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

Nuclear Reactors for Thorium
Table 5.1 shows the nuclear properties of the fissile isotopes U-233, U-235 and Pu-239 averaged
over thermal energies. The large η-value allows the breeding of U-233 even at thermal energies.
The effective fraction of delayed neutrons (βeff) of U-233 is less than half of that of U-235, reducing
the time constant of the feedback system for controlling a critical reactor.
Table 5.1: Comparison of the Nuclear Properties for the Fissile Isotopes U-233, U-235 and Pu-239.
Altogether, the favorable nuclear properties of U-233 are counter balanced by a loss of neutrons
due to the residual Pa-233 and the lack of fast-neutron fission in Th-232 compared with U-238. As
a consequence, it seems extremely difficult to design a thermal breeder reactor out of a Light
Water Reactor fuelled with Th-232/U-233 under industrial conditions.
5.3 Past Experiences
In the 1960s and 70s, the development of thorium nuclear fuel was of great interest worldwide. It
was shown that thorium could be used practically in any type of existing reactors. A large amount
of work was carried out and resulted in many interesting developments, among these were
prototype High Temperature Reactors (HTRs), Light Water Reactor (LWRs) and Molten Salt
Reactors (MSRs).
In the mid 1970s the U.S. Electric Power Research Institute (EPRI) carried out a study of the
prospects for improvements in the nuclear fuel cycle of modern LWRs by introducing thorium
[42]. Several reactor types other than the LWRs have tried using thorium [43]. The most notable
among these are the first gas cooled, graphite moderated reactor in the US (Peach Bottom, 40
MWe, 1967 - 1969) and the first pebble bed reactor in Germany (AVR, 15 MWe, 1966 - 1972) [44].
Table 5.2 gives an overview of the experimental reactors and the power reactors that have
utilized thorium as nuclear fuel.
41