Measurements of Fission Cross Sections for the Isotopes relevant to the Thorium Fuel Cycle

The present concern about a sustainable energy supply is characterised by a considerable uncertainty: the green house effect and foreseeable limits in fossil fuel resources on the one hand, the concern about the environmental impact of nuclear fission energy and the long term fusion research on the other hand, have led to the consideration of a variety of advanced strategies for the nuclear fuel cycle and related nuclear energy systems. The present research directories concern such strategies as the extension of the life span of presently operating reactors, the increase of the fuel burn-up, the plutonium recycling, and in particular the incineration of actinides and long-Lived fission products, the accelerator driven systems (ADS), like the "Energy Amplifier" (EA) concept of C. Rubbia, and the possible use of the Thorium fuel cycle. The detailed feasibility study and safety assessment of these strategies requires the accurate knowledge of neutron nuclear reaction data. Both, higher fuel burn-up and especially waste incineration options require improved and partly new data on actinides. It is well known that only two fertile elements, i.e. $^{232}$Th and $^{238}$U, exist in nature, with the first showing major radiotoxicity advantages over the second. In the thorium cycle shown in Figure 1-1, the fissile element $^{233}$U is regenerated by the $^{232}$Th(n,$\gamma$)$^{233}$Pa($\beta^{-}$)$^{233}$U process. The basic $^{232}$Th(n,$\gamma$) and $^{233}$U(n,f) reaction cross sections have to be known very precisely. The other fundamental parameter of this cycle is the cross section of the $^{233}$Pa(n,$\gamma$) reaction, which due to the high specific activity of $^{233}$Pa can only be indirectly determined. In the uranium cycle the main fissile element is $^{239}$Pu, that is generated via $^{238}$U(n,$\gamma$)$^{239}$Np($\beta^{-}$)$^{239}$Pu. The (n,$\gamma$) and (n,f) reactions are the most important in developing waste incinerators, since they allow to transform long-lived radioactive isotopes into short-lived ones. Charged particle producing reactions, such as (n,p) and (n,$\alpha$), reactions have generally much lower cross sections, usually below 100 mb, and are therefore of lesser importance. However, the (n,n') and (n,xn) reactions with cross sections attaining several barns are of great interest in the conception of ADS projects, since they govern the neutron flux and a large part of the energy density . The LLHE$^{a}$ component of the radioactive waste comes from parasitic reactions on the different constituents of the fuel. For the Th-cycle the parasitic reactions are $^{233}$U(n,2n)$^{232}$U, $^{232}$Th(n,2n)$^{231}$Th($\beta^{-}$)$^{231}$Pa and $^{231}$Pa(n,$\gamma$)$^{232}$Pa($\beta$)$^{232}$U leading to the production of $^{232}$U, which is responsible for a large part of the short term (few centuries) radiotoxicity, and $^{231}$Pa responsible for the long term radiotoxicity . The importance of the thorium fuel cycle is twofold: as an $\textit{alternative fuel}$, it is the basis for safe and sustainable energy generation through accelerator driven systems, as well as the design and development of $\textit{nuclear waste incinerators}$, based on the Th-U fuel. Indeed studies for incinerating the radioactive waste from LWR's$^{b}$ and for an alternative fuel with limited waste production allowing in addition a long-term sustainability have led to new interest in the accelerator assisted Th fuel cycle.