The second experiment in the Phebus-FP series, namely FPT1, was chosen as the basis for the International Standard Problem n°46, realised with the support of the European Commission DGResearch. This provides for the first time the opportunity to assess the capability of systems-level severe accident modelling codes in an integral manner, covering core degradation through to the late phase (melt pool formation), hydrogen production, FP release and transport, circuit and containment phenomena, and iodine chemistry, in one experiment. The possibility also existed to compare the performance of detailed-level codes covering one aspect of the accident progression, for example core degradation, with the systemslevel codes, thus providing benchmarking of the different levels of modelling.
The ISP was conducted as an open exercise, with all the relevant experimental results being available to the participants. The areas covered by the experiment, and therefore by the Standard Problem, are fourfold:
- Fuel degradation, hydrogen production, release of fission products, fuel, and structural materials ('bundle' part of the ISP);
- Fission product and aerosol transport in the circuit ('circuit' part of the ISP);
- Thermal hydraulics and aerosol physics in the containment ('containment' part of the ISP);
- Iodine chemistry in the containment ('chemistry' part of the ISP).
Participants were encouraged to perform integral calculations covering all four aspects of the exercise. However, the ISP was so organised that it was also possible for participants to calculate any of the above phases in a stand-alone manner, using detailed-level mechanistic codes that treat for example core degradation or containment thermal hydraulics and aerosol physics on their own.
The ISP was organised following normal practice on an approximately two-year timescale. A key event was the general release of the Phebus FPT1 Final Report worldwide in September 2001, thus making the results available outside the Phebus project partnership. The issue of a preliminary version of the Specification Report in advance of the Preliminary Workshop allowed time for comment, particularly relevant to those participants outside the Phebus project who might not have analysed Phebus experiments before. The most intensive phases were preparation of the specification (4 months), participants' calculations and submission of their results (7 months), and the co-ordinators’ analysis of the results and draft of the Comparison Report (7 months).
The ISP was well supported, with participation from 33 institutes, companies etc. in 23 countries and international organisations. The latter comprised EC-JRC, Austria, Belgium, Bulgaria, Canada, Croatia, Czech Republic. France, Germany, Greece, Hungary, Italy, Japan, Korea, Mexico, Russia, Slovenia, Spain, Sweden, Switzerland, Turkey, UK and USA. The participating organisations included utilities, regulators, research institutes and private engineering comsultancy companies, thus providing a good range of backgrounds to the technical work. Fifteen different codes were used: ASTEC, ATHLET-CD, COCOSYS, CONTAIN, ECART, FEAST, IMPACT/SAMPSON, ICARE/CATHARE, IMPAIR, INSPECT,