Source Term Evaluation and Mitigation (STEM) Project
Joint project

STEM phases

First phase (2011-2015)

The STEM Project was initiated with a first phase in 2011 to improve the general evaluation of the fission-product (FP) source term in the event of a severe accident of a water-cooled reactor in relation to two major FPs, iodine and ruthenium. The STEM Project is conducted at Institut de radioprotection et de sûreté (IRSN) facilities at Cadarache, France. The first phase of the project, which ended in 2015, addressed three main issues: experiments on radioactive-iodine release due to irradiation of iodine-bearing aerosols that would contribute to mid- and longer-term source in the containment; a literature survey on interactions between iodine and paints; and experiments on transport of volatile ruthenium species through steel pipes representing the reactor coolant system (RCS).

Second phase (2016-2019)

The second phase (STEM-2) pursues experimental investigations of iodine and ruthenium issues undertaking the following investigations concerning iodine:

  • assessing to what extent molecular and organic iodine-release kinetics can be modified by the dose received by paint before and during an accident since paint ageing by irradiation may induce significant chemical alterations in paint
  • measuring the production of molecular and organic iodine (gas/vapour), and studying the influence of the dose, temperature and specifically higher humidity rates on the radiolytical decomposition of iodine-oxide species (solid particulate)
  • explaining the radiolytical oxidation of representative, multi-component iodine-bearing aerosols that would be produced in the reactor coolant system and lead to the production of volatile iodine;
  • assessing the decomposition of iodine oxides by carbon monoxide and/or hydrogen, leading to the production of volatile iodine.

Regarding ruthenium, experiments in more representative conditions than in STEM will be performed on simulations of ruthenium transport in the RCS. In particular, this means more representativity for the deposition surface, the use of stronger oxidising conditions characteristic of air radiolysis products (such as ozone and nitrous oxides) and the use of representative gaseous and/or aerosol “pollutants” (i.e. seed particles, silver aerosols, aerosol deposits) that could influence ruthenium behaviour.

STEM also has strong scientific links with the Behaviour of Iodine Project (BIP), including complementary objectives and many common partners.

STEM-2 members' area (password protected | reminder)


STEM: Canada, Czech Republic, Finland, France, Germany, Korea and United States

STEM-2: Canada, Finland, France, Germany, Japan, Korea, Sweden, United Kingdom and United States.

Project period

STEM: July 2011 to Sept 2015
STEM-2: Jan 2016 to Dec 2019


STEM: EUR 3.5 million
STEM-2: EUR 2.5 million