Neutron reaction cross section data for advanced nuclear applications

Full text of publication follows: Worldwide major research efforts are currently being carried out in order to develop a new concept of nuclear power generation, so-called accelerator driven systems (ADS) for energy production and transmutation of radioactive nuclear waste. A suggested approach is the energy amplifier (EA), which is a sub-critical reactor using a powerful proton accelerator and a spallation reaction as neutron source. Since the EA is based on the thorium-uranium fuel cycle, where the natural resources of the main fuel thorium are estimated to last for hundred thousands of years, it is considered to provide clean and almost inexhaustible nuclear energy. Apart from necessary new technical developments, the realization of these concepts depends strongly on the availability of accurate nuclear reaction data. In particular, precise knowledge about cross sections for fission, neutron capture and scattering is required for the nuclides involved in the Th-U fuel cycle. Among the first priority isotopes the IAEA had pointed out 231Pa and 233Pa. The latter one, 233Pa, is of specific interest, since it plays an important role as an intermediate nucleus in the formation of the fissile 233U from the fertile 232Th. With its half life of 27.0 days for β-decay, 233Pa is not a 'long-lived' nucleus, but it still requires careful attention in the design and operation of thorium-fueled reactors. When a thorium-fueled reactor is stopped, the present amount of 233Pa will continue to decay into 233U, leading to an increase in reactivity, which may even cause criticality. This mechanism is known as 'protactinium effect' and is proportional to the power level of the reactor. Also the precise knowledge of the fission cross section of 231Pa (above 1 b for fast neutrons) is essential for simulations of the balance of nuclei in and, thus, the reactivity behavior of the reactor. We present recent cross section data from direct, energy resolved measurements of the neutron-induced fission of 231,233Pa and compare the experimental results with previously known values from the evaluated data bases ENDF/B-VI and JENDL-3.3 as well as new evaluations in the frame work of the statistical model code STATIS. (authors)