I remember being a kid in the 1960s when there was a lot more talk in the everyday media about nuclear power, bombs, and radiation generally, and commentators explaining about the three different kinds of radiation–alpha, beta, and gamma. While they made it clear that gamma radiation was the most lethal, I don’t remember what the effects of the other, lesser radiations were, if any.
I know what they are–alpha particles are helium nuclei and beta particles are electrons, but can they do anything good or bad to an organism? IIRC it doesn’t take much to stop these particles.
As I understand, yeah, they are potentially damaging – being hit by any radioactive particle is going to cause some molecule a world of hurt. But they’re nowhere near as penetrating as gamma, and (IIRC) don’t carry as much energy on average.
Alpha, in particular, is quite easy to buffer against. Protecting against beta is a bit more difficult but quite doable. Gamma, on the other hand…
Right you are. In Navy ELT school (water chemistry & radiation tech), they taught us that alphas would be blocked by the layer of dead skin before it gets to live skin. Betas can be stopped by a sheet of paper. Our yellow cotton “anti C’s” clothing was sufficient to block either quite nicely.
The real danger was if the beta or alpha emittor was allowed to get close enough to live cells to cause damage. In comparison to gamma radiation, alphas are great huge bowling balls that smash up everything they touch. That’s why they stop so quickly. Who cares if a gamma ray or a neutron goes clean through you? It didn’t hurt you if it made it out the other side. Alphas will hit something and damage it.
The typical route for this is by inhalation or ingestion of alpha/beta emittors or getting radioactive contamination in an open wound. Consequently, it’s quite important to wear a gas mask if one is in a contaminated environment.
I love this topic. I used to explain it to students like this:
There are gamma rays and other stray stuff going through your body all the time. Nearly every sample of anything you can find on the ground has some extremely miniscule (for example, 1 part per billion) ratio of radio-active isotopes in it. The base assumption of Carbon-14 tests is that this “Background Radiation” we are all exposed to above ground is roughly constant - so that as Carbon-14 atoms decay, more are replaced on exposed Earth Surface (C02 is the best indicator). If one were to sheild a sample by burying it for a while, one might be able to use decay constants to guess how long it has been buried.
(1) GAMMA PARTICLES: Our bodies are built to handle a stray gamma or two, periodically. Sometimes they pass through without hitting anything. Sometimes, they pass through and put a temporary hole in a cell membrane (not too horrible). Sometimes they nail a cell nucleus and kill the cell. Sometimes, the nail the nucleus and the cell survives - passing on any random modifications. Sometimes the gamma ray his the right gene within the nucleus, randomly affecting growth patterns - resulting in CANCER. If a few cells are killed every few minutes, our body takes care by simply replacing them or working around them.
Strong gamma sources are like tiny molecular-sized machine-guns. If you get to close, it will start taking out more cells than your body can handle - Radiation burns. If you remain too close, death will occur as your body can’t deal with the massive gangrene, distributed evenly thoughout the body.
Lead is the favorite sheilding for gamma rays, but every metal has its own work function.
(2) ALPHA PARTICLES: They are often the result of Nuclear fusion reactions, like that take place in the sun. A great percentage of the solar wind is composed of alpha particles. As those particles are bent and focused by the Earth’s magnetic feild, they spiral in at the poles, producing the Aurora Borealis (Northerm Lights). Our thick layer of atmosphere will stop them. Also, Polycarp is right - any substantial barrier will stop alphas, but heat up in the process. As long as dissipation is at equilibrium, the barrier will not melt.
(3) BETA PARTICLES are between. They are far less massive, but can have odd effects. Under just the right (or wrong) conditions, you can get +Betas (positrons) which, as they collide with normal matter (e-), they produces multiple gammas. -Betas can also produce X-rays under the right acceleration conditions.
(4) NEUTRON RADIATION is potentially the worst. When you have large amounts of neutron radiation, many horrible things can happen.
(a) A high energy neutron will decay into 1 gamma + 1 high energy proton + 1 high energy Beta and neutrinos.
(b) Normally stable atomic nucleii can pick up stray neutrons and begin to fission, producing more gammas, betas and Alphas.
(5) People are such safety freaks these days… and cures for cancer is sooo expensive, I can just see the news-media reel: “it can enslave a whole generation if it doesn’t kill them first”… and Nuclear powerplants have been looked down upon by Cons, libs and libarts in US, so why should anyone want to talk about it. Only independants, futurists and scientists seem interested in the subject.
Ok. That was probably more than you ever wanted to know.
Alpha particles are basically a helium nucleus; since they’re so heavy they generally have low energies and can’t even penetrate skin or more than a few feet of air. Betas, which are electrons (or positrons, for beta-minus) are slightly more penetrating but can be stopped by a foil-thickness of metal. Neither are much of a threat in the ambient environment, but if one injests a substance which emits these it can be absorbed into organs like the liver or bone marrow which can lead to rapid tissue ionization and cancer.
Gammas, on the other than, are just high energy photons (above the x-ray range). Since they are very low mass (no rest mass) they readily penetrate deep within a body and through a considerable amount of buffer mass; a moderate exposure over a short timespan isn’t going to do significant damage but long-term exposure or a high rate of exposure can result in radiation poisoning, immune system failure, and massive tissue damage.
One form of radiation you don’t mention is neutron radiation. It isn’t technically an ionizing radiation (as it doesn’t create ions or affect electrons) and doesn’t occur in nature on modern Earth in any significant quantities, but exposure to it can be quite hazardous. Unlike ionizing radiation, it is best absorbed by lighter elements such as hydrogen and helium; hence, per unit mass, water is a better shield than lead (which is why atomic piles are often suspended in water…it absorbs and moderates the neutron flux). The damage done by free neutrons is relatively independent of their energy levels, too; in fact, lower energy neutrons are more likely to be absorbed, so there is no “low energy threshold” of safety as their is with photon or alpha radiation. The neutrons create radioactive isotopes of normally radiologically inert elements within the body (a process called neutron activiation) which then emit ionizing radiation as if they were absorbed as above. It also does direct damage to the body tissues, of course, but if you absorb enough neutrons for that to be a problem then you can pretty much guarantee on dying in the recent future from radiation poisoning and/or massive carcinoma growth.
So, don’t go standing in front of any particle accelerators, hey?
There is an old saw that goes: You have an alpha, beta, gamma, and neutron source. You have to eat one, hold one in your hand, put one in your pocket, and throw one away. What do you do?
The answer is eat the gamma, because it doesn’t make much difference if it is inside you or outside; hold the alpha, it will be mostly stopped by the skin; put the beta in your pocket, your clothes will shield it; and throw the neutron source away, neutrons are bad for you.
On the subject of neutron radiation and moderators: the operative mechanism here is transfer of energy rather than ionization. A good example of this is shooting a cue ball onto a huge pool table that is full of bowling balls, marbles, and other billiard balls. The cue ball represents our neutron. If it hits marbles, it simply shoves them aside without losing much energy; if it hits a bowling ball, it bounces right off, still retaining most of its energy. If it hits a billiard ball, it transfers much of its energy to the other ball. Since neutrons have a similar mass to Hydrogen’s proton, this serves as a very good means of slowing down the speeding neutron. Now it has a better chance of being absorbed by something else.
In the case of a nuclear reactor, we like our neutrons to be absorbed by nearby U-235 atoms, but they really only like “thermal neutrons” (at ambient speeds). This is where your “moderates the neutron flux” comes into play, Stranger. Though someone could read your correct statement to imply that moderation is part of shielding (and slowing-down does happen in the shielding), the job of the true moderator is to reduce the velocity of neutrons to a point where they are more able to cause fission, thereby increasing the reaction rate.
In order to better shield the reactor for neutrons, they use layers of water and borated polyethylene. The water and poly slow down the neutrons (as described above); the boron absorbs the neutrons.
My favorite tidbit about fission (and the one I frequently share in threads like these) is that modern naval reactors use the temperature of the reactor coolant to automatically control the reaction rate. As the throttleman whips open the throttles, the increased steam demand draws down the temperature of the reactor coolant; the coolant becomes more dense; it becomes a better moderator; the reaction rate goes up to meet the new demand. As the throttles are shut, the excess energy that is no longer used to generate steam causes the coolant to heat up; it becomes less dense; it becomes a poorer moderator; the reaction rate drops down to the new power level.
This is a fairly critical mechanism since ships engines change their power output quite frequently and rapidly as the ship maneuvers, as opposed to civilian power plants that have much more even loads.
Thanks for clarifying; that previous post was not an exemplar of clarity.
Naval powerplants are actual a marvel of efficiency; under most regimes they don’t even require an active motor to pump working fluid and coolant, as they are designed to use natural convection to effect flow. And the US Navy has an utterly phenomenal record of nuclear power plant safety owing to its rigourous standards and assessments. The private/state nuclear industry could learn much by studying the methods of the Nuclear Navy and Admiral Rickover.
I would have to slightly disagree here. Each source has a 1/r^2 intensity dependance… So tossing the Neutron Source - ok. But I don;t know if I could swallow the gamma – I’d put it in my hand and hold it at the largest distance away from me as possible. I imagine a GREAT deal of damage done within the body. I would probably put the alpha source in my pocket - it might get hot - I might turn the pocket away from my privates. And I would swallow the beta emitter: if it were e-, without question - I’ll risk a few hours of stray X-Rays, but if it were a steady e+ emitter, I might reconsider and swallow the alpha emitter instead.
The problem you have is that ALL of the energy emitted by an alpha or a beta emitter will be absorbed by your body if you have swallowed it, while a substantial portion of the gammas will escape without hurting you.
What you have decided to do, in putting the alpha in your pocket, is a good idea. However, swallowing either an alpha or a beta emitter is a really bad idea.
In fact, I swallow gamma emitters all the time. They’re called bananas.
Seriously. The potassium-40 in bananas emits lots of gammas.
Everything has pretty much been covered except I’m surprised nobody mentioned “beta burns”. Yes, beta particles are fairly easily blocked by materials as thin as clothing. But without any shielding, it produces burns. A number of Pacific islanders suffered beta burns after being coated from fresh fallout after A-bomb tests in the 50’s and 60’s. So don’t underestimate the potentially harmful effects of beta emitters.
Not all naval power plants are able to function without reactor cooling pumps running. In fact, none of the plants I worked at were designed to function in this way (a.k.a. natural convection). IIRC, one plant I was at sported the ability to remove a small amount of decay heat via natural convection in case of a complete loss of flow accident – even after one drops the rods to shut down the reactor, fission products continue decaying, producing a substantial amount of decay heat that must be removed to prevent meltdown.
One can imagine that a natural-convection-capable plant would have some key design features:
o The reactor vessel outlet would be higher than the vessel inlet.
o The steam generator would be at the top of the coolant loop.
o They would probably need to provide a bypass around the reactor coolant pump (why waste natural convection flow energy spinning a turned-off pump?)
I can’t imagine why one wouldn’t want to design all naval power plants to be able to run in this fashion, but the truth is that only a portion of the plants have this feature. Perhaps it is tricky business indeed with lots of engineering tradeoffs to get a plant to successfully run with its coolant pumps turned off. It is either much more costly or inefficient or both.
Though all navy plants are very similar pressurized water reactor design, there are several variants in use that span decades. It stands to reason: a cruiser plant will not necessarily be like a submarine plant, and neither will closely resemble an aircraft carrier plant. In addition, they are expensive assets, so one doesn’t dispose of a 1960s-era plant just because it’s a dated design.