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

Thorium as an Energy Source - Opportunities for Norway
products that produces the heat required to melt the reactor. The Energy Amplifier as shown in
figure 5.9 will probably not melt if the heat removal from the lead-bismuth coolant stops, because
it contains around 8000 metric tonnes of passively cooled metal that will absorb the heat. But, if
the coolant leaks from the tank the core will certainly melt.
5.4.4.3 Sub-criticality and the Accelerator
In an ADS, the sustainability of the nuclear fission reactions is made possible because of the
presence of an external source of neutrons provided by the proton beam; the working point of the
core itself is far below criticality. Two alternatives, based on well established technologies, can be
envisaged in order to fulfil the indicated requirements for the accelerator:
1. Super-conducting cavity LINACs, for which a vast experience exists in Europe, including on
the industrial production of the cavities and of the associated RF sources, especially as a
consequence of the realisation, now dismantled, of LEP200 8 at CERN and of the work at
DESY (Germany) and elsewhere (Italy, France) in association with the TESLA 9 program. The
most powerful LINAC is now being commissioned at the Spallation Neutron Source facility at
ORNL.
2. Isochronous cyclotrons, for which the world reference with respect to the ADS question is PSI
(Switzerland). There is also a strong experience accumulated at GANIL (France), CAL
(France) and LNS (Italy) facilities. The guiding magnetic field can be produced either by
ordinary magnets (PSI, GANIL, CAL) or super-conducting coils (LNS).
The system needed to drive an ADS represents only a reasonable extrapolation of what has
already been achieved in current accelerator technology [108], [109], [110], [111], [112] and [113].
An important element for the accelerator is a high reliability in continuous operation and in
particular the absence of “beam glitches”, namely of short interruptions of the beam current. A
beam interruption will induce very strong stresses in the window between the accelerator vacuum
and the target and produce a fatigue of the reactor elements, reducing the lifetime of the system.
So this topic is of first importance. The LEP200 super-conducting LINAC has demonstrated a
remarkable level of continuity of operation, since a single short glitch would imply the immediate
loss of the colliding beams. At PSI, however, some beam instabilities due to discharges in the RFfeed
through to the cavities have been observed. It is believed that this problem can be overcome
with a more advanced design of such components 10 , which are known to operate without such
inconveniences in other installations (CERN).
It is expected that both accelerator schemes (LINAC and cyclotron) will offer highly reliable and
redundant methods to control the beam current and to switch it off extremely quickly (a few
microseconds) and reliably if and when required by the sub-critical unit. However, it seems that
the LINAC option is favoured by the international accelerator community [114].
5.4.4.4 Technology Issues of Medium-to-High Power Spallation Targets for ADS
The spallation target has to provide the highest possible neutron yield, be transparent to
neutrons, and at the same time sustain a large beam power of 10 to 20 MW. In this respect,
molten lead is almost an ideal candidate since it has also excellent thermodynamic properties and
can contribute to cooling. The use of liquid targets is a tendency which is presently developing in
8 The LEP200 programme ultimately consists of about 3.6 GeV gradient for the continuous acceleration of
relativistic electrons and positrons with a current of up to 7 mA.
9 The TESLA programme aims at the realisation of advanced super conducting cavities, of high gradient and lower
cost in order to open the way to the realisation of a Linear Collider in the 1/2 TeV range and eventually beyond.
10 New cavities are under development also at PSI.
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