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

Thorium as an Energy Source - Opportunities for Norway
5.2 Properties of the Fissile Material Uranium-233
The fission cross section of U-233 is comparable to that of U-235 in the thermal neutron energy
region. However, there are resonances above an energy of around 1eV (Figure 5.4). This has to be
considered if shifts in the spectrum during massive transients in a thermal reactor are regarded.
U-233 exhibits further fission and absorption resonances in the epithermal energy region which
could influence the dynamic behaviour of a reactor loaded with large amounts of Th-232 and U-
233. Like other fissile materials, U-233 has a small fission cross section at high energies.
Figure 5.4: Fission Cross Sections of the Isotopes Uranium-233 (U-233), Uranium-235 (U-235) and
Plutonium-239 (Pu-239) in the Thermal Energy Region.
The uranium isotope U-233 has very attractive features as a fissile material especially in the
thermal energy region. For thermal neutron energies, U-233 has a higher η-factor than U-235
and Pu-239 (Figure 5.5). The η-factor is the number of neutrons produced per neutron captured
by the fuel, defined by the expression:
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Figure 5.5: The η -Values of U-233, U-235 and Pu-239.