Of all boogey men in society, the Radiation Boogey Man is probably the most feared. The threat posed by radiation from nuclear power is discussed here. First, it is necessary to know what ionising radiation is.
Ionising radiation is the flow of particles of sufficient energy to ionise atoms. When an atom is ionised, it is stripped of one or more of its electrons and develops a net electric charge. It has become an ion. Because ionised atoms are at a higher energy level, ionisation of a material causes its temperature to increase slightly. This is how nuclear decay can be a source of heat. It is not usually difficult for an atom to regain its missing electrons from the surroundings so ionisation of an inanimate material will not cause any long-term effect aside from a small increase in temperature. However, if ionisation happens in delicate molecular structures, it can cause permanent disruption. In modern computers, which depend on microscopic wiring in the form of microchips, the discharges caused by ionisation can destroy information and otherwise damage the hardware. In biological material such as DNA, ionisation can cause unwanted chemical processes to happen, which can destroy and change the genetic code and this can manifest itself in cancer and radiation poisoning if the cells' own self-repair mechanisms do not fix the damage.
Figure 1- Absorption of nuclear radiation. Alpha radiation will be absorbed by a piece of paper, beta by a plank of wood and gamma and neutron radiation by a block of concrete or lead.
The main culprits are photons from high energy ultraviolet and high frequency electromagnetic radiation, protons, neutrons, electrons and helium-4 nuclei. Of course, any high energy particle can ionise. Nuclear radiation principally applies to alpha, beta and gamma radiation released through decay and neutron radiation released through fission and some types of fusion. There is also proton radiation, but that is only significant in cosmic radiation and solar wind and so is only of importance to spacecraft.
To recap, an alpha particle is simply a helium-4 nucleus emitted from a decaying radioisotope. The designation is purely historical because the true nature of nuclear radiation was not at first understood. Alpha radiation is the most ionising of the decay radiations because it is heavy and highly charged. The corollary of that is that it is the easiest to shield. Alpha particles will not be able to penetrate a piece of paper, a few centimetres of air or the dead outer layers of skin.
An alpha emitter with little gamma like plutonium-240 is highly manageable because as long as you do not eat it or handle it unprotected, you will not be affected by its radiation. However, radon-222 is also an alpha emitter with no gamma and is gaseous, so we are constantly breathing it in and having our lungs ionised. Exposure to radon accounts for more than half our total exposure and is natural in origin.
Beta particles are electrons (beta-negative) or positrons (beta-positive) but both ionise in the same way. They are very light and have half the charge of an alpha particle and so are not as ionising. Similarly, they are more penetrating. Beta particles will be stopped by about 20cm of air, or a plank of wood.
Gamma rays are electromagnetic radiation, like light, but of considerably higher energy. They have no charge and as such are far less ionising than alpha or beta but are also far more penetrating. Lead or concrete are necessary to suitably attenuate gamma rays, but overall, they do not represent the same ionisation hazard as alpha or beta.
Neutrons are heavy but have no charge. They do not directly ionise, but they can be absorbed by nuclei to form a new isotope that is one nucleon heavier, called an activation product. Frequently, these heavier isotopes are very unstable and will decay. In this way, they represent a larger hazard than radiation from decay, although because they are produced only in specific circumstances, in power reactors, measures can be taken to provide adequate shielding.
Radiation dosage
You should already be familiar with the term activity, which describes the rate of decay in a sample of radioactive material. At this point, we must introduce two other terms: absorbed dose and dose equivalent.
Absorbed dose is basically a measure of how much energy is deposited in a mass of some material by ionising radiation. The SI unit is the gray, which the dose of one joule per kilogram. The older English unit is the rad, equivalent to a hundredth of gray.
Dose equivalent is the energy deposited in a mass of human tissue corrected for the quality factor of the culprit particles. The quality factor refers to the different effect that different ionising particles have depending on their type. Some particles are better at depositing their energy in human tissue and so the quality factor describes their relative potential. Dose equivalent is measured in sieverts mostly but the older English unit is the rem, equivalent to a hundredth of a sievert.
How much is too much?
It is important to understand that radiation dosage given in becquerels or curies is meaningless in terms of assessing risk, at least without further information. For example, radon-220 decays by alpha emission, with each alpha particle having on average 6.29MeV. So to receive a dose of 1Gy each second by having a sample of radon-220 ionise 1 kilogram of your tissue (don't try this at home as it is the energy equivalent of dropping an apple from one metre above each second on this 1kg of your tissue), the activity of the sample would need to be less than 28Ci. On the other hand, tritium decays by beta emission, with the beta particles having on average 0.0186MeV. So to get the same dose, the activity would need to be more than 9000Ci. Depending on the isotopes observed, activity can be significantly higher while actual dose is much lower.
So when someone tries to scare you by saying "1000Ci of radiation were released from Conway nuclear power station! Run for the hills!" you have to ask yourself what this value means in terms of health hazard. It could be alarming if it were radon-220 (not that alarming because it would be dispersed over a wide area so only a minute fraction of the energy would actually ionise the tissues of the public), but if it was tritium, it represents an insignificant amount of energy.
Even more insignificant is the quantity (so many tonnes or cubic metres). 1g of uranium-239 has an activity of 34 million curies, while 1g of uranium-238 has an activity of just 12kBq or a third of a microcurie.
Beyond quantity and activity, there is absorbed dose, measured in grays. But again, there is a problem with using this figure. In order to say how this will affect human tissue, the value must be adjusted for the quality factor of the radiation involved. So 1Gy of neutron radiation is more threatening than 1Gy of beta radiation. Correct values are the dose equivalent.
| Maximum permissible dose rate for the general public | 5mSv per year |
| Maximum permissible dose rate for radiation workers | 50mSv per year |
| Natural background dose rate | 1.25mSv per year |
| Dose rate due to industrial, medical and agricultural use | 0.12mSv per year |
| Maximum dose rate due to atmospheric bomb testing (1954-61) | 0.012mSv per year |
| Average dose rate due to nuclear reactors (general population, 1980) | 0.002mSv per year |
| Threshold for the induction of cataract | 15,000mSv life total |
| Threshold for nausea | 1,000mSv in a few hours |
| Threshold for fatality | 1,500-2000mSv in a few hours |
| 50% fatality within 30 days (from infection) | 3,000mSv in a few hours |
| Gastro-intestinal death within 3-5 days | 10,000mSv in a few hours |
| Central nervous system death within a few hours | 20,000mSv in a few hours |
Table 1- Important radiation figures according to the American Institute of Physics Handbook
Only when values of dose equivalent are given can there be an accurate picture of radiation hazard. Be aware this is not a simple calculation. A certain quantity of a given isotope has a given activity and energy that can be absorbed, but how much of this energy will be absorbed by an innocent bystander rather than some brick or oxygen molecule is where the uncertainty lies. A release of tritium might be able to give you a theoretical dose of 1mSv per year, but that may be the case only if all the radiation from this release goes to ionise your tissue. Make sure you know to what specifically dose equivalent figures refer. Even then, radiation dosage is a science of statistics. A certain dose may kill one person within a few days while another might live to a ripe old age and die from a shorting toaster (not likely but possible).
Now we get to the point. What dose equivalent is threatening? Table 1 shows the figures published by in the American Institute of Physics Handbook. This is by no means uniquely authoritative and other sources give different estimates, but there are approximate to an order of magnitude. The lethal doses are the most controversial because there is no direct evidence for them.
There is however a greater controversy over the extrapolation to low doses.
The linear no-threshold hypothesis
Radiation protection is currently based on the principle of ALARA, As Low As Reasonably Achievable. This states that all radiation exposure must be avoided where practical. It based on the linear no-threshold hypothesis.
Following the bombing of Hiroshima and Nagasaki, much research was done into the health effects of those exposed to radiation from the blasts. The victims under study had all received high doses of gamma radiation, with exact amount depending on their location relative to the blast. This was an opportunity to study to the effects of radiation on risk to human health, particularly cancers like leukaemia. The conclusions were that the risk of getting cancers was directly proportional to the radiation dose. This is the linear relationship.
However, the empirical evidence only applied to high doses, not the low doses we encounter in every day life and the kinds of doses we would expect to be brought upon through various human activities. The only way at the time to reach a conclusion about the effects of low doses, was simply to extrapolate and assume the linear relationship continued all the way down to the bottom. This is the no-threshold attribute of the hypothesis.
Hence the linear no-threshold hypothesis was adopted, which stated that all radiation is harmful and that harm is proportional to the dose. And so ALARA has been used ever since.
To illustrate, if someone takes a hundred aspirin, he is likely to die. If we applied the LNT to this, we would expect a death for every two hundred aspirin taken regardless of how sensibly they are used.
However, evidence since then has come out against the LNT. It has been found that people in high altitude areas, less shielded against cosmic radiation, have a lower risk of contracting radiation induced cancers than those in the low radiation environments of the coastal areas. Further controversy was created studying the aftermath of Chernobyl. While the bombing of Japan resulted in a short high level exposure, Chernobyl caused a long term, elevated but still low level exposure to the affected areas. Studies, including from UNSCEAR themselves have shown the dramatic cancer predictions spectacularly failed to materialise and studies on animals have shown no sign of chromosomal damage.
This has lead radiation scientists to suspect that there is indeed a threshold at low levels below which radiation exposure does not cause cumulative harm. Some go even further. Observing animals from the Chernobyl exclusion zone and the populations of high radiation environments, such as Ramsar in Iran, there is evidence that low level exposure to radiation triggers cell repair mechanisms, increasing resistance to cellular damage, leading to a reduction in cancer risk and increasing health. This process is called hormesis.
While the exact effects of low level radiation exposure remain the subject of much debate, the increasing body of evidence suggests that the assumption of the LNT was a rash decision and that the risks of low level exposure are overestimated.