The disaster at the Fukushima Daiichi nuclear power stations in Japan could have long-lasting effects that go beyond the radiation exposure measured directly from the site, according to one anti-nuclear group.

At a press conference held by Physicians for Social Responsibility, three speakers outlined the health risks from the current and possible releases of radiation from the stricken power station.  

Even small doses of radiation, PSR said, can result in health problems, as it depends on what organs in the body are exposed to radioactive elements and ionizing radiation.

The relationship isn't linear, said Dr. Ira Helfand, past president of Physicians for Social Responsibility whose research focuses on the effects of radiation. You can have a small amount of plutonium and still get cancer.

He said the amounts of plutonium that can cause cancer are measured in micrograms, and wouldn't necessarily show up when one measures a whole-body dose. That is because there is a difference between radiation and the radioactive isotopes that cause cancers. Radiation often describes the gamma rays generated by nuclear reactions.

Gamma rays go through the body but don't contaminate anything, so the damage they do is immediate and short-term. However, blocking them requires heavy shielding. Alpha particles, which are essentially high-energy helium nuclei, can be more dangerous, but they have to be ingested and can be blocked by the skin.  

Cancers are caused when radioactive elements that are by-products of fission reactions are released into the environment and get into the body. Radioactive iodine, for example, has a short half-life and is concentrated in the thyroid gland. Strontium-90 has a longer half-life - on the order of decades - and it tends to concentrate in the bones. While there, it emits radiation in small amounts, which can cause bone marrow cancers and leukemia.

While a certain amount of radiation is always present, Dr. Marvin Resnikoff, an international consultant on radioactive waste issues, said the issue is how much greater the radiation levels are when compared to normal levels. Hourly doses, he said, need to be compared to what a person is exposed to over the course of a year.

Radiation dosage is measure in microsieverts. Reports yesterday indicated radiation levels in the Tokyo area were measured at 20 times the background level on Tuesday, at 0.8 microsieverts per hour. That had fallen to 0.06 microsieverts later. A typical chest X-ray is 600 microsieverts.

A big issue, Resnikoff said, is that the radiation released would not be in a single burst as it was in the bombing of Hiroshima and Nagasaki. It would be released as particles that would blow with the winds. If the prevailing winds are from the west, then much of the contamination would go out to sea, but if it does not then it could contaminate large areas.

Another problem is the spent fuel stored at the site. If plant operators fail to keep it submerged in water, which functions as a radiation shield and prevents the fuel from overheating. Overheating fuel could cause fires, as well as generating radioactive isotopes that can be released into the air.

In the case of the Fukushima reactor, the spent fuel stored on the site may be a worse problem than the reactor cores. The reactor cores have two layers of containment: the reactor vessel and the containment building. There are reports of damage to one of the reactor vessels, but for the most part they have stayed intact.

The spent fuel storage areas have only the containment building itself. In one part of the facility the fuel is stored in a building that isn't designed for the same kind of containment as the main reactor building. Two of the reactors, numbers 1 and 3, have lost parts of the containment building as a result of steam explosions.

The water the spent fuel is submerged in has to be pumped in. If it isn't, the temperature gradually rises until it starts to evaporate. When that happens, the spent fuel can melt, just as the fuel inside a reactor can. Overheated fuel rods could generate plumes of radioactive material, said Resnikoff. The heating could drive chemical reactions that generate hydrogen and damage the zirconium cladding that contains the radioactive fuel. Fires also can turn some radioactive elements into aerosols, tiny particles that can travel a long way before they settle to the ground.

Bryant Kinney, a spokesperson for the Nuclear Energy Institute, disputed that exposed cannisters of nuclear fuel would necessarily be a problem as it would depend on whether the cladding was damaged. If not, then much of the release of radioactive particulates would be as noble gases such as Xenon and Radon, which do not tend to settle. Without water around the cannisters boiling away, there is no mechanism to carry away radioactive isotopes of cesium, iodine or strontium.

There are six nuclear reactors at the Fukushima plant. Four were damaged in the earthquake. Three have suffered explosions and one, reactor number 4, has had a fire. There were attempts to pump water to the spent fuel but those were unsuccessful. Reactor 4 had no nuclear material in it as it was being refueled. Reactors no. 5 and 6 were already shut down for maintenance; plant workers are pumping water into them to keep their spent fuel from overheating.