HONG Kong will wait a year before conducting an emergency exercise to test its contingency plans for a major fallout from the Daya Bay nuclear plant which opens tomorrow. The Secretary for Security, Alistair Asprey, said the Government would consider arranging a joint safety exercise with China. It is known that Hong Kong made an informal proposal to the Chinese side some time ago. But officials admitted it would take a long time to work out a joint plan because of the different ways of handling emergencies in the two areas. Principal Assistant Secretary for Security Frankie Lui Kin-fun said the plan was tested and revised for the first time in 1990. A second exercise was held last year. Experts from the International Atomic Energy Agency have attended both exercises and advised that every two years would be a suitable interval for the drill to take place. A hotline between Hong Kong and Daya Bay has been set up to inform Hong Kong immediately of any off-site emergency which would have radiological consequences beyond the site boundary. Both sides will exchange information on radioactivity levels and meteorological conditions as well as the timing to activate food and water supply controls. Mr Lui said in case of accidents, the public should keep calm and follow instructions issued by the Government. He said they should be safe since Hong Kong had a comprehensive arrangement to prevent any indirect contamination through water and food. In addition to notification by China, the Royal Observatory has been monitoring the radiation level in the territory through its 10 stations to detect leakage. Senior scientific officer Yeung Kai-hing said an alarm at the Royal Observatory would be sounded if the radiation level was abnormal at any of the stations. They would inform the Security Branch as soon as possible of any alarm together with their assessment of the causes. To undertake a more comprehensive contingency plan, Mr Lui said they hoped to reach agreement with the Chinese side to inform Hong Kong promptly of any on-site incident. A meeting between the two sides will be held later this month on the subject. ''It is important for us to have the facts on any on-site incidents or accidents to clarify any rumours especially as the public is easily alarmed,'' Mr Lui said. Before any agreement, the Government has to depend on information supplied by the China Light and Power Company to determine whether evacuation would be needed for residents on Ping Chau and Mirs Bay which are within 20 kilometres of the Daya Bay nuclearplant. China Light systems control centre at Tai Po is able to detect immediately any interruption of electricity supply from the nuclear plant. Such an interruption is usually the first indication of an incident and the company will alert the Government and then submit a written report, as information becomes available, on the possible cause of the incident. While it is an international practice that a nuclear power plant discloses only off-site accidents, United Democrats legislator Fung Chi-wood said the plant should show its sincerity by not attempting to cover up any accidents. ''People in Hong Kong would feel safe if we were well informed of what was going on in the plant,'' Mr Fung said. The legislator said immediate reports from the plant on any accidents could eliminate any rumours about leaks there. Another Daya Bay watcher, executive committee member of the Conservancy Association, Hung Wing-tat, said the disclosure of on-site accidents as well as off-site problems would make Hong Kong residents feel reassured. Mr Hung also said the disclosure would allow outside institutions to take part in giving the plant suggestions on how to operate safely. ''Outside institutions, not necessarily green organisations, would have the information to study and could give them alternative views in improving safety measures. ''That's why we want the plant to disclose its on-site accidents, and we can have the information about what's going on inside the plant,'' he said.