A FAULT in a starter motor at the Daya Bay power station has led to increased maintenance tasks but will not delay restart of unit two this week, the Hong Kong Nuclear Investment Company said. During re-commissioning tests ready for the start-up, engineers found a problem in the starter motor for the generator in the conventional part of the station, senior technical adviser Philippe Mauger said yesterday. The reactor had already been powered up to criticality - when the nuclear reaction becomes self-sustaining - for tests and was currently in a cold shutdown mode, he said. Increasing power again to criticality could be done within a day for coupling to the grid on March 14, Mr Mauger said. First the primary pumps heat up the reactor coolant over a matter of hours to about 280 degrees Celsius and a pressure of 155 atmospheres - called hot shutdown. Then the control rods are lifted out of the reactor and the boric acid coolant diluted over another few hours to increase the reaction to criticality. Coupling to the grid is done at 15 per cent output, after which escalation to 100 per cent power output - 980 megawatts - would take about five days, Mr Mauger said. Guide tubes for the safety control rods which drop into the reactor in an emergency to stop the reaction have all been changed. Mr Mauger said the slowest drop time for the rods was 1.425 seconds and the fastest, 1.335 seconds - faster than when the unit was first tested during commissioning in late 1993. The shutdown for refuelling of unit one is scheduled to start on March 24.