The Energy Amplifier
Nils Johan Engelsen
March 24, 2011
Submitted as coursework for
Physics 241,
Stanford University, Winter 2011
Introduction
A nuclear reactor producing no long-lived nuclear
waste whilst running at subcriticality sounds like a dream come true.
Add that the same reactor can use the abundant material thorium as fuel,
and you have Carlo Rubbia's proposed Energy Amplifier. In 1993, the
Nobel laureate held a talk outlining how such a reactor could be built
and has since lobbied for resources to construct a test reactor. [1] The
proposed design entails significant technical and economic challenges,
involving a particle accelerator to generate the necessary neutrons to
drive the nuclear reactions. However, the potential benefit of such a
reactor could be vast, providing enough energy for mankind for up to
200,000 years. [2]
Mechanism of Energy Production
Nuclear fission is the process in which a heavy
nuclei splits into two smaller nuclei with the release of energy and
more neutrons. We distinguish between fissile and fertile nuclei.
Fissile nuclei can be split directly by bombarding them with neutrons,
while fertile nuclei need to absorb a neutron first, to then decay to a
fissile nuclei. In a conventional nuclear reactor, we drive the reactor
at criticality, meaning there are enough fissile nuclei in the reactor
to achieve a chain reaction where an equal amount of neutrons are
generated and absorbed. However, Carlo Rubbia suggested that we can
supply the neutrons from a particle accelerator, and thus run the
reactor subcritically. The processes occurring after the neutrons are
produced are well understood and were tested in CERN in 1995. [3] The
neutrons would be generated by bombarding lead with high energy protons.
Herein lies the main technological and economic challenge of the design,
building a high energy and high power proton source. In his original
paper, Carlo Rubbia suggested using a three-stage cyclotron design seen
in particle accelerators. [2] However, no cyclotron with as high output
power and energy as the one required for the Energy Amplifier has ever
been built. Recent developments at Fermilab may make such a cyclotron
more feasible. [4]
Reduced Waste
The Energy Amplifier can run on any kind of nuclear
fuel, both fissile and fertile. It can potentially be used to reduce
plutonium stockpiles, by burning plutonium, but it can also tap into the
vast reserves of thorium found on Earth. Thorium is a fertile material
that can be transmuted into fissile U-233 by a decay chain after neutron
absorption. However, the Energy Amplifier can also incinerate actinides,
which are the main components of long-lived nuclear waste. Elimination
of long-lived nuclear waste would make long-term geological storage like
the Yucca Mountain project unnecessary. The Energy Amplifier has very
attractive waste characteristics, assuming the fuel is reprocessed. When
reprocessing, fission products are separated from the rest of the fuel,
then the remains are reformed with some extra thorium to create new fuel
rods. The small amount of actinides created would therefore be recycled
and always kept in the reactor. [2] The chemical processes to reprocess
fuel from the Energy Amplifier have not yet been fully developed. [5]
Safety Features
A conventional nuclear reactor has a risk of going
supercritical, meaning that more neutrons are generated from fission
reactions than are absorbed. This leads to an uncontrolled chain
reaction, that can lead to disasters similar to Chernobyl. Since the
Energy Amplifier runs subcritically, this risk is eliminated. The Energy
Amplifier is also designed to use a lead convection cooling system,
which would enable cooling of the reactor without supply of power. This
would eliminate the risk of loss of cooling power accidents, such as the
ongoing accident at the Fukushima Daiichi Nuclear Power Plant in Japan.
[2] Through a series of passive safety features, the reactor will shut
itself down if it overheats, even with no human intervention Because the
fuel efficiency of the Energy Amplifier could significantly reduce the
amount of fuel required, it could reduce the demand for mining. The
majority of the public exposure to radioactivity due to nuclear power is
from mining. The Energy Amplifier would therefore expose the public to
less radioactivity than both conventional nuclear power plants and coal
power plants. [2]
Protection Against Proliferation
A major concern of nuclear power plants today is
proliferation of nuclear weapons. The traditional pressurized water
reactors allow for breeding of Plutonium which can easily be separated
by the chemical PUREX process, thus eliminating the need for expensive
centrifuges. In the Energy Amplifier, only small amounts of Plutonium
would be generated. Making an atomic bomb from spent fuel rods the
Energy Amplifier would be challenging due to the presence of various
radioactive elements - both bomb poisons that make bomb yields smaller
and very radioactive elements making handling difficult. [2] However,
the greatest proliferation danger of the Energy Amplifier seems to be
from breeding fissile fuel using the strong proton beam driving the
reactor. Rubbia proposed that the Energy Amplifier could be sealed until
the fuel rods have to be changed every five years, and then have
international teams from the IAEA change the fuel rods. [2] This does
not seem to address the main problem, namely that the proton beam would
still be accessible. It would be simple to direct the beam into a lead
target to generate neutrons for breeding Plutonium from 238U that could
then be used to make nuclear weapons. The Energy Amplifier does
therefore not solve the proliferation issue.
Challenges
The main challenge of the Energy Amplifier is
economic - the great risk of investing a very large sum of money in an
unproven technology, where significant problems may occur during
development. The main economic barrier seems to be the construction of a
high energy and high power proton source to generate neutrons. Rubbia
estimated that a prototype would cost $500 million in 1995. [6] The
Norwegian company Aker Solutions recently bought Rubbia's patent and is
working on raising funding to make a prototype reactor. [7,8] Their cost
estimate for a first reactor is $3.2 billion, and they are trying to
raise $100 million for the next stage of development. Some parameters
have been adjusted from Rubbia's original design to enable use of a
smaller and thus cheaper proton accelerator. The new design seeks to run
the reactor closer to criticality, thus requiring fewer neutrons from
the accelerator to produce power. [8] However, Aker Solutions do not
clarify whether running nearer criticality could have a negative impact
on the safety of the reactor. It also seems that a conventional Thorium
reactor would provide more barriers to proliferation due to the
contamination of bred fissile fuel with highly energetic gamma-ray
emitters. [5] There are also unsolved problems like corrosion from lead
- Rubbia claims that corrosion is not significant until the reactor runs
at a higher temperature than the 600C proposed. [2] Further long-term
experiments are required to see whether the corrosion problem can be
solved or not. The THOREX process for reprocessing spent nuclear fuel
necessary to get the full benefits of the Energy Amplifier is also not
well-explored, since the Thorium fuel cycle has never been fully
implemented. [5]
Conclusions
The Energy Amplifier is certainly a promising
prospect for nuclear power as it potentially solves the major problems
of both long-term fuel supply and long-lived radioactive isotopes.
However, the proliferation problem does not appear to be solved as it
should be a simple matter to re-engineer the neutron beam to breed
Plutonium for nuclear weapons. There also appears to be significant
technical problems remaining, but these seem to be possible to overcome
given a substantial one-time investment. 15 years after the original
paper by Rubbia, the Energy Amplifier seems to have caught the eye of
Aker Solutions and perhaps they can raise the money required for this
novel reactor technology.
© Nils Johan Engelsen. The author grants permission
to copy, distribute and display this work in unaltered form, with
attribution to the author, for noncommercial purposes only. All other
rights, including commercial rights, are reserved to the author.
[1] B. James, "Nuclear
Energy: A New, Safer Way?," New York Times, 27 Nov 1993.
[2] C. Rubbia et al, et al., "Conceptual
Design of a Fast Neutron Operated High Power Energy Amplifier", European
Organization for Nuclear Research,
CERN/AT/95-44, 29 Sep 95.
[3] S. Andriamonje et al., "Experimental
Determination of the Energy Generated in Nuclear Cascades by a High
Energy Beam", Phys. Lett. B 348, 697 (1995).
[4] L. Pham, "Considering
an Alternative Fuel for Nuclear Energy," New York Times, 19 Oct 09.
[5] "Thorium Fuel Cycle - Potential Benefits and
Challenges," International Atomic Energy Agency,
IAEA-TECDOC-1450, May 2005.
[6] C. Rubbia et al, et al., "Some Preliminary
Considerations on the Economical Issues of the Energy Amplifier,"
European Organization for Nuclear Research,
CERN/AT/95-45, 29 Oct 95.
[7] A. Evans-Pritchard, "Obama
Could Kill Fossil Fuels Overnight With a Nuclear Dash for Rhorium ",
The Daily Telegraph, 29 Aug 10.
[8] V. Ashley et al., "Aker's
ADS uses Thorium," Nuclear Engineering International, 06 Nov 10.
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