Like millions of other Japanese, Hiroko Tanaka has been glued to television footage of workers frantically trying to prevent disaster at a crippled nuclear plant. She has more reason to be worried than most: as an atom-bomb survivor, or hibakusha, Mrs Tanaka knows the impact of radiation first hand, and she has faced discrimination all her life in a country where little is still known about its effects.

She waited 50 years before applying for the government certificate that proves she is a surviving victim of the 1945 US atomic bombing of Hiroshima. "Radioactivity is so scary because it's invisible," she says. Her husband, Takeo, sitting beside her, adds: "She kept her status hidden from me. At the time, there were all sorts of rumours. People in Tokyo said 'Don't marry someone from Hiroshima, you will be contaminated'."

Japan's roughly 227,000 hibakusha have suffered higher cancer rates as a result of the fallout from the twin bombings of Hiroshima and Nagasaki. The drama unfolding at the Fukushima nuclear complex fills many with growing alarm. Although little is known about the technicians, fire-fighters, pilots and soldiers battling to prevent a meltdown at the six-reactor complex, hundreds of them are believed to have been irradiated, some heavily. In the 20km zone around the plant, thousands more have been exposed.

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Exactly how much radiation has leaked is unknown, and experts say its effects won't be clear for years. Fears of contamination have already prompted several countries to impose controls on imports from Japan. At South Korean airports, passengers arriving from the country are being provided with a test to check their degree of exposure to radiation. Koichiro Maeda, the director of Hiroshima's Peace Memorial Museum, says the concern is natural. But like others with experience of nuclear issues, he worries that lack of government transparency and public ignorance will create what he calls an "excessive reaction".

"The hibakusha faced an extreme degree of discrimination based on unfounded ideas," he said. "If the public was given a clear explanation of the effects of radiation, this problem would no longer exist."

Many observers say the Japanese government has downplayed the problems at the Fukushima plant and the fallout from a series of explosions since the earthquake and tsunami struck on 11 March.

"There is incredible information control and manipulation going on at this critical time," said Satoko Norimatsu, the head of the Vancouver-based Peace Philosophy Centre, an anti-nuclear group.