Probably a stupid car question...

[Lissa]

For gamma / neutron radiation, here is source for computing necessary shielding.

And you'll note that there are given thicknesses required to stop any gamma or neutrons. It's well known that you need 7+ thicknesses to cut the radiation to near 0 levels and it's going to depend on how powerful a gamma or neutron source is (case in point, when I was at the University of Arizona, we had a 5 Cu Co60 source. We had a couple feet of lead to act as shielding, but even still, when the source was raised to irradiate a sample, you could still measure a slight rise in radiation outside the room where the source was stored (when placing samples in the irradiation chamber while the source was lowered, we were required to wear finger dosimeters and we had to make sure that when we reached in, we had to do so that the lead shielding blocked our body and head). Given the amount of lead we needed for just a 5 Cu source, the amount of lead/uranium/other dense material for a reactor, even a small one, would be considerable.

It appears to me that Co60 is as energetic, if not more so. I also wonder if the shielding requirement would be unique to that isotope since the release of gamma radiation happens as a secondary product from the decay to nickel-60. Therefore, Lead is less effective than depleted Uranium to stop gamma radiation, and there are better dense barriers. Lead and huge walls of concrete are cheaper than using depleted Uranium, or alloys of Osmium and other dense rare materials. Most decisions regarding shielding favor excessive cheap mass, rather than super dense expensive materials.

Lead is cheap and dense. Uranium 238 is one of the densest materials known to man, but it's also difficult to get in quantity, especially for an institution that does nuclear research. It also has some nasty, non-radioactive qualities to it as well. Because it's expensive to get and difficult to obtain even if you have the money people don't use it for shielding on irradiators. The other aspect is lead is a lot easier to mold and work with than Uranium 238.

And no, concrete is not a good choice for shielding a gamma source as it's not that dense and you need a dense material to adequately shield against gammas. You would have to have a similar amount of weight of concrete to get the same stopping power as lead (and it's worse if you can actually use Uranium). You have to have a lot between you and the radiation source and concrete just as too many voids between the atoms to do much good.

If you look at densitys of common materials, the best gamma shielding you could get is Pt (which will cost you a lot), Tungsten is slightly better than Uranium (19.6 Mg/cubic m vs. 18.9 Mg/cubic m), with lead coming in at a respectable 12.6 Mg/cubic m, and concrete coming in at a pitiful (in comparison to the above) of 1.75 to 2.4 Mg/ cubic m. So, in order to stop as effectively as lead, you would need about 5 times as much concrete than lead to shield a similar source.

The problem isn't better engineering, we're designing reactors now that are passively safe, it takes an act of sabotage to make them meltdown now. The problem still remains, depending on your source strength, you're going to need a certain amount of shielding and that shielding can weigh a lot.

Another aspect is this, the Russians have created suitcase and hand gernade size nukes, we know minaturization is possible, the problem is the shielding and it always will be. You can't use coulombic forces to stop a neutron or a gamma like you can with a beta or an alpha, so the only way to really stop them is material that will increase the chance of interaction and potentially back scatter to the source till the harm said radiation is nil.

Yes, but... There is very little research into crafting a solution and the ones that have been built rely on the same technologies as were available in the 1940's. There are some advances due to the use of radiation in medicine. My link to the Toshiba 4S reactor was to show that you can scale down a TMI facility from 800MW to a Toshiba 10S at 10MW. It is incrementally more difficult to shrink it further, and as Pete suggested, a small part of their safety is housing the core at the bottom of a thirty foot shaft. However, they could not allow the soil under the house to become irradiated, so the shielding provided within the 6 foot diameter shaft must be sufficient to block the large bulk of the excess radiation. There may also be ways of containing rogue energetic particles using field effects to force their interactions into smaller spaces with less weight.

And... There is a difference in engineering required to create a safe 30-50 year sustained reaction, as opposed to releasing it in a fraction of a second. Well, now we get back to my original suggestion. Once you conceive it is in the realm of physical possibility, it is our fear of misuse that squelches further consideration.

One of my main beliefs of our age is that our process of educating people crushes out their ability to imagine the impossible, and then make it a reality. Consider the mobile devices we carry in our pockets... portable communicators, super computers, GPS...

Then consider the reasoned advice of some people without imagination.

"I think there is a world market for maybe five computers."
-- Thomas Watson, chairman of IBM, 1943

"There is no reason anyone would want a computer in their home."
-- Ken Olson, president, chairman and founder of Digital Equipment Corp., 1977

"640K ought to be enough for anybody."
-- Bill Gates, 1981

"Heavier-than-air flying machines are impossible."
-- Lord Kelvin, president, Royal Society, 1895.

Once again, you continue to negliect that you must have a certain amount of material between you and the radiation source, this is something you cannot work around. Neither gammas nor neturons can be effected by coulombic forces to stop them (which is why it's easy to shield against alphas and betas since they're charged particles). The only way to stop gammas and neutrons is to have them interact with an atom and you have to have them transfer their energy to those atoms through scattering or absorbtion (and absorbtion could potentially lead to more neturons with a heavy enough atom as you could get a fission event or you may get a decay event that leads to a neutron release as well). This is the only way to shield against these two forms of radiation, there's no way around not putting materials between you and the gamma or neutron source.

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Ah, but it only takes one, or perhaps a small group, to do vast amounts of damage.

--Pete

Although I agree with what you wrote it wasn't directed to what I meant.

Nuclear weapons and biological weapons sophisticated enough to 'kill billions' are indeed luckily not easy to make for anyone with an internet connection.

If they have the materials, it's actually a bit easier to make them if you have an internet connection as the information is available and in the public domain if you know how and where to look.

I think potential terrorists would not wait with their attack until a nuke becomes available. They would probably use something more low-tech.
So although I share your concern about 'it only takes 1 or 2 people to create disaster' I think we could better use large part of our anti-terrorist resources for other things (like telling the 90% of people with a drivers license that can't drive safely to stay at home for example).

Terrorist are looking to cause as much terror as possible. Conventional means we desencitize a population eventually (look at the Israelis for this). A WMD, be it a nuclear, chemical, or biological weapon, will cause no end of terror and, depending on who it's used upon, cause said population to lash out wildly causing a greater catastrophe (if you ask anyone in the military how you respond to having a WMD use upon you, you will see that almost universally they will tell you, you respond in kind with a WMD of your own). This is why using a WMD can be devestating to the world because the populous it's used upon will likely lash out (the Japanese were actually smart when we used the two atomic bombs on them, Bushido required them to fight to the end, but the atomic bombs gave them a way out to save face as you can't really fight an enemy like that although there were some still in the Japanese military that wanted to fight it out to the bitter end, but Hirohito won over the majority of Japanese and was able to end the war while retaining some face as demanded by Bushido).