, so I would like to make a proposal that would be a turning point."
Upon receiving the proposal, TEPCO plans to examine its feasibility over the next one to two years and decide on a method for retrieving the debris.
What is the filling solidification method?
The filling and solidifying method proposed for use this time involves injecting filler such as cement material into the reactor and the containment vessel surrounding it, solidifying the nuclear fuel debris together, and then excavating from the top of the building. This is a method of removing it.
These filling materials have been used to solidify and store radioactive waste.
▽By covering nuclear fuel debris with filler material, it is expected to be effective in blocking radiation and suppressing the scattering of dust containing radioactive materials.
▽Compared to other methods, the equipment used for retrieval is smaller and the preparation time is shorter. It is said that by shortening the length of time, it is possible to start the retrieval process as soon as possible.
However, there is no evidence that it has been used to retrieve nuclear fuel debris, and technical issues remain, such as whether it is possible to establish filler materials and injection methods that suit various locations and conditions.
Shunichi Suzuki, a senior researcher at the University of Tokyo, has been researching this method for eight years.
Mr. Suzuki is focusing on a cement-based material called geopolymer, which has a high ability to block radiation and is resistant to high temperatures.
In January this year, using a device 1/6 the size of the actual containment vessel that simulated the bottom of the containment vessel, ``geopolymer'' was successfully placed in room-temperature water close to the containment vessel. We conducted an experiment to see if it solidified.
In the experiment, the geopolymer, which had been poured into the device for a month, was hollowed out by boring or cut with a wire to check its condition.
The results showed that there were almost no cracks or cracks in the cut geopolymer, confirming that it would harden as intended under the conditions.
Since there are still many things we don't know about the conditions inside the reactor, Suzuki plans to repeat the experiment by changing conditions such as temperature and radiation dose, and hopes to encourage as many researchers as possible to participate in the research.
Mr. Suzuki said, ``Geopolymer has a high radiation shielding ability and is a material that can withstand the high temperatures of fuel debris, so we can expect that it will be possible to safely remove it.It also has the property of trapping radioactive materials, so it can be removed. It is also easy to manage when storing it as waste later on.However, since there is little evidence of its use, it is important for various people to collect and share as much data as possible to deepen our understanding of this material.'' I was there.
Background of debris construction method study
After the Fukushima Daiichi Nuclear Power Plant accident, the government and Tokyo Electric Power Company established a timetable to complete the decommissioning of the reactor within a maximum of 40 years, with the goal of starting the removal of nuclear fuel debris by 2021, 10 years after the accident.
Initially, we aimed to use the ``submergence method,'' which had been proven in the decommissioning of the Three Mile Island nuclear power plant in the United States, which involves filling the inside of a nuclear reactor with water to block radiation.
However, as internal investigations progressed, it was discovered that the damage to the nuclear reactor and containment vessel containing the debris was more severe than expected, and in 2017, a national specialized agency decided to take out the debris in the air, saying that flooding would be difficult. It was recommended that consideration should be centered on the "medium construction method."
On the other hand, the ``partial air method'' is unprecedented in the world, and requires the remote control of robots and other equipment to carry out most of the work in an environment with extremely high radiation levels that is inaccessible to humans. yeah.
For this reason, it was necessary to develop new equipment just to conduct preliminary surveys to consider specific construction methods, which took a long time.
Furthermore, prior to full-scale retrieval, the internal investigation is progressing and even for the trial retrieval scheduled to be carried out at Unit 2, it took time to develop the robot arm to be used for retrieval, and the piping in which the arm would be installed was difficult to implement. The mission has been postponed three times due to difficulty in removing the clogged sediment, and the start of debris removal is about three years behind the original plan.
In light of this situation, in March last year, a national specialized agency announced that in order to begin full-scale removal in the 2030s, with the aim of completing decommissioning by 2051, it is necessary to accelerate the study of construction methods. A committee of experts was set up, and the committee has been considering the possibility of completely new construction methods, which are unprecedented in the field of decommissioning nuclear power plants.
In his interim report last summer, former Nuclear Regulation Authority Chairman Toyoshi Fuketa, who assumed the top position of the commission, said, ``We want to accelerate the study so that it is not too late to meet the decommissioning schedule of 30 to 40 years.'' I was there.