Abstract: The distribution in the plasma of coolant-Helium impurity leaked from the first wall has been calculated and numericall results have given. The peak of the distribution is within the scrape-off layer (SOL). When Helium comcentration at the plasma center is required to be less than 5 percents, permitted Maximum-Leak-Rate (MLR) N_max is about 0.35×10^-3 mol/s and the biggest flaw allowed is with the length of about 3.1mm and the width of 0.63 mm. The main factors effecting the impurity distribution and concentration are such as the temperature and density at the edge of plasma, the temperature and density scrape-off lengths in SOL and the wall temperature.

Key words: Impurity distribution, Scrape-off-layer (SOL), Maximum-Leak-Rate (MLR)