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12 October 2000: the staff
at the Institut de protection et de sûreté nucléaire
(IPSN) are busy in the control room of the Phébus experimental
reactor. After long months of preparation, the time has come for
the 'FPT-2 test'. At 09.23, the operators start up the reactor,
progressively increasing the power. The flow of neutrons heats up
the main 'guinea pig' in the experiment: a 'bundle' of 20 nuclear-fuel
rods - containing uranium oxide pellets - placed at the centre of
the reactor, where the temperature starts to climb to over 2 500°C.
This simulation causes a degradation in the materials tested similar
to that caused by the very worst nuclear accidents. The process
ends at 14.52 when, having consulted the many instruments placed
in the device, it is decided that the fuel degradation and related
emissions of dangerous materials have reached the expected levels.
The drama of the experiment
was followed by a variety of operations and physico-chemical examinations
of the aerosols emitted and the chemistry of the radioactive products.
Throughout the test, samples were taken - at regular intervals and
at different locations inside the circuits and the container shell
- of the elements released as a result of the intense heat and the
melting of part of the test fuel bundle. These operations continued
until 17 October. The researchers then started to collect the samples
taken for analysis and to examine the cluster and its components.
The post-experimental phase is still continuing. It will be another
two or three years before the final conclusions of the FPT-2 are
available, when all the experimental results have been acquired,
analysed and interpreted.
Six global experiments
This simulation was carried out under the European
Phébus FP programme,(1)
the most important international research project in the field of
nuclear safety. The aim? To study as closely as possible what happens
to fission products - the substances created by the fission reactions
of the atomic fuel core - in a situation that simulates a serious
accident in which the reactor fuel melts, in full or in part. In
other words, it is a question of evaluating what quantity of extremely
harmful radioactive products are likely to be emitted into the environment
and in what form - gas or dust.
Phébus FP is distinctive in that it carries
out research on safety by means of global experiments at an installation
(see box) which reproduces on a reduced scale,
in the region of 1/5000, the principal configurations of a pressurised
water reactor (the most common type in the West). Phébus
FP therefore complements experimental studies that focus on specific
effects but are incomplete and on a smaller scale.
FPT-2 is the fourth in a series of six simulation
exercises, the first of which was carried out in 1993. The main
differences between them lie in the fuel tested and the flow rate
of the steam. The irradiated fuel (i.e., fuel which has already
been used in a reactor) employed in last October's FPT-2 simulation
is equivalent to that found in operational nuclear power stations,
and produces greater quantities of fission products. The variation
in the steam-flow rate determines the degree to which the fuel and
its zircaloy (an alloy of zirconium) cladding is oxidised. The oxidation
of zircaloy is an exothermal reaction which emits hydrogen and thus
poses two major threats in a damaged reactor where the core is excessively
hot: the physical degradation of the fuel, and the production of
hydrogen which can lead to the build-up of excess pressure in the
container building.
Confirmations and surprises
The results obtained to date by the Phébus
FP programme confirm much of what was already known, but also indicate
a number of important effects which were not expected and had not
been taken into account by the modelling and computing codes used
to study serious accidents: the hydrogen produced by the oxidation
of the cladding had been underestimated; the melting at the core,
that is the liquefaction of the fuel rods, occurred at temperatures
several hundred degrees lower than anticipated; and a small part
of the radioactive iodine - one of the principal products of highly
radioactive fission - penetrated the container shell in the form
of a gas (bringing an increased risk of emissions in the environment)
when it was believed that it would be in the form of solid caesium
iodide particles. On the other hand, the silver produced by the
melting of the control rod proved to have a positive effect as it
trapped the rest of the iodine in solution in the sump water, thereby
reducing the quantities released in gaseous form.
These results demonstrate the benefits of the
research. They are already being included in the accident computing
codes used throughout the world, such as the ICARE 2 core deterioration
code developed and used in France by the IPSN, or the ASTEC global
code which the IPSN developed in cooperation with its German partner
the GRS (Gesellschaft für Anlagen und Reaktorsicherheit).
The next experiment (FPT-3) is scheduled for 2003.
It will be similar to the test which has just been carried out,
but with a control rod made of boron carbide (B4C) - as
used by many reactors - rather than Ag-In-Cd alloy. The aim is to
study the deterioration of the fuel in the presence of boron carbide
as well as changes to the chemistry of the fission products, particularly
the iodine. Scheduled for 2006, the final test is still being defined.
It should simulate an accident scenario in which the air penetrates
the interior of the reactor core (following an accident when recharging
the core or rupture of the vat by the melted core, for example).
Once all six Phébus FP experiments have been carried out,
analysed and interpreted, the world community will have the knowledge
necessary to significantly improve the prevention, evaluation and
management of serious nuclear accidents. But Phébus FP will
not have the last word. The Phébus-2000 programme, currently
being developed, will continue the work.
(back to text)
Boxes
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Phébus,
technical specifications
Phébus is a small experimental reactor
(40 megawatts), able to hold a zircaloy-clad test bundle of
20 one-metre fuel rods in a centralised watertight cell and
to heat it by neutronic irradiation. An absorbent rod in a
silver-indium-cadmium alloy lies at the centre. This represents
the control rods which are inserted to various depths into
the heart of a nuclear reactor to absorb the neutrons and
thereby regulate or stop the fission reactions.
The test bundle is contained in a sealed
cylindrical envelope, cooled by circulating water, and irrigated
by steam which collects the products formed and carries them
to a section of the cooling circuit. This represents the principal
elements of the primary cooling circuit of a pressurised water
reactor. The steam carries the fission products and other
materials which escape from the fuel cluster to a containment
system measuring 10 m³, which represents the container
building in a nuclear reactor. This is a way of reproducing
a rupture in a reactor's primary circuit resulting in the
steam and the substances it is carrying being released into
the container building. In Phébus, the fission products
are deposited on painted surfaces (making it possible to study
the action of the paint which reacts to the iodine) or in
the sump at the bottom of the container.
The circuit, the container and the sampling
devices are placed in a watertight housing known as a 'caisson'
measuring 350 m³ , which isolates them from the exterior
and prevents any emissions into the environment. The caisson
itself is situated in an experimentation hall protected by
concrete one metre thick. The experiment installation is also
equipped with approximately 250 measuring and sampling instruments:
several dozen thermometers distributed throughout the fuel
cluster and enveloping structures (to monitor the state of
degradation of the test cluster), gamma ray spectrometry devices
(to monitor the length of the circuit, and in the container
changes to the fission products carried by the steam), and
devices measuring the aerosols and the composition of the
gaseous and liquid phases in the container.
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A programme
of global interest
The Phébus FP programme, launched
in 1988, is being implemented by the IPSN in cooperation with
the European Commission's Joint Research Centre (JRC). It
brings together 35 bodies representing most of the countries
which operate nuclear power stations. About 100 engineers
and technicians are permanently employed on the project at
the CEA (Commissariat à l'énergie atomique)
in Cadarache (FR), with a dozen radiochemistry laboratories
worldwide involved in the analysis phase.
Phébus FP's total budget is in the
region of 150 million euros spread over 15 years, most of
it provided by the IPSN, with the European Commission contributing
about 25%. Of the 12.8 million euros devoted to research on
nuclear safety, 4.5 million euros goes to Phébus FP
under the Fifth Framework Programme (1999-2002).
There are also several specific projects related to the prevention
and mitigation of severe accident consequences, co-sponsored
by the EU under the Fourth and Fifth Framework Programmes(1).
Some of them are directly linked by Phébus issues and
are aimed at supporting it, for example, by contributing to
pre- and post-test analyses, and by applying validated numerical
models to reactor assessment studies
Contacts
Roland Zeyen
Centre de Cadarache - Saint-Paul-lez-Durance
(FR)
Fax +33 4 42 25 70 78
roland.zeyen@ipsn.fr
Alejandro Zurita,
European Commission
alejandro.zurita@ec.europa.eu
Internet sites
www.ipsn.fr
www.cea.fr
cordis.europa.eu/fp5-euratom/home.html
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