Whoever built the twin reactor domes of the San Onofre nuclear power plant seems to have had something against right angles.

Since San Onofre's Unit 2 and 3 reactors were first opened in 1982 and 1983, their distinctive shape has caused many to wonder: What's up with that?

Plant spokesman Ray Golden said engineers decided to enclose the plant's two 1,100-megawatt reactors inside 170-foot-tall domes because that particular shape offers much more strength than simple, square-cornered buildings.

"It's purely for strength," Golden said.

The ruins of ancient civilizations, from the Greeks and Romans to the Babylonians and Assyrians, show that mankind has long recognized the superior strength of the arch. Because they can hold up much greater loads than simple post-and-beam construction, arches are still used today wherever strength is paramount, from bridge supports to dam walls.

Golden said a dome is simply an arch rotated 360 degrees and that it offers the same load-bearing advantages of an arch. He added that San Onofre's concrete containment domes —— which range in thickness from 4 to 7 feet —— are laced with steel rods and are crisscrossed with high-tension steel cables, adding additional strength to an already strong design.

All that strength is necessary to contain the reactors' nuclear reactors in an emergency.

The plant's reactors use heat generated by splitting atoms to create steam that turns turbines to generate electricity. But some of that hot water is radioactive. If either reactor suddenly sprung a big enough leak, it would release thousands of gallons of radioactive steam. And that steam must be stopped from entering the environment.

To contain any leaks, the engineers who designed San Onofre, and many other reactors operating worldwide, chose a dome. The domes are airtight and are designed to contain every drop of radioactive steam if there's ever a leak. Golden said a peek inside either dome would show a comparatively small reactor and lots of open space.

"About 100 feet of each 170-foot dome is just open space to allow the steam somewhere to go without getting out," Golden said.

Those who have carefully scrutinized San Onofre's reactor containment domes may notice that each is capped with a smaller round structure at the very top.

Golden said the caps are ventilation ducts that can bring fresh air into the domes when they are not sealed.

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Contact staff writer Paul Sisson at (760) 901-4087 or psisson@nctimes.com.