Last update on September 2014
The MIRE (Mitigation of Releases to the Environment in the event of a nuclear accident) project launched in January 2014 by IRSN aims to study and improve filtration of radioactive releases during a reactor meltdown accident (referred to as a severe accident). It is one of seven projects run by the Institute and selected by the National Research Agency (ANR) for the call for research projects related to nuclear safety and radiation protection (RSNR).
Context and objectives
When a core meltdown occurs in a nuclear reactor, radioactive substances are released from the fuel rods and transported into the containment in the form of particles (aerosols) or gases. In order to reduce the risk of a massive release of these substances into the environment in the event of an excessive increase in pressure in the containment, some reactors, and in particular French reactors, are equipped with a venting system. Such systems provide for the depressurization of the containment and are generally equipped with a filter to limit the release of radioactive substances into the environment.
In France, the filtering system is made up of a metallic pre-filter inside the containment and a sand filter placed outside the containment. This system, which is relatively efficient for the filtration of aerosols, is not as efficient for the retention of 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. The project consists in comparing the effectiveness of filtering systems available throughout the world today and in acquiring useful knowledge for the development of innovative, effective and robust filtering systems.
So as to better identify the form and quantity of radioactive substances which have to be filtered, the MIRE project also aims to improve existing knowledge concerning the resuspension or revolatilization of radioactive substances that could occur during venting.
Research areas and methods
The MIRE project includes three research areas using both experimentation and modeling.
1/ Source term studies
The goal is to improve the knowledge concerning the types and form of radioactive substances that could be in suspension in the containment when the venting is operated. There are two 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 in 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 are analyzed using tests involving revolatilization (cesium, iodine and ruthenium) from surfaces of the reactor coolant system. The main parameters studied are the type of carrier gas and thermal conditions.
- Characterization of iodine oxides
During an accident, iodine oxide particles form in the reactor containment from the oxidation of volatile iodine by the products of radiolysis of air (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 are being conducted to obtain qualitative and quantitative information (size, mass, distribution, composition, etc.) on the interactions between iodine oxides and the products resulting from the radiolysis of air, as well as on the chemical stability of these oxides in the medium term.
2/ Evaluation of the effectiveness of existing filtering systems
Research conducted in this area aims to determine the effectiveness of filters intended to trap molecular iodine and organic iodine, as well as ruthenium in gaseous form. The filters tested are those already installed in nuclear facilities in France and elsewhere in the world:
- Sand filters
- Liquid filters (scrubbing filters)
- Metal filters
Adsorption tests for the considered gaseous species are performed on a small scale for each type of filter. The parameters studied include adsorption speed, maximum trapping capacity, temperature and moisture content. The reversibility of this capture under radiation conditions is also evaluated. These tests are supplemented by larger-scale tests on a new IRSN facility, the PERSEE test bench, designed for the study of filtering and purification systems.
This experimental data will then be analyzed, and corresponding models will be developed. The models established will be integrated into the ASTEC software program developed by IRSN, or the MAAP program used by EDF, for the simulation of severe accidents.
3/ Research and development of new filtering systems
This research area consists in the evaluation of the effectiveness and properties of new types of materials or media in an effort to design filters that are more effective and robust under severe accident conditions. The robustness sought includes in particular resistance to high temperatures, humidity and radiation. Two types of filtering media are being examined: zeolite and MOF (Metal Organic Framework) filters.
These filtering media will be tested under flows of gaseous iodine and/or ruthenium, and several parameters will be studied: concentration, presence of any pollutants, gas flowrate, temperature, etc. They will also be evaluated according to their capacity to retain irreversibly the radioactive substances. The most promising filtering media will then be tested on the PERSEE test bench.