A war of fiery words

The myr­iad chal­lenges of a suc­cess­ful nu­clear strike

Los Angeles Times - - FRONT PAGE - By Ralph Vartabedian

Be­fore the age of com­pact cars, lap­top com­put­ers and pocket tele­phones, there were minia­ture nu­clear war­heads.

For as long as there have been en­gi­neers, they have been work­ing on mak­ing com­pli­cated things smaller and bet­ter. Weapons are no ex­cep­tion.

Now, North Korea ap­par­ently has fig­ured out how to make a very big ex­plo­sive small enough to sit atop one of its mo­bile-launched mis­siles, a de­vel­op­ment that could threaten much of the U.S., ac­cord­ing to a U.S. in­tel­li­gence re­port that sur­faced this week.

North Korea is mak­ing progress, show­ing it can put to­gether com­pe­tent teams of sci­en­tists and solve tech­ni­cal prob­lems, but it is far from prov­ing that it is ca­pa­ble of launch­ing a pun­ish­ing nu­clear strike on the U.S., ac­cord­ing to U.S. weapons ex­perts.

Mak­ing a minia­ture nu­clear weapon that has a large ex­plo­sive force in­volves a lot of sci­en­tific and en­gi­neer­ing know-how.

The “Lit­tle Boy” bomb that the

U.S. dropped on Hiroshima on Aug. 9, 1945, weighed as much as two 2017 Cadil­lac Es­calade SUVs, about 9,700 pounds. Three days later, the “Fat Man” bomb, slightly heav­ier at 10,300 pounds, was dropped on Na­gasaki.

Since then, the weight of U.S. atomic bombs has shrunk con­sid­er­ably as sci­en­tists have re­fined the physics of the de­vices and stream­lined how they are armed.

With the last gen­er­a­tion of nu­clear weapons de­signed in the 1980s, en­gi­neers at Los Alamos Na­tional Lab­o­ra­tory pro­duced the W88, weigh­ing only 800 pounds de­spite hav­ing an ex­plo­sive force equal to 475,000 tons of TNT — in other words, less than one­tenth the weight of the first atomic bomb but 400 times more pow­er­ful.

What tech­ni­cal ca­pa­bil­ity is nec­es­sary to build a mis­sile-ready nu­clear bomb?

The first step is un­der­stand­ing how to re­duce the amount of con­ven­tional high ex­plo­sives that sur­round a hol­low pit of highly en­riched ura­nium or plu­to­nium. A nu­clear det­o­na­tion oc­curs when the high ex­plo­sive im­plodes the hol­low sphere of fis­sile ma­te­rial next to it to start an un­con­trolled chain re­ac­tion.

Af­ter the war, work pro­gressed on smaller bombs. One of the cru­cial de­sign steps was to cre­ate a small, pre­cisely uni­form air gap be­tween the con­ven­tional ex­plo­sives and the sphere of nu­clear fuel, am­pli­fy­ing the force of the con­ven­tional ex­plo­sion and re­duc­ing the amount needed to trig­ger a nu­clear chain re­ac­tion.

It’s un­clear that Py­ongyang has mas­tered that pre­cise con­struc­tion, said Jef­frey Lewis, a nu­clear weapons an­a­lyst with the James Martin Cen­ter for Non­pro­lif­er­a­tion Stud­ies in Mon­terey, Calif.

What Py­ongyang has said so far is that its weapon is a “Korean-style mixed charge” de­vice, in­di­cat­ing “they don’t have a lot of plu­to­nium, so they are mix­ing it with ura­nium,” Lewis said.

It is pos­si­ble the North Kore­ans are also in­ject­ing tri­tium gas into the hol­low sphere to get some fu­sion en­ergy out of the bomb as well, he said.

“The con­cept is well known, but you can’t know with­out test­ing. But North Korea tests, so they would know,” he said.

North Korea has prob­a­bly not suc­ceeded in build­ing a light­weight, minia­tur­ized bomb, as the U.S. and Rus­sia have, but only a more com­pact weapon that isn’t sig­nif­i­cantly lighter.

The big­gest stride in minia­tur­iza­tion in­volved the hy­dro­gen bomb de­sign pi­o­neered by two Eastern Euro­pean im­mi­grants, Ed­ward Teller and Stanis­law Ulam.

The Teller-Ulam con­fig­u­ra­tion cre­ated two or three stages in a weapon, in which a fis­sion trig­ger causes Xrays to com­press a sec­ondary stage of the weapon con­tain­ing fu­sion fuel. The sec­ondary stage can trig­ger a third stage that con­tains more fis­sion fuel.

The first full-scale demon­stra­tion of such a ther­monu­clear weapon was con­ducted in 1952, just seven years af­ter the first atomic bomb test of the Man­hat­tan Project. It cre­ated an un­ex­pect­edly large blast equal to 19 mega­tons of TNT.

The “bomb” was ac­tu­ally a ma­chine that weighed 82 tons, in­clud­ing cryo­genic cool­ing equip­ment.

The en­su­ing decades led to re­fine­ments that dras­ti­cally re­duced its weight. By the time of the Rea­gan ad­min­is­tra­tion, the U.S. was able to field Peace­keeper MX mis­siles that could carry 10 war­heads each and drop them on sep­a­rate tar­gets any­where in the world.

North Korea is be­lieved to have been work­ing on nu­clear weapons only since the 1990s. The na­tion lacks the in­dus­trial in­fra­struc­ture of the other nu­clear pow­ers, not only the U.S., but also France, Bri­tain, Rus­sia and even In­dia.

In Septem­ber, the coun­try det­o­nated a weapon es­ti­mated to have a nu­clear yield of 15 kilo­tons to 25 kilo­tons, ac­cord­ing to Siegfried Hecker, for­mer di­rec­tor of the Los Alamos Na­tional Lab­o­ra­tory.

North Korea said it had stan­dard­ized the de­sign and would be­gin pro­duc­tion.

Philip E. Coyle III, a re­tired nu­clear weapons de­sign ex­ec­u­tive for the En­ergy Depart­ment and a for­mer se­nior Pen­tagon of­fi­cial, said North Korea’s twostage Hwa­song-14 mis­sile, which flew 45 min­utes and reached an al­ti­tude of 1,850 miles in space in late July, is sig­nif­i­cantly smaller than the three-stage U.S. Min­ute­man III.

Although it ap­pears that the un­armed Hwa­song-14 could reach the U.S. main­land, a heavy nu­clear war­head would sig­nif­i­cantly re­duce its range.

“I as­sume what North Korea means by minia­ture is that it is small enough to be car­ried by their rocket,” Coyle said. “But this North Korean rocket does not have a lot of pay­load-car­ry­ing ca­pac­ity.”

The other key ques­tion is whether the North Kore­ans have an ad­e­quate reen­try ve­hi­cle. A war­head trav­els about 4 miles per sec­ond, glow­ing red hot, as it en­ters the at­mos­phere.

The nose cone of the mis­sile has to pro­tect the weapon from the heat and aero­dy­namic forces of reen­try. Its shape has to be nearly per­fect to avoid drift­ing off tar­get.

“The U.S. spent years and years to de­velop nose cones that would ab­late uni­formly so the reen­try ve­hi­cle wouldn’t drift off tar­get,” said David Wright, a weapons ex­pert at the Union of Con­cerned Sci­en­tists. “The mis­sile has to know where it is and where it is go­ing. All the er­rors add up like crazy. My guess is that they would be lucky to land 10 miles from their tar­get­ing point.”

Kim Won-jin AFP/Getty Im­ages

NORTH KORE­ANS mass in Kim Il Sung Square in Py­ongyang in sup­port of leader Kim Jong Un’s stance against the United States. The Korean Peo­ple’s Army, in an es­ca­lat­ing ex­change of threats, has vowed to “turn the U.S. main­land into the the­ater of a nu­clear war.”

Korean Cen­tral News Agency

ALTHOUGH it ap­pears that North Korea’s un­armed Hwa­song-14 could reach the U.S. main­land, a heavy nu­clear war­head would sig­nif­i­cantly re­duce its range.

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