Probably a stupid car question...

[kandrathe]

On a (slightly) unrelated note, just how dense does the shielding have to be to impart Bremsstrahlung from Nitrogen-16 decay? I mean, just how energetic is that beta particle event?

From Wolfram|Alpha... Nitrogen 16. During its decay back to O16, it gives off 5-7 MeV high energy gamma radiation. From Wiki, "Condensate from the condenser is typically retained for 10 minutes to allow for decay of the 16N. This eliminates the need to shield and restrict access to any of the feed water piping or pumps."

Errr...there's no gamma with the decay, just a beta. Given, you have an electron moving at relativistic speeds, but it's just an electron which will be stopped by your skin if the decay happens external to your body. If the N16 decays in your lungs however, there is definite damage potential.

Ah, well then, the Wikipedia is wrong about that as well.

"Radioisotope N16 is the dominant radionuclide in the coolant of pressurized water reactors or boiling water reactors during normal operation. It is produced from O16 (in water) via (n,p) reaction. It has a short half-life of about 7.1 s, but during its decay back to O16 produces high-energy gamma radiation (5 to 7 MeV)."

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.

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.