Some radiation and material questions

[BrunoPuntzJones]

These are probably pretty easy/dumb questions, but I have a masters in Accounting, and my engineering experience is from the college of Wile E. Coyote.

My curiosity stems from reading on Radioisotope Thermoelectric Generators used since the 60s or so on spacecraft. Neat-o stuff.

1. RTGs use thermocouplers and other means to directly convert heat into electricity. How inefficient is this? It must be pretty huge. Why aren't there devices to say...convert that 1000 watt SLI quad core rig back into some usual energy from the heat dissapation? Granted that's a simplistic example, but is it due to cost, inefficiency or what?

2. In regards to half-lives. Elements that have short half-lives, where do they come from? Some of these have a half life of days or years. I assume they have to be some sort of lab created isotope. In nature they'd have to have relatively long half-life, otherwise none of it would exist. If the planet is several billion years old...then the whole thing would have to been made of uranium at some point, which can't be true.

3. On another forum some guy had one of those "Nuke Detectors" that the DOD buys, little keychain device that goes off when hit with certain amounts/types of radiation. The guy says he lives next to a highway, and a tractor/trailer drives by and a few seconds later the detector starts going off. After about 15 minutes it quits.

He claims that the detector is functioning fine, that it should only stop beeping when the detected radiation drops. Wouldn't this be pretty much not possible in a 15 minute span? I mean if the truck was transporting some sort of radioactive stuff, it would have already gone *poof* before he even arrived at his destination. What's the deal with that?

 

[silverpig]

1. They're horribly inefficient. A few percent at best. The reason why they're used in spacecraft is because they have no moving parts. Slap your TEG on your hot piece of metal and presto, power. You could use one on your computer if you really wanted but it's not a good idea. First being about 2% efficient, you'd get 4 W off of your cpu. That's enough to say, run a few fans. Sounds great right? Nope. First you assume that all 200W from your cpu is going into the TEG. This only happens if you have it in direct contact with the cpu. That means it has to sit between your cpu and your heatsink so one side is heated and the other side is cooled. This makes your cpu hotter by being less efficient at heat transfer. Secondly, even if you were to go through all this hassle, the cost to buy and set one of them up isn't worth the savings in your power bill.

2. Most are lab created sure, but they are also being created in several ways. First atoms high up in the atmosphere are exposed to highly energetic particles from space. This can transmute them into other elements, sometimes unstable ones. This is how C14 is created. Secondly, when a somewhat stable isotope of a heavy element decays, it can decay into daughter products which might be very unstable themselves.

3. I'm not sure what this could be, but I'll hazard a few guesses. Alpha particles don't penetrate through much of anything. Beta particles will, but are stopped usually by something as thin as aluminum foil. Gamma rays can penetrate a lot of things. If the detector is set to pick up gamma rays, then I don't really have much of an explanation. If it's set to detect beta particles, then what could be happening is some small amount of radioactive material is diffusing into the air. This diffusion is fairly slow so that's why there's the delay of a few seconds. It then takes some time for this radiation to dissipate, hence the 15 minute lag. This is all purely speculation though as I really have no idea.

 

[QuantumPion]

Originally posted by: BrunoPuntzJones
These are probably pretty easy/dumb questions, but I have a masters in Accounting, and my engineering experience is from the college of Wile E. Coyote.

My curiosity stems from reading on Radioisotope Thermoelectric Generators used since the 60s or so on spacecraft. Neat-o stuff.

1. RTGs use thermocouplers and other means to directly convert heat into electricity. How inefficient is this? It must be pretty huge. Why aren't there devices to say...convert that 1000 watt SLI quad core rig back into some usual energy from the heat dissapation? Granted that's a simplistic example, but is it due to cost, inefficiency or what?

2. In regards to half-lives. Elements that have short half-lives, where do they come from? Some of these have a half life of days or years. I assume they have to be some sort of lab created isotope. In nature they'd have to have relatively long half-life, otherwise none of it would exist. If the planet is several billion years old...then the whole thing would have to been made of uranium at some point, which can't be true.

3. On another forum some guy had one of those "Nuke Detectors" that the DOD buys, little keychain device that goes off when hit with certain amounts/types of radiation. The guy says he lives next to a highway, and a tractor/trailer drives by and a few seconds later the detector starts going off. After about 15 minutes it quits.

He claims that the detector is functioning fine, that it should only stop beeping when the detected radiation drops. Wouldn't this be pretty much not possible in a 15 minute span? I mean if the truck was transporting some sort of radioactive stuff, it would have already gone *poof* before he even arrived at his destination. What's the deal with that?

3. "Nuke Detectors" are gamma ray detectors. Since gamma rays are highly penetrating, a small key-chain detector would have extremely low sensitivity. Low enough that incident cosmic rays or natural decay products in the ground or in concrete could set it off at random.

If the source of the radiation the guy in your anecdote was describing was the passing truck, the detector would only beep while the truck is in line of sight and close enough to the detector. If the detector was beeping for 15 minutes after the truck passed, then almost certainly it was just a spurious natural source and is not coincident with the truck.

 

[Mark R]

1. RTGs are desperately inefficient. Figures vary, but I seem to recall coming across 1% efficiency as fairly typical. The advantage of RTGs is extreme reliability and long life - e.g. a Strontium-90 source (readily available in huge quantities as a by-product of nuclear power or weapons production) has a half life of around 30 years, fairly respectable heat production, and limited gamma/X-ray emission. As a result, such devices have been widely used (particularly in Russia & the old Soviet Union, where cost and accessibility were issues, and safety less so) in military and special industrial applications (e.g. powering sensors at remote military bases, powering scientific equipment on marine buoys, etc.

For space use, higher performance isotopes tend to be used - Plutonium 238 is the typical isotope, as it has very high spontaneous heat production and as it's an alpha emitter requires less shielding than beta sources like Sr90. Pu238 is difficult to obtain and rare - I think it normally has to be made by irradiating other artificial elements in a reactor under controlled circumstances - it's not obtainable in useful quantities as a by product of nuclear power or weapons.

Aside from the fact that the thermoelectric effect is pretty inefficient - there are fundamental limits to efficiency of conversion of thermal to non-thermal energy. The Carnot limit is the application of the 2nd law of thermodynamics, which relates the maximum efficiency at which energy can be harnessed to the temperatures of the 'hot' and 'cold' side of a device. The larger the temperature gradient, the better the efficiency. For example, a device which has a heat input at 100C, and a heatsink temperature of 40C, would have a theoretical perfect efficiency of 19%. In practice, this level cannot be reached. This is why power plant designers go to such enormous efforts to get the temperatures up as high as possible (and is why nuke plant efficiency is in the high 20s, low 30s per cent, whereas nat gas is high 50s - you can't let the reactor get as hot as a gas turbine).

Of course, if your heat source is plentiful enough, then it may be worth building a device to make use of that gradient. There's a power plant technology called OTEC (ocean thermal energy conversion) - this makes use of the fact that in the tropics, deep ocean water is much colder than the surface. Even though it's only 20 degrees or so, and the Carnot efficiency is only a few per cent, the amount of heat that you can extract through your plant is large enough to make this viable.

2. Long half life elements were present when the earth was formed (e.g. uranium and throium). Uranium-238 has a half life of 4.5 billiion years, U235, about 1.5 billion, and thorium about 10 billion. Resources of uranium were significantly richer in the young earth. In fact, the isotopic properties were richer - with more of the fissile U235. In fact, about 1.5 billion years ago, natural uranium then would be what we currently regard as 'slightly enriched uranium'. While current natural uranium won't support a nuclear reaction without careful engineering, enrichment makes it much easier. In fact, in Gabon, there was a rich uranium ore deposit that did undergo a spontaneou nuclear reaction (at about that time, when it got flooded with water).

Shorter half life isotopes may be part of the decay chain of other elemetns. E.g. Uranium decays via Radium, etc. Radium has a relatively short half life about 1500 years, so you get an equilibrium forming according to the ratio of half lives - so uranium will contain about 0.3 ppm of radium, which can be concentrated. Same story with Radon, which is an intermediate of uranium and thorium decay.

Short half life elements can be made in a variety of ways - fission of uranium, and subsequent purification; decay of a parent isotope; irradiation of an element with heavy particles (e.g. neutrons, protons or deuterium nuclei) in a particle accelerator. The technique used depends upon the existance of a suitable reaction, and the degree of purity needed.

E.g. Technetium 99m is a widely used medical isotope used for many types of body/brain scan. It has a half life of about 6 hours, so is usually made on site. This is done in an elution generator: This is a vial filled with resin beads containing the isotope Molybdenum-99. The Mo99 decays into Tc99m, which is not electrochemcially attracted to the resin, so will leach out if the vial is filled with water. The Mo99 itself is made by purification of fission products of uranium that has been irradiated in a reactor.

Another medical example is Fluorine 18 which is used for PET scans. This is made by irradiating heavy water (containing Oxygen 18) with protons in a particle accelerator (cyclotron). F18 has a half life of under 2 hours, which makes PET scanning logistically difficult and very expensive (as the F18 must be shipped from the accelerator facility to hospitals by the quickest method possible).

3. I'm not sure what that would be detecting. Most pocket radiation detectors detect high energy beta and gamma rays. These rays are emitted from the source material - the material itself doesn't have to escape. Similarly, gamma rays can never be completely blocked, some will always get through - but you can cut them down to such low levels that it doesn't really matter. Beta (and alpha) particles have a defined range - beta can go up to several yards in air, alpha may only manage a few inches. And there is a further problem, if you want to detect these rays - you need to stop them *in* your detector. If your detector is small (key chain sized) it's not going to do a very good job, and will have terrible sensitivity.

One of the things that a lot of people forget is that radiation sources obey the inverse square law - so if you're 10 times as far away, you receive 1% of the radiation. This means that radiation levels fall off very, very fast - if you want a good chance of detecting a radiation source, you need to be close. If you were able to detect radioactive sources on a truck, from a house near a highway - you can bet that it will be doing a good job of cooking the driver.

Indeed, it's not clear what your friend would have been detecting on a truck. Most shipments of industrial radioactive materials are heavily shielded (there are very strict laws on the amount of shielding) and it would be unlikely that such a source would be reliably detectable even standing directly outside the truck. Even high-level nuke waste from power stations is shipped so heavily shielded that you're unlikely to detect it from more than about 30 or 40 feet.

Indeed, the fact that the detector kept alarming for 15 minutes, suggested it detected something in the nearby environment (or it was simply a false positive) rather than anything related to the truck, as the truck would move rapidly out of range.

 

[BrunoPuntzJones]

Originally posted by: Mark R
snip

Whoa, you know stuff 😀

Those were perfect answers I was looking for. Nothing too technical, but simple enough to understand.

I'll have to do some reading on enrichment as that seems to be a big buzz word in politics now.

All very interesting stuff.

Here's a link to the device the guy had http://www.nukalert.com/

He wasn't a friend, was just on another message board I read.

I appreciate the detailed response, far better than I was expecting.

 

[Mark R]

Originally posted by: BrunoPuntzJones

Here's a link to the device the guy had http://www.nukalert.com/

Interesting device.

Anyway, as I suspected, the device is pretty insensitive (but then, all it's intended to protect against, is immediately dangerous exposures. It's minimum trigger level is about 1000x the natural background exposure). This means it's unsuitable for verifying that the local environment is safe for long-term residence, and has not been contaminated.

For example this device would not have protected against the following event: There was an incident in Taiwan about 15 years ago, where an industrial Cobalt-60 source was accidentally sent with a load of steel to a steel plant for recycling. There it was melted down with hundreds of tons of scrap, to form new construction steel, which was used to build apartment blocks. About 10,000 people were exposed, over a period of about 10 years before the contamination was discovered, to what, in total, were very significant exposures. (*)

This means its unlikely to detect any radiation at all where legally required precautions are in operation. E.g. you could place the detector on top of a nuke shipping container filled with 15 tons of high level nuclear waste, and it wouldn't alarm, or you could walk through the restricted area in a nuke plant where the waste is stored in casks, and it wouldn't alarm.

Similarly, this device would not detect low activity materials like depleted uranium or fresh nuclear fuel even on direct contact. It's also unlikely that the device would be able to detect a nuclear warhead, also even on direct contact. (but this is a guess, as this sort of info isn't readilay available).

Again, this puts perspective on the truck anecdote. If the detector started alarming, as the result of a truck driving past - there must have been one heck of an unshielded source on that truck. Some back of envelope calculations suggest that a truck carrying a load of unshielded high-level waste straight out of a reactor would just about do it. Clearly a completely absurd situation - not least because the driver of said truck would be incapacitated by the radiation in the cab within minutes. And that doesn't explain the fact that the detector kept alarming for 15 minutes.

It's most likely that the detector picked up some local radiation - possibly from some low-grade environmental contamination (relatively unlikely), some other radioactive material (e.g. radioactive glow-in-the dark materials, e.g. in watches or gun sights - again relatively unlikely), or that it was some sort of detector malfunction. However, any positive reading from that thing would concern me, and if I had one, I'd probably like to go and get a proper radioactivity survey meter to check.

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(*) Current theories on radiation exposure would expect about 300 deaths and dozens of birth defects as a result of this accident. So far, studies have not shown these increased levels of cancer or birth defects in these exposed people - explanations for this are controversial, but include the possibility that the current theories of radiation effects greatly exaggerate the effects of low-level exposure.