Radioactive contamination

[Kaeroll]

Not sure if this belongs in chemistry or physics, but I figured it'd get more exposure here.

 

In brief, my question is: what is the mechanism of radioactive contamination?

 

Bit of context to what I mean, at a university in the UK there has recently been a controversy over the use of what was formerly Ernest Rutherford's lab. Several people working in the building have recently died from cancer, and there's fear it's still contaminated from his work.

 

I'm familiar on a basic level with radioactivity, but it's not my specialty at all. How can radiation linger for nearly a century? My guess would be that nuclei in (for example) the walls, floor, etc. are converted to radioactive nuclides with longer half-lives.

 

Anyone care to fill me in?

[Klaynos]

Your guess is pretty much my understanding.

 

This leads to the radiation spreading out, it can be quite large distances...

[swansont]

Contamination (the noun) is an uncontained radioactive substance, and this can be spread, or can accumulate. An insidious thing is that it can start out as a gas and then decay, as in Radon decaying to Polonium and on along its decay chain. The half-lives involved can be many years. Pb-210, for example (in the Rn-222 decay chain) has a half-life of 22.3 years.

 

An unsealed sample can be spilled and the particulates can be spread quite easily. Especially so in the early days of radioactivity experimentation, I would imagine, when radiation safety was essentially nonexistent.

[GDG]

There's a Wiki entry for it.

 

As a practical matter, contamination occurs from handling radioisotopes and their packaging/shielding materials (which become radioactive). The quantity that is toxic for some materials is just incredibly miniscule. For example:

• Polonium: "The maximum permissible body-burden for ingested polonium is only 0.03 microcuries, which represents a particle weighing only 6.8 x 10^-12 grams. Weight for weight it is about 2.5 x 10^11 times as toxic as hydrocyanic acid."
  
• Plutonium: "The maximum permissible body burden, or the amount that can be maintained indefinitely in an adult without producing significant body injury, for Pu239 is now set at 0.04 microcurie (0.6 micrograms). It is recommended that the concentration of plutonium in air not exceed 0.00003 micrograms per cubic meter. Plutonium, therefore, is one of the most dangerous poisons known."
  
• Americium: "Americium must be handled with great care to avoid personal contamination. As little as 0.02 microgram of Am241 is the allowable body burden (bone). The alpha activity from Am241 is about three times that of radium. When gram quantities of Am241 are handled, the intense gamma activity makes exposure a serious problem."
  
• Uranium: "Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint. Finely divided uranium metal, being pyrophoric, presents a fire hazard. The maximum recommended allowable concentration of soluble uranium compounds in air (based on chemical toxicity) is 0.05 mg/cu meter; for insoluble compounds the concentration is set at 0.25 mg/cu meter of air. The permissible body level of natural uranium (based on radiotoxicity) is 0.2 microcurie for soluble compounds; for insoluble compounds the level is 0.009 microcurie, or in air 1.7 x 10^-11 microcurie per milliliter."
  

Source: The CRC Handbook of Chemistry and Physics, 56th Edition.

 

U and Pu oxidize readily in moist air.

 

You can imagine how difficult it is to avoid letting such a tiny amount escape...

 

FWIW, I'm a chemist by education.

Edited March 12, 2009 by GDG
last comment

[Kaeroll]

Cheers for the replies.

 

My first port of call was the Wiki article, and while it has plenty of information about how it can occur and what's so nasty about it, it doesn't say how it can hang around afterwards, even if the offending (bulk) material is cleaned up.

 

So I guess your suggestion is that trace amounts left over are responsible, gdg? (FWIW - I'm also a chemist, in training..)

 

W/r/t Po in particular - yikes!

Edited March 12, 2009 by Kaeroll
Consecutive posts merged.

[GDG]

Trace amounts that have escaped, and other materials that have been rendered radioactive through exposure to radiation.

 

There are a number of ways that radioisotopes can escape.

• "Polonium-210 is a low-melting, fairly volatile metal, 50% of which is vaporized in air in 45 hours at 55oC." *** "The energy released by its decay is so large (27.5 calories per curie per day or 140 watts/gm) that a capsule containing about half a gram reaches a temperature above 500oC."
  
• Thorium has a high melting point, but decays to Radon - a radioactive gas.
  
• Uranium and Thorium are pyrophoric: when machined or filed, the filings can spontaneously burst into fire.
  

One can imagine handling these materials very carefully, in a glove box, and then going to clean the glove box (or even just to dispose of it). How does one make sure that there is less than 10^-11 g of a material left inside the box?