The testes and sperm of bulls abandoned in the evacuation zone around the battered Fukushima nuclear plant were not affected by chronic exposure to radiation, a Japanese academic study has found.

The work provides crucial data to scientists and public policy advisers on the possible impact of the reactor meltdowns on human -- and especially reproductive -- health, two-and-a-half years after the tsunami-sparked disaster.

A team of researchers examined two bulls caught in September 2011 and January 2012 within a 20-kilometre (12-mile) radius of the plant, an area that was completely evacuated because of fears over radioactive risk.

They also looked at a male foetus from the area to help determine the effects of prolonged radiation exposure associated with the disaster, the world's worst since Chernobyl in 1986.

"Since the testis is a relatively radio-sensitive organ, we considered that radiation exposure would lead to changes in the morphology or the function of this organ," the study noted.

Researchers from Tohoku University and other schools found concentrations of caesium 134 and caesium 137, substances of concern because of their relatively slow rates of decay, were broadly similar in all organs, but were sharply higher in muscles.

"Radioactivity concentration of caesium in the testis was about more than half of that in the skeletal muscle and the level was the same as in other organs," the study said.

Examination of the bull's sperm showed their total number and their structure and size were normal.

"Adverse radiation-induced effects were not observed in bull testes" following exposure of up to 10 months, researchers said.

"The effect of radiation on farm animals in the evacuation zone... provides information about the health risks of livestock and can also be extrapolated to humans," the study said.

However, they cautioned, their particular study had a small sample size and further work was required.

The Fukushima plant hurled radioactive substances into the air, soil and sea in the days and weeks after its reactors went into meltdown when their cooling systems were swamped by the March 2011 tsunami.

Tens of thousands of people fled homes and farmland, many because of compulsory orders, but some because they did not trust government information that they were safe to stay where they were.

The study noted the evacuation zone was established in April 2011, leaving 3,400 cows, 31,500 pigs and 630,000 chickens behind. There were also hundreds of cats, dogs and other pets, as well as many thousands of wild animals like boar, previous reports have said.

The government ordered the culling of cattle in the zone in May 2011, to avoid the possibility of contaminated animals entering the human food chain. Many evaded capture for months.

While those creatures that were confined to buildings invariably died, many others went feral and roamed over a large area, free from human interference.

Scientists say cattle would have received high doses of radiation by eating and drinking contaminated plants and water in large volumes.

"Abandoned animals now have formed an invaluable model for studying the effects" of a chronic intake of radioactive substances, it said, adding an analysis of surviving animals could provide urgently needed data affecting both human health and the livestock industry.

Researchers found caesium levels in the organs of the bull that had stayed in the zone for 315 days after the disaster were higher than those in the bull captured after 196 days.

Caesium levels found in the organs of the bulls and the foetus were mostly well above the official limit for consumption, currently set at 100 becquerels per kilogramme (2.2 pounds).

The study was published online Tuesday in Scientific Reports, a peer-reviewed journal from the publishers of the respected Nature.

The researchers are conducting other studies to analyse whole genome sequences among bulls in the evacuation zone and foetuses obtained by fertilisation using sperm from bulls in the zone.