MIRE project

Context and objectives

When a core melt accident occurs in a nuclear reactor, the radioactive elements contained in the fuel rods enter the containment in the form of particles (aerosols) or gases. In order to reduce the risk of a massive release of these elements into the environment in the event of an excessive increase in pressure in the containment, certain reactors, and those in French nuclear power plants in particular, are equipped with a venting and filtration system. This system provides for the depressurization of the containment via a filtration system which limits the release of radioactive elements into the environment.

In France, the filtration system used is made up of metallic prefilters and sand filters arranged inside and outside of the containment, respectively. This system, which is relatively effective for the filtration of aerosols, cannot retain gaseous forms of two elements presenting a significant health risk: iodine and ruthenium.

The main objective of the MIRE project is to reduce radioactive releases that could result from the intentional venting of the containment. It has an initial phase (Phase 1 - 2013-2019) and a project extension phase (2019-2022).

Phase 1 outline and research themes (2013-2019)

Phase 1 of the MIRE project was conducted based on three research areas using both experimentation and modelling.
 

1/ Source term studies ⁽¹⁾

 The goal was to build on existing knowledge concerning the inventory and form of radioactive elements that could be suspended in the containment. So as to better identify the form and quantity of radioactive elements that could travel through these venting and filtration systems, this phase of the MIRE project aimed to improve existing knowledge concerning the resuspension or revolatilization of radioactive elements that could occur during venting or subsequently, over the longer term. There were two main issues within this research area about which much is still unknown:

•  The resuspension of adsorbed species

Changes in the thermal hydraulic conditions in the containment or the composition of its atmosphere could lead to the resuspension of radioactive elements deposited on, or adsorbed by, the surfaces of the containment, the steam generator and the reactor coolant system. As part of the MIRE project, these processes were analysed using tests involving revolatilization (caesium, iodine and ruthenium) from surfaces of the reactor coolant system. The main parameters studied were the type of carrier gas and thermal conditions.

It was shown that in some conditions, deposits can be remobilised with the formation of gases and aerosols.

This theme is linked to experimental observations during the Fukushima-Daiichi accident, and will be further studied as part of the OECD ESTER (Experiments on Source TErm for delayed Releases) project starting in autumn 2020.

• Characterization of iodine oxides

During an accident, iodine oxide particles form in the reactor containment from the oxidation of volatile iodine by air radiolysis products (ozone formed under radiation, for example). Although these particles can in certain cases represent a large portion of the iodine particles suspended in the containment, little information on them is available (size, composition, stability, etc.). For the MIRE project, experiments were conducted to obtain qualitative and quantitative information (size, mass, distribution, composition, etc.) on the interactions between iodine oxides and air radiolysis products, as well as on the chemical stability of these oxides in the medium term.

These studies concluded that iodine oxides are formed by (UV and radiolytic) oxidation of volatile iodine species, and are found in the form of small particles (several hundreds of nanometres) whose chemical composition can vary as a function of temperature and humidity. These particles are fairly unstable and can be partially redecomposed into gaseous iodine (I2), depending on the chemical form of the oxides.

 2/ Evaluation of the effectiveness of existing filtration systems

The studies performed aimed to evaluate and compare the effectiveness of filtration systems currently available worldwide (see the OECD Status Report on Filtered Containment Venting).

Research conducted in this area focused on determining experimentally the effectiveness of filters intended to trap molecular iodine and organic iodine, as well as ruthenium in gaseous form. The filters tested were those already installed in nuclear facilities in France and elsewhere in the world:

• sand filters,
• liquid filters (sparge filters),
• metal filters.

Adsorption tests for the gaseous species considered were performed on a small scale for each type of filter. The parameters studied included adsorption rate, maximum trapping capacity, temperature and moisture content. The reversibility of this capture under radiation conditions was also evaluated. These tests were supplemented by larger-scale tests on a new IRSN facility, the PERSEE test bench, designed for studying filtration and purification systems.

The data acquired confirmed that for volatile iodine species, metal, sand or liquid filters do not effectively trap organic iodides, and CH3I in particular.

 

3/ Research and development of new filtration systems based on the use of dedicated porous materials

This phase aimed to study and develop innovative porous materials for trapping iodine gaseous species (I2 and CH3I) and gaseous ruthenium as ruthenium tetroxide (RuO4).

Two types of filtering media were examined: silver zeolite and MOF (Metal Organic Framework) type filters.

Trapping of different iodine species was studied along with associated mechanisms (modelling and experimental studies). Analytical tests in conditions as close as possible to severe accident conditions (radiation, temperature, steam, inhibitors, etc.) were performed using the SAFARI (measuring the filtration efficiency of materials on ruthenium and iodine) test bench and PERSEE test bench (see the installation description). The most promising materials were also tested on gaseous ruthenium.

For gaseous iodine, theoretical studies using density functional theory (DFT) molecular dynamic calculations confirmed that silver dispersed in zeolite is the best candidate for trapping iodine. For gaseous pollutants, carbon monoxide (CO) can have an inhibiting effect on the adsorption of iodine species, even at low concentrations. There was high water content in the gas, which can also have a significant effect. From an experimental perspective, of the zeolites tested, adsorbents prepared by ion exchange with silver in Y-faujasite (particularly with around 20% of silver by mass) showed the highest performance for CH3I and I2 trapping. A commercial Ag-X zeolite with 35% silver by mass also showed good trapping capacities. For MOFs, some materials offer good radiation resistance, but at this stage, we have not been able to find a structure that can efficiently trap iodomethane.

For gaseous ruthenium, the results obtained show variable trapping capacities depending on the porous materials studied. The series of tests performed provides an initial ranking of existing porous solid materials. MOFs and organically modified silica had the highest trapping capacities of the compounds tested. Additional tests are required to explore their performance in conditions that are as representative as possible of severe accidents.

(​1) The expression "source term" refers to all the information that characterizes the release of radioactive materials into the environment, including the chemical species released, the isotopes in question, their physical and chemical forms (gases, aerosols), the quantity released for each species, and the release kinetics.

Extension phase research themes (2019-2022)

The MIRE project extension phase aims to assess the behaviour of these innovative materials (silver zeolites) in conditions representative of a severe accident. It also focuses on research into an "alternative" material (a "less complex" porous material mainly doped with silver or without silver) which can easily be used on an industrial scale (easy implementation, maintenance, etc.). The University of Lorraine, which led most of the studies into zeolites in the first project phase, EDF and the microbusiness SOMEZ, working primarily on research into an alternative porous material to zeolites, are taking part in this research programme phase.

The project extension is organised as follows:

• Task 1. Define the best volatile iodine trapping concept: limit the final choice to no more than 2 porous materials on the basis of laboratory testing and molecular modelling.
• Task 2. Consider the issue in the light of factors of scale, including forming, and test to study potential ageing.
• Task 3. Purification coefficient measurement tests at laboratory level in SAFARI and "semi-pilot" level in PERSEE, and trapping stability tests in the EPICUR facility with supporting modelling.
• Task 4. Summary report with proposals for improving the U5 system, the filtered venting system which provides containment decompression and filtration in the event of a severe accident.

The results obtained under the project extension will:

• provide detailed information on potential improvements to the U5 system on the current reactor fleet;
• provide technical solutions (selected by testing), that have been tested under conditions as close as possible to severe accident conditions and including factors of scale, for mitigating iodine release in the event of containment venting for future GenIII/III+ reactors;
• after a cost-benefit analysis, the materials developed through this approach could be implemented in facilities where it is difficult to control filtration conditions and in which activated carbon is less efficient (repositories, chronic releases, etc.).