Engineering and construction giant Bechtel Corp. is expected to disclose Wednesday that it will partner with nuclear vendor Babcock & Wilcox Co. to bring a small, commercial reactor design to market.

Closely held Bechtel declined to reveal its investment in the venture other than to say it is "substantial" and affirms its optimism about prospects for new plant designs that could make nuclear power affordable to smaller utilities and get new plants into operation faster.

Under...