TOKYO (Reuters) — The amount of radioactive materials released in the first days of the Fukushima nuclear disaster was almost two and a half times the initial estimate by Japanese safety regulators, the operator of the crippled plant said in a report released on Thursday.

The operator, the Tokyo Electric Power Company, said the meltdowns it believes took place at three reactors at the Fukushima Daiichi plant released about 900,000 terabecquerels of radioactive substances into the air during March 2011. The accident, which followed an earthquake and a tsunami, occurred on March 11.

The latest estimate was based on measurements suggesting the amount of iodine-131 released by the nuclear accident was much larger than previous estimates, the utility said in the report. Iodine-131 is a fast-decaying radioactive substance produced by fission that takes place inside a nuclear reactor. It has a half-life of eight days and can cause thyroid cancer.

It is difficult to judge the health effects of the larger-than-reported release, since even the latest number is an estimate, and it does not clarify how much exposure people received or continue to receive from contaminated soil and food. Experts have been divided on the health impacts since the disaster because the studies of assessing radiation risks are based mainly on a different type of exposure — the large doses delivered quickly by the atomic bombs in Japan in 1945.