An 11-year-old girl who evacuated from the town of Futaba after the 2011 Fukushima nuclear disaster was likely exposed to radiation levels near the government-set standard, despite assurances that no children were exposed to such high doses.

The girl is said to have been exposed to a radiation dose of about 100 millisieverts, the threshold for enhanced risk of cancer, following the triple meltdown at the Fukushima No. 1 nuclear power plant.

The previously undisclosed case, which was reported to The National Institute of Radiological Sciences (NIRS) after the disaster, contradicts the central government's statement that "there has been no confirmed cases of children exposed to radiation doses of 100 millisieverts or higher."

According to the NIRS, the case was not disclosed at the time because the institute considered that the estimate was based on information from the site using a simple monitoring instrument and that the figures were not calculated precisely.

The Fukushima Prefecture town of Futaba co-hosts, along with Okuma, the crippled nuclear plant, which was inundated by massive waves triggered by the megaquake on March 11, 2011.

On around March 17, 2011, a radiological technician of the Fukushima prefectural government office engaged in radiation check-up tests on residents detected 50,000 to 70,000 cpm of radiation when checking the girl's thyroid gland using a radiation monitoring device at a gym in Koriyama, according to the NIRS and other sources.

Cpm, or counts per minute, is a measurement of radiation emitted per minute from radioactive substances detected by such a device.

No documents regarding the case remain, but the figures were conveyed to a team from Tokushima University that traveled to the site to provide support for the tests.

The team estimated that the radiation level in the girl's thyroid gland was likely a dozen kilobecquerels on the assumption that all the radioactive substances were absorbed by her thyroid gland and reported the estimated figures to the NIRS.

A becquerel is a measurement unit that indicates the ability of a radioactive material to emit radiation, or the intensity of radioactivity.

A sievert, in contrast, is a unit that focuses on the effects of radiation on human health.

The NIRS shared the information on the case among its staff members and left memos indicating the dose that the girl may have been exposed to a radiation dose of around 100 millisieverts.

Children are said to be particularly vulnerable to thyroid gland cancer due to radiation exposure.

In March 2011, a government survey of 1,080 children in the three municipalities of Iwaki, Kawamata and Iitate in Fukushima Prefecture found a maximum level of 35 millisieverts of exposure, far lower than the 100-millisievert standard.