|
Among the solutions considered
for the treatment of nuclear waste, the incineration of long-lived waste
in a sub-critical reactor(1) is a very promising research
avenue. An experiment has just been successfully carried out by the Laboratoire
de Physique Subatomique et de Cosmologie of Grenoble (LPSC) (CNRS/IN2P3(2)),
the Université Joseph Fourier and the INPG(3),
during which it was possible to control the power of a sub-critical reactor
for the first time ever with an external source of neutrons and to directly
measure the parameters that are responsible for its operation. This study
was carried out within the framework of the European MUSE program, using
the Masurca experimental reactor of the DEN/CEA(4) in
Cadarache, driven by the Genepi neutron generator designed by the LPSC.
What can be done with long-lived nuclear waste? The Bataille law, voted
in December 1991, defined three main axes of research for stocking highly
active and long-lived waste produced in nuclear reactors (~ 15 tons/year
in France) and specified 2006 as the year to take final stock of the different
proposals. Public research institutes, including the CNRS, have made a
major commitment to this research through programs involving many different
disciplines and many different organizations.
Long-lived nuclear waste, made up essentially of elements that are heavier
than uranium (neptunium, plutonium, americium, curium), can generally
be destroyed in the intense neutron flux that exists in some nuclear reactors
(referred to as transmutation or incineration) but under conditions that
do not make it possible to satisfy established safety requirements. To
satisfy these requirements, researchers are considering operating these
reactors for the purpose of incineration in sub-critical mode, a promising
research avenue that could also lead to a new type of reactor.
An experimental research program known as MUSE (MUtiplication par Source
Externe), devoted to the study of sub-critical reactors, was launched
in 1995 by the French Atomic Energy Commission (CEA) whose aim was the
experimental study of the behavior and of the physics of a hybrid system
made up of the DEN/CEA Masurca experimental reactor in Cadarche, France,
coupled with an external source of neutrons (accelerator + target). Initial
experiments made it possible to define and optimize the characteristics
of this source, which was then developed at the Laboratoire de Physique
Subatomique et de Cosmologie of Grenoble (LPSC) and given the name of
GENEPI (high-intensity pulsed neutron generator). In May 2001, within
the framework of the same program, which had become European(5)
in 2000, Genepi was coupled with the Masurca reactor, an important step
that was followed by a series of experiments dealing with the measurement
of the characteristics and the reactivity of the reactor.
In February 2003, the LPSC then carried out an experiment during which
the power of the reactor could be controlled (between 2 and 30 Watts)
by an external source, by driving the frequency of the accelerator to
modulate the intensity of the external neutron source. Normally, the reactor's
power is driven by control rods that are moved from place to place within
the reactor and which absorb neutrons to varying degrees, depending on
their position. However, there are considerable time constants related
to these movements and the presence of these absorbers is highly disruptive
to the neutron flux. On the other hand, and herein lies the originality
of the experiment, a sub-critical reactor behaves like an amplifier of
the number of neutrons injected by the external source. Therefore, a modification
of the intensity of this source leads to an instantaneous modification
of the reactor's power without disturbing its core. The idea of controlling
the reactor's power by modulating the intensity of Genepi was thus born.
It was then possible to make the reactor suddenly change from a given
power to a power 15 times lower and to slowly increase it (for safety
reasons) to its maximum value for a new measurement cycle. Thanks to this
highly encouraging experiment, it was possible to make a direct measurement
of the neutron generation time. This type of measurement opens the way
to an in situ control of the parameters that underlie the operation
of a hybrid system, thus demonstrating the merits of the external pulse
source, not just for the study of waste incineration but also for the
reactors of the future.
(1)Reactor
coupled with an accelerator responsible for providing the surplus of neutrons
necessary to maintain the chain reaction and drive the reactor.
(2) Institut National de Physique Nucléaire et
de Physique des Particules
(3) Institut National Polytechnique de Grenoble
(4) Direction de l’Énergie Nucléaire
du Commissariat à l’Énergie Atomique, French Atomic
Energy Commission
(5) Participants in the MUSE European network: CEA/DEN
Cadarache (France), Université Joseph Fourier de Grenoble (France),
SKN-CEN Mol (Belgium), FZK Karlsruhe (Germany), FZJ Julïch (Germany),
BNFL Preston (U.K.), ENEA Rome (Italy), NRG Petten (The Netherlands),
TUDelft.IRI (The Netherlands), CIEMAT Madrid (Spain), KTH Stockholm (Sweden),
Chalmers University Goeteborg (Sweden), UMMET.FPNT.DEP Krakow (Poland)
Researcher
contact:
Roger Brissot
Tel: +33 4 76 28 40 68
e-mail: brissot@lpsc.in2p3.fr
IN2P3 contact:
Dominique Armand
Tel: +33 1 44 96 47 51
e-mail: darmand@admin.in2p3.fr
Press contact :
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
|