An Environment Ministry decision to allow reuse of contaminated soil emanating from the Fukushima nuclear disaster under road pavements came despite an estimate that it will take 170 years before the soil's radiation levels reach safety criteria, it has been learned.

According to the revelation, an Environment Ministry panel approved the recycling of tainted soil generated from Fukushima decontamination work despite an estimate presented during a closed meeting of a working group that it will require 170 years for radioactivity concentrations in the contaminated soil to drop to legal safety standards, shelving a decision over whether such soil should be put under long-term management.

The ministry is planning to allow reuse of the tainted soil in mounds beneath road pavements, asserting that radiation will be shielded by concrete covering such mounds. However, an estimate presented at the closed meeting of the working group on the radiation impact safety assessment states that such mounds would be durable for just 70 years, suggesting that the soil would need to be managed for another 100 years after its road use ends.

"There's no way they can manage the soil for a total of 170 years without isolating it," said an angry expert.

The working group is a subgroup of an Environment Ministry panel called "the strategic panel for technical development of volume reduction and reuse of removed soil in temporary storage," and the two groups share some of their members. According to the working group's in-house documents obtained by the Mainichi Shimbun, the closed meetings were held six times between January and May, with the attendance of over 20 people including eight group members and officials from the Environment Ministry and the Japan Atomic Energy Agency (JAEA).

Under the Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors, the safety standards for recycling metals and other materials generated from the decommissioning of nuclear reactors are set at up to 100 becquerels of radioactive cesium per kilogram. Meanwhile, the special measures law concerning decontamination of radioactive materials, which was enacted after the 2011 Fukushima No. 1 Nuclear Power Plant crisis, classifies materials whose radiation levels top 8,000 becquerels per kilogram as designated waste, and stipulates that waste whose radiation levels are 8,000 becquerels or lower can be put to ordinary disposal.

According to working group chairman and Hokkaido University professor Tsutomu Sato, the group served as a forum to "prepare itself for theoretical argument" over setting the upper radiation dose limit for reusing contaminated soil at 8,000 becquerels.

The Environment Ministry set forth the plan to reuse contaminated soil in public works such as in mounds beneath road pavements and in coastal levees on the grounds that the "radiation levels can be contained to levels on par with clearance levels" by covering tainted soil with concrete and other materials. During the second meeting of the working group on Jan. 27, a member pointed out, "The problem is what to do with tainted soil after use (in roads and other structures). If such soil is allowed to be dug over freely, it would be difficult to convince the upper limit of radiation levels (for soil reuse)."

A JAEA official presented the aforementioned estimate, saying, "For example, it will take 170 years for radiation levels to reduce to 100 becquerels if tainted soil of 5,000 becquerels is put to reuse. Because the durable life of soil mounds is set at 70 years, a total of 170 years will be required to manage that soil -- both when the soil is being used in mounds and after that."

Discussions on the soil management period never went any further, and the strategic panel overseeing the working group on June 7 approved recycling such contaminated soil on condition that the maximum radiation levels of such soil be 8,000 becquerels and that the levels should be no more than 6,000 becquerels if the soil is covered with concrete and no more than 5,000 becquerels if the soil is planted with trees.

The Environment Ministry is set to begin a demonstration experiment possibly later this year, in which radiation levels will be measured in mounds using soil with different radioactivity concentrations at temporary storage sites in Fukushima Prefecture.

Working group chairman Sato, who also serves as a member of the strategic panel, admitted the existence of the 170-year estimate, but said, "We have discussed the matter but haven't decided anything. We just presented our initial idea for reuse (of tainted soil) this time, and we will examine the feasibility of the plan later."

Hiroshi Ono, who headed the Environment Ministry's decontamination and interim storage planning team, said, "We have yet to decide what to do (with the tainted soil) in the end (after reuse), but the Environment Ministry will take responsibility for that."

Another working group set up under the strategic panel, whose members primarily comprise those from the Japan Society of Civil Engineers, has presented a view, stating, "It will be in no way easy to secure the traceability (of recycled tainted soil)."