Neutronics experiment on a mock-up of the ITER shielding system at the Frascati neutron generator (FNG)

Absolute activation reaction rate (RR) measurements were carried out in a mock-up of the inboard shield of the International Thermonuclear Experimental Reactor (ITER) reactor at the Frascati neutron generator (FNG). Since the mock-up has a thickness of about 1 m, a flux gradient of six orders of magnitude is observed for the high-energy part of the neutron spectrum. This required, for the high threshold reaction (e.g.93Nb(n,2n)92Nb) to be measured, the use of ultra-low background germanium detectors. These detectors allowed activity to be measured, even in the deepest positions of the mock-up, with a total error lower than ± 15%. The measured reaction rates (E) are also compared with the results of the transport calculation (C) performed using the MCNP code and the FENDL-1 library. The C/E ratios for some of the measured quantities are reported. These results show a systematic underestimation of the calculated quantities with respect to the measurements. The underestimation is as high as 30% in the deepest positions.