The entire article written by Helen Caldicott should be read at the Guardian.
Here is an excerpt: "Internal radiation, on the other hand, emanates from radioactive elements which enter the body by inhalation, ingestion, or skin absorption. Hazardous radionuclides such as iodine-131, caesium 137, and other isotopes currently being released in the sea and air around Fukushima bio-concentrate at each step of various food chains (for example into algae, crustaceans, small fish, bigger fish, then humans; or soil, grass, cow's meat and milk, then humans). 
After they enter the body, these elements – called internal emitters – migrate to specific organs such as the thyroid, liver, bone, and brain, where they continuously irradiate small volumes of cells with high doses of alpha, beta and/or gamma radiation, and over many years, can induce uncontrolled cell replication – that is, cancer.
Further, many of the nuclides remain radioactive in the environment for generations, and ultimately will cause increased incidences of cancer and genetic diseases over time. The grave effects of internal emitters are of the most profound concern at Fukushima.
It is inaccurate and misleading to use the term "acceptable levels of external radiation" in assessing internal radiation exposures...
Nuclear industry proponents often assert that low doses of radiation (eg below 100mSV) produce no ill effects and are therefore safe. But , as the US National Academy of Sciences BEIR VII report has concluded, no dose of radiation is safe, however small, including background radiation; exposure is cumulative and adds to an individual's risk of developing cancer....