* U.N. scientists draft report on Fukushima fallout
* Expects no higher cancer rates as people left area
* In contrast, many children were affected after Chernobyl (Adds press statement, details on cancers from Chernobyl)
By Fredrik Dahl
VIENNA, May 31 (Reuters) - The evacuation of tens of thousands of people helped prevent rising cancer rates and other health problems after Japan's Fukushima nuclear disaster, the world's worst in 25 years, U.N. scientists said on Friday.
Radiation exposure following the reactor meltdowns more than two years ago did not cause any immediate health effects, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) said after its annual meeting.
That would be in contrast to Chernobyl, the 1986 Soviet reactor explosion which sent radioactive dust across much of Europe and is believed to have caused thyroid cancer in some children.
A magnitude 9 earthquake and tsunami on March 11, 2011, killed nearly 19,000 people and devastated the Fukushima Daiichi nuclear plant, spewing radiation and forcing about 160,000 people to flee their homes.
Actions to protect inhabitants in the area, including evacuation and sheltering, significantly reduced the exposure to radioactive substances, the scientific body said after the session to prepare a report for the U.N. General Assembly.
"These measures reduced the potential exposure by up to a factor of 10," said senior UNSCEAR member Wolfgang Weiss.
"If that had not been the case, we might have seen the cancer rates rising and other health problems emerging over the next several decades," he said in a statement.
Weiss, who chairs work on UNSCEAR's Fukushima report, told reporters that dose levels were "so low that we don't expect to see any increase in cancer in the future in the population".
UNSCEAR's findings appeared to differ somewhat from a World Health Organisation (WHO) report published in February which said people in the area worst affected have a slightly higher risk of developing certain cancers.
NO RADIATION-RELATED DEATHS
Weiss suggested the UNSCEAR study, carried out by 80 experts and with the involvement of five international organisations including the United Nations health agency, was based on information covering a longer period after the accident.
UNSCEAR's 27 member states scrutinised the draft during this week's session in Vienna, it said, adding it would be the most comprehensive scientific analysis of the issue so far.
While a few received very high doses, no radiation-related deaths or acute effects were observed among nearly 25,000 workers - including employees of the operator Tokyo Electric Power Co - involved at the accident site, it said.
Highlighting the differences between Chernobyl and Fukushima, Weiss said people close to the then Soviet plant were exposed to radioactive iodine that contaminated milk.
The thyroid - a gland in the neck that produces hormones that regulate vital body functions - is the most exposed organ as radioactive iodine concentrates there. Children are deemed especially vulnerable.
"In Chernobyl, many children used milk which had high iodine concentrations, resulting in high thyroid doses, resulting in an increase of thyroid cancer," Weiss said, adding that the doses in Japan were "much, much lower".
In Belarus, Russia and Ukraine, the countries most affected by Chernobyl, more than 6,000 cases of thyroid cancer had been reported by 2005 in children and adolescents who were exposed at the time of the accident, UNSCEAR says on its web site.
"Many of those cancers were most likely caused by radiation exposures shortly after the accident," it adds. (Editing by Alistair Lyon)