A
subcritical reactor is a nuclear
fission reactor that produces fission without
achieving
criticality. Instead of a
sustaining
chain reaction, a
subcritical reactor uses additional
neutrons
from an outside source. The
neutron
source can be a
nuclear fusion
machine or a
particle
accelerator producing neutrons by
spallation.
Such a device with a reactor coupled to an accelerator is called an
Accelerator-Driven
System (
ADS).
Motivation
The long-lived
transuranic
elements in
nuclear waste can in
principle be
fission, releasing
energy in the process and leaving behind the
fission products which are
shorter-lived. This would shorten considerably the time for
disposal of radioactive waste. However, some isotopes have
threshold fission
cross section and have a small
effective fraction of
delayed
neutrons and therefore require a
fast
reactor for being fissioned, and for safety reasons preferably
a subcritical reactor if they constitute a significant fraction of
the fuel. The three most important long-term radioactive isotopes
that could advantageously be handled that way are
Neptunium-237,
Americium-241 and Americium-243. The
nuclear weapon material
Plutonium-239 is also suitable although it can be
expended in a cheaper way as
MOX fuel or inside
existing fast reactors.
Principle
Most current ADS designs propose a high-intensity
proton accelerator with an energy of about 1
GeV, directed towards a
spallation target made of
thorium that is cooled by liquid
lead-
bismuth in the
core of the reactor. In that way, for
each proton interacting in the target, an average 20
neutrons are created to irradiate the surrounding
fuel. Thus, the neutron balance can be regulated such as the
reactor would be below
criticality if
the additional neutrons by the accelerator were not provided. The
main advantage is inherent safety, even if the
nuclear fuel under consideration lack
Uranium's nice self-regulating properties—like
delayed neutrons and doppler coefficient—that make standard
nuclear reactors safe. Whenever the
neutron source is turned off, the reaction ceases.
Technical challenges
There are technical difficulties to overcome before ADS can become
economical and eventually be integrated into future nuclear waste
management. The accelerator must provide a high intensity and be
highly reliable. There are concerns about the window separating the
protons from the spallation target, which is expected to be exposed
to stress under extreme conditions. The chemical separation of the
transuranic elements and the fuel manufacturing, as well as the
structure materials, are important issues. Finally, the lack of
nuclear data at high neutron energies
limits the efficiency of the design.
Some laboratory experiments and many theoretical studies have
demonstrated the theoretical possibility of such a plant.
Carlo Rubbia, a nuclear physicist and former director of CERN
, was one of
the first to conceive a design of a subcritical reactor, the
so-called "energy
amplifier". In 2005, several large-scale projects are
going on in Europe and Japan to further develop subcritical reactor
technology.
Economics and public acceptance
Subcritical reactors have been proposed both as a means of
generating
electric power and as a
means of
transmutation of
nuclear waste, so the gain is twofold.
However, the costs for construction, safety and maintenance of such
complex installations are expected to be very high, let alone the
amount of research needed to develop a practical design (see
above). There exist cheaper and reasonably safe waste management
concepts, such as the long-term
geological disposal. However, the
solution of a subcritical reactor might be favoured for a better
public acceptance—it is considered
more acceptable to burn the waste than to bury it for hundreds of
thousands of years. For future waste management, a few
transmutation devices could be integrated into a large-scale
nuclear program, hopefully increasing only slightly the overall
costs.
Subcritical Hybrid Systems
While originally thought that an ADS would be a part of a
light water reactor design, other
proposals have been made that incorporate an ADS into other
generation IV reactor
concepts.
One such proposal calls for a
gas cooled fast reactor that is
fueled primarily by Plutonium and Americium. The neutronic
properties of Americium make it difficult to use in any critical
reactor due to neutronic properties that tend to make the
moderator temperature coefficient more
positive, decreasing stability. The inherent safety of an ADS,
however, would allow Americium to be safely burned. These materials
also have good neutron economy, allowing the pitch-to-diameter
ratio to be large, which allows for improved natural circulation
and economics.
External links
See also
References
- UIC
- MYRRHA (Belgium) [98138]
- Multiple authors. "A Subcritical, Gas-Cooled Fast Transmutation
Reactor with a Fusion Neutron Source", Nuclear Technology, Vol.
150, No. 2, May 2005, pages 162–188. URL:
http://www.ans.org/pubs/journals/nt/va-150-2-162-188
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