Abstract
Subcritical thorium reactors
Carlo Rubbia, 1984 Nobel Laureate Physics, CERN, Geneva, Switzerland
In the sixties,” atoms for peace” promised cheap, abundant and universally available nuclear power, where the few “nuclear” countries would ensure the necessary know-how to the many others that had renounced nuclear weaponry. Today, the situation is far from being acceptable: the link between peaceful and military applications has been shortened by the inevitable developments and the corresponding widening of the know-how of nuclear technologies.
However, to make nuclear energy freely and abundantly available in more countries, some totally different nuclear technology must be developed. A major improvement of future fuel availability, problems related to proliferation especially in the developing countries and the security of long-term waste disposal demand radically new solutions.
Particularly interesting are fission reactions on Th-232 or U-238 progressively converted into a readily fissionable, energy generating, daughter element, where the totality of the initial fuel can be burnt and the only waste is constituted by the much more short lived fission fragments and the spent cladding materials.
The natural Thorium abundance on the earth’s crust is similar to the one of Lead and adequate for many tens of centuries at a level several times larger than today’s primary fossil production. To give an example, the continuous production of 1000 MWatt of electric power requires 3,5 million t/y of Coal, 200 t/y of natural Uranium, but only 1 t/y of Thorium. A Thorium driven reactor will be essentially proliferation free.
However, unlike the U-235 nucleus, two neutrons must be produced in the case of Thorium, one to maintain the chain reaction and the other to create the fertile material. Such very small neutron excess is essentially incompatible with the requirements of a critical reactor. An external neutron supply has to be added to ensure the balance. A modern high energy accelerator permits the production of this needed complementary neutron flux with the help of the spallation of a heavy nucleus. The so-called Energy Amplifier (EA) and other possible Thorium-based scenarios will be described.
The very same EA technology would permit to alleviate the problem of long lived actinide nuclear waste of ordinary reactors by “burning” it with very fast neutrons. In this method also the simultaneous breeding of U-233 could be ensured in order to permit the start-up of the Thorium driven reactor.