Fig. 6-5 A cross section of the experimental layout Many radioactive dose sensors are inserted into the experimental holes.

Fig. 6-6 The neutron flux as a function of the distance from the surface of the test domain A 5 order-of-magnitude reduction is analogous to identifying a 0.4 m length from the top of a 4,000 m high mountain

In ITER plasmas, D-T reactions will produce 14 MeV neutrons which will decelerate in the shielding blanket and creating heat. This shielding should reduce the neutron flux to levels such that no biological impact on human bodies can occur outside of the tokamak. To meet this condition, the required reduction factor is more than 5 orders of magnitude between the inside and outside of the blanket. As a consequence, there is some concern that a trivial error in a shape factor could accumulate, producing a large error in estimation, even if a single collision process is known precisely. To address this issue, we constructed an imitation of the shielding in ITER, and we experimentally measured the energy broadening and intensity reduction rate of 14 MeV neutrons and of gamma rays. The schematic diagram of the neutron source and the stainless steel structure are shown in Fig. 6-5. The measured neutron fluxes are shown in Fig. 6-6 as a function of the distance from the surface of the test domain, showing parameters of particular energy ranges. These results demonstrate that the uncertainty in the nuclear data calculation is less than 40%. Before these experiments, there was no confirmation of this result, and an uncertainty factor of 2 to 3 has usually been taken as a design margin. The high accuracy measurement performed here, over a wide range of the neutron intensity, overturns traditional knowledge of the design margin.

Reference C. Konno et al., ITER Bulk Shielding Experiment at FNS, JAERI-Conf 96-005, 102 (1996).

Select a topic in left column