* No significant change in cancer rates expected, UN scientists say
* Risk of thyroid cancer may increase for most exposed children
* But risk still low; no Chernobyl-style rise in cases seen (Adds quotes from press conference, details from report on ecosystems)
By Fredrik Dahl
VIENNA, April 2 (Reuters) - Japan's Fukushima nuclear disaster is unlikely to lead to a large number of people developing cancer like after Chernobyl in 1986, even though the most exposed children may face an increased risk, U.N. scientists said on Wednesday.
In a major study, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) said it did not expect "significant changes" in future cancer rates that could be attributed to radiation exposure from the reactor meltdowns.
The amounts of radioactive substances such as iodine-131 released after the 2011 accident were much lower than after Chernobyl, and Japanese authorities also took action to protect people living near the stricken plant, including evacuations.
However, some children - estimated at fewer than 1,000 - might have received doses that could affect the risk of developing thyroid cancer later in life, UNSCEAR said, making clear that the probability of that happening was still low.
UNSCEAR chair Carl-Magnus Larsson said there was a theoretically increased risk among the most exposed children as regards to this type of cancer, which is a rare disease among the young.
But, "we are not sure that this is going to be something that will be captured in the thyroid cancer statistics in future," he told a news conference.
On March 11, 2011, a magnitude 9 earthquake and tsunami devastated the Fukushima Daiichi nuclear plant, 220 km northeast of Tokyo, spewing radiation and forcing about 160,000 people to flee their homes.
It was the world's worst nuclear disaster since the Chernobyl reactor explosion sent radioactive dust across much of Europe. People close to the then Soviet plant were exposed to radioactive iodine that contaminated milk.
In contrast, UNSCEAR's Fukushima report said it expected a low impact on cancer rates of the population and that this was largely due to "prompt protective actions" by Japanese authorities following the accident.
A 30-km radius around the plant was declared a no-go zone, while areas where radiation was not so critically high took steps such as replacing the earth in parks and school playgrounds, decontaminating public spaces like sidewalks, and limiting children's outdoor play time.
"No discernible changes in future cancer rates and hereditary diseases are expected due to exposure to radiation as a result of the Fukushima nuclear accident," it UNSCEAR said in a statement accompanying its nearly 300-page study.
LOW RISK AMONG CHILDREN MOST EXPOSED
The thyroid - a gland in the neck that produces hormones that regulate vital bodily functions - is the most exposed organ as radioactive iodine concentrates there. Children are deemed especially vulnerable.
UNSCEAR said the normal thyroid cancer risk for young children was very low.
"The occurrence of a large number of radiation-induced thyroid cancers as were observed after Chernobyl can be discounted because doses were substantially lower," it said.
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 website.
UNSCEAR said about 35,000 children aged up to five lived in districts where the average absorbed dose to the thyroid was between 45 and 55 milliGrays (mGy), a radiation measurement.
But doses varied considerably among individuals, from about two to three times higher or lower than the average.
UNSCEAR "considered that fewer than a thousand children might have received absorbed doses to the thyroid that exceeded 100 mGy and ranged up to about 150 mGy," the report said.
"The risk of thyroid cancer for this group could be expected to be increased," it said.
UNSCEAR's press statement made clear it was still not seen as a big risk, however, with a headline saying: "Low risk of thyroid cancer among children most exposed."
The report also said that any effects of radiation exposure on land and marine ecosystems would have been "transient".
"For marine ecosystems, the possibility of effects on flora and fauna was limited to the shoreline area adjacent to the power station and the potential for effects over the long term was considered insignificant," it said.
UNSCEAR said 80 leading scientists had worked on the report - "Levels and effects of radiation exposure due to the nuclear accident after the 2011 great east-Japan earthquake and tsunami" - and that the material was reviewed by its 27 member states. (Editing by Hugh Lawson)