一颗核弹的破坏性有多大，能否毁灭北京？

核弹分类：

　　核弹按属性分为原子弹、氢弹、特殊性能核武器（中子弹、冲击波弹等等）。氢弹原理是核聚变（就是太阳），比原子弹的核裂变杀伤力更大。中子弹的冲击波破坏力并不大，但爆炸产生大量电离性辐射，可以在保留建筑的情况下大量杀死有生力量，导致空城、鬼城。

核弹威力：

　　一般情况下，衡量核武器威力的单位是“当量”。只核爆炸中所释放的能力，相当于多少吨TNT炸药爆炸所释放的能量。当下的主流核弹中，万吨级都是相当普遍了，超级弹头甚至可达千万吨甚至亿吨级。核爆炸造成的破坏性，不只取决于核弹当量，还与核弹属性、弹头数量、爆炸高度相关，甚至于风力、风向、晴雨等天气因素相关。

核弹杀伤：

　　早期的核弹都是核炸弹，大多采用了触地引爆。日本广岛和长崎的房屋又多是木质结构、层高较小。所以造成的结果就是把城市抹平了。而现在的城市充满了钢筋混凝土的摩天大楼，高密度高强度的建筑会极大削弱核弹的冲击波威力。但是别高兴的太早了。因为：（a）现代核弹都是装载在导弹甚至洲际导弹上，是拥有强大的制导系统、精准的定时引爆器战术导弹。核导弹通过机动变轨、超低空飞行等方式到达指定城市上空千米处自动引爆，冲击波不是直线击中建筑，威力大大增加。（b）核空爆造成的冲击波不仅仅是狂风。首先是静态超压，高压和真空会急速的拉扯建筑物，造成长达数秒的高强度反复，从而导致建筑承重结构疲劳、松散，而随后的狂风一扫就讲爆炸中心部位抹平了。在冲击波覆盖范围内，基本不可能有人存活。（c）核弹的最主要杀伤不是冲击波，而是辐射云。作为流体的冲击波衰减是很快的，影响范围有限。而核爆后，中心的真空会吸取大量的尘埃和土壤，随着核爆废料一起蒸发，上升成为放射云。而后的放射性沉降将带来致命的高辐射，并持续数月甚至几年。所有放射云覆盖面积内的生物，都会被大量的核辐射摧毁，即便侥幸不死，也不会安然无恙。（d）核弹会摧毁城市的交通、供电、供水系统。大威力核弹空爆导致的高温、高强度热辐射，将点燃整个城市，随之而来的大火会烧掉剩下的东西。持续的大火和长期的高辐射，使躲在地下防御工事人也很难存活。只能祈祷离核爆中心足够远。

核弹实战：

　　在只考虑“当量”情况下，来看几个核弹的威力：1945年8月6日，美国空军B-29超级堡垒轰炸机“艾诺拉·盖”在广岛投掷核武器“小男孩”。城市中心12平方公里内的建筑物全部被毁，全市房屋毁坏率达70％以上。 据日本官方统计，死亡和失踪人数达71379人，受伤人数近10万。“小男孩”的当量是1.3-1.6万吨，如果扔到北京，重灾区基本在二环以内。是下面的效果

（注：以下贴图均来自于这个网站http://nuclearsecrecy.com，<此网站部分内容附后>。可以在google地图上的任意点扔核弹，然后爆炸效果就会展示在地图上面）：

模拟“小男孩”当量的核弹威力覆盖范围

　　中心半径的0.1公里范围的任何物体将直接被火化；红色半径0.7公里的圆内是冲击波范围，存活率基本为0；然后绿色区域半径1.4公里，50%-90%的死亡率，数小时和数周内会有持续的死亡；然后灰色区域半径1.85公里内，大部分建筑物都会倒塌，死伤无数；最大的半径2.14公里的圆内，暴露在外的人会因为核弹的强光辐射导致皮肤三度烧伤，同时引发大量火灾。

　　“B61”战术核弹，目前服役的约有三千枚，34万吨当量，基本可以摧毁四环内所有目标。W59核弹，100万吨当量，可以覆盖几乎所有五环，三环以内基本不会留下任何东西。

模拟“B61”战术核弹的威力覆盖范围

　　至于目前实验过的最大当量核弹“沙皇炸弹”，最初设计是1亿吨当量，堪称炸弹之王。其恐怖的杀伤力将把五环以内夷为平地！威力将覆盖密云、延庆，廊坊也逃不掉。可以说北京将彻底在地球上消失：

模拟“沙皇炸弹”核弹的威力覆盖范围

　　上海？一千万吨当量就可以说拜拜了！通过上面对比可以计算出，沙皇炸弹的当量是小男孩的6.2万倍，然而冲击波半径仅仅增加了不到9倍。但是覆盖面积就增加了81倍。所以即便增加核弹当量，对造成的破坏影响有限，但架不住疯狂的人类堆出重达27吨的变态玩意（威力减半的沙皇炸弹），其破坏力是小男孩的3864倍。

核战争:

　　真正的核战争，沙皇炸弹这种畸形胖子基本没法用，但是还有多弹头巡航导弹。俄国的一艘核潜艇上可以一次发射3-5枚，每个核弹将有5-8个分导式多弹头，每个弹头都有万吨当量，可以覆盖美国所有的主要城市。如果不予拦截，这么一下子就可以让美国回归石器时代。对一个城市来说，如果天气合适，先大风扩散，再小雨沉降，人口密集度高，不考虑地下防御工事，核弹的杀伤力将翻倍，死亡人数无法估计。

不扩散：

　　核弹真的是毁灭性武器。根据《不扩散核武器条约》，至2010年全世界依然有22000枚活跃核弹头，如果丢的位置合适，真的可以把90%以上的人类都杀死了。

　　所以说，必须坚决的反对核武器这种超级大杀器！否则二十一世纪真过不去了。

附：http://nuclearsecrecy.com部分内容

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VISIONS

Operation Crossroads at 70

by Alex Wellerstein, published July 25th, 2016

This summer is the 70th anniversary of Operation Crossroads, the first postwar nuclear test series. Crossroads is so strange and unusual. 1946 in general ought to get more credit as an interesting year, as I’ve written about before. It was a year in flux, where a great number of possible futures seemed possible, before the apparently iron-clad dynamics of the Cold War fell into place. Crossroads happens right in the middle of the year, and arguably made a pretty big contribution to the direction that we ended up going. Such is the subject of my latest article for the New Yorker‘s Elements blog, “America at the Atomic Crossroads.” Today is the anniversary of the Baker shot, which Glenn Seaborg dubbed “the world’s first nuclear disaster.”

There are a lot of things that make Crossroads interesting to me. The bomb was still in the hands of the Manhattan Project. The Atomic Energy Act of 1946 had not yet been signed into law (Truman would sign it in August, and it would go into effect in January 1947), so the Atomic Energy Commission did not yet exist.

There were these amazing interservice rivalry aspects: the whole backdrop is a Navy vs. Army tension. The Manhattan Project, and the Army Air Forces, had gotten all the glory for the bomb. The Navy didn’t want to be left out, or seen as irrelevant. Hence them hosting a big test, and glorying in the fact that a Nagasaki-sized atomic bomb doesn’t completely destroy a full naval squadron. (Which was no surprise to anybody on the scientific or military side of things.)

The US had only about 10 atomic bombs at the time. So they expended about 20% of their entire nuclear arsenal on these tests, for relatively little military knowledge gained. The Los Alamos scientists were pretty lukewarm on the whole operation — it just didn’t seem like it was getting them much. One wonders, if the bomb had not still be under military control, whether it would have happened.

Photograph of the early mushroom cloud of Crossroads Able by LIFE photographer Frank Scherschel, with a darkened filter to compensate for the brightness of the flash. Source.

The first shot, Able, was something of a flub. The fact that it missed its target meant that for public relations purposes it was seen as very ineffective, but it also means that their scientific observations were largely pretty useless. In fact, it missed its target and blew up over one of the main instrumentation ships.

If you read most sources about Crossroads they will say that the source of the Able miss was undetermined, but if you dig down a little deeper you find some pretty plausible solutions (and the reason why the official verdict was “undetermined”). Paul Tibbets, the captain of the Enola Gay and overall head of the atomic delivery group, was pretty clear that it was human error. He said that even before the shot they realized that the crew of the B-29 which dropped it, Dave’s Dream, had gotten bad information about the weather conditions, but that they ignored attempts at correction. Tibbets would re-run (with a dummy bomb) the drop with the correct information (and got very close to the target), and also re-ran it with the wrong information (which missed by nearly the same amount as the Able shot). But the USAAF really didn’t want to throw their bombardier and plane crew under the bus. So they hinted it might be a problem with the ballistics of the weapon (which were indeed a bit tricky), which infuriated the Manhattan Project officials. Anyway, everyone seems to have been satisfied by just saying they couldn’t figure out where the error was. But Tibbets’ account seems most plausible to me.

Crossroads was not secret operation, though there was much classified about it. There were full-spread articles about its purpose in national news publications both before and after its tests. There was probably no test series so publicly conducted by any nuclear power — announced well in advance, covered by the press in real-time, and then heavily publicized afterwards. The fact that the Soviets were invited to a US nuclear test operation (something that would not happen again until the late-1980s) opens up whole other dimensions.

Mikhail Meshcheryakov  in 1946. At right he is on the USS Panamint, at the Crossroads test. Source: Mikhail Grigorivich Meshcheryakov, on the 100th-anniversary of his birth(Dubna, 2010).

The Soviets had three observers at the test: Professor Semyon P. Aleksandrov, a geologist who had worked on the prospecting of uranium; Mikhail G. Meshcheryakov, an experimental physicist; and Captain Abram M. Khokhlov, who attended as a member of the international press corps (he wrote for the Soviet periodical Red Fleet). I found a really amusing little anecdote about the Soviet observers from one of the men who worked the Manhattan Project security detail on Crossroads: Aleksandrov was someone they knew already (he was a “dear old geologist”), but Meshcheryakov was someone “whose name was known, but no one had met personally leading some of us to support he was really an NKVD agent watching Aleksandrov.”

I found nothing in the Russian source materials (mentioned below) that would indicate that Meshcheryakov was NKVD, though he was definitely the one who wrote up the big report on Crossroads that was given to Beria, who summarized it for Stalin. Meshcheryakov’s report is not among the declassified documents released by the Russians, so who knows if it has any political commentary on Aleksandrov in it. Meshcheryakov ended up having a rather long and distinguished physics career in the USSR, though there is almost no English-language discussion of him on the Internet. Aleksandrov, the “dear old geologist,” was actually a major Soviet big-wig in charge of mining operations, which at that time meant he was high in the Gulag system, which was run by the NKVD. For what it’s worth.

Radiation from the Crossroads Baker shot — the radiation went up with the cloud, and then collapsed right back down again with it, resulting in a very limited extent of radiation (the entire chart represents only 4.5 miles on each axis), but very high intensities. Chart source: DNA 1251-2-EX. Collapsed cloud picture source: Library of Congress.

It was also something of the real birth of “atomic kitsch.” There are some examples from before Crossroads, but there is just a real flourishing afterwards. It seems to have taken a year or so after Hiroshima and Nagasaki for enough time to have passed for Americans to start to regard nuclear weapons entirely frivolously. With Crossroads in particular, a deep connection between sex and death (Freud’s favorites) circled around the bomb. This is where we start to see the sorts of activities that would later result in the “Miss Atomic Bomb” contests, the release of the really kitchy songs, and, of course, the Bikini swimsuit, named after the “atomic bomb island,” as LIFE put it.

The key fulcrum of my article is a meditation on the “crossroads” metaphor, and I should probably note that it was, to some degree, intentional. Vice Admiral William Blandy was reported by the New York Times to have told Congress, that the name was chosen for its “possible significance,” which the Times writer interpreted to mean “that seapower, airpower, and perhaps humanity itself — were at the crossroads.”

An unusual color (but not colorized!) photograph of the Crossroads Baker detonation, from LIFE magazine. Source.

What’s interesting to me is that Blandy clearly saw some aspects of the “crossroads,” but there was much he couldn’t have seen — the atomic culture, the arms race, the contamination, the nuclear fears. He knew that “crossroads” was a good name for what they were doing, but it was an even better name than he could have known, for both better and worst.