DAYA BAY'S troubled reactor Unit One has had its safety control rods changed for the second time - back to the old ones. Engineers are now completing the fitting of 16 extra control rods to both nuclear reactors at the Guangdong power station to help solve the problems that have kept the station out of action for a month. Yesterday, the vessel head on Unit One was being replaced after eight rod assemblies had been added to the usual 53 and the old set of rods put back, said Hong Kong Nuclear Investment Company senior technical adviser Jacques Pretti. On Unit Two, the annual refuelling has been completed. Spent uranium fuel, which makes up one-third of the reactor's 157 fuel rods, has been replaced with new fuel. Extra control rods have also been added to the previous set. The rods had been polished to remove surface abrasions with a special remote-control device because they were radioactive, Mr Pretti said. Now each rod in Unit Two was being connected to their guidance system, he said. Tests would be conducted next week and, assuming approval from China's National Nuclear Safety Administration, the company hoped to have the reactors back on line by the beginning of June. The plan to add control rods was proposed by nuclear equipment supplier Framatome after the rods in Unit One failed a test of their speed to fall and stop the reactor in an emergency. The control rods make up the main safety mechanism, absorbing the reactive neutrons that keep the nuclear fission going. Framatome believed the problem was due to friction with the tubes that guide the rods into the reactor, and it replaced all 53 rod clusters with a new design incorporating a harder outer coating. But some of these rods fell even more slowly, leading to a further study that showed the new design of the guide tubes was to blame. The company has now installed the extra rods to counteract the seven rogue, slow-falling rods. This interim solution includes running the reactors at reduced power, and fitting monitoring devices to the rods to ensure that any further problems are soon detected.