An appraisal of deuterium retention in JET

The amounts of deuterium retained within the JET vessel after campaigns in the years 1985 to 1989 have been reevaluated and shown to indicate that codeposition of D and wall material is the principal retention mechanism. During the period covered Be was introduced into JET by evaporating thin layers over the first wall and as a limiter material: the addition of Be is shown to have little effect on the rate of accumulation of the D inventory. The amount retained per pulse ( / 4 × 10 20 D atoms per pulse on average) is shown to fit well with a model wherein the amount of carbon (or other wall material) passing through the plasma during the discharge determines the maximum retention (which can only be realised if first wall temperatures do not exceed 270°C). Such a model may be adapted to predict hydrogen isotope retention in other large machines.