[ RadSafe ] Chernobyl Exclusion Zone Radioactive Longer Than Expected

[Cary Renquist]

Thanks, John & Peter.

I knew that cesium was taken up by trees and vegetation -- I just wasn't
sure if it was incorporated in leaves and thus released...

c.

---
cary.renquist at ezag.com


-----Original Message-----
From: Peter Bossew [mailto:Peter.Bossew at reflex.at] 
Sent: Tuesday, 15 December 2009 16:02
To: Cary Renquist; radsafe at radlab.nl
Subject: Re: [ RadSafe ] Chernobyl Exclusion Zone Radioactive Longer
Than Expected

"Cary Renquist" <cary.renquist at ezag.com> writes:
>Sounds like Cesium is moving around...
>
>Could something like deciduous trees be serving as a cesium store and
>then releasing cesium via their leaves?


Yes this phenomenon exists. In forest ecosystems, particularly with poor
soils (like in Chernobyl: practically pure sand, so called poleskoye), 
minerals are being recycled quite efficiently. The same can be observed
in
tropical forests. Dropped leaves disintegrate and form a narrow humic
layer of OL and OF, but often little OH because the nutrients (incl Cs)
are eagerly recycled by the living compartment of the soil.

Apart from this, downward migration depends on the chemico-physical
speciation of the radionuclides. In parts of the Chernobyl zone, most of
initial fallout was contained in hot particles, which disintegrate
(erode)
slowly, thus Cs which can migrate is set free only slowly. Erosion
velocity of hot particle depends on soil chemistry and on the type of
particles (oxidized or not; the former set free during the fire, the
latter by the initial explosion). 

These effects lead to an apparent migration velocity, and hence
ecological
half life, which is lower than would be expected for true migration of
condensed fallout like in most agricultural soils.

Horizontal transport due to resuspension exists, but is negligible to my
knowledge.

There is much literature about these phenomena (I also did some research
about it). 

Btw., one radionuclide is building up instead of decaying: 241Am (from
214Pu), whose maximum will be reached only in about 2050.

Peter Bossew



>
>
>
>Cary
>--
>Cary.renquist at ezag.com
>
>
>Chernobyl Exclusion Zone Radioactive Longer Than Expected | Wired
>Science | Wired.com
>Short URL:
>http://bit.ly/7xDfHE
>
>
>SAN FRANCISCO - Chernobyl, the worst nuclear disaster in the human
>history, created an accidental laboratory to study the impacts of
>radiation - and more than twenty years later, the site still holds
>surprises.
>
>Reinhabiting the large dead zone around the accident site may have to
>wait longer than expected. Radioactive cesium isn't disappearing from
>the environment as quickly as predicted, according to new research
>presented here Monday at the meeting of the American Geophysical Union.
>Cesium 137's half-life - the time it takes for half of a given amount
of
>material to decay - is 30 years, but the amount of cesium in soil near
>Chernobyl isn't decreasing nearly that fast. And scientists don't know
>why.
>
>It stands to reason that at some point the Ukrainian government would
>like to be able to use that land again, but the scientists have
>calculated the cesium's ecological half-life - the time for half the
>cesium to disappear from the local environment - is between 180 and 320
>years.
>
>"Normally you'd say that every 30 years, it's half as bad as it was.
But
>it's not," said Tim Jannick, nuclear scientist at Savannah River
>National Laboratory and a collaborator on the work. "It's going to be
>longer before they repopulate the area."
>
>In 1986, after the Chernobyl accident, a series of test sites was
>established along paths that scientists expected the fallout to take.
>Soil samples were taken at different depths to gauge how the
radioactive
>isotopes of strontium, cesium and plutonium migrated in the ground.
>They've been taking these measurements for more than 20 years,
providing
>a unique experiment in the long-term environmental repercussions of a
>near worst-case nuclear accident.
>
>In some ways, Chernobyl is easier to understand than DOE sites like
>Hanford, which have been contaminated by long-term processes. With
>Chernobyl, said Boris Faybishenko, a nuclear remediation expert at
>Lawrence Berkeley National Laboratory, we have a definite date at which
>the contamination began and a series of measurements carried out from
>that time to today.
>
>"I have been involved in Chernobyl studies for many years and this
>particular study could be of great importance to many [Department of
>Energy] researchers," said Faybishenko.
>
>The results of this study came as a surprise. Scientists expected the
>ecological half-lives of radioactive isotopes to be shorter than their
>physical half-life as natural dispersion helped reduce the amount of
>material in any given soil sample. For strontium, that idea has held
up.
>But for cesium the the opposite appears to be true.
>
>The physical properties of cesium haven't changed, so scientists think
>there must be an environmental explanation. It could be that new cesium
>is blowing over the soil sites from closer to the Chernobyl site. Or
>perhaps cesium is migrating up through the soil from deeper in the
>ground. Jannik hopes more research will uncover the truth.
>
>"There are a lot of unknowns that are probably causing this
phenomenon,"
>he said.
>
>Beyond the societal impacts of the study, the work also emphasizes the
>uncertainties associated with radioactive contamination. Thankfully,
>Chernobyl-scale accidents have been rare, but that also means there is
a
>paucity of places to study how radioactive contamination really behaves
>in the wild.
>
>"The data from Chernobyl can be used for validating models," said
>Faybishenko. "This is the most value that we can gain from it."
>
>
>Citation: "Long-Term Dynamics of Radionuclides Vertical Migration in
>Soils of the Chernobyl Nuclear Power Plant Exclusion Zone" by Yu.A.
>Ivanov, V.A. Kashparov, S.E. Levchuk, Yu.V. Khomutinin, M.D. Bondarkov,
>A.M. Maximenko, E.B. Farfan, G.T. Jannik, and J.C. Marra. AGU 2009
>poster session.
>