The nuclear energy revival is so far proceeding slowly, but the reactor models that utilities around the country are now contemplating are about to gain a competitor.

Babcock & Wilcox, a large construction and energy development company, has scheduled a “major commercial nuclear power product announcement” for Wednesday. Two nuclear industry experts said it was a reactor, albeit one that is different from those currently on the market.

Reactor designs in the 1960s and 1970s — the period when today’s reactors were ordered — tended to grow larger and larger. And the four large reactor projects now widely touted as ripe for orders are even bigger.

The new reactor, in contrast, will be “modular,” meaning relatively small and probably mostly built in a distant factory rather than on site, a technique that could be faster and cheaper — and one that requires a smaller bet by the buyer. Additional modules could be added fairly quickly, as demand developed — addressing one of the disadvantages of reactors now on the market: extended construction times that can take years, requiring the buyer to predict electricity demand far into the future.

The idea of small reactors seemed promising in the 1990s. Westinghouse decided that one reason for the drought of orders in that decade was that reactors were too big for the lower-growth environment of the era. It designed a 600-megawatt model intended to be faster and simpler than those finished in the 1980s, and also, only about two-thirds as large.

But it had no orders, so it redesigned the new model as an 1,100-megawatt reactor.

Some experts have predicted that a smaller reactor would be a good choice for third-world countries. Big reactors can only be installed on big grids; otherwise, each unscheduled reactor shutdown means a blackout. But so far, reactors of any size have not sold well in the third world.

In the United States, most builders think big reactors have more output per worker, and lower capital costs per unit capacity. And security costs, which were high even before the attacks of Sept. 11, 2001, are higher now, and do not vary much according to reactor size.

Three or four decades ago, Babcock & Wilcox sold reactor models roughly on the same scale as its competitors. But now, as Westinghouse, General Electric, Hitachi and Areva push toward ever larger units, Babcock appears to be cutting its own path.

The announcement Wednesday is to expected to be made jointly with the Tennessee Valley Authority and the Nuclear Energy Institute, an industry trade group.