CIVIL Aid Services (CAS) volunteers will be called upon to act as dispatch riders carrying air samples from around the territory in the event of a nuclear accident at the Daya Bay plant. The Government's contingency plan also calls for the members to perform crowd control duties outside swimming pools used as decontamination areas. Speaking at a CAS exercise in Chai Wan yesterday, senior training officer (technical force), Au Yin-shan, said their members would also be deployed in helping to man Agriculture and Fisheries Department control points to monitor radiation. Last month Hongkong organised a ''command post'' drill of a nuclear accident but little has been made public about the Government's contingency plan. It is believed the biggest threat to humans would come from contaminated food, thus sufficient monitoring of radiation levels would be needed. A test of contingency plans in 1990 found the number of personnel monitoring radiation levels was inadequate, and CAS members could help reinforce this role. However, CAS principal training officer (special operations), Fung Kwok-him, said communication problems and the fact people had full time jobs, could make it difficult to assemble a group of volunteers. But despite this, he said he was confident of getting people together in the shortest possible time. The CAS's annual large scale exercise yesterday, which simulated a landslide in a squatter camp, involved more than 800 of its 3,724 members and required them to rescue casualties and administer first aid. In an assessment of the exercise and the service as a whole, Mr Fung said: ''I believe we can cope with the kind of disaster that may require our services.''