The CHIP (chemistry of iodine in the reactor coolant system) programme was designed to study the transport of iodine in the reactor coolant system in order to assess gaseous and aerosol fractions that might enter the containment. This gas/aerosol distribution impacts calculations of short-term releases in the event of a major accident.
This programme was launched in the 2000s after observation of a gaseous iodine fraction in the PHEBUS-PF tests, which had not been predicted by the computer codes, with the fraction varying significantly depending on the test conditions.
The first phase of the CHIP programme was led by IRSN as part of the International Source Term Programme
, co-funded by the CEA, EDF, IRSN, the European Commission, the US Nuclear Regulatory Commission, Atomic Energy of Canada Limited, the Korea Institute of Nuclear Safety (representing a South Korean consortium), the Paul Scherrer Institute and GDF-Suez-Tractebel, and ran from 2005 to 2012. The CHIP programme focused on chemical systems involving iodine and other volatile fission products (Cs, Mo), with the aim of determining the proper conditions for the persistence of gaseous iodine in the event of a loss of coolant accident.
The second phase, known as the CHIP+ programme, ran from 2013 to 2018, in partnership with EDF, and focused more specifically on the role of control rod elements (Silver-Indium-Cadmium and Boron).
The experimental programme took place over a long period due to the technical challenges that needed to be resolved in order to identify and develop experimental facilities that met requirements, and in order to introduce the elements in a properly controlled manner.
Content 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, after travelling through the reactor coolant system (RCS). Analysis of the PHEBUS FP tests showed that the widely-held assumption that iodine was fully transported through the reactor coolant system as CsI to enter the containment in aerosol form was oversimplified.
Very early, it was suspected that the assumption of thermochemical equilibrium for gaseous species was flawed due to the very high temperature gradient in the reactor coolant system (~2000°C in the top of the vessel and down to 150°C for a breach in the cold leg) and the limited time fission products spend in this system.
The CHIP programme therefore focused on studying iodine chemistry, potentially far from thermodynamic equilibrium (impact of chemical kinetics) in the reactor coolant system of a water reactor in the event of a core melt accident.
It was organised around two major areas, which respectively aim to:
- Identify the physico-chemical elements that may react with iodine during its transfer from the core to the containment (short transfer time, fast cooling especially in the presence of water vapour), and also identify the chemical species that impact the presence of volatile iodine at low temperatures (~150°C)
- Obtain kinetic data for the main reactions involved.
To achieve this, the experimental programme included an experimental facility, known as the CHIP test bench, which was modified over time, and laboratory-scale test benches for support studies (GAEC test bench, laminar flame reactor, etc.). These support studies were more fundamental and often carried out under doctoral research. This includes the following research by:
- Fatima Roki, PhD thesis at the Grenoble Institute of Technology, defended on 29 January 2009.
- Marion Lacoue-Nègre, PhD thesis at Lille 1 University, defended on 6 December 2010.
- Yathis Délicat, PhD thesis at Lille 1 University, defended on 5 June 2012.
- Melany Gouello, PhD thesis at Grenoble Institute of Technology, defended on 6 November 2012.
In support of the CHIP programme, close cooperation between the IRSN LETR and the University of Lille PC2A laboratories made it possible to perform numerous modelling exercises using theoretical chemistry tools, either to determine and reassess thermodynamic data for gaseous species, or to develop kinetic mechanisms by studying the kinetics of elementary reactions. Notable work includes doctoral research by R. Vandeputte whose thesis defence was on 18 December 2013.
CHIP experimental test bench at Cadarache
After over ten years of research under the CHIP and CHIP+ programmes, the key results obtained relating to the transport of fission products in the reactor coolant system under severe accident conditions can be summarised as follows:
- The reactivity of gaseous-phase iodine is kinetically controlled, especially in oxidizing environments; this point was demonstrated through tests on the iodine-oxygen-hydrogen system. Experimental data can be described by the reaction mechanism developed by IRSN, which is present in the ASTEC/SOPHAEROS severe accident code.
- As expected in severe accident scenarios, when caesium largely exceeds iodine, it causes quick and complete formation of caesium iodide (Csl), preventing the formation of any gaseous iodine. The tests involving caesium and iodine did not demonstrate any kinetic effects.
- Molybdenum (Mo) and boron (B) act as Cs traps, thereby promoting primarily the formation of gaseous iodine. This occurred during one of the PHEBUS PF tests (FPT3 with a boron carbide (B4C) control rod, a species present in 1300/1450 Mwe reactors). The gaseous-phase formation of caesium molybdates by reactions with molybdenum trioxide (MoO3) strongly reduces the possibility of Csl formation, thereby promoting the transport of gaseous iodine into the reactor coolant system. The chemistry of molybdenum is especially sensitive to reducing conditions, with the formation of Mo(IV) and Mo(V) species that are less reactive with caesium. The reactivity of the Mo-CsIOH system is simulated quite well by the ASTEC/SOPHAEROS code, by assuming that all gaseous species are in chemical equilibrium, with the exception of the I-O-H system controlled by kinetics. Compared to molybdenum trioxide, boron oxides are less reactive, but their reactivity is less sensitive to the oxydo-reducing conditions of gas composed of an H2/H2O mixture in variable proportions.
- Taking into account the addition of a control rod element in SIC (Silver-Indium-Cadmium, an alloy used in the control rods of all reactors, especially in 900 MWe reactors, and tested in the PHEBUS FPT0 FPT1 and FPT2 tests) to the Mo-CsIOH system, indium is the least reactive element and Mo still acts as a Cs trap in oxidizing environments (water vapour) with the formation of gaseous iodine. Due to the presence of Cd or Ag, elements released into the reactor coolant system in large quantities during a severe accident, significantly less gaseous iodine forms due to the formation of Cdl2 or Agl, even though silver iodide is not the main metal iodide formed. The experimental results have shown that, in line with the formation of metal molybdates (with Cd, Ag), which compete with the caesium molybdates, and the reducing role of metals (Cd, Ag), molybdenum plays a smaller role than Cs.
- Adding silver and cadmium to the Mo-CsIOH system causes low temperature transport of iodine almost exclusively in aerosol form, a mix of Csl and Cdl2 metal iodide, with less than 10% in Agl form. Adding boron to these systems has no impact on the quantity of gaseous iodine transported, which remains low.