If the families are successful in what is expected to be a mammoth six-month hearing, 40 others will also sue the plant's owners, British Nuclear Fuels (BNFL), for compensation.
But the implications of the case go much further than the world- wide nuclear industry. This is the first time that the courts will have been asked to decide on an issue of personal injury allegedly resulting from genetic damage - in other words, that the children suffered diseases triggered by their father's exposure to radiation.
The same argument could be extended to cases of, for example, exposure to chemicals, pesticides and solvents which may cause sperm damage or injury in the womb. Indeed, a group of pesticide claims are about to be launched.
The first of the two test cases today is being brought by Elizabeth Reay whose 10-month-old baby, Dorothy, died of leukaemia in 1962. George Reay, the baby's father who died of cancer in the mid-1980s, had suffered one of the highest radiation doses of any of the Sellafield workers.
The second is brought by Vivien Hope, 23, who has been treated using chemotherapy for nonHodgkin's lymphoma, a leukaemia-related illness, diagnosed in 1988. Although there has so far been no re-occurrence it has left her disabled and sterile. Her father, David, was also a fitter at the plant for more than 20 years.
A major difficulty in these cases will be proving 'on the balance of probabilities' that the diseases resulted from exposure and not from some other cause. Causation has so far proved a barrier to those seeking to sue for brain damage in the whooping cough vaccine claims.
During the months of the Sellafield case over 50 geneticists, epidemiologists, environmental chemists, toxicologists and experts in radiation dosage representing both the families and BNFL - which is to fiercely contest the claims as 'totally unfounded' - will give evidence. A spokesman for BNFL said yesterday that there was no medical evidence linking Sellafield to leukaemia.
A series of studies have indicated a cluster of childhood cancers around Sellafield, but evidence of what caused them remains subject to argument.
Richard Meeran, the solicitor for the families, will be relying heavily upon a study published two years ago by Martin Gardner, the head of environmental epidemiology at Southampton University, who found that the children of fathers working at Sellafield ran twice the risk of developing leukaemia than other children in the area. Some men who received somewhat higher doses in their working life had a six to eight times greater chance of fathering children with cancer, he found.
But BNFL will also be producing research from around the world which it says disproves any theory that leukaemia can be passed to offspring through the genes. Lawyers for BNFL are expected to argue that research from Nagasaki and Hiroshima shows that the children of the victims showed a smaller incidence of leukaemia.
The case is unique in both its complexity and costs - estimated to reach up to pounds 10m. It has taken over three years to prepare, and more than 100 scientific reports from Japan, Germany, the US, eastern Europe, Holland, Sweden and elsewhere will be produced.Reuse content