Virgin steel

Great question. Whether radiation levels in the post-nuclear world have increased appreciably or not depends on what year we are talking about and what you are doing (also, I suppose, what one considers "appreciable"). For the present day, I would say the answer is "no" unless you live in certain parts of Europe or Japan, and unless you are performing very low background physics experiments.

I have pointed out in at least one other thread that the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has estimated the average exposures to people from different radiation sources, including natural sources, weapons testing fallout, Chernobyl fallout, and the nuclear fuel cycle. In 1963, when atmospheric weapons testing was ended, the average dose from weapons testing peaked at approximately 0.11 mSv/year (UNSCEAR Report 2008, Annex B, paragraph 247). This has decreased to about 0.005 mSv/year currently. The annual dose from natural sources is approximately 2.4 mSv/year (primarily due to radon, cosmic radiation, and potassium-40). So in 1963, the average dose from weapons testing was nearly 5% of the natural dose, while today it is less than 0.3%. One of the reasons for ending atmospheric testing was that many people thought that the dose from anthropogenic radiation was becoming appreciable.

A very rough way to see this is in our recent soil tests. We found Cs-137 in older soils from previous fallout. The highest measurement we have is about 2.5 Bq/kg, as opposed to ~500 Bq/kg of the natural isotope K-40 in the same sample. So anthropogenic isotopes are definitely present in our environment but in very small amounts.

As for "virgin" steel: When metals like steel are forged, there are always trace impurities that get in the metal from the environment. Since the first nuclear weapons explosions in 1945, forged metals started including trace impurities of anthropogenic radioactive elements. These include Cesium-137 (30 year half-life) and Cobalt-60 (5.3 year half-life). There also also natural radioactive isotopes that are incorporated into the metal as it is forged, and these can cause problems as well.

Only in the most sensitive experiments are these trace amounts detectable. There are a number of physics experiments that require very low background environments, such as the deep underground experiments searching for dark matter or exotic phenomena such as neutrinoless double beta decay. Even though these experiments are deep underground, they still need radiation shielding because rocks can contain uranium, thorium, and potassium. But if regular steel or lead is used to shield these experiments, the experiments will actually see the radiation from the "dirty" steel or lead.

To get around this problem, you can either forge the steel or lead and wait around for centuries until the trace isotopes decay to an acceptable level for your experiment, or you can search for old sources of the metals. Battleship steel from before 1945 is particularly abundant and will not have Cs-137 or Co-60.

Another fascinating solution to the shielding problem is that some experiments use lead found in shipwrecks from the Roman era. Not only does this lead not have anthropogenic isotopes, but the natural stuff has also had plenty of time to decay away. An especially troublesome natural isotope is Lead-210 (22 year half-life), which is ubiquitous. Here is a great article about one recent experiment that uses Roman lead. The article shows a spectrum of the radioactivity of new lead, low-background lead, and Roman lead.

In the end I suppose it's a judgment call whether the manmade sources are "appreciable" or not. I have never said that the levels are zero, just that the amounts today are very tiny in comparison to natural sources.

Mark [BRAWM Team Member]