How Radiation From Atomic Energy Programs
To understand why there is grave concern about nuclear electricity generation, it is necessary to know just how nuclear electric power exposes human beings and other living things to the danger of being irradiated. And, it is essential to understand a few extremely simple points about radioactivity, if we are to bypass the confusion on safety points generated by well-planned propaganda campaigns.
The nuclear fission of uranium or plutonium releases enormous energy. This is the energy, ultimately converted to heat, which produces steam to drive turbines and produce electricity. If this were the only energy released when uranium or plutonium atoms split, nuclear electricity might well have been a real boon to mankind, providing electric power for decades or centuries. Unfortunately, the fissioning itself is only the beginning. There is another source of energy involved—after the fissioning is completed—a source which creates the extreme radiation hazard that accompanies nuclear electricity generation.
The uranium itself decays or disintegrates very slowly. This is why uranium is still present on earth so long after the earth was formed. Uranium is radioactive, but only feebly so. The kind of uranium (uranium-235) which maintains the chain reaction in a nuclear reactor decays by emitting so-called alpha particles very slowly, so slowly that it takes 710 million years for one-half of the uranium-235 atoms to disintegrate. For any radioactive substance, the time required for one-half of it to disintegrate is called the half-life. Thus, uranium-235 has an extremely long half-life and, consequently, the radioactive hazard of uranium-235 is very small.
When a uranium-235 nucleus splits, two (usually) "fission fragments" of the original nucleus are produced. What are these "fission fragments"? They represent what is left after a neutron has disintegrated the uranium-235 nucleus and after some additional neutrons have escaped during the fission process itself. It has been discovered that these fragments are nothing more or less than variant forms of elements commonly occurring in nature.
|A fission reaction.|
The vast majority of the variant forms of the elements found in the earth's crust are referred to as stable. This means that they do not decay radioactively or create radiation as does uranium.
The variant forms of any particular element are called nuclides of that element. So, we can say that in nature we find several stable nuclides for one particular element.
The fissioning of a uranium-235 nucleus can and does produce stable nuclides. But it also produces radioactive nuclides of many chemical elements. Any particular uranium nucleus has a certain probability of splitting, or fissioning, to produce any one, of many hundreds, of stable nuclides or radionuclides, of many different chemical elements. Thus, while one cannot predict which stable nuclide or radionuclide will be produced by the splitting of a particular uranium nucleus, we do know with great accuracy, now, how many times out of a million uranium fissions a radionuclide—for example, tin-123 or iodine-131—will result.
Remember that the stable nuclides produced, when the uranium atom splits, are the same as stable nuclides already present in nature. Thus, biologically they create no problem whatever. But, the radionuclides produced at the same time, represent most unwelcome by-products of the operation of nuclear reactors for electric power generation.
So long as the radionuclides remain in the nuclear reactor, they create no problem. True, some of the decaying radionuclides within the reactor do produce gamma rays which can penetrate through steel or concrete, but sufficient shielding and distance can reduce any risk from this source to a negligible level. But, should such radionuclides get outside the reactor—they can and do so—a host of problems can ensue. The problems arise because humans and other living things can be irradiated by them.
A human can be irradiated by such radionuclides from outside his body (external radiation), or from inside his body (internal radiation). There is no difference in biological effect, whether the radiation is external or internal, provided the same amount of radiation is absorbed in a particular part of the body.
The common forms of radiation resulting from uranium fission all have the same effect upon living tissue which absorbs them.
It also makes no difference, biologically, whether the tissue receives a specific amount of radiation, in a given time period, from a radioactive particle of long half-life (like thousands of years) or from one of short half-life (like a few days).
Knowing these basic facts, we can see that the only real issue that concerns us, about any radioactive material, is how much radiation is absorbed by a single portion of tissue plus the length of time in which this absorption occurs. In order to systematize all radiation measures, radiant energy absorption is measured by the RAD unit. A substance is said to receive 1 RAD of radiation when it absorbs a given amount of energy per gram of that substance.
One rad delivered to a piece of muscle will have essentially the same effect whether it comes from a dental x-ray machine, from radioactive atmosphere irradiating the muscle from the outside, from gamma rays from radioactive substances in the earth, or from radioactive substances taken into the body with food or water. And it matters not at all, whether the source of radiation is a nuclear power reactor or any other source. One rad is one rad for nearly all types of radiation, from any source for all practical purposes.
If only the rads that are absorbed matter, why bother about external versus internal radiation, or about the differences from one radioactive element to another?
(a) External versus internal radiation
The various radioactive elements produced by uranium fission emit beta rays of varying speeds. The speed determines how far the beta ray can penetrate into the body. Thus, a radionuclide may emit only very low-speed beta rays that don't even penetrate beyond the deep layers of the skin. The internal organs are safe when such a radionuclide is placed outside the body. However, the damage to skin from such an external source can be severe.
|This rat is one of a group of experimental rats that were each exposed to a single dose of beta rays. Six months after irradiation, the group of rats began to develop skin tumors. Tumors were still appearing at 14 months after the single dose of beta rays. Chance of tumor rises with radiation dose.|
On the other hand, if this same radionuclide were ingested with radioactively contaminated food or water and distributed uniformly throughout the body tissues, all body organs can be irradiated by it.
The case is the same for x-rays or gamma rays. Low-energy x-rays or gamma rays from outside the body may not affect a deeply-situated internal organ. On the other hand, if the radionuclide emitting those same x-rays or gamma rays is taken into the body, it can affect any organ its x-rays or gamma rays reach.
(b) Concentration in organs
We must recall that the radioactive elements produced in a nuclear reactor behave almost precisely as do their non-radioactive counterparts. For example, radioactive iodine-131 behaves chemically and biologically just as does stable, or non-radioactive, iodine.
But chemical elements do differ from each other in how they distribute themselves once they are inside the body. Iodine is interesting because the thyroid gland has a special affinity for iodine. As a result, the thyroid accumulates far, far more iodine from an ingested dose than any other body organ does. The thyroid uses iodine to manufacture its major active hormone, thyroxin.
The radioactive forms of iodine (iodine-131 is one) behave just as non-radioactive iodine would when taken in with food, accumulating preferentially in the thyroid gland. As a result, that tissue receives a far higher radiation dosage in rads from the decaying radioactive iodine than other tissues of the body do. Naturally, radioactive iodine has its major biological effects on the thyroid gland, compared with its effect on other cells in the whole body.
There is nothing special about the radioactivity of iodine-131 that makes the thyroid gland vulnerable. Precisely the same injurious effect to the thyroid gland could come from x-rays out of an x-ray machine, or from a source of external radioactive cobalt. Only the rads that accumulate in the thyroid gland cells during a particular time matter in assessing possible damage. If half the radiation comes from the outside (as from an x-ray machine) and half from ingested iodine-131 the injury to thyroid cells will be the sum of the rads delivered by both radiation sources.
Certain chemical elements taken into the body along with food or water do not concentrate in specific tissues or organs. Instead, they distribute themselves throughout the body. Cesium, produced abundantly in its radioactive form, cesium-137, by uranium fission, does precisely this. It is said to produce whole-body radiation. Again, the radiations from cesium-137, radioactive iodine-131, or any other radionuclide, or x-rays from a machine are comparable. The effect upon cells anywhere in the body depends solely upon the number of rads they absorb in a particular time period.
(c) The role of chemical similarity between elements
Certain chemical elements are grouped together because they are particularly similar to each other in chemical (and, hence, biological) properties. Lithium, sodium, potassium, rubidium, and cesium represent one such chemical group. Though these elements are by no means identical in chemical or biological behavior, they do show numerous marked similarities in properties, including the way living things use them in their bodies.
Potassium is a prominent, vital constituent of the interior of every living human cell. Fish living in fresh water, where the concentration of potassium is very low, may be forced to concentrate the potassium 1000-fold, in order to maintain the concentration necessary to sustain life. Because cesium is chemically quite similar to potassium, the same mechanism also concentrates cesium from such a fresh water source approximately 1000 times. If the cesium in the fresh water happens to be radioactive cesium-137, from a nuclear reactor or other source, then the fish will contain 1000 times more cesium-137 than the fresh water itself, on a weight-for-weight basis.
So here is an illustration of how the remarkable similarity in behavior of certain chemical elements can lead to massive concentration of radioactive substances in living tissues, over that existing in the inanimate environment. This can be very serious, not because the particular radioactive nuclide is different in kind from others, but because concentration through such a mechanism finally leads to a high dose in rads to the tissues exposed. Drinking the water might expose one to very little radiation. Eating fish from that water would expose one to 1000 times more cesium-137 radiation, on a weight-for-weight basis.
(d) The role of half-life of the radioactive nuclide
Some of the radioactive substances that occur as a result of nuclear fission in a nuclear reactor are extremely unstable, half of the radionuclide decaying or disintegrating in a matter of seconds. In contrast, radioactive strontium-90 has a half-life of 25 years, radioactive cesium-137 a half-life of 33 years. But what counts, in terms of biological injury to living beings is the number of rads absorbed in a tissue per unit-time, not the half-life of the particular radioactive nuclide which is irradiating tissue. Thus, 10 rads from a radionuclide having a half-life of 2 seconds is the same as 10 rads from a radionuclide of 30 years half-life.
However, there is an important difference between the very short-lived radioactive element and the very long-lived one, in terms of potential harm. For the short-lived nuclide, 10 rads delivered in the course of 5 minutes may be all the radiation that will ever be received by the tissue, simply because essentially all of the radioactive atoms of that radionuclide have decayed away.
However, for the long-lived radionuclide, not only can it deliver 10 rads in the course of 5 minutes, but it can continue to deliver this amount, or nearly this amount, of radiation every 5 minutes for years and years. This is the essence of the difference in potential hazard for long-lived versus short-lived radionuclides. Rad for rad, however, they are identical.
Another way that half-life of the radionuclide becomes important for biological reasons concerns the chance it has to get out of a nuclear power facility in time to do biological harm. If a radionuclide has a half-life measured in seconds, it is clear that if it can be contained for an hour before being released, essentially all of it is gone before reaching any living tissue. On the other hand, radiostrontium-90 or cesium-137 have half-lives of the order of 25-35 years; not only must we worry about them getting out of a nuclear reactor, but we must worry about them for several centuries!
Such radioactive elements must be kept from intersecting with living things, man in particular, at the reactor, during spent fuel transportation and at the fuel reprocessing plant. And finally, the highly radioactive waste must be guarded for something like 500 years. When a radionuclide has a half-life of 30 years, an appreciable amount of radioactivity will persist several hundred years.
(e) Whole body radiation versus partial body radiation
Obviously, irradiation of the entire body with any number of rads is more serious than irradiation of a particular part, some organ for example. It is not easy to state flatly whether irradiation of one specific part of the body is worse than irradiation of some other specific part. In the case of the male and female reproductive organs, however, the damage done during reproductive years guarantees great harm.
Since the gene-containing cells for future generations of humans reside in these organs, irradiation here will cause the genetic (inherited) alterations which can produce mental and physical deformities, and a host of serious diseases in future generations. And since genetic injury is the most serious injury produced by radiation, it is true that irradiation of ovaries and testes is almost as serious as irradiation of the whole body (which, of course, includes the ovaries and testes).
For all the remaining organs, in adults at least, cancer is a major hazard and is almost certainly due, in a specific organ, to irradiation received by that organ. At first thought, it might appear that the organs could be ranked by weight to determine the seriousness of the irradiation for cancer induction. The cancer risk for any specified amount of radiation appears more related to the spontaneous cancer risk for that organ than to its size.
As we have seen, in most cancer induced by radiation, the radiation directly affects the cells which may become cancerous later. However, from certain animal studies, it appears that radiation in one part of the body can result in a lymph cancer developing elsewhere. This is not common.
(f) Age an important factor
No factor is of greater importance in considering the implications of delivery of radiation to humans than is age. Direct evidence has been provided by Dr. Alice Stewart of Oxford, England that developing embryos are vastly more sensitive to the cancer and leukemia producing effects of radiation than are adults. In fact, a given amount of radiation increases the risk of future cancer or leukemia 50 times more if delivered to the embryo during gestation than if delivered to adults. Next to the sensitivity of the fetus in utero are children, and then come adults. Unfortunately, even the sensitivity of adults to cancer production by radiation is 10 to 30 times more than "expert" bodies of scientists thought up until the last few years.
The embryo presents other special problems too. Radiation, received at a time where the various organs are being formed, can cause a whole organ system to be deformed. For example, early radiation can lead to serious brain injury with resultant mental infirmities. This was seen in Hiroshima.
|This is the hand of a physician who was exposed to repeated small doses of x-ray radiation for 15 years. The skin cancer appeared several years after his work with x-rays had ceased. Cancer incidence depends on radiation dose. From Meissner, William A. and Warren, Shields: Neoplasms, In Anderson W.A.D. editor; Pathology, edition 6, St. Louis, 1971, The C.V. Mosby Co.|
Both from the point of view of injuring whole organ systems during pregnancy and that of producing massive increases in the risk of future cancer and leukemia, irradiation early in pregnancy is an extremely serious matter. This would be true for any source of radiation—medical x-rays, nuclear electricity or other activities. Since a woman often does not realize she is pregnant during these critical early stages, it seems extremely unwise for women of child-bearing age to be associated, in any way, with the nuclear power industry or other activities where the chance of radiation exposure is high.
(g) Other radiations: Alpha particles, "Hot" particles, Plutonium
For the broad group of hundreds of radioactive elements produced in nuclear fission, the radiations are beta particles, x-rays, or gamma rays or various combinations. For all these we can focus our attention on the number of rads absorbed by an irradiated tissue or organ. This simplification helps.
But one type of radioactivity associated with nuclear electricity deserves special consideration; alpha radioactivity. Alpha particles, electrically charged nuclei of helium atoms, affect material (including human tissues) in their paths so extensively that they travel only short distances before they have expended all their energy. In the process they provide intense radiation to the tissue in their path.
What is more, it appears that for some biological effects, including cancer production, alpha particles are worse per rad absorbed in tissue than any of the radiations discussed above, possibly several times worse.
Much confusion has been generated by some so-called authorities (in AEC) concerning alpha particle radiation. These "authorities" have stated repeatedly that, since alpha particles transfer so much of their energy in such a short distance and are then stopped, it follows that alpha particles are not serious. This assumption is false. It is true that a radionuclide (emitting alpha particles) lodged on the skin cannot irradiate internal tissues, simply because no alpha particles get any deeper than the skin. But they can provoke skin cancer.
Much much worse is the inhalation of nuclides which emit alpha particles. Once inhaled, the radionuclide can be distributed along the lining of the respiratory tract and there irradiate those cells especially prone to develop cancer. Indeed, this is the source of lung cancer induced by radioactive exposure of uranium miners, one of the truly unnecessary tragedies that has already occurred in the nuclear electricity industry.
"Hot" particles are very small dust-like particles that are made up of alpha-emitting substances. One of the prominent ones, plutonium-239, is widely heralded as the "nuclear fuel of the future." Fine particles of pure plutonium-239 oxide (formed when plutonium burns) are very intense sources of alpha particles.
Geesaman and Tamplin have shown that such fine particles, referred to as "hot" particles because of their extremely high alpha particle emission in a localized region, may be 10 to 1000 times more effective in producing cancer than would be expected if the same number of rads were delivered in a more diffuse manner to an organ, such as the lung.
It is this "hot" particle problem associated with plutonium-239 that makes the contemplated, future, widespread use of this radionuclide as a fuel in the nuclear-electricity-generation plants such an unmitigated nightmare for mankind. Not only may the hot particles of plutonium oxide be super-cancer producers, but with a half-life for plutonium-239 of 24,000 years, such plutonium oxide can be spread about the earth, re-suspended in air, and produce lung cancers in generations of humans for 100,000 to 200,000 years.
Manufacture of plutonium-239 and its widespread use in nuclear electric power may represent man's most immoral act.
Aside from the alpha-emitting radionuclides and the "hot" particle problem, the vast majority of other radionuclides can be considered to have equivalent effects, provided one considers simply the rads delivered to a particular tissue.
These points are stressed because so much confusion has been generated in the public's mind concerning possible special importance of one or another particular radionuclide. The question is commonly asked, "Which radionuclide, associated with nuclear electricity generation, is to be most feared?" Aside from the special case of plutonium and the "hot" particle problem, the answer is any or all the radionuclides in direct proportion to the radiation they deliver to a particular organ. Important differences in their chemical behavior can determine how, and if, they will wend their way through the food chain to man. If the radionuclide gets to deliver its radiation to man's tissue, only the rads delivered count in assessing hazard.
In addition to the rad unit, it has proved convenient to use a unit 1000 times smaller, known as the millirad. 1000 millirads equals one rad. You may commonly encounter both the rad and the millirad in public discussions of radiation hazard. Thus, for example, the Federal Radiation Council allows Americans to receive an average dose of 0.17 rads per year. This can also be stated as 170 millirads per year. Both will frequently be referred to in public writings and discussions.
other units that may be encountered are
the rem and the roentgen. For almost all purposes
involving beta rays, x-rays, and gamma rays,
(and this encompasses all the fission products from
uranium ) we can say the rad, the rem, and the
roentgen are roughly equivalent. We shall use only
the rad, or the millirad, in this book. Should the reader
encounter radiation hazards discussed elsewhere in
units of rems or roentgens, there will be no significant
error in considering these units to be the same as rads.