Preface
Since the discovery that nuclear reactions produce energy, 90 years have already
passed. Now fission reactors are well established as energy sources, while a fusion
reactor seems to need still several decades to be realized. Even though the assembly
of ITER (the International Tokamak Engineering Reactor) has started on July 28,
2020, it will not be used as an energy source. Why has such a long time been required
to establish a fusion reactor as an energy source compared to fission reactors? The
answer could be found in different energy conversion mechanism between the two.
In a fission reactor, the energy required to start nuclear chain reactions and to
keep them in steady-state operation is quite small. Energy is used just for adjusting
the position of control rods including neutron absorbers or the density of neutron
absorber in cooling water to maintain steady operation and to stop the reactions
by inserting control rods in the reactor core. In order to convert the output power
produced by fission reactions, a massive flow of coolant, i.e. water in most operating
fission reactors, should be circulated and this requires a significant amount of energy,
similar to that required by normal power stations using oil or coal as a fuel and would
be required by a fusion reactor as well.
Unlike the fission reactor, a huge power is required for starting and keeping the
burning plasma in a fusion reactor. Accordingly, the power used is dissipated to
plasma-facing surfaces (PFS). In particular, the power load to the divertor area is
extremely high so that power exhaust is one of the critical issues to establish the
fusion reactor. The power load to PFS appears as plasma–materials interactions
(PMI), which are similar to what the surface of a rocket running into the sun would
be exposed to and, at present, the physical and chemical phenomena expected in PMI
in a fusion reactor would be very difficult to study directly.
Among presently operating plasma apparatus, only JET can realize similar or a
little lower levels of power load. Therefore, understanding of PMI in a fusion reactor
must be extrapolated from observations on the currently operating apparatus whose
power load is still too low. Hence special apparatus such as linear plasma machines
to realize a heat load like the divertor region of a fusion reactor are being constructed.
Heat exhaust is also one of the most important technical issues in the development
of a fusion reactor.
v
Since the discovery that nuclear reactions produce energy, 90 years have already
passed. Now fission reactors are well established as energy sources, while a fusion
reactor seems to need still several decades to be realized. Even though the assembly
of ITER (the International Tokamak Engineering Reactor) has started on July 28,
2020, it will not be used as an energy source. Why has such a long time been required
to establish a fusion reactor as an energy source compared to fission reactors? The
answer could be found in different energy conversion mechanism between the two.
In a fission reactor, the energy required to start nuclear chain reactions and to
keep them in steady-state operation is quite small. Energy is used just for adjusting
the position of control rods including neutron absorbers or the density of neutron
absorber in cooling water to maintain steady operation and to stop the reactions
by inserting control rods in the reactor core. In order to convert the output power
produced by fission reactions, a massive flow of coolant, i.e. water in most operating
fission reactors, should be circulated and this requires a significant amount of energy,
similar to that required by normal power stations using oil or coal as a fuel and would
be required by a fusion reactor as well.
Unlike the fission reactor, a huge power is required for starting and keeping the
burning plasma in a fusion reactor. Accordingly, the power used is dissipated to
plasma-facing surfaces (PFS). In particular, the power load to the divertor area is
extremely high so that power exhaust is one of the critical issues to establish the
fusion reactor. The power load to PFS appears as plasma–materials interactions
(PMI), which are similar to what the surface of a rocket running into the sun would
be exposed to and, at present, the physical and chemical phenomena expected in PMI
in a fusion reactor would be very difficult to study directly.
Among presently operating plasma apparatus, only JET can realize similar or a
little lower levels of power load. Therefore, understanding of PMI in a fusion reactor
must be extrapolated from observations on the currently operating apparatus whose
power load is still too low. Hence special apparatus such as linear plasma machines
to realize a heat load like the divertor region of a fusion reactor are being constructed.
Heat exhaust is also one of the most important technical issues in the development
of a fusion reactor.
v