Myth of initial loading tritium for DEMO—Modelling of fuel system and operation scenario

•Start up of fusion plant without external source of tritium for initial loading is needed.•Realistic Power Ascension Tests (PAT) of fusion DEMO is explained and analyzed.•Typical PATs require years of operation from minimal power and pulsed power output with long dwell time.•Tritium plant is continuously operated to recover all the tritium produced by the DD and low DT burn.•Collected tritium is sufficient for the tests at virtually no additional cost or operation time.