THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Nuclear Reactors for Thorium
• Additional measurements of the absorption cross sections of the salt constituent Li, Be, and F
could reduce the uncertainty further.
• Verifying the temperature coefficients of reactivity, including Doppler coefficients.
Chemistry: An extensive program of molten-salt chemical research over the past 40 years has
established a sound basis for understanding the chemistry of the pertinent fluoride salts.
Additional work is desirable, however, in several areas:
• A better understanding is needed of the behaviour of the noble metal fission products in MSBR
systems.
• Alternative processes for rare earth removal should be investigated, and chemical information
on the protactinium removal process needs to be extended to a wider range of conditions.
• The phase behaviour of PuF3 in molten salts ought to be studied more thoroughly to provide a
firmer chemical basis for the use of plutonium.
• Further measurements are needed of the physical and chemical properties of molten salts, and
particularly of the sodium fluoride/sodium fluoroborate salt mixtures which are the presently
proposed secondary coolants for MSBRs.
Fuel Processing: Obtaining good breeding in a single-fluid reactor requires rapid on-site
reprocessing to keep protactinium out of the high neutron flux as well as to remove fission
products. The investigation of most of the steps needed to effect rapid reprocessing has not
proceeded past preliminary laboratory tests.
Materials: Damage from high-energy neutrons eventually leads to deterioration of graphite at
temperatures of interest for molten-salt reactors, and improved grades need to be developed and
tested. The Hastelloy-N used in the past for containing molten salts is embrittled by helium
produced in the metal when it is irradiated. Although the present material has been satisfactory
for the Molten Salt Reactor Experiment (MSRE), an improved alloy is needed for power reactors.
The resistance of the modified alloy to corrosion by fuel and coolants salts, particularly the
fluoroborate, must be demonstrated at all reactor operating conditions.
Molten Salt Reactor Experiment (MSRE): MSRE operation has shown the compatibility of
graphite with fissioning fuel at low power densities and low burnup, and a variety of in-pile
capsules and loops have provided additional information. However, the experience to date has not
included exposures at the power densities of breeder reactors. In-pile loop experiments thus need
to be performed using appropriate materials at the peak power densities of high-performance
breeders.
5.4.4 Accelerator Driven System (ADS)
An Accelerator Driven System (ADS) is a subcritical reactor, i.e. a reactor that cannot sustain a
chain reaction on its own. The reactor can only run when neutrons are supplied from an external
source. In the case of the ADS, the neutrons come from spallation of heavy nuclei like lead. In this
context, spallation means that a high energy proton (500 - 1000 MeV) hits an atomic nucleus in a
target (e.g. lead) and rips off a large number of neutrons (15 - 30), depending on the target
material and the proton energy. The collision between the proton and the target nucleus may also
result in fissioning of the nucleus, or the production of nuclei of light elements. Further, the
neutrons that are produced will induce fission and nuclear transmutation reactions in the fuel.
The high energy protons are supplied by a powerful particle accelerator which may be either a
linear accelerator (LINAC) or a cyclotron.
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