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Enhancing Nuclear Safety


Research

Theses in progress

Biogeochemical cycle of iodine in forest ecosystems


Host laboratory: ​Biochemistry, bioavailability and radionuclide transfer laboratory (L2BT)

Beginning of the thesis: October 2015

Student name: Marine Roulier


Subject description

This thesis is included in the CYCL part (Cycle of radionuclides in Forest ecosystems) of the AMORAD project focused in the improvement of dispersion forecast and impact assessment of radionuclides in the environment.

Iodine is a trace element naturally present in the earth’s crust with a mean concentration of 0.45 mg kg-1. Natural iodine is composed of stable isotope 127I (99.99 %) and single long-lived radioisotope 129I (0.01 %, radioactive half-life 15.7 106 years). 129I is also present in high-level and long-lived radioactive waste and thus considered in environmental impact studies of nuclear installations because of its potential mobility. It is assumed that nuclear waste storage for long periods in geological formations could become a potential source of iodine in environment through ground and atmosphere. Indeed, part of the iodine contained in soils and plants can be transformed into volatile fraction and released into the atmosphere. Forest ecosystems which represent about one third of the surface territory in France and in Europe, need to be considered in case of environmental iodine dissemination. Forests are characterized by a strong interception of radionuclide atmospheric deposition and take part into radionuclide speciation modifications and distribution related to the organic matter cycle. Specifically, forest vegetation can recycle and accumulate significant amounts of 129I radioisotope following transfer processes similar to 127I stable isotope, which could be used as radionuclide analogue considering long term studies. However, few studies have been conducted on iodine interception by forests and related behavior within this ecosystem. Monitoring studies of iodine in forests is necessary for better understanding the processes that govern its distribution and fluxes.

Iodine uptakes into trees come from wet or dry deposits on leaves and needles and from roots according to iodine bioavailability in soil and from litter recycling. Within the forest ecosystem, the soils remain the main pool of iodine. In forest soil, where surface layers are strongly enriched in organic matter, iodine can exist as different inorganic and organic forms: iodide (I-), elemental iodine (I2), iodate (IO3-), alkyl compounds as iodomethane (CH3I) having different environmental behaviors. Although organic matter is a parameter controlling the speciation of iodine in soil, its influence on the persistence and bioavailability of iodine in forest ecosystems is not well documented. In addition, there is a lack of knowledge on iodine speciation into tree compartments although this is an essential information to understand the cycle (accumulation and distribution) of iodine in trees.

Therefore, the objectives of this thesis are to study the iodine cycle in forest ecosystem to better understand and model its behavior for medium and long term. To reach these objectives two approaches were chosen. Firstly, the global cycle of iodine in forest is caracterized by measuring its distribution in the different forest ecosystem compartments (soil, wood, foliage, humus, litterfall, etc.) to calculate fluxes and their variation with time. This study is performed by a three years in situ monitoring, carried out on the Montiers forest site (OPE - Andra). In addition, other monitoring sites from the RENECOFOR network (ONF) are studied to obtain parameters of the iodine distribution on French territory for different combinations of tree species, humus and soils. Secondly, a focus on the evolution of nature and availability of iodine compounds during the mineralization of organic material in humus is conducted on some selected RENECOFOR sites. This study will provide an initial classification of forests relatively to their impact on the (bio)availability of iodine.


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