A Nuclear Power Primer: Part 3: How Does Radiation Hurt Us and How Much Does it Take?
We'll start with neutron radiation first, because it's really the simplest. First of all, neutron radiation is the only one that impacts the nucleus of the atom. The other three cause damage through ionization of the atom's electrons. To understand neutron radiation, imagine a pool table set for the start of a game. 15 balls are in the middle of the table, with the cue ball set for the break. The cue ball is a free neutron. When the neutron hits the nucleus, one of three things might happen. First, if the cue ball doesn't have enough energy, or hits at the wrong angle, it caroms off, barely disturbing the pack of balls. Second, if the ball has too much energy, it slams through the pack. breaking it up. This is fission, and results in fission products, more free neutrons, and energy. Third, if the ball has just the right amount of energy, it just makes it to the pack and joins in, becoming another neutron in the nucleus. Here is where our analogy breaks down, because many times, when a nucleus gets another neutron, it becomes unstable, and begins to decay, emitting alphas, betas, or gammas. This is called activation, and is one of the trickier problems with neutron irradiation an the physical properties of the irradiated matter can be quite different from the original.
So biologically, neutron irradiation can deposit energy in cell tissues, can break up molecules that might be important to cellular function, like DNA for example, and can activate materials in our bodies, giving off secondary radiation and causing additional damage.
The other three types of radiation are often referred to as ionizing radiation. Remember, an ion is an atom that has gained or lost an electron, making it a charged particle instead of neutral. This charge causes it to ionize other atoms, leading to cellular damage both by the deposit of thermal energy, and through changes in molecular structures. In the cases of alpha and beta radiation, the mechanism is easy to understand. These charged particles, when close to other atoms, either attract or repulse the electrons surrounding them. If the energy is right they can strip those electrons from the atoms, creating new ions. What is not so obvious is how photons cause ionization. They have no charge to attract or repulse electrons, and no mass to knock them loose, so how do they cause ionization?
The answer gets a little bit complicated, but the short answer is that when conditions are right, the photon can give it's energy to the electron, which causes it to move to a higher level. If the energy is enough, it can free the electron completely, causing ionization.
The damage caused by ionization is can be critical. The thermal energy alone can cause damage, similar to a sun burn. The cell heats up and dies. The bigger issue is the long term damage done to cells that survive the initial deposit of energy. Ionization can cause damage to the cells DNA, resulting in mutations that can result in cancerous tumors, dead tissues, and other problems, not to mention the burden on the body in disposing of these wasted tissues.
Radiation sickness is the term we use to describe the immediate damage to body tissues on exposure to radiation, and the longer term illnesses as the body tries to repair the damage.
Gamma and neutron radiation present the biggest problems since, like we discussed earlier, they penetrate the whole body, causing damage everywhere. The distributed nature of the damage makes treatment nearly impossible. The best doctors can do is support the patient and hope that the damage is not so severe that the body can't recover.
So how much is too much?
The standard unit of exposure, or dose, is the rem, or the metric version, the sievert. To convert, not like you'd want to, 1 sievert is equal to 100 rem. The LD50, or the dose expected to kill 50% of the people exposed is roughly 450 rem, or 4.5 sieverts.
And that tells you almost nothing since you don't have anything to compare it to.
Normal background radiation in the US, what we are exposed to everyday from cosmic rays, sunshine, and naturally occurring radiation from sources like radon gas, runs about 0.35 rem per year. If you smoke a pack of cigarettes a day, you're gaining an additional dose of roughly 2 rem per year. A full dental x-ray gives you a dose of .004 rem. The occupational limit for exposure to radiation is 5 rem per year. My personal exposure after nearly five years of working in and living on a nuclear reactor was about 0.5 rem.
I feel fine.
So, that should give you a little bit of a feeling for how much radiation exposure it takes to cause significant troubles.
Now that we have a short background on radiation and how it hurts us, we can start to take a look at how it helps us, as in, how it generates power for us. Next, we're going to look at the differences between radioactive decay and nuclear fission. Nuclear fission is where the big power comes from.
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