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i24

Limited knowledge on radioactivity

Not all radioactivity is measured

Nuclear power is inseparably coupled to the generation of large amounts of man-made radioactivity, distributed over dozens of different radionuclides (radioactive isotopes of the chemical elements) [more i08]. Each radionuclide has its own physical and chemical properties and biological behavior.

Unavoidably a substantial part of the man-made radioactivity escapes into the environment, partly by authorized routine releases, partly by leaks and small accidents, and partly by large disasters, such as Chernobyl and Fukushima. Via these pathways all kinds of radionuclides are released into the human environment [more i17]. A part of the radionuclides are dispersed in gaseous form, another part as aerosols, a third kind solved in water. The radionuclides are entering the food chain and drinking water supply. A number of kinds of radionuclides tend to accumulate in living organisms, locally resulting in high radioactivity. Some kinds of radionuclides are biologically very active in the human body, such as tritium (radioactive hydrogen, symbol T or H-3), carbon-14 (radioactive carbon, symbol C-14) and radioactive iodine (symbols I-129 and I-131).

Troublesome detection of radioactivity

The presence of radionuclides is not always easily detectable. Several radionuclides, including tritium and carbon-14, cannot be detected by handheld detectors. Some dangerous alpha emitters, such as plutonium, are also difficult to detect, the more so in the presence of strong gamma emitting radioaclides. All these difficult radionuclides require special equipment. Radiometric surveillance after a severe accident such as Chernobyl and Fukushima focuses on the detection of cesium-137, an easily detectable radionuclide. From the measured intensity of gamma rays emitted by cesium-137 a dose rate (usually measured in millisieverts per unit time) is deduced: the amount of radiation absorbed per unit time by an individu at that location.

Apparently the nuclear industry assumes that the concentration of cesium-137 at a given location is a good measure of the health risks of the local people. This simplification ignores two important issues:

• Due to their very different physical and chemical properties , the dispersion of the various radionuclides during and after an accident is widely different. Absence, or near-absence of cesium-137 does not warrant the absence of other, probably more dangerous radionuclides in soil, water and air.

• Even if the dispersion of cesium-137 would be a good criterion for the dispersion of other radionuclides, then the simplification ignores the different pathways of radionuclides into the body (inhalation of gases and aerosols, ingestion via food and/or water) and the widely different biological activities of the various kinds of radionuclides in the human body.

By not monitoring continually the presence of a number of dangerous radionuclides in food, water and air, including the difficult ones, a false sense of safety could be roused. Necessary actions might remain undone, causing avoidable harm to people. Another serious consequence of the absence of monitoring is the fact that no data are collected for future epidemiological studies. In this way it becomes impossible to prove unambiguously the adverse health effects of a nuclear accident. The nuclear world then can keep playing down the health effects of a given accident.

No measurements, no knowledge.

Biomedical unknowns

A number of radionuclides has been investigated to some extent, other nuclides (e.g. carbon-14) practically not. The empirical database on effects of radonuclides in the human body seems to be very meager. Synergistic effects are unknown basically. What are the effects of several radionuclides together in a biological system?

The classical explanation for radiation's effects was that they were mostly caused by structural DNA damage which resulted in mutations in the cell's genetic information that, without repair or elimination, would end eventually in cancers. This is the target theory of radiation effects, the target being specific sequences in DNA and chromosomes. The doses causing non-targeted effects are too low to cause structural DNA damage. The dose-response curve of these effects is often not linear, with substantial increases at very low doses followed by a levelling off at higher doses. Presently there is no mechanical explanation for how the non-targeted effects actually occur. The target for radiation damage is greater than the initial tissue volume irradiated [more i22].

The observed phenomena pose many fundamental questions to be answered and result in a paradigm shift in the understanding of radiation biology.

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