BOTH nuclear reactors at the Daya Bay power plant were switched off yesterday as a 'cell' of several dozen top engineers in Paris got to work on correcting possible flaws in the reactor design. The experts in hydraulics, computer programming and nuclear engineering from equipment supplier Framatome and operator Electricite de France (EdF) were working with data from Daya Bay. Top managers of the design and construction, and nuclear generation divisions of both companies held an all-day meeting to map out a rescue strategy, EdF officials said. Unit two would be left to cool for a few days before the reactor could be opened for refuelling to begin, the company said. The engineers were concentrating on the design of the control rod mechanism - the reactor's main safety system - and the cooling water flow, both slightly different at Daya Bay than at similar French plants, said Framatome general manager for nuclear operations Dominique Vignon. The 3.5-per-cent faster water flow, which increases the power output, could lead to greater resistance when the control rods dropped in, said Hong Kong Nuclear Investment Company senior technical adviser Jacques Pretti, but he was unsure why only some rods were affected. 'The programme of analyses and investigations now under way is aimed notably at determining more precisely whether there is any connection between these design changes and the increased rod drop time,' the company said yesterday. Control rods on the unit one reactor, closed since refuelling began in December, have twice failed international tests of drop speed. The rods are used to slow the reaction and, in an emergency, to stop it. During a test in mid-February, seven of the 53 control rods fell slower than the required 2.15 seconds, leading to all the rods being changed for a newer design. However, last week the new rods failed the speed test. The latest troubles at the unit-one reactor, which has suffered 13 shutdowns in its first year of operation, also led to speculation that the same Daya Bay team of EdF, Framatome and UK-French conventional power plant supplier GEC-Alsthom might be forced to renegotiate the deal to build the station's sister plant down the coast at Lingao. Contracts must be signed by mid-July, following a memorandum of understanding signed in January between the Beijing Government and the companies, said one source. The source said the Chinese were 'very annoyed' about the shutdown, estimated to cost millions of dollars a day in lost power. Nuclear engineers worldwide said they had never experienced this problem with control rods. Legislative Councillor the Reverend Fung Chi-wood said the problem showed that nuclear plants were not as reliable as those powered by fossil fuel.