A United Nations panel says Japanese authorities underestimated by 20 percent the amount of radiation workers were exposed to in the initial phase of the Fukushima Daiichi nuclear reactor cleanup.

The 11 March 2011 earthquake and tsunami devastated the nuclear plant, causing three reactors to melt-through. The operator, Tokyo Electric Power Company (TEPCO) has been struggling to contain the spewing radiation ever since.

The UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) analyzed radiation doses in 25,000 people who worked at the plant on or before October 2012.  The panel determined that the tests used on workers did not take into account radiation from certain types of isotopes with short half-lives.

The UN didn’t give any additional insight into the long-term health implications of the effected workers.