THE nuclear reaction at the Daya Bay power plant is expected to begin today - honouring the most significant date on the French calendar with a unique symbol of Sino-French collaboration. The ceremonial opening will be held later, but today's national day celebration will have a special feel to it. It began last night with a party for about 400 physicists, engineers, wives and children who were joined by 100 of their Chinese counterparts at a party. Among those celebrating their national day out of France is Marie-Helene Thomas, a nuclear engineer who recently arrived in Daya Bay on an 11-month posting. Ms Thomas is senior shift engineer, overseeing the start-up stage and the early months of power generation. ''I have given some years of my life to this project, and now I want to see it brought fully on-stream,'' she said. The road to Daya Bay, up the Guangdong coastline, is alive with activity - modern construction against a backdrop of villages that seem not to have changed for centuries. The final approach to Daya Bay is a long, straight, dusty road lined with trees and Chinese-run restaurants which serve good French food. It has been dubbed the Champs Elysees. Instead of the Arc de Triomphe stands the nuclear plant. The man who organised last night's party is Thierry Piron, the Administrative Representative for Electricite de France (EdF), which is the largest of the many French companies involved in the project. Despite being thousands of kilometres from les negociants de vin, the boulangeries and the charcuteries of home, catering was not a problem. Apart from the Chinese girls at the check-outs, the supermarket in the main expat village could have been transported directly from any small French country town. The wine, pate, cheese, frozen meat and yoghurt is imported through Hongkong. Only milk, fruit croissants, ficelles and baguettes are locally produced. Mr Piron's programme for the open-air celebration began with dinner and was followed by fireworks, dancing and champagne at midnight. Mr Piron has been at Daya Bay for 21/2 years with his wife, Isabelle, and their two children, aged eight and five. ''I will have mixed feelings when we return home in a year,'' said Mr Piron. ''I will have the professional satisfaction of having helped this community to operate well; to be, in effect, a real French village, with a real spirit about it. ''The sad thing is that we will not have anything like the standard of life in France as we have here. ''This is a paradise for young children. ''There is a good school, a clinic, plenty of friends, the beach, the clubhouse and the pool.'' EdF has recognised his efforts. He has had one promotion while here, and another is due on his return. Jean-Christophe Delvallet, the Hongkong-based managing director for EdF in the Far East, also has one more year before returning home. ''Before I leave, I want to see EdF investing capital in projects in China,'' he said. ''So far, we have only provided consulting and training services.'' EdF's fee for the Daya Bay project will be more than US$300 million. ''The Daya Bay project will, ultimately, have two pressurised water nuclear reactors of the 900-megawatt class,'' said Mr Delvallet. ''Seventy per cent of the power will go to Hongkong and 30 per cent to Guangzhou. ''It is one of the largest joint ventures in China, having a total budget of HK$28.7 billion.'' There are four major overseas companies involved. EdF has overall technical responsibility for the building and operation of the plant, plus the training of the Chinese management. Framatome and Spie Bagnolles provided and erected the nuclear parts of the plant. GEC-Alsthom, an Anglo-French company, provided the turbines and generators. The French community at Daya Bay is immense - at its peak, staff number about 1,750 people. This represents half the total number of French people in China. Include the expats from other nations - the United States, Britain, South Africa, Zimbabwe and Japan - and the rough total of 2,000 people accounts for 10 per cent of China's overseas community. The nuclear idea is awesome, but the process, apart from the physics in the reactor core itself, is quite simple. The 75 tons of uranium fuel rods were installed in the reactor core two months ago, together with the control rods of solid boron which prevent any nuclear activity. Subject to final preparations and testing, the rods will be lifted from the core to start the chain reaction. As each uranium atom naturally decays, it emits two neutrons. As long as at least one of these strikes another atom and hastens its fission, the reaction will be sustained. The by-product of this process is intense heat. The nuclear energy heats water in one sealed and pressurised system. This, in turn, heats a second closed loop, producing steam which drives a turbine, feeding a generator and a transformer. Then the power will be distributed into Guangzhou and Hongkong through the miles of transmission cabling looping across the countryside. The plant should be connected to the grid and producing electricity in six weeks.