Design and performance characteristics of the radiation shield developed for the TFTR charge exchange diagnostics

The high source strength of 2.45-MeV neutrons (∼1.2 x 10/sup 16/ n/s) and neutron-induced gamma rays produced by the Tokamak Fusion Test Reactor (TFTR) have substantially degraded the performance of many diagnostics. In particular, data obtained by the charge exchange neutral analyzer (CENA) were seen to degrade at modest production levels of ∼10/sup 15/ n/s. To reduce the radiation-induced noise to acceptable levels, massive shielding was required. A design and cost-effectiveness study was performed for the TFTR CENA which showed that a double-layered enclosure consisting of a 10-cm-thick inner housing of lead surrounded by a 23-cm-thick layer of 1.0% borated polyethylene was required to reduce the neutron and gamma noise level by two orders of magnitude. This design was recently implemented on TFTR and was shown to perform as predicted. The details of the enclosure will be presented and the contributing factors that led to the choice of this particular design will be reviewed