Last update on October 2015
The Source Term Evaluation and Mitigation (STEM) project, initiated by IRSN under the aegis of OECD/NEA, is aimed at learning more about the behaviour of radioactive materials that could be released to the environment in the event of a core melt down accident at a nuclear facility. The first phase of the project began in 2011 and lasted four years. It has just come to an end and a second four-year phase is scheduled to begin in early 2016.
Context and objectives
Significant uncertainties affecting the evaluation of releases to the environment (or source term1) remain regarding some of the complex phenomena encountered during a core melt down accident in a pressurised-water reactor (PWR). Examples are the behaviour of iodine in the containment and of ruthenium in the reactor coolant system. The STEM experimental project seeks to reduce these uncertainties. STEM has two main objectives:
- develop enhanced numerical simulation tools and improve knowledge, with the ultimate aim of helping the teams in charge of managing a nuclear accident to make a more robust diagnosis and prognosis of the progress of the accident;
- identify additional measures that could be taken to further reduce (or mitigate) the release of radioactive materials to the environment.
Project outline and research themes
The first phase of the STEM project was carried out from mid-2011 to mid-2015 and was devoted to three research themes.Medium- and long-term radioactive releases
Research work on the source term carried out during the 2000s mainly focused on assessing potential radioactive releases to the environment in the short term, in other words, during the very first days of a reactor core melt down accident, which is when the most significant releases occur. The next task was to work on quantifying and understanding delayed and continuous release over a relatively long period (several days or even weeks after the start of the accident), whence the introduction of "medium or even long terms releases" in the STEM project.
The ability to assess changes in gaseous iodine releases (as molecular iodine I2 and organic iodine RI) over a period of a week is considered important for accident management as it determines in particular the length of time during which emergency measures such as evacuating or sheltering the population must be maintained within a given area. Recent progress in R&D in the field has confirmed some of the assumptions made regarding possible new phenomena that could significantly modify this "medium-term releases". For example, particles of iodine aerosols deposited on the inner walls of the containment building or trapped in filtration devices might not be stable when exposed to ionising radiation, in which case they would change into volatile species. One aim of the experiments conducted as part of the STEM project was therefore to acquire the necessary data to determine the laws governing the decomposition kinetics of the different species of iodine in aerosol form.
As well as acquiring experimental data for the medium term, the STEM project seeks to extend the domain of validation of the models in the ASTEC computer code that describe the behaviour of gaseous iodine in the containment beyond the standard 24 h reference period, which is characteristic of "short-term" releases.
Ruthenium chemistry in the reactor coolant system
For some accident scenarios, air may come in contact with the fuel damaged during the accident and quantities of gaseous ruthenium (RuO4) can be released as a result.
While data on the behaviour of RuO4 in the containment is relatively plentiful, less is available on the behaviour of RuO4 as it is transported through the reactor systems from the fuel to the containment. It therefore appeared necessary to perform tests as part of the STEM project to cover all the various accident situations that might occur.
This "ruthenium issue", which is historically related to PWR safety issues under accident conditions (typically a reactor loss-of-coolant accident followed by ingress of a gaseous oxidising mixture from the containment atmosphere following vessel failure), can also be very important for other types of accident situations such as spent fuel pool uncovery or those relating to fuel handling. These various cases involve different temperature ranges and chemical hardening of the Ru04, which is why these parameters have also been studied as part of the STEM project.
The analytical and parametric experiments of the "ruthenium" part of the STEM project have yielded enough data to develop new models that will be implemented in the ASTEC computer code.Paint ageing and the impact on interactions with iodine
It is well-known that organic materials, especially paints, are a crucial factor to be considered when studying the behaviour of iodine in the containment. The next step of the STEM project, which involves conducting a bibliographical study of paint interactions with iodine, has now been undertaken, revealing gaps in knowledge and identifying a series of experiments to be given priority. These concern the study of iodine adsorption and desorption phenomena and the production of volatile organic iodine, taking paint ageing into consideration.
The results presented concern tests performed on CHROMIA, an experimental chemistry and radiochemistry platform and, more specifically, at the EPICUR-LEAR facility and on the START test facility (see photos below).Medium- and long-term iodine releases
The "Iodine" part of the first phase of the STEM project consisted of 19 tests involving iodine-131 and covering two themes:
- Irradiation tests covering the "medium-term" phase (30 to 100 h), were performed on steel test specimens, painted with molecular iodine (I2) deposits, to assess gaseous iodine (I2 and CH3I) release kinetics from these paints. The different kinetics quantified over time ultimately made it possible to estimate the medium-term conversion rate of molecular I2 to organic iodine with a satisfactory confidence level.
- Tests on the stability of irradiated caesium iodide and cadmium iodide were also performed over periods of time consistent with the "medium-term" phase. They revealed the production of gaseous molecular iodine (I2). This phenomenon, although suspected, had never been revealed before. It can have a significant impact on the stationary concentration of gaseous iodine in the containment and eventually on the iodine source term, in that it represents an additional term concerning delayed releases. In other words, the quantity of iodine liable to be released to the environment could be significantly increased in the event of a late containment venting-filtration procedure.
Based on these experimental observations, models of the key phenomena governing iodine behaviour in the reactor building have been added to the ASTEC simulation software:
- Interaction of I2 and CH3I species with paints under irradiation;
- Formation and radiolytic decomposition (i.e. under the effect of ionising radiation) of iodine aerosols and iodine oxides when exposed to radiation;
- I2 - CH3I radiolytic conversion.
These new developments will be used in calculations for application to the reactor case for more precise quantification of the impact on the source term.
EPICUR facility, used for iodine test © IRSN
Ruthenium chemistry in the reactor coolant system
Following the qualification of experimental devices (test reactor and related sampling and measuring devices) and the systems used to reproduce the thermal gradients to be studied, which represent the different accident conditions as faithfully as possible, more than 20 tests were performed on the START test facility.
START test facility © IRSN
The following main parameters of interest were studied:
- Ruthenium transfer kinetics in the thermal gradient tube (TGT), which represents a PWR reactor coolant system with temperature decreasing from 800°C to 150°C : tests reproducing direct transport representing vaporisation from the core and tests referred to as Ru revaporisation tests from the deposits in the system, "indirect transport";
- The thermal gradient ("abrupt" or "smooth" gradient) applied in the TGT;
- TGT material (quartz vs stainless steel).
The main conclusions can be summed up as follows:
- At the end of the different phases of vaporisation and revaporisation of deposits in the system, release containing a few per cent of gaseous ruthenium was observed (under test conditions) at the system outlet.
- During the revaporisation tests, almost all the ruthenium transported was in gaseous form;
- The revaporisation process is as much important as the vaporisation one as regards the releases whether the TGT is made of stainless steel or quartz. This confirms the experimental approach adopted (many tests carried out on a system made of quartz, a material that is easier to use in post-test examination, to perform a greater number of analytical tests and use of stainless steel (which, although representative of PWR reactor coolant systems, is less practical for post-test analysis) for the sole purpose of ensuring representativeness.
Scheme of the START test facility © IRSN
While the experimental results of the STEM project will allow validation work on the models implemented in the ASTEC simulation tool to continue, it has been deemed necessary to extend the experimental programme as of January 2016 to study test conditions that are even more representative of real accident conditions involving core melt down scenarios. The two historical parts of the project, one devoted to iodine chemistry in the containment and the other to ruthenium chemistry in the reactor coolant system, will be maintained. The different research issues will be:
- Iodine behaviour under irradiation
- Ruthenium chemistry focusing on revaporisation phenomena
- Effect of the deposition surface and of its surface condition
- Effect of applying super-oxidising conditions
- Effect of the presence of "gaseous pollutants"
- Effect of the ruthenium speciation entering the reactor coolant system.
1. The expression "source term" refers to all the information that characterises the release of radioactive materials to the environment, including the chemical species released, the isotopes concerned, the physical-chemical forms (gases, aerosols), the quantity released for each species, and the release kinetics.