Nuclear Radiation — How Toxic is it?

All nuclear power applications have resulted in an increase of at most only ~0.1% in overall radiation exposure, over previous natural background radiation. And this is just for the small number of people living right next to nuclear power plants. For the great majority of people, there is no measurable radiation exposure from nuclear power. Also of note is the fact that radiation exposure from coal plant emissions is 100 times higher than those of nuclear plants; that along with a host of much more serious pollutants from coal.

Curies are a measure of how many radioactive decays are occurring each second, and the rate at which energetic particles (alphas, betas, gammas, or neutrons) are being emitted within the material. The dose (in Rem, or milliRem) is a measure of biological damage to the body from such particles. More specifically, it is a measure of energy deposited in human tissue, which biological damage is roughly proportional to. The "dose rate" refers to the rate at which dose is received, or the amount of dose received over a given period of time (e.g., per hour or per year, etc.). For doses above a certain (very high) threshold, radiation sickness or perhaps even death occurs. At a lower (medium) levels, cancer risk, in proportion to the dose received, may result. Whether or not low levels of radiation dose, within the range of natural background, have any health effects at all (cancer risk increase, etc...) is the subject of hot debate. Regulatory bodies assume that the linear risk-vs.-dose function applies all the way down to zero, even though there has never been any evidence to date showing a correlation between radiation dose and cancer risk, for dose rates within the range of background (i.e., ~1 Rem per year or less).

The dose rate is a function of many things, the curies of the radioactive material being just one of those. The particles can externally impact the body, if one were standing next to a concentrated pile of unshielded waste. One can also inhale or swallow radioactive material, in which case the material emits particles into the body until the material passes out of the body, or it decays away into a stable material. The dose rate received when standing next to radioactive material is a function of the materials configuration, the shielding that is present, and the energy of the emitted particles (which governs the amount of damage they do and how easily shielded (blocked) they are). Over decades, rigorous "dose conversion factors" have been developed for the inhalation or ingestion of all types of radioactive isotopes. These factors give the total eventual dose (in Rem or millirem) on a per-curie-ingested/inhaled basis. Of course, for a radioactive material to cause exposure, it must get to the population before it can be inhaled or ingested, etc... So how it is stored or buried, and how mobile it is in the environment, etc... all play major roles in how much public health impact it will have. A large pile of material tucked away in a remote place like Yucca Mountain, inside sealed containers, will not have any effect at all on public exposure, no matter how many curies there are (i.e., how many particles are emitted). None of the particles will reach human tissue, and cause dose.

In short, public health risk is proportional to dose (or dose rate), not curies. And the maximum possible public doses from nuclear energy applications (including normal operation, or accident conditions like a plant accident or a leaking Yucca Mountain) have all been rigorously calculated. The results clearly show that nuclear power applications do not increase annual radiation exposure (dose) over natural background sources by any meaningful amount ( << 0.1%). It should also be noted that the total amount of curies in the radioactive materials naturally occurring in the earths crust, is orders upon orders of magnitude greater than the curie content of all nuclear waste materials. Not only that, much of these natural materials are less isolated from human contact than are the nuclear waste materials (inside nuclear plants or in the Yucca Mountain Repository) a greater fraction of their emitted particles will contact human flesh, resulting in dose.

Betas are high energy electrons. They are the most common radiation emitted by fission products. As atomic weight increases, the ratio of neutrons to protons for stable atoms (or semi-stable atoms, at least), increases. Thus, the higher neutron/proton ratio required for stability at high atomic numbers would actually be too high, and cause instability, and lower atomic numbers. When uranium-235 (which has a high neutron proton ratio, with 143 neutrons and 92 protons) fissions, it produces two smaller (~half-size) atoms with the same high neutron/proton ratio. At these smaller atom sizes, this neutron/proton ratio (~1.5) is unstable, so the fission products are radioactive. An atom with too many neutrons generally decays via beta emission. Instead of sending off a neutron, one of the neutrons in the atom’s nucleus emits a negatively charged electron (i.e., a beta particle) which changes its electric charge from neutron to positive. This causes the neutron to change into a proton, which greatly reduces the neutron/proton ratio. In fission products, electrons (betas) are emitted until an acceptable neutron/proton ratio (and thus, stability) is achieved. Betas are charged particles, which have a speed which increases with the energy of emission. As they are smaller than alphas, they are harder to stop, and travel farther in matter. Thus, more shielding is required to stop betas (perhaps a thin metal sheet or foil, or a larger distance in air).

Gammas are a form of radiation, like visible light, but with a much higher photon energy (and shorter wavelength). Gammas always travel at the speed of light, but they have various energy levels, with higher frequencies (and lower wavelengths) being associated with higher energy. Gammas are much harder to shield than any charged particle, as their lack of charge reduces their level of interaction with matter. As the energy level gets higher, they get much harder to shield. Thick layers of steel, lead, or concrete are required to shield gammas, and air provides negligible shielding. Sometimes, after various nuclear reactions, the nucleus of an atom winds up in an energized state (think of it as the neutrons and protons having a lot of kinetic or potential energy within the nucleus). Such nuclei decay down to the “ground” energy state by emitting gammas, which carry away the excess energy. In such a reaction, no particles are lost from the nucleus, or change their charge. They just lose some energy.

Finally, there are neutrons, which are only emitted by fission itself (2 or 3 neutrons are emitted along with the two fission product atoms). The only other sources of neutrons are the spontaneous fission of certain rare, exotic atoms (Cm-252, etc...), and some other rare nuclear reactions. For this reason, neutron sources are rarer, and the number of neutrons emitted (in spent nuclear fuel, for example) is several orders of magnitude smaller than the rate of gamma or beta emissions. The down side is that neutrons are hardest of all too shield (hydrogen-bearing materials work best) and it is the only type of radiation that can actually make material it is absorbed in radioactive.

For alphas, betas, and gammas, the damage is done through the deposit of the particle's energy in the body. When a person is struck by alphas or betas, ALL of the energy will be deposited in their body, as it is clear that the particles will not make it through the body. For gammas, most energy is likely to be deposited, and the gamma is likely to be fully absorbed, but there is some chance that the gamma will exit the body (on the other side) while retaining some of its energy. As I discussed earlier, radiation dose is a measure of energy deposited in the body. Thus, the amount of dose received is primarily a function of the total energy of the particles that actually strike one’s tissue.

Neutrons are an exception to this, as they can cause significant damage regardless of their energy level. This is because, unlike the other particles, they mainly cause damage by causing nuclear reactions within the body. When they are absorbed by an atom in the body, a nuclear reaction occurs and energy is released (in the form of betas or gammas). In some cases, the neutron can transmute the atom into another element, which can also cause additional damage from the chemical change (which may disrupt the chemical properties or processes of various protein or DNA molecules).

For the most part, alpha and beta emitters are only harmful if ingested or inhaled. They are so easy to shield (by the air, if nothing else) that it is very difficult for a person to get a direct dose from a pile of such material. Even if alphas or betas struck the body’s exterior, they would mostly be absorbed in the outer skin layer (including a thin dead skin layer that we all have). If they reside inside the body, however, all of the energy in these emitted particles is clearly deposited in the body. For gammas and neutrons, however, shielding is difficult and direct doses are a problem when dealing with higher level wastes (such as spent fuel assemblies). For instance, most of the (small) dose to the public that would occur from spent fuel storage or transport casks would be in the form of direct gamma and neutron radiation, that makes it through the shielding and directly impacts the body (from the outside). That is, unless the casks were breached in some (unlikely) hypothetical severe accident.

As I stated before, health effects are generally proportional to dose (or perhaps dose rate), which in turn is defined as an amount of energy deposited in the body by emitted particles. For direct radiation, the energy deposited is a function of the number of particles hitting the body, and their energy level. The overall “whole body” dose is basically proportional to the amount of overall energy in the particles that strike the body (neglecting the rare effect of particles that pass all the way through). For high-level waste sources, these direct doses are calculated based on the amount of radioactivity present, the types of radiation and their energy level, the shielding configuration around the source, and the proximity and residence time of the exposed individual(s).

Unlike the direct radiation case, it is known that for inhaled or ingested radioactive material, ALL of the energy in the emitted particles will be deposited inside the body. Thus, the eventual overall dose is a function of the rate at which that radioactive material emits energy (in the form of emitted particles) and the time period over which the material resides within the body. Let’s say that the ingested material initially emits energy at a given rate. Two factors will act to reduce this release (and energy deposit) rate over time. First of all, the radioactivity level falls over time as the material decays, with the rate of fall off being governed by the half-life of the material. The other means by which the energy deposit rate is reduced is the elimination of the material from the body via various biological processes. There is a “biological half-life” defined for each radioactive material, which is a function of its chemical properties. The two half-lives are combined to form an overall effective half-life, which determines the rate at which exposure (or energy deposit) falls off. These factors determine the overall eventual dose (i.e., amount of deposited energy) that will occur from the ingestion of a given amount of radioactive material.

Scientific reference documents list “dose conversion factors”, for each isotope, which give the total eventual whole body dose, for each unit of radioactivity level of that isotope. Thus, the factors convert, for each isotope, an activity level in curies to an eventual total dose, in Rem (or milliRem). Such factors are given for ingestion and inhalation (i.e., the eventual dose in Rem, per curie ingested or inhaled). These factors account for all the effects above, including the isotope’s half-life, as well as its biological half-life (i.e., its rate of elimination from the body). There are factors for “whole body dose” as well as specific factors for individual organs. This accounts for effects where certain radioactive substances tend to concentrate in certain organs (like radioactive iodine, in the thyroid, for example). Thus, there is a relatively high thyroid gland conversion factor for iodine-131. The whole body doses (calculated using the whole body dose conversion factors) are compared to whole body dose limits, and specific-organ doses (calculated using the organ-specific factors) are compared to organ-specific dose limits. With nuclear regulation, both the whole body limits and the limits for all individual organs must be met.

The direct radiation effect is the one thing that is different about nuclear waste, as compared to other toxins, which may be the source of some of the fear and mystique. All other toxins required ingestion or inhalation for harm to occur. Radioactive material is the only toxin that can “strike from a distance”. This is because chemical toxins need to be in the body to cause chemical changes that harm cells and biological processes, whereas radioactive material emits high energy particles that can travel over distances.

It should be noted, however, that the direct radiation effect is never a significant player in terms of risk to the public from nuclear energy applications. Due to the long residence time, and the fact that there is no shielding effect, ingesting a given amount of radioactive material is vastly more damaging than simply standing next to that same amount of material. Thus, radioisotopes represent much more of an ingestion and inhalation hazard. Direct radiation only becomes a factor when enormous quantities of radioactivity are present within a small volume or area, enough to kill someone a million times over if that amount of activity were ingested or inhaled. Thus, direct radiation is only really a factor for personnel who work at nuclear plants, where spent fuel assemblies, and other massive activity sources are handled. Even with spent fuel cask transportation, the direct doses to the public are tiny, due to the heavy shielding and the short times over which people are near casks. For casks, the only real public risks involve the extremely small likelihood of cask breach. Even these risks, however, are negligible compared to the risks from chemical shipments.

Whether in the case of a plant accident (meltdown), a cask breach, or from a leaking repository, direct radiation is never a significant factor with respect to total public health impact. Instead, the effects come from dispersal of radioisotopes onto the land, air, and water, and the subsequent ingestion or inhalation of those isotopes. In all cases, the concentrations of radioisotopes are far too small for the soil, water, or air in question to cause a significant direct radiation dose to a nearby person. Instead, these isotopes are ingested or inhaled, and they then spend a significant residence time in the body, causing various health effects. But in this respect, radioactive material does not behave any differently from any other toxin. It basically has to be inhaled or ingested to have effect. Once inside the body, it does damage, in proportion to amount ingested, by causing various forms of chemical damage (to proteins, DNA, etc…). The radioactive material can NOT impact a person from a distance, i.e., just from standing next to the material, because the concentrations or amounts present are simply millions of times too small. Thus, although the mystique exists, it will never come into play in any real way, in any real situations. For all intents and purposes, radioactive material does not behave any differently than any other toxin.

All studies show that health effects are a function of energy deposited in the body from emitted particles. They are not a function of anything else, although the rate at which energy is deposited may play some role (with a large amount of energy deposited over a short time period likely having a greater effect, whereas a slow constant rate of exposure, beneath some threshold, is likely to have no effect). Doses of hundreds of Rem over a short time period will cause radiation sickness and even death. Dose rates of ~10-100 Rem, over extended periods, are shown to increase cancer rate, with the cancer risk proportional to the overall exposure. However, no studies have ever shown a measurable increase in cancer rate for exposure rates of fewer than 10 Rem/year. Common natural background dose rates vary from 0.1-1.0 Rem/year, with dose rates of over 10 Rem/year in some locations. No studies have ever shown any correlation between cancer rates and natural background dose rates.

The dose rates to people living near nuclear plants are ~0.1 milliRem (i.e., 0.0001 Rem) per year, less than 0.1% of natural background. No measurable dose rates occur for people living further away. Even in the case of a worst-case plant meltdown, the dose rates for all but a very few people would STILL remain within the range of natural background. The land area over which the contamination is such that the dose rate would fall outside the range of naturally occurring background levels is quite small (very small, if a 10 Rem/year standard were used). It is actually clear that any health impacts would be quite limited, even in this worst-case scenario, especially when all the emergency planning measures that are in place are considered.

For Yucca Mountain, the whole issue is that EPA set a maximum allowable dose rate standard of 4 milliRem per year (i.e., ~1% of natural background) and required that it be proven that not even a single person (even under the most hypothetical of conditions) be exposed to more than 4 milliRem over the entire repository life. That used to mean 10,000 years, but now with the court decision it means at any point over all time. Never mind that such a standard has never been used (with an arbitrary 10,000 year limit being applied for all toxic chemical dumps, etc…). This is all quite astonishing, since no health effects are observed under 10,000 milliRem, and since millions of people have lived in 1,000 milliRem background areas all their lives, throughout history, with no resulting health effect. A tiny chance of having a handful of people being exposed to 5 or more milliRem is considered unacceptable, but nothing is ever done about (or even talked about) the millions of people living in 1,000 milliRem of natural background, or the 100 million in the US that routinely get hundreds of milliRem per year from radon in homes, or tens of milliRem that millions of people get from flying each year, etc…… The hypocrisy and double standards are beyond belief. Even the absurd 4 milliRem standard was doable with the 10,000 year time limit. Now, with the court action, the limit will have to be raised to a remotely reasonable value, or the project will fail, for NO reason.

The amount of radioactivity in nuclear waste is tiny compared to the total radioactivity on earth (i.e., in the earth’s crust). Thus, it does not appreciably add to overall radioactivity level. However, as stated above, the mere presence of radioactive material is irrelevant, since only materials that enter human bodies has any effect. It is more a question of how much nuclear waste material gets inside humans, as compared to natural radioactive material. In other words, what is important is the comparison of dose, i.e., of collective exposure. Here, the results are even clearer. Doses from nuclear energy are one thousandth of those from natural sources, even for the most exposed people. For the average person, it is more like a million times smaller. The increase in overall collective exposure to radiation for the world’s population from nuclear energy is absolutely negligible compared to the doses that they get from other sources, mostly Mother Nature. And it will always be this way, as no set of events or circumstances (i.e., hypothetical meltdowns, repository failures, etc…) will ever come even close to changing this situation.

Recommended reading:

Radiation And Modern Life: Fulfilling Marie Curie's Dream
—Written by Alan E. Waltar

Introduction by Dr. Hélène Langevin-Joliot, granddaughter of Marie Curie

“Radiation has existed since the very beginning of the universe...”