Families at the upscale Traum Haus condominiums sleep a little easier thanks to a basement bomb shelter with 3-foot-thick walls.

It's a safeguard unavailable to all but a handful of elite South Koreans _ including those willing to pay at least $5 million for one of the condominiums _ but North Korea's announcement this week that it successfully detonated its first-ever atomic bomb has the whole country rethinking its defenses.

"The threat always exists because North Korea exists,"said Chris Chae, managing director of Traum Haus, as he showed off a Swiss-made filter designed to purify radioactive air for up to 200 lucky survivors hunkering in the bunker 45 feet below ground.

Ever since the 1950-53 Korean War ended in an uneasy cease-fire rather than a peace treaty, South Korea has lived in the shadow of the North's formidable military juggernaut, now the world's fifth-largest, with 1 million soldiers.

Defenses have traditionally focused on thwarting an onslaught of invading infantry. Highways leading into Seoul, the South's sprawling, neon-lit capital, are fitted with tank traps; stretches of the nearby Han River are lined with razor-wire.

While the South has long suspected its former enemy of having atomic bombs, Monday's test claim finally made the threat impossible to ignore.

Shortly after the reported blast, South Korean Defense Minister Yoon Kwang-ung said his country would bolster its conventional arsenal to deal with a nuclear-armed North.

Yoon said the South will also enhance its ability to conduct precision strikes against the enemy, and to intercept potentially nuclear-tipped missiles.

The remark coincided with media reports that South Korea's military was checking its readiness for a nuclear attack _ including verifying and improving the troops'equipment.

The beefed-up defense will likely affect the South's alliance with the U.S. military, which stations about 29,500 troops in South Korea as a deterrent against the North. Those ties, too, have been based on threats of a conventional war.

Already, it is evolving to include the deployment of U.S. PAC-3 missile intercepters.

The top U.S. commander in South Korea said defenses were sufficient.

"Be assured that the alliance has the forces necessary to deter aggression, and should deterrence fail, decisively defeat any North Korean attack,"said U.S. Army Gen. B.B. Bell."U.S. forces have been well trained to confront nuclear, biological and chemical threats."

Military experts doubt North Korea's ability to accurately deliver a nuclear bomb, because it is believed to lack the miniaturization technology needed to fit one on a missile.

South Korea has a mere 23 atomic bomb shelters, enough to protect only 1,500 people, the Dong-a Ilbo newspaper reported, citing an opposition lawmaker privy to classified information from the Emergency Planning Commission.

What's more, those shelters are reserved only for military and government elites.

"Finland, Switzerland and other developed countries have shelters that protect citizens from nuclear attacks while our citizens will be in a defenseless state when North Korea fires a nuclear bomb,"legislator Kim Jung-hoon was quoted as saying.

Though shelters can protect those inside from radiation after a nuclear attack, they would be all but useless as a shield against a direct hit.

Still, officials at Traum Haus, which translates as"Dream House"in German, are proud to bill their five-building complex as having the only private A-bomb shelter in Seoul.

Accommodations are, unsurprisingly, more rustic than the plush penthouses above. Beds are plastic-backed cots stacked three-high. Electricity streams from a hand-turned crank, and toilet relief comes in the form of a plastic bucket.

"I hope we never have to use it,"Chae said."But after the test, we've seen a lot of interest."

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