The PHEBUS FP (Fission Products) international research programme was conducted between 1988 and 2010. Its purpose was to improve the understanding of the phenomena occurring during a core meltdown accident in a light water reactor and to validate the computational software used to represent these phenomena in reactor safety evaluations. The report of the last test was published in December 2010 and was the topic of a closing seminar organised in June 2012.
Context and objectives
Since the core meltdown accident in the Three Mile Island reactor in 1979, a series of experimental safety research programmes has been conducted by a number of international research organisations, including the IRSN, which manages the European SARNET network
. Simulation models have been developed to calculate the sequence of events in an accident of this type, evaluate its consequences and assess the efficacy of the various measures that can be implemented to limit its effects.
The PHEBUS FP research programme was launched in 1988 by the IRSN (under the former name IPSN – Institut de Protection et de Sûreté Nucléaire, Nuclear Protection and Safety Institute) in partnership with the European Commission and EDF. France collaborated in this programme with the United States, Canada, Japan, South Korea and Switzerland, and five experiments were conducted between 1993 and 2004, involving approximately 80 persons per year.
The main objective was to reduce the uncertainty in evaluating the release of radioactive products in the event of a core meltdown accident in a pressurised water reactor (PWR). To do this, “global” experiments were conducted, that is, experiments in which all the phenomena were represented, from melting of a fuel assembly to release of fission products and structural materials inside a simulated containment vessel, duplicating as closely as possible the conditions that would apply in an accident of this type. The programme had the simultaneous aim of developing and validating the simulation software used to calculate the progression of the accident. This research was to contribute to improving IRSN crisis management by optimising the activities and procedures that would be implemented in the event of a nuclear accident to protect the population and the environment.
Five tests were conducted in an experimental facility representing a PWR set up in the PHEBUS reactor (see the Installation section). They aimed to reproduce the physical phenomena that govern reactor core meltdown, the transfer of fission products from the fuel to the containment vessel, and their fate there.
A test took place in two successive phases:
- a "degradation" phase, lasting several hours, during which, through an increase in the power for the PHEBUS core, the temperature of the test fuel increased to the point of liquefaction and displacement of materials (between 2300 and 2500°C), leading to release of fission products and transport into the circuit and the containment vessel. At the end of this phase, the PHEBUS reactor was shut down;
- a “containment” phase, lasting several days, during which the quantities of interest in understanding transport phenomena, materials deposits and iodine chemistry in the circuit and the containment vessel were measured.
The first three tests (FPT-0 in 1993, FPT-1 in 1996 and FPT-2 in 2000) studied the effect of radiation levels and the environment (oxidising or reducing) on fuel degradation, release and transport of fission products, and their behaviour in the containment vessel. These three tests were conducted with an Ag-In-Cd alloy control rod representative of Westinghouse-type PWRs (900 MWe), for which the influence on the behaviour of fission products has been demonstrated.
A fourth test (FPT-4) in 1999 involved the final phase of the accident, with study of the release of low-volatility fission products and transuranics from a debris bed and a bath of melted fuel.
The fifth test (FPT-3, in 2004) determined the influence of a boron carbide (B4C) control rod on fuel degradation and behaviour of the FPs. Boron carbide is a material used in the most recent PWRs, but also in the boiling water reactors in service in Europe and around the world, as well as in some Soviet-type reactors in service in Eastern Europe.
Figure 1. Tests chart (in French)
The installation used for the PHEBUS FP programme was located at the Cadarache Study Centre and is currently being dismantled by the CEA (Commissariat à l'énergie atomique, Atomic Energy Commission). It was made up of two main components:
- the PHEBUS reactor, of pool type, with a 40 MW capacity, used to provide the neutron flux intended to heat the test fuel by fission;
- the experimental circuits composed of three main portions: the in-pile section comprising the test system, a circuit portion simulating the primary circuit of a reactor up to breach, and a reservoir simulating the containment vessel of the reactor.
Figure 2. Layout of the PHEBUS installation
The installation reproduced a 900 MWe pressurised water reactor at the 1/5000th scale.
At the centre of the PHEBUS reactor was a sealed cylindrical cavity into which the fuel rods bundle were introduced. The latter was composed of 18 irradiated fuel rods and two instrumented rods. In the centre was a rod simulating the features of a reactor control rod.
The primary circuit contained a steam generator simulated by an inverted U-tube. The containment vessel was simulated by a 10 cubic metre reservoir including a volume filled with water as a sump, a gaseous volume and painted surfaces.
The fuel bundle, the experimental circuit and the containment vessel in which the fission products were collected at the end of the tests were instrumented with gamma spectrometry devices allowing on-line measurement of variations in activity along the circuit and in the vessel, on the walls, on the condensing surfaces and in the sump. An opacimeter allowed the variation in suspended aerosol concentrations to be followed, and coupons exposed for predetermined periods of time collected fractions of the aerosols that were deposited by sedimentation. These instruments were supplemented by sensors for pressure, humidity levels, hydrogen concentration, wall temperatures, gas and water in the sump. Maypack-type selective filters were used to differentiate the forms of iodine collected in the atmosphere and measure their concentrations.
The various tests in the PHEBUS FP programme provided scientific information on core meltdown, the composition of the aerosols, their lower-than-predicted retention in the circuits, the re-evaporation of some fission products and hydrogen production. The observations showed that for a reactor equipped with silver alloy control rods, the primary circuit of the reactor constituted the principle source of volatile iodine in the vessel. When the control rod is of boron carbide, the release of gaseous iodine is even greater. The test programme clearly revealed a link between the kinetics of fission product release and the phenomena of fuel degradation. Moreover, the programme showed that the concentration of volatile iodine in the reservoir simulating the containment vessel beyond 24 hours depends mainly on physico-chemical processes in the gaseous phase. It thus depends on the concentration of volatile iodine coming from the primary circuit or formed in the containment vessel, which itself depends on the affinity of the iodine for the surfaces of the vessel (paint, material, etc.)
Figure 3. Fuel rods radiographies after tests
A database that can be consulted by scientists around the world to assess their computational software was established. The experimental data collected during the tests are used by the IRSN to improve evaluation of the releases in the event of a severe reactor accident and are used in the simulation software for severe accidents, in particular ASTEC, developed by the IRSN in partnership with its German counterpart Gesellschaft für Anlagen- und Reaktorsicherheit mbH (GRS), or the American Melcor software (developed by Sandia National Laboratories) or MAAP (developed by the Fauske & Associates, Inc. company used by EDF). The European Eursafe project, the objective of which was to develop a realistic evaluation of possible releases into the environment, prioritised the research to be conducted with a view to reducing the remaining uncertainties. Part of this research is conducted during the international SOURCE TERM programme), which includes a series of analytic tests, since 2005. A new international programme, STEM, has completed the research since in 2011.
The general overview of the PHEBUS FP tests was presented at the closing seminar in June 2012 on core degradation and release of fission products, transport of aerosols and fission products in the primary cooling circuit, and iodine chemistry. A significant portion of the seminar was devoted to use of the results of the programme for safety studies by the various partners.