MSR Review

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MSR Review
An Introduction to Liquid Fuelled MSR Technology
Nuclear power is harnessed from nuclear chain reactions where a captured neutron fissions ▲ a
fuel nucleus, releasing new high energy neutrons to sustain the reaction. Fission of a uranium
atom releases a hundred million times more energy than the combustion of a carbon atom.
Fission is induced only when a nucleus is bombarded with neutrons, gamma rays, or other
radiation. Fission produces atoms of lower atomic mass. These fission products are usually
radio-active. The mass of the fissioned atom is greater than the mass of the fission product
atoms. This difference in mass is converted into energy according to Einstein’s equation E=mc 2
where E is energy, m is mass and c is the speed of light. This energy provides the heat to
ultimately produce electricity.
Slow moving neutrons have a larger probability of inducing fission than fast ones. Many reactor
configurations slow down the emitted high energy neutrons to ensure efficient capture to sustain
the reaction. An effective moderator used for this purpose is graphite as it can be placed in
contact with the fuel, whether solid or liquid. Other moderators, including water, need protective
cladding.
Most molten salt-fuelled reactor designs operate with slow, that is thermal spectrum ▲ , neutrons.
They have employed blocks of graphite as moderator through which the fuel is pumped.
Neutron economy requires that any chemical that absorbs neutrons should be avoided both
in construction and in the fuel supply. This presents many challenges. One such challenge is
procurement of graphite of a nuclear grade. Recent surveys identify a shortage of supply of
the high quality special coke produced by oil companies which is needed as starting material
for suitable graphite manufacture. Because demand for this is very small there is at this time
insufficient interest in providing a supply.
Materials for many reactor components and design materials utilize the characteristics of hafnium
and zirconium, which occur naturally together and are physically very similar. However hafnium
absorbs 600 times more neutrons than zirconium and so is used in reactor control rods. Pure
zirconium is almost transparent to neutrons and hence is desirable as a construction material
for many reactor components (this has included metallic cladding of fuel rods for solid fuelled
PWRs).
A thorium/uranium fuel cycle in an MSR, when configured appropriately, can breed more fissile
fuel than it consumes. This class of reactor is known as a breeder reactor (see Thorium ▲
for
further explanation). This operation requires reprocessing of the fuel salt. Less moderated
reactors increase actinide burn-up in the fuel salt. Un-moderated fast reactors work well with
plutonium fuel. A fast, high neutron energy spectrum reactor is effective at burning long lived
actinides. Using thorium in the reactor fuel produces less long lived actinide waste than the
uranium/plutonium cycle. These reactor characteristics are considered in the review process of
this study. Most designs can be re-configured to accommodate different fuel cycles. Further
advantages are discussed in the next section.
Except at start up and shut down molten salt reactors operate in a near equilibrium steady state.
With negative reactivity coefficients they slow down or speed up to follow load. Fuel quantity is
adjusted by breeding or replenishing. Refuelling is often planned as part of on-line reprocessing.
Molten salt reactors employ several methods of fission product removal. Noble gases are
removed either by bubbling out independently or assisted with a helium sparge. Fluorination
▲ see Glossary
Energy Process Developments Ltd. 20