FRENCH engineers are concentrating on the design of the tubes that guide the main safety rods into nuclear reactors as they prepare a report on the troubles at the Daya Bay plant. One Japanese engineer said yesterday the whole central core of the reactor might have to be replaced, including the fuel into which the control rods slide to slow or stop the reaction in an emergency. Some reports have been prepared by experts from nuclear supplier Framatome and plant operator Electricite de France following the repeated failure of the control rods on Daya Bay's unit one reactor to pass international speed drop tests after refuelling. Framatome representatives in France said solutions were expected to be put to the Daya Bay owner, the Guangdong Nuclear Power Joint Venture Co, next week. But officials quoted by Xinhua (the New China News Agency) yesterday gave no date for a resumption of power generation from the reactor. The agency indicated Chinese technicians and the plant's French designers were groping for answers. The metal control rods - making up the reactor's main safety mechanism - slow the reaction by absorbing the neutrons produced by the uranium fuel. After the year-old reactor's first refuelling, the first set of rods failed the speed test. They were thought to be scratched and were replaced. But these also failed the test last week, leading to a team of French engineers flying to the plant and another working in Paris to try to solve the problem. Japanese engineer Jinzaburo Takagi said yesterday that if parts of the guide tubes needed changing, the company might decide to replace the entire mechanism.