LAS VEGAS -- As many as 500 workers at the proposed nuclear waste dump at Yucca Mountain will receive notices next week that they might be laid off at the end of September.

Officials said Friday that the layoffs were part of an ongoing reorganization at the site 90 miles northwest of Las Vegas.

Notices are being prepared for about a fourth of the work force employed by managing contractor Bechtel SAIC and for its commercial and federal laboratory subcontractors, Bechtel spokesman Jason Bohne said.

Many of the employees affected are scientists, engineers, computer modelers and technical workers. Some of the workers are expected to be retained by Bechtel as it repositions its work plans, Bohne said.

Others are expected to be offered jobs by the Sandia National Laboratories, which is taking over portions of the Yucca project from Bechtel.

Bohne and Sandia representative Kate Rivera said they did not know how many workers might be offered new jobs and how many might face layoffs in the fall.

The Energy Department announced the reorganization in January, saying it expected the transition to be complete by October.

Under the reorganization Sandia will assume control of science and technical components, including projections of how long the underground repository might prevent residue of highly radioactive and decaying nuclear waste from escaping into the environment.

Sandia performed a similar role in coordinating the Waste Isolation Pilot Project in Carlsbad, N.M., a repository that began receiving transuranic nuclear waste for disposal in 1999.

DOE managers said the reorganization was designed to improve the project's credibility with scientists and regulators. The department wanted to open the dump in 2010, but allegations that government scientists skirted quality control requirements and a federal court's invalidation of the government's proposed radiation safety standards have pushed back the opening date.