The measurement of tritium activity in water in the GBq/litre range is an important topic in fusion and other areas. In this work a scintillator detector based on the BC-408 plastic scintillator was built up and tested in the mentioned range. The structure of the detector was simplified to ease maintenance. Memory effect and scintillator damage were investigated by means of experiment and simulation. The results are analyzed in view of further detector development, and conclusions are drawn concerning the scintillator material.