The Cambridge Electron Accelerator's experimental hall, scarred by an explosion six months ago, will re-open to Harvard and M.I.T. physicists in February.

Two teams from Harvard and one from M.I.T. will start projects in the hall shortly after Feb. 15, Robert E. Cummings, CEA assistant director, said yesterday. It will be the first major experimental work in sub-atomic physics at either school since the July explosion.

Reconstruction in the hall has cost the U.S. Atomic Energy Commission, which sponsors all projects at the CEA, about $1.3 million, Cummings said.

After the explosion, the CEA had hoped that work in the hall would resume as early as November. But more time was needed. Cummings explained, to design and install a steel roof on the hall. The original concrete roof was lifted up by the force of the explosion and shattered on the way down on its own girders.

The explosion was set off, it is now believed, when highly flammable liquid hydrogen broke through beryllium windows on the CEA bubble chamber, which was being filled for the first time. The CEA has decided since the explosion to abandon the apparatus and bubble chamber research.

A final report on the causes of the blast will probably be issued by the Atomic Energy Commission within the next few weeks.