Chernobyl Still Radioactive After 23 Years
ADIT, décembre 2009
Chernobyl Still Radioactive After 23 Years
Even more so than originally expected

The Chernobyl nuclear power plant after the meltdown, showing reactor 4 with its roof blown off

     Speaking at the annual meeting of the American Geophysical Union (AGU) on Monday, experts revealed a troublesome fact about Chernobyl, the Ukrainian nuclear power plant that blew up in 1986. Recent measurements in the exclusion zone, where no humans can go without protective equipment, have revealed that the radioactive material that was spilled in the area was nowhere near the decay level that was predicted for it. In other words, the scientists are saying that it will take a lot more time for the land to be cleansed than originally believed, Wired reports.
     Previous estimates, based on the fact that the Cesium 137's half-life is 30 years, estimated that the restriction zone could be lifted, and then re-inhabited soon. But experiments reveal that the radioactive material is not decaying as fast as predicted, and scientists have no clue as to why this is happening. The April 26, 1986 accident was the largest nuclear accident in the world, and only a level 7 event on the International Nuclear Event Scale. Its fallout was made worse by the Soviet Union's attempt at covering up the incident, which saw a lot of people exposed to lethal doses of radiations.
     "Normally you'd say that every 30 years, it's half as bad as it was. But it's not. It's going to be longer before they repopulate the area," Savannah River National Laboratory nuclear scientist Tim Jannick, who has also collaborated in the new investigation, said at the AGU conference. The new estimates point out that the half-life for ecological Cesium 137, the variety out in the open, is between 180 and 320 years. This pours a stream of cold water on the Ukrainian authorities' plans to start reusing the land.
     "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," Lawrence Berkeley National Laboratory (Berkeley Lab) nuclear remediation expert Boris Faybishenko says. In the greater scheme of things, the fallout and nuclear disaster provided researchers with a window of opportunity into studying the possible effects of radiation poisoning on the environment and ecosystems. For the past 20 years, teams have been analyzing how strontium, plutonium and cesium isotopes infiltrate the ground around Chernobyl several times annually.
Chernobyl Exclusion Zone Radioactive Longer Than Expected

SAN FRANCISCO — Chernobyl, the worst nuclear accident in history, created an inadvertent 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 that what they call 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.
Voir aussi :

Vertical migration of radionuclides in undisturbed grassland soils
     Gerald Kirchnera, Friederike Streblb, Corresponding Author Contact Information, E-mail The Corresponding Author, Peter Bossewc, Sabine Ehlkend and Martin H. Gerzabeke
a) BfS Federal Office for Radiation Protection, D-38201 Salzgitter, Germany
b) Austrian Research Centers GmbH (ARC), Department of Radiation Safety and Applications, A-2444 Seibersdorf, Austria
c) Institute for Environment and Sustainability, DG Joint Research Centre, European Commission, I-21020 Ispra, Italy
d) Klinikum Bremen-Mitte, D-28177 Bremen, Germany
e) University of Natural Resources and Applied Life Sciences Vienna, Peter-Jordan-Strasse 82b, A-1190 Vienna, Austria

     Literature data on numerical values obtained for the parameters of the two most popular models for simulating the migration of radionuclides in undisturbed soils have been compiled and evaluated statistically. Due to restrictions on the applicability of compartmental models, the convection–dispersion equation and its parameter values should be preferred. For radiocaesium, recommended values are derived for its effective convection velocity and dispersion coefficient. Data deficiencies still exist for radionuclides other than caesium and for soils of non-temperate environments.

1. Introduction
2. Models for the transport of radionuclides in soils
   2.1. Compartmental models
   2.2. Convection–dispersion models
   2.3. Links between Kd-values and migration parameters
3. Parameters for vertical migration in undisturbed soil profiles
   3.1. Time dependence of 137Cs vertical migration
   3.2. Effect of soil texture on 137Cs migration
   3.3. Migration rates of 137Cs in different contamination scenarios
   3.4. Influence of fuel hot particles on migration rates in soil
   3.5. Vertical migration of radionuclides other than 137Cs
4. Conclusions