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See also Multimegaton Weapons: the largest nuclear weapons.
High-yield thermonuclear tests: overview
The U.S. and U.S.S.R. have conducted a total of 23 nuclear tests of at least 4 megatons each. The U.S. from 1952 to 1962 detonated 11 such devices above ground plus one underground in 1971. The total yield of these was 105 mt for an average of 9 mt each. The largest of these was a 15 mt test in 1954. In turn, the U.S.S.R. detonated 12 such multi-megaton devices above ground in 1961 and 1962, plus one underground in 1973. Their total yield was 200 mt, for an average of 15 mt each. The largest Soviet test was a 58 mt test in 1961. The total fission yield of all 22 above ground tests was about 124 mt (54 mt from U.S. tests and 70 mt from Soviet tests). In addition, one 4 mt atmospheric test was conducted by the People's Republic of China in 1976.
U.S. nuclear tests with at least 4 megaton yield
|31 Oct 1952|
|1.9 km crater|
|28 Feb 1954|
|2.0 km crater|
|26 Mar 1954|
|25 Apr 1954|
|0.9 km crater|
|04 May 1954|
|10 Jul 1956|
|20 Jul 1956|
|1.2 km crater|
|28 Jun 1958|
|1.75 km crater|
|12 Jul 1958|
|27 Jun 1962|
|DOMINIC I-Bighorn||Christmas Island|
|30 Oct 1962|
|DOMINIC II-Housatonic||Johnston Atoll|
|06 Nov 1971|
|1.1 km crater|
Explanation: GMT=Greenwich Mean Time, lat/long=latitude and longitude, HOB=height of burst above ground/water surface (negative height indicates depth below surface), % fission=percentage of yield from fission, notes includes crater diameter if applicable, "barge" indicates detonation on off-shore barge, "experimental" indicates device not closely related to a warhead design. See text for sources.
The largest U.S. nuclear tests
The U.S. conducted 11 nuclear tests with yields exceeding 4 megatons. These had a total yield of 105 megatons (54 mt fission). All were conducted from 1952 to 1962 in the Pacific Ocean. The last was an underground test, Shot Cannikin of Operation Grommet, described above with the W71 warhead.
The largest U.S. nuclear test was Shot Bravo in Operation Castle. This test was one in a series of tests in the Marshall Islands with the goal of testing prototypes of the first weaponized thermonuclear weapons ("emergency capability" weapons). Bravo was the first shot in the series and was a test of the "Shrimp" TX-21 device, with a predicted yield of 6 megatons. The device had been placed at the end of a causeway extending onto the reef 970 meters from the southwest tip of Namu island in Bikini Atoll. It was detonated at 6:45 AM on 1 March local time (18:45 28 February Universal Time). Designers had significantly underestimated the efficiency of reactions involving lithium-7 in the lithium deuteride solid fuel (one of the design innovations being tested); the actual yield was 15 megatons, 67% from fission.
The higher-than-predicted yield combined with a wind shift shortly after detonation to cause casualties from fallout. On two U.S. ships, 37 U.S. naval personnel--21 on the USS Philip and 16 on the USS Bairoko--received beta burns from fallout particles, producing lesions which all healed without complications. A Japanese fishing boat, the Fukuryu Maru (Lucky Dragon) was just outside the 130-km radius restricted zone and received heavy fallout beginning about 1.5 hours after detonation. The 23 crewmembers did not recognize the falling material as hazardous and made little effort to minimize their exposure to it; some crew members tasted the fallout. Some crew members began developing radiation sickness within three days, and the entire crew developed acute radiation sickness before the boat returned to Japan on 14 March. On return to Japan, the boat's owner recognized the crew was ill and called a hospital, which referred the men for treatment the following day. One member of the Fukuryu Maru crew died of a liver disorder, a complication from radiation sickness, on 23 September 1954.[4, 5]
Significant fallout also fell on inhabited islands west of Bikini under U.S. jurisdiction, exposing native islanders on Rongerik, Rongelap, Ailinginae, and Utirik atolls, all of which were evacuated on 3 March. The highest doses were to the 64 inhabitants of Rongelap Atoll (about 170 km from ground zero), some of whom received 175 rads before their evacuation 44 hours after the detonation. Some reports indicate that Rongelap inhabitants (unaware of the nature of the fallout) did little to minimize exposure, including inadvertent ingestion of fallout, contributing to acute radiation injury. Some evacuated inhabitants of downwind atolls later developed thyroid hypofunction; thyroid exposure for some who were children at time of exposure is estimated at 700-1,400 rad and at 325 rad for some adults. Those who were children at Rongelap show high frequencies of thyroid anomalies, and one 19-year old male died in 1972 of leukemia (age 1 year at time of exposure).[4, 5, 6, 7, 8]
The W71 was the high-yield warhead developed for the Spartan ABM. The W71's yield was too large for underground testing at the Nevada Test Site, so Amchitka Island in the Alaskan Aleutians was selected as a site. To evaluate concerns over this site, a test of 1.2 megatons was conducted at Amchitka on 2 October 1969 (Milrow). Political opposition to the W-71 test (and the Safeguard ABM system in general) included an appeal to the U.S. Supreme Court attempting to block the test on the scheduled day; the Court rejected the appeal 4-3, allowing the test to procede. On 6 November 1971 the Spartan's warhead, the W71, was tested at full yield in shot Cannikin of Operation Grommet. At the bottom of a 1.76 km-deep shaft, the warhead's yield was reported as "approximately" 5 mt or "less than 5 megatons", estimated here as about 4.8 megatons.
Soviet nuclear tests with yield over 4 megatons
Note: all the tests listed below were conducted at the Novaya Zemlya test site.
|06 Oct 1961|
|23 Oct 1961|
|30 Oct 1961|
|31 Oct 1961|
|05 Aug 1962|
|25 Aug 1962|
|27 Aug 1962|
|19 Sep 1962|
|25 Sep 1962|
|27 Sep 1962|
|22 Oct 1962|
|24 Dec 1962|
|27 Oct 1973|
The largest Soviet nuclear tests
In October 1961 there were four Soviet tests of at least 4 mt (although the yield of one is uncertain). All were conducted at the Novaya Zemlya test site. They are listed below with the detonation altitude and yield reported by Russia's MINATOM, and the yield reported by Western sources:
The 30 October test was that of the RDS-220 and was the highest yield nuclear detonation ever conducted. The largest nuclear weapon ever developed by any nuclear power was the Soviet RDS-220, also nicknamed "Big Ivan", "Vanya" or "Tsar Bomba" (King of Bombs). It was a three-stage weapon weighing 24.8 metric tons and was 8 meters long. Its 2-meter diameter required a specially modified version of the Tu-95 Bear bomber for delivery. The single such Tu-95V carried the RDS-220 partially protruding from the bomb bay.[15, 16] The U.S.S.R. tested this design in an airdrop over Novaya Zemlya on 30 October 1961 at a yield of about 58 megatons.[17, 18, 22] However, this was a reduced yield "clean" version: the uranium sleeve on the tertiary stage was replaced with lead, and the fission yield was only 3% of the total yield. The full yield version had a yield variously reported as 100 mt  or 150 mt . About 80% of the fallout from the "Tsar Bomba" test was deposited as global fallout.
Some sources report the burst altitude as 3500 m, while Russian sources give the burst altitude as 4000 m. It was dropped from the Tu-95V bomber from an altitude of 10,500 m by parachute, allowing the bomber to reach a distance of 45 km before detonation. Reportedly buildings were damaged by the blast "hundreds of kilometers" from ground zero.
The yield of the 30 October 1961 test remains the subject of some debate. Best estimate here is that the actual yield was 57-58 mt, based on the following:
The 1962 test series included up to eight tests over 4 mt (the yield of several are uncertain). These high yield tests were all conducted at Novaya Zemlya. MINATOM-reported burst height and yield and Western yield estimates are:
The 19 September and (probably) the 27 September tests were competing designs for a high-yield warhead, probably for ICBM delivery. Andrei Sakharov describes his unsuccessful efforts to stop the test of the competing design on the grounds that it would unnessecarily increase global fallout. The last multimegaton test, that on 24 December, was a reduced yield version of the Chelyabinsk high-yield weapon design (yield of the full-yield version was 50 mt).[12, 25]
Two high yield tests were conducted underground at the southern island of Novaya Zemlya in 1973. At least one probably exceeded 4 mt in yield. The yield of these and other Soviet underground tests were the subject of debate in the West for years, with some sources suggesting that published yield estimates were too high. Based on recent information from Russian sources, it appears if anything that the Western estimates had been too low. MINATOM has reported a total yield of 7.8 mt for the two 1973 tests at Novaya Zemlya. The first test, on 12 September, involved a salvo detonation of one device reported as 1.5 to 10 mt in yield plus two with yields between 0.15 and 1.5 mt. The total yield for this test was about 4 mt. The test on 27 October is reported by MINATOM as between 1.5 and 10 mt in yield. Western estimates have ranged from 2.8 to 4.9 mt; recent reports place the yield at 3.5 mt. If this is correct, the 12 September test yield was about 4.2 mt, of which about 3-3.5 mt was the larger device.[12, 13, 25, 30, 31, 32] Both tests were probably reduced yield versions of warheads for ICBMs nearing deployment.
The largest PRC nuclear test
On 17 November 1976 the P.R.C. conducted its largest nuclear test. A thermonuclear device was dropped from an H-6 bomber over Lop Nur test site and detonated with a yield of 4 mt. This was probably a test version of the warhead for the DF-5 ICBM, possibly at reduced yield.
© 2003-2006, 2008 by Wm. Robert Johnston.
Last modified 2 February 2008.
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