ThorEA - Towards an Alternative Nuclear Future.pdf

Chapter 1: Thorium-fuelled ADSRs and their potential UK and global impact
1.1 The ADSR concept and the advantages of
thorium fuel
The term accelerator-driven subcritical reactor system, or
ADSR, refers to a device formed by coupling a substantially
subcritical nuclear reactor core with a high energy proton
accelerator, as illustrated in Figure 1. The core of the reactor
is designed such that it is unable to sustain fission processes
(i.e. a chain reaction) in the absence of an externally
induced neutron flux. The necessary supply of neutrons
to the ADSR fuel is provided by high energy protons from
the accelerator impacting a heavy metal target embedded
Subcritical
ADSR core
Extracted proton beam
Figure 1. Schematic view of an Accelerator-Driven Sub-critical Reactor
As an example (see Appendix I for details), a 10 MW proton
accelerator producing a proton current of 10mA with proton
energies of 1GeV, will generate 1550MWth in a reactor core
with a criticality factor as low as k=0.985. Assuming typical
efficiency factors for thermal to electrical energy conversion,
such a system might be expected to produce 600MWe of
electrical power, comparable to a conventional power station.
The 10MW accelerator will require approximately 20MWe to
sustain its operation. This power can, of course, be taken
from the ADSR itself, with the remaining 580MW of electrical
power being fed to the grid.
The enormous multiplication of power afforded by the
coupled accelerator-core system has given the ADSR its
synonymous label of Energy Amplifier (or EA).
One consequence of the subcritical operation of the ADSR
and its effective decoupling of the neutron source (producing
spallation neutrons) from the fissile fuel (generating fission
10 Towards an Alternative Nuclear Future
fraction of power,
f (~5%), fed back
to accelerator
within the core, and “spalling”, or chipping, neutrons from
the nuclei of the atoms within the target. This spallation
process is particularly efficient, with approximately 25
neutrons being generated by each proton. The resulting
spallation neutrons then go on to induce fission in the fuel
of the core. A particular and evident advantage of the ADSR
is that if the proton accelerator is switched off the nuclear
processes also instantly shuts down.
High energy,
high current
proton accelerator
energy extraction
with efficiency (~40%)
power, (1- f),
fed to the Grid
neutrons), is that the range of potential nuclear fuels broadens
significantly: minor actinides (MAs), high levels of plutonium
and long-lived fission products (LLFPs), which would degrade
the neutronic characteristics of conventional critical nuclear
cores to unacceptable levels, can be incorporated with
standard nuclear fuel and burnt within an ADSR.
Significantly, and perhaps even more importantly, this
characteristic enables ADSR technology to exploit fully the
enormous and virtually untapped potential of thorium as
nuclear fuel (Appendix II).
Natural thorium (100% Th-232 isotopic composition) is not
fissile and thus cannot, in its natural state, be used as nuclear
fuel. However, thorium is fertile and can be converted into a
source of fissile material (U-233) through neutron capture.
This fertile-fissile conversion, and subsequent fission of
U-233, can be effected by the spallation neutrons and
induced fission neutrons within the core of an ADSR.