Duration of Radioactive Contamination

[SilverStud]

My question has a few sub-questions.

How long does an area stay contaminated, say after a bomb? A reactor leak?

Would different radioactive materials leave an area contaminated longer than others?

What would be the most scientifically plausible for an author to explain the 1000-year contamination of an area?

Thanks in advance!

 

[mfb]

Radioactivity comes from different isotopes.
Each isotope has its own half-life and total amount. After one half-life, half of the isotope is decayed. After 2, half of the remaining amount is decayed (so 25% is left), and so on. After 10 times the half-life, 99.9% are decayed. The shorter the half-life, the quicker it will decay - but this leads to a higher initial intensity. There is no fixed time after which everything decayed, the activity just goes down forever.

If you plot activity as function of time, you get something like this.
In addition, you have to take into account how the material gets distributed in the environment.

Now, some practical examples: At Hiroshima and Nagasaki, rebuilding began quickly (~months) after the war. There was significant fallout, but most of the material was short-living, or spread in the environment quickly (the bombs exploded in the air). Today, radiation level (above the usual background) is negligible.
Chernobyl, on the other hand, still has significant radioactivity in the area around it, mainly from Cesium IIRC. This will slowly decrease in the next decades to centuries. A significant release of Cesium (+other stuff) in a reactor leak might be a good scenario.

 

[Astronuc]

My question has a few sub-questions.

How long does an area stay contaminated, say after a bomb? A reactor leak?

Would different radioactive materials leave an area contaminated longer than others?

What would be the most scientifically plausible for an author to explain the 1000-year contamination of an area?

Thanks in advance!

There are nuclear and chemical aspects to the questions. The nuclear aspect involves both the fission products and the activated materials. The fission products originate with the fission process, and have a characteristic isotopic (radionuclide) vector. The activation products depend on the materials of the ground and structures that absorb neutrons and become radioactive. Nuclear reactors have a different isotopic vector than detonated nuclear weapons.

In either case, there are short-lived radionuclides that decay in seconds, minutes or hours, somewhat moderate-lived radionuclides that decay in days, weeks or months, and long-lived isotopes that decay over years, decades, centuries or millenia.

In 10 half-lives, the activity decreases by 2¹⁰ ~ a factor of 1000, in 20 half-lives, the activity decreases by a factor ~ 1,000,000, and in 30 half-lives, the activity decays by a factor of 1 billion. So short lived radionuclides can disappear in minutes or days or up to a year or so, whereas moderate-lived take several years or decades, and the longer-lived isotopes can be present for centuries. The longest-lived are persistent.

The other side of that is that the longer the half-life, the lower the activity for a given quantity, that is the lower the specific activity (or activity per unit mass).

The other aspect is the chemical behavior. Cs, for example behaves like Na or K, so it can be taken up by plants, animals and humans. Similarly, I is taken up by plants, animals and humans, and the concern is radio-iodine that is absorbe by the thyroid gland of animals and humans. Radioisotopes of Sr and Ba are a concern for bone uptake, since they behave chemically like Ca. Some of the transurancis, e.g., Pu are also 'bone-seekers'. Various other radionuclides of the transitional metals are a concern because they may be absorbed into the body and used in essential vitamins.

The other side of the chemical behavior is the transport in the environment whereby the radionuclides dissolve and/or form compounds that act to dissipate the radiation.

After the nuclear weapons testing and accidents like Chernobyl and Fukushima, trace levels of radionuclides are found in various parts of the world, but at relatively low levels, although, the levels are higher in the area around the accident site.

 

[anorlunda]

Your question implies that you think contamination is a binary choice. Either something is or is not contaminated. That's not the way things work.

In reality, there is a continuum. The longer the elapsed time, the less the emitted radioactivity.

In terms of health risks due to radiation, safety too is a continuum, not a binary choice. To say that a site is unsafe on day 365,000 but safe on day 365,001 would be arbitrary, not scientific.

 

[SilverStud]

Wow, that was some fantastic information! And thanks for the idea about cesium.
I just now figured out my real question. It is this: can I have a place contaminated for a thousand years, or would smart people like you guys read it and be like "Bull chips!"? Should I find a different excuse for an area to be off limits?

 

[mfb]

If the area has a contamination which is significant for 1000 years, the radioactivity will go down very slowly after some centuries - there will be no year or even decade where someone can (reasonably) decide "it was too dangerous before, but now it is safe".
This does not take engineering into account - you can clean an area manually, reducing the radioactivity a lot during that cleaning campaign. So if you want some specific end date, let humans remove/clean/whatever the upper layer of the ground.

Should I find a different excuse for an area to be off limits?

Flooded?