A brief overview of the four types of ionizing radiation, followed by a discussion of the EPA's proposed new rules for "emergency" radiation limits after an accidental release of radioactive materials.
There are four types of ionizing radiation: Alpha, Beta, Neutron and Gamma (and x-ray, which is a lower-energy emission than gamma rays, and thus less damaging, but otherwise identical to gamma ray emissions).
There are 20 levels or "weighting factors" of radiation damage from various emissions, depending on energy levels of the emission and type of emission/penetration capabilities (alpha, beta, neutron and gamma/x-ray each penetrate differently).
Beta particles, gamma rays and x-rays are all classified as level 1 -- the least damaging for a given energy level. Alpha particles are level 20 -- the most damaging. Neutrons are classified at level 5, 10, or 20 depending on their energy level. Interestingly, the highest level of damage from neutrons is neither at the highest or lowest energy level, but in what might be called the middle energy level (100 keV to 2 MeV).
Additional classifications of radiation damage depends on what, if anything, the radioactive isotope "targets" (bone, thyroid, etc.). All such classifications are generally based on damage to an adult healthy (white) male.
Most biological damage from radiation is probably due to the creation of "free radicals" in the body: Free radicals are molecules which become electrically unbalanced ("charged"). A radioactive emission can knock an electron out of its orbit around a nucleus. The unbalanced atom will then grab an electron from something else, which may then do the same thing to something else, and so on through lower and lower electron bonding energy levels.
If a DNA strand is involved, such events can permanently mutate that DNA strand. RNA can also be damaged, which might cause a cell to start producing a poison instead of a useful protein. If the damage simply causes cell death that's usually not so bad unless it's a heart or brain cell, which are not replaced during a lifetime. Most other cells in the body have limited lifespans anyway and die (self-destruct might be a better word) with various average life spans. Intestinal lining cells, for example, only live a few days, taste bud cells live about two weeks. However, if the damage causes either rapid (or slowed) cell division it can be much more dangerous.
A radioactive isotope is not the same thing as a radioactive emission. A radioactive isotope has a "half-life." A half-life is the amount of time it takes for half of a given quantity of a given type of isotope to decay. A radioactive emission is ejected from a radioactive isotope at the moment of decay. Different radioactive isotopes decay with different radioactive emissions, and those emissions have different energy levels.
There is no way to predict what the precise energy level will be, nor when the emission will occur, or what direction it will take as it leaves the radioactive isotope. Many types of radioactive decays result in another radioactive isotope being created from the original isotope. Sometimes as many as 20 different elements are created and then altered again, before a stable isotope (such as lead) is reached.
For alpha and beta particles, the emission lasts only until the particle (alpha or beta) slows down from about 98% of the speed of light for alpha particles, and 99.7% for beta particles at the moment of emission, to "terrestrial" speeds. This takes very little time: on the order of a billionth of a second (give or take a few orders of magnitude).
After they slow down, alpha particles become helium atoms, but initially without their electron shells. They will grab other atom's electrons very quickly, though, since most atoms hold their outermost electrons much less tightly than helium atoms hold theirs.
Beta particles become electrons when they slow down.
What slows alpha and beta particles down (and does the damage to biological systems) is their interactions with electrons, atomic nuclei, and/or molecules. Alpha and beta particles are "charged" particles and only have to be near another charged particle to have an effect, and to be effected by other charged sub-atomic particles. Alpha particles are thousands of times larger than beta particles, and twice as strongly charged (in the opposite direction: Positive instead of negative).
Whereas alpha particles "blunderbuss" into electrons, atoms and molecules, beta particles are so small and travel so fast that when they are initially ejected that they pass by other electrons, atoms and molecules so fast that they don't have time to do much damage. It's when they slow down a bit, having passed thousands of charged particles at nearly the speed of light (each charged particle they pass acts as a little brake) that beta particles can do the most damage. For this reason, the nuclear industry's oft-repeated claim that "low energy beta particles" such as from tritium aren't very damaging is utterly false!
Gamma and x-ray emissions are neutrally charged and don't slow down; instead their energy is dissipated by one of three methods: 1) Crashing into an electron and knocking it out of its orbit (this can make the electron a beta particle). The gamma ray disappears. This is known as the photoelectric effect. 2) At higher energy levels, a lower-energy gamma ray or x-ray might also be produced. This is known as the Compton effect. 3) At very high energy levels, gamma rays can also produce a positron when it collides with an electron. This is known as electron-positron pair production.
Neutrons are electrically neutral (hence the name). This neutral charge allows them to interact more directly with the nucleus of an atom and/or with electrons, since they are neither repelled nor attracted to other (charged) sub-atomic particles. Neutrons usually decay into a proton, an electron and an "electron anti-neutrino." The half-life of a free neutron is about 10 minutes.
Nuclear reactors depend on neutron emissions to operate: The neutrons split other atoms, giving off more neutrons in a "chain reaction." In a light water reactor such as all American reactors (both Pressurized Water Reactors and Boiling Water Reactors) the neutrons are slowed with normal ("light") water which acts as a moderator. The reason reactors use a moderator is because at higher speeds the neutrons won't split ("fission") other atoms.
Only a few isotopes of a few types of atoms can be fissioned, including several isotopes of Uranium and Plutonium. Although Thorium cannot be split, Th-232 can absorb a neutron, then the Th-232 transmutes, first becoming Protactinium-233 by beta emission, then the Pa--233 transmutes (also by beta decay) into a fissile isotope of Uranium, U-233.
Spent fuel also emits neutrons, and special "neutron absorbers" are placed around the spent fuel to prevent the neutrons from getting out. If the spent fuel assemblies are crushed together (for example, by an earthquake, terrorist bomb or airplane strike) and water or some other moderator is present to slow the neutrons down, a "criticality event" becomes possible -- an uncontrolled chain reaction, producing enormous amounts of heat and fission products in a few thousands or even millionths of a second.
Neutrons are very damaging to biological systems but fortunately, isolated radioactive particles in the environment do not emit neutrons.
Setting permissible levels of radiation:
The EPA's proposed changes are specifically for accidental releases, so that at worst (so to speak) only the immediate area needs to be evacuated. This is to aid the nuclear industry so that it can keep operating with barely a blip. Any reasonable person looking at the future of nuclear power can see that A) A meltdown somewhere in America is practically inevitable sooner or later, and B) A major accidental released at, say, Indian Point would require long-term evacuation of New York City, whereas with the new limits, they probably would not evacuate NYC at all, even after a full-blown meltdown (or two) at Indian Point.
There are worse accidents possible than even a meltdown, however: A fire hot enough to burn the uranium dioxide fuel pellets, for example. Such an event at Indian Point would almost surely require the permanent evacuation of New York City and the surrounding area of lower New York state, as well as all of Connecticut and New Jersey, perhaps an even larger area.
These proposed new EPA guidelines do nothing to protect the citizens of NYC, and will be responsible for a plague of cancers in the decades after an accident. Radiation levels equivalent to 250 chest x-rays per year will be permissible during the period after an accident. Granted, moving all those millions of people to "temporary" shelters would also be hazardous to their health, especially their mental well-being, which is probably the underlying justification for the EPA's new rulings.
Of course, no careful studies of "hot spots" after an accident will be done -- they never are done after a radioactive release -- so individual dose assays will be impossible, and there will be no follow-up of individuals as they move around the country to get away from the depressed local area -- again, there never are such studies.
If any government research is done later, it will be of the "healthy survivors," not the miscarriages, stillbirths, and non-fatal ailments such as inflammation, lowered IQs, deformities, etc.. They might study lung cancer deaths, but that would be about it, and those studies would probably be done in the first couple of years, long before most lung cancer deaths would even appear.
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|Allen L. Jasson|