JOHANNESBURG — THE first trickle of what will become a flood of 30,000 political exiles who fled apartheid rule have begun to return to South Africa under the auspices of the United Nations.At Johannesburg's Jan Smuts Airport on Dec. 12, there was a joyous welcome for about 120 exiles airlifted from Tanzania to mark the official beginning of the repatriation process administered by the UN High Commissioner for Refugees (UNHCR). Another group of 120 from Tanzania were expected to land in Johannesburg by Dec. 18. The process is expected to be completed by April. Most of the Dec. 12 group were teenagers, and one elderly African woman greeted them with the cry: "Welcome home, my babies." The wife of African National Congress chairman, Mrs. Oliver Tambo, who was among those welcoming the returning exiles at the airport, said: "We would like to try our best to make them feel happy and comfortable." But the initial joy at being home for Christmas was soon tempered by statements from the ANC, and others, that the immediate future for most of the exiles is likely to be grim. Already, more than 6,000 exiles have returned in advance of the UN program, only to have their high expectations dashed by the reality of massive unemployment and the prolonged recession now being experienced in South Africa. A further three flights from Tanzania under UN guidance will be undertaken before the end of the year, bringing to 7,000 the number of exiles who have returned since 1990. Many are unlikely to find homes. An official with the National Coordinating Committee for the Repatriation of South African Exiles, Moss Chikane, said many exiles who previously returned this year were still homeless. Some in the first UN-guided group from Tanzania did not know if they would find their parents. They were being housed temporarily at a hotel in Johannesburg's mixed race inner-city suburb of Hillbrow, while attempts were made to trace their parents and relatives. One of those was Kabelo Dikgatle, 26, who left the country last year, and said he had lost contact with his parents, and doubted if they were still alive. His sense of foreboding contrasted with the celebrations of Abe Nameng, whose efforts were justified after he came to the airport on the "chance" of finding his brother Johannes, 35, who had "just disappeared" in 1983, among the group of returnees. Also among them was Azana Thwala, 13, who was born in Tanzania, and had never been to South Africa, and was accompanied by her parents. Many of the first group who arrived on Dec. 12 were from the Solomon Mahalngu Freedom College in Tanzania, and complained that they would now be forced to complete their studies under the "bantu education" system. This system, which still divides black and whites in many areas into two groups for schooling, had been one of their driving reasons for fleeing South Africa. The UNHCR, which has predicted that the repatriation program will be completed by April, and the reintegration process by October 1992, has already declared it has insufficient funds to meet its tasks. Winnie Mandela, the controversial head of the ANC's welfare department and wife of ANC President Nelson Mandela, said: "We have an extraordinary situation here. Normally it would have been the government's responsibility to assist us with accommodation and the various social problems we are faced with. We have not got the resources to afford this process ourselves." The UNHCR chief of mission in South Africa, Kallu Kalumiya, said his organization would be going back to donor nations to obtain more than the $28 million already pledged. He sounded more optimistic than the ANC official. He said the pilot program from Tanzania had presented no hitches, and added: "This is a momentous moment for the returnees and the UNHCR. Many of these South Africans left years ago; some were born in exile. It is a historic day." The return of the exiles was a necessary condition before the ANC would agree to sit down to formal negotiations on a new political order, at the historic multiparty talks which begin on Dec. 20 and 21. But as long as mistrust remains between the ANC, the government's security establishment, the exiles also have the potential to add a new element of volatility into the explosive world of South African politics. There are risks for both sides. The exiles could face persecution from elements in the security establishment sticking to an outdated political agenda. On the other hand, even moderates in the South African government will be acutely aware that they have decided to allow back those who were once the most dedicated to overthrowing the white-controlled administration. Many of the exiles had military training in African countries, Cuba, and the Soviet Union. It will be up to the ANC to ensure any extremists are prevented from continuing to try and bring down the government through armed action, a policy that was suspended last year to make way for this week's historic negotiations.