The Effects of Radiation on Lead: Examining the Tsar Bomb

[Sean Conley]

I've read that the Tsar Bomb used a lead casing for the second and third stages of the bomb instead of a uranium casing. A uranium casing would have resulted in fast fusion but would have created too much fallout. Typical Hydrogen bombs use uranium casings. A uranium casing would have indeed resulted in fast fusion but would have created too much fallout therefore the soviets chose to use lead instead.

My question is, what happens to lead after it has absorbed so much radiation? Does it change and become another element?

 

[ChrisVer]

I am not sure if that's what applied on nukes...but, lead is a good absorbing matterial for gamma rays.
The gammas are either absorbed (photoelectric effect) or scatter off (compton effect) the electrons. So the gamma rays mainly interact with the lead's electrons and not nucleus,so they don't create other elements.
Lead has a relatively large attenuation (or absorption) parameter $\mu$ which suppresses/brings down the intensity of radiation $I_0$ exponentially with distance:

$I(r) = I_0 e^{-\mu r}$

 

[mfb]

The lead nuclei will absorb some of the neutrons that get emitted, and change their isotope. Some neutrons convert stable isotopes to other stable isotopes, but some produce radioactive isotopes that decay afterwards (mainly to bismuth). Still much better than uranium.

 

[ChrisVer]

For the neutrons:
https://www.physicsforums.com/threa...ead-nuclei-compared-to-uranium-nuclei.613203/
So I doubt that lead can be a good choice (except for the cases mentioned in Post#3 in the above link)...

 

[mfb]

A good choice for what? For simulating additional mass of the bomb and absorbing gamma rays, it is not bad (everything more dense is much more expensive).

 

[Sean Conley]

Thank you for your responses guys, it's much appreciated. I read that Uranium was the initial choice for the Soviets for the simple fact that it would have resulted in fast fusion, thus increasing the yield of the bomb to somewhere around 80 MT. Their final choice of using lead instead brought the actual yield to about 50 MT. I couldn't help but think the nuclei would absorb some of the radiation, which made me wonder what would happen to the lead atoms at that point. Mfb, your explanation answered this. I can't help but wonder if the lead shielding used in radioactive laboratories eventually becomes spent and less effective. Your explanation makes believe the answer would be yes.

 

[mfb]

Outside of nuclear weapons and reactors, you never get enough irradiation to change a significant fraction of your material.

 

[e.bar.goum]

Outside of nuclear weapons and reactors, you never get enough irradiation to change a significant fraction of your material.

Further, the places you find lead shielding in most nuclear labs is for creating "low background environments" for sensitive gamma-counting experiments, where the amount of radiation is very small. So the possibility of activating the lead enough to be significant is even lower.

Now, there are situations where you do worry about activating your material - in vacuum chamber design. You're never going to "change a significant fraction of your material", but you don't need to do much to make something radioactive, and in nuclear experiments, you tend to get a fair amount of neutrons (not a lot, but enough to worry about).

So to deal with this, in general, vacuum chambers for nuclear physics experiments are made from aluminium rather than stainless steel, as aluminium has a smaller probability of neutron capture than steel.

 

[mfb]

In addition to the lower cross-section, aluminium has nice (non-)activation chains. Natural aluminium is ²⁷Al, if it catches a neutron the ²⁸Al quickly (minutes) decays to ²⁸Si which is stable even if it captures one or two additional neutrons.

Natural iron has about 5% ⁵⁴Fe, if it catches a neutron to become ⁵⁵Fe it has a half-life of 2.7 years. Too long to wait, too short to ignore. A small fraction of ⁵⁹Fe with a half-life of 44 days does not help either. And steel has tons of other elements in it, some of them produce other nasty isotopes when activated.