'Clean' nuclear weapon tests: Navajo and Zuni
Time magazine, Monday, Jul. 08, 1957
THE PRESIDENCY: The Clean Bomb
"Three of the nation's leading atomic scientists were ushered into the White House one morning last week by Atomic Energy Commission Chairman Lewis Strauss for a 45-minute conference with the President. The scientists: Edward Teller, credited with the theoretical discovery that led to a successful H-bomb, Ernest O. Lawrence, Nobel Prizewinning director of the University of California's radiation laboratory at Livermore, Calif., and Mark M. Mills, physicist and head of the lab's theoretical division. They brought a report of grave but potentially hopeful meaning. In the lab at Livermore, they told the President, scientists have found how to make H-bombs that will be 96% freer from radioactive fallout than the first models."
Above: clean-nuclear weapons physicist Dr Mark M. Mills testifying during his testimony before the Congressional Joint Atomic Energy Committee hearings on The Nature of Radioactive Fallout and Its Effects on Man, 1957 (the testimony is linked here in PDF format). Dr Mills was tragically killed in a helicopter accident during torrential rain on April 6, 1958, during the test preparations at Eniwetok Atoll, so the successful secret 1956 very "clean" test of the 4.5 Mt, 95% fusion, 5% fission Redwing-Navajo test was never publically demonstrated in the scheduled repeat Pinon test in 1958 (for comprehensive technical details of the Pinon, see Dr Gerald Johnson's 1958 Handbook for United Nations Observers, Pinon Test, Eniwetok, report UCRL-5367, PDF file linked here). After Dr Mills died, the public proof-testing of the clean bomb scheduled as Hardtack-Pinon was cancelled for weak reasons:
“[Lithium-7 deuteride and lithium-6 deuteride fusion fuel] costs can be estimated from the market price for lithium – with a lithium-6 content of 7.5 % - and with the advertised prices for heavy water [containing deuterium]. The latter sells for $28 per pound. ... the separation cost for lithium-6 ... should not be excessive since the isotopes Li-6 and Li-7 differ in mass by as much as 15 % and are therefore relatively easy to separate. My estimate for Li-6 D is $400 per pound. ... Making conservative assumptions about the fission yield in the [dirty U-238 fission] jacket, one concludes that a ton of TNT equivalent can be produced in the jacket for a fraction of one cent. ... It would undoubtedly be more expensive to construct a bomb without [a U-238 fission jacket]. And it would certainly be a much more difficult technical undertaking, since the success of Stage II [fusion of lithium deuteride] strongly depends upon the presence of the jacket. The neutron linkage and the cyclic nature of the multi-stage bomb make for a marriage between fission and fusion.”
- Dr Ralph E. Lapp, “The Humanitarian H-Bomb”, Bulletin of the Atomic Scientists, September 1956, p. 264.
Lapp's cynical complaint of the high relative cost of lithium-6 to U-238 in thermonuclear weapons ignores the fact that on average 50% of the yield of ordinary "dirty" stockpiled thermonuclear weapons comes from fusion anyway! By replacing the U-238 with lead, you're making no difference to the cost of the weapon: you're simply reducing the total yield and increasing the percentage due to fusion by a factor of 10 or more! In addition, Lapp ignores the fact that you simply don't need lithium-6 deuteride in a thermonuclear bomb: you can use natural lithium cheaply instead! In 1954, the highly efficient 15 Mt Castle-Bravo test used lithium only enriched to 40% lithium-6, while the successful 11 Mt Castle-Romeo test used only natural lithium deuteride (7.5% lithium-6 and 92.5% lithium-7), with no lithium enrichment. Sure, the heat released in the fission of the U-238 pusher by fusion neutrons acts as a catalyst to boost the fusion stage efficiency, and you lose that boost when you remove the U-238 jacket. But the successful tests of clean weapons prove that this is not an insuperable objection. Dr Samuel Glasstone, author of the secret nuclear weapon design physics report WASH-1037/8, 1962, 1963, 1972a and 1972b, wrote in his 1985 Funk & Wagnalls Encyclopedia (incorporated into Microsoft's Encarta 1998 multimedia encyclopedia) article on Nuclear Weapons:
"(E) Clean H Bombs
"On the average, about 50 percent of the power of an H-bomb results from thermonuclear-fusion reactions and the other 50 percent from fission that occurs in the A-bomb trigger and in the uranium jacket. A clean H-bomb is defined as one in which a significantly smaller proportion than 50 percent of the energy arises from fission. Because fusion does not produce any radioactive products directly, the fallout from a clean weapon is less than that from a normal or average H-bomb of the same total power. If an H-bomb were made with no uranium jacket but with a fission trigger, it would be relatively clean. Perhaps as little as 5 percent of the total explosive force might result from fission; the weapon would thus be 95 percent clean. The enhanced-radiation fusion bomb, also called the neutron bomb, which has been tested by the United States and other nuclear powers [1 kt, 500 metres altitude air burst] is considered a tactical weapon because it can do serious damage on the battlefield, penetrating tanks and other armored vehicles and causing death or serious injury to exposed individuals, without producing the radioactive fallout that endangers people or structures miles away."
Above: This newspaper article, "Clean H-Bomb Test Junked as U.S. Fears Mammoth Propaganda Dud", in The Deseret News, July 30, 1958, highlights the controversy in 1958 that fusion neutrons escaping into the atmosphere turn some nitrogen atoms into radioactive carbon-14, just as nuclear radiation from the sun does. But neutron-induced activities are a trivial hazard compared to the fission products from a "dirty" (high fission yield) surface burst. (See the declassified report USNRDL-TR-215, linked here, which contains the accurately measurements of the very small ratios of atoms/fission for neutron-induced Co-60 and other radionuclides in the 95% clean 1956 Navajo nuclear test fallout.) Another claim was that communist countries declined to attend the clean proof-test. But American could have still gone ahead and published the facts about clean nuclear weapons tests.
Above: neutron induced activities in atoms per fission depend upon bomb construction, particularly fission yield fraction ("cleanliness"). This table of data is based on the same source as Harold A. Knapp's 1960 table of accumulated doses from neutron induced activities in fallout (shown below), and is taken from the 1965 U.S. Naval Radiological Defense Laboratory report USNRDL-TR-1009 by Drs. Glenn R. Crocker and T. Turner. (This report is available as a 10 MB PDF download here. For Crocker's report on the fission product decay chains, see this link.)
Above: neutron induced activity gamma doses are smaller than fission product gamma doses, so "clean" nuclear weapons - despite releasing neutrons and creating some neutron induced activity - do eliminate much of the fallout problem.
Above: fallout from 95% 'clean' bomb test Navajo, Bikini Atoll, 1956 (WT-1317 ). Surface burst in the lagoon on a barge, the yield was 4.5 Mt, and it was only 5% fission. Each square in this and the next map has side of 20 minutes of latitude/longitude (= 20 nautical miles or 37 km). The radiation levels are relatively low, 20 times smaller than a fission weapon of similar yield.
Above is the best fallout pattern (from U.S. weapon test report WT-1317 by Drs. Terry Triffet and Philip D. LaRiviere) from the Zuni shot of 3.53 megatons, 15% fission at Bikini Atoll in 1956. It combines all available data, unlike the data in report DASA-1251, which gives unjoined data for the lagoon and the ocean. The ocean data was obtained in three ways, since fallout sinks in water. First, ships lowered probes into the water and measured the rate the fallout sank with time. Second, ships took samples of water from various depths for analysis. Third, the low level of radiation over the ocean was measured by both ships and aircraft, correcting for altitude and shielding of the geiger counter.
This particular test is unusual, as it was a surface burst on land (coral island), and was extensively studied; they even fired rockets through the different parts of the cloud at 7 and 15 minutes after burst, containing miniature radiation meters and radio transmitters, to map out the radioactivity distribution (it worked, showing toroidal distribution!). Ships were located in the fallout area at various locations to determine the fallout arrival time, build up rate (which was slow, due to the huge mushroom cloud which took time to pass overhead and diffused lengthways), decay rate after fallout arrival, mass of fallout and visibility of fallout deposit, and the chemical abundances of the various nuclides in fallout at different locations. Near the burst, large fallout particles arrive which fallout of the fireball before gaseous nuclides in decay chains have decayed into solids and condensed, so the biggest fallout particles, near ground zero, have relatively little I-131, Cs-137, and Sr-90. Gaseous precursors like xenon and krypton prevent Cs and Sr decay chains from condensing early, while iodine is volatile itself. Smaller fallout particles, while posing an overall smaller radiation hazard, have relatively more of these internal hazards (I-131 concentrates in the thyroid gland if ingested, say by drinking milk, while Cs-137 goes into muscle and Sr-90 goes into bone, assuming it is in a soluble form, which is of course not the case if the ground burst is on silicate-based soil, because the radioactivity is then trapped inside glass spheroids).
Here is a report of Dr. Hans A. Bethe, working group chairman, originally 'Top Secret - Restricted Data', to the President's Science Advisory Committee, dated 28 March 1958, defending 'clean nuclear weapons tests', courtesy of Uncle Sam:
Pages 8-9 defend clean nuclear weapons! As stated, Zuni was only 15% fission, so it was 85% clean. The dose rates given on these fallout patterns are extrapolated back to 1 hour, before the fallout had completely arrived anywhere, so are far higher than ever occurred anywhere! The true dose rates are lower due to decay during wind-carried transit. The dose rates also refer to the equivalent levels on land, which are about 535 times higher than over ocean at 2 days after burst, because the fallout landing on the ocean sinks steadily, and the water shields most of the radiation. The average decay rate of the fallout was t^-1.2 for all weapons tests. It is amazing how much secrecy there was during the cold war over thecivil defence data in WT-1317. The point is, fallout is not as bad as some people think, just like blast and cratering.
Co-60 bomb research
Wikipedia insert: Extensive residual radioactivity experiments and civil defence fallout studies were made during these tests. The Antler-1 test contained normal cobalt-59 which upon neutron capture was transmuted into radioactive cobalt-60 . This provided a way to measure the neutron flux inside the weapon, although it was also of interest from the point of view of radiological warfare.
The then Science Editor of the New York Times, William L. Laurence, wrote in his 1959 book Men and Atoms (Simon & Schuster, New York, p. 195):
‘Because the cobalt bomb could be exploded from an unmanned barge in the middle of the ocean it could be made of any weight desired ... Professor Szilard has estimated that 400 one-ton deuterium-cobalt bombs would release enough radioactivity to extinguish all life on earth.’
The total amount of gamma ray energy emitted from cobalt-60 is only 2.82 MeV and this meagre energy release is spread over a statistical mean time of 1.44 times the 5.3 years half life of cobalt-60. (The number 1.44 is given approximately by 1 over the natural logarithm, i.e., the log to the base e, of 2, since this is the conversion factor between half-life and mean life time for radioactivity.) For comparison, every neutron used to fission an atom of U235, Pu239, or U238 releases 200 MeV of energy, including 30 MeV of residual radioactivity.
Hence, fission is by far the most efficient way to create radioactive contamination. The dose rate from Co-60 in the Antler-1 fallout was insignificant until most of the fission products had decayed, and only a few large pellets of Co-60 were found afterwards. The overall contribution of Co-60 to the fallout radiation was trivial compared to fission products and shorter-lived neutron induced activities in the bomb materials.
A study was done into the penetration of the fallout gamma radiation from the Antler tests by British Home Office and Atomic Weapons Research Establishment scientists A. M. Western and H. H. Collin in Maralinga. The results in their AWRE paper Operation Antler: the attenuation of residual radiation by structures, were published in Fission Fragments No. 10, June 1967, and showed the the long-term integrated fallout gamma radiation doses were reduced by a factor of 5 for a mass shielding of 312.4 kg per square metre, which is equivalent to a thickness of 15 cm of earth. A mass shielding of 781.1 kg per square metre stopped 96.6 % of the gamma rays, and this is equivalent to a protection factor of more than 29 by a shield of 38 cm of earth. America also performed studies which showed how fallout problems can be avoided.
NEUTRON CAPTURE-INDUCED NUCLIDES IN FALLOUT
Dr Terry Triffet and Philip D. LaRiviere, Operation Redwing, Project 2.63, Characterization of Fallout, U.S. Naval Radiological Defense Laboratory, 1961, Secret – Restricted Data, weapon test report WT-1317, Table B.22: some 21 radioactive isotopes of 19 different radioactive decay chains from neutron induced activity are reported for megaton range tests Navajo (lead pusher, 5 % fission), Zuni (lead pusher, 15 % fission) and Tewa (U-238 pusher, 87 % fission) are reported. Summing all the 19 separate decay chains abundances (of radioactive capture atoms per fission) gives results of:
15.6 atoms/fission for Navajo (5 % fission),
7.03 atoms/fission for Zuni (15 % fission), and
1.25 atoms/fission for Tewa (87 % fission).
(These data are computed from a full table which includes some nuclides not listed in WT-1317. I'll give that complete listing later. At present data tables do not seem to format properly on this blog site.)
But even for a very 'clean' bomb like Navajo, fission products dominate the fallout radiation dose. The sodium isotope Na-24 (15 hours half life) is generally considered the most important environmental form of neutron induced activity, and the abundance of Na-24 was only 0.0314 atom/fission in Navajo, 0.0109 atom/fission in Zuni, and 0.00284 atom/fission in Tewa. (These tests all involved large quantities of sea water being irradiated, Navajo and Tewa were water surface bursts and Zuni was on a small island surrounded by ocean.)
Far more important were U/Np-239, -240 and U-237 (which is created by a reaction whereby one neutron capture in U-238 results in two neutrons being emitted). The capture atoms/fission for Navajo, Zuni and Tewa were respectively 0.04, 0.31 and 0.36 for U/Np-239, 0.09, 0.005, and 0.09 for U/Np-240, and 0.09, 0.20 and 0.20 for U-237. (See also USNRDL-466.) These can emit as much radiation as fission products at the intensely critical early times of 20 hours to 2 weeks after detonation. They emit very low energy gamma rays, so the average energy of fallout gamma rays for a bomb containing a lot of U238 is low during the sheltering period, 0.2-0.6 MeV, and this allows efficient shielding to be done far more easily than implied by most civil defence calculations (which are based on gamma radiation from fission products with mean gamma energy of 0.7-1 MeV).
This was first pointed out based on British nuclear test fallout data (for Operation Totem and other tests) by George R. Stanbury in a Restricted U.K. Home Office Scientific Advisory Branch report in 1959, The contribution of U239 and Np239 to the radiation from fallout (although this paper originally contained a few calculation errors, the point that the average fallout gamma ray energy is lower than for fission products stands). You get much better shielding in a building that American calculations show, due to their incorrect use of 0.7-1 MeV mean gamma ray energy. The mean gamma ray energy at 8 days after Castle tests was only 0.34 MeV (WT-934 page 56, and WT-915 page 145; see also WT-917 pages 114-116, and also see of course WT-1317).
When tritium fuses with deuterium to produce helium-4 plus a neutron, the neutron’s mass is 20% of the total product mass, so the complete fusion of a 1 kg mixture of deuterium and tritium yields 0.2 kg of free neutrons, which – if all could be captured by cobalt-59 – would create 12 kg of Co-60. This was Professor Szilard’s basis for a ‘doomsday’ device.
However, Dr Gordon M. Dunning (b. 1910) of the U.S. Atomic Energy Commission, who was responsible for radiological safety during 1950s American tests, published calculations for such ‘cobalt-60 bombs’ (Health Physics, Vol. 4, 1960, p. 52). These show that a 100 megaton bomb with a thick cobalt-59 case, burst at a latitude of 45 degrees North, would produce an average Co-60 infinite-time gamma radiation exposure outdoors of 17 Roentgens in the band between 30 and 60 degrees North, around the earth. This ignores weathering of fallout, and assumes a uniform deposition.
The maximum rate at which this exposure would be received (outdoors), is 0.00025 Roentgens per hour, only 12 times greater than background radiation. Choosing a longer half-life reduces the intensity by increasing the time lapse between each particle emission; so the longer the half-life, the lower the intensity. If it is decaying rapidly, you can shelter while it decays. If the half-life is long, you can decontaminate the area before receiving a significant dose. No problem!
Creating Co-60 inside a weapon uses up precious neutrons, without releasing any prompt energy to help the nuclear fusion process, unlike U-238 fission, which releases both prompt energy and neutrons. Every neutron captured by Co-59 to produce radioactive Co-60 will lead to the release of only 2.82 MeV of radiation energy (one beta decay and two gamma rays). However, every neutron induced fission of uranium-238 releases about 200 MeV of energy, including more residual radiation energy than that released from Co-60. Therefore, fission gives a greater hazard than that from Co-6o and other neutron capture activities.
All of the escaping neutrons in an underwater or underground burst are captured in the water or soil, but only about 50% are captured by the water or soil in a surface burst. The amounts of neutron induced activity from the environment generally have a small effect, the highest activity being due to Na-24. In bombs containing U-238, the major neutron capture nuclides are Np-239 and U-237, which give off low energy gamma rays for the first few days and weeks. Shielding this radiation is easy.
The use of tungsten (W) carbide ‘pushers’ for clean nuclear weapons led to the discovery of W-185 (74 days half-life) in fallout from the 330 kt Yellowwood water surface burst at Eniwetok, 26 May 1958. It emits very low energy (0.055 MeV) gamma rays. Yellowwood produced 0.32 atoms of W-185 per fission, based on the ratio of W-185 to Zr-95 (assuming 0.048 atoms of Zr-95 per fission) in the crater sludge at 10 days after burst. (Frank G. Lowman, et al., U.S. Atomic Energy Commission report UWFL-57, 1959, p. 21.) W-185 was discovered on plankton and plant leaves, but was not taken up by the sea or land food chains. In fallout from the 104 kt, 30% fission Sedan shot at Nevada on 6 July 1962, W-187 (24 hours half-life) gave 55% of the gamma dose rate at 24 hours after burst, compared with 2% from Na-24 due to neutron capture in soil.
The ocean food-chain concentrates the neutron-capture nuclides iron (Fe) and zinc (Zn) to the extent that Fe-55 and Zn-65 constituted the only significant radioactivity dangers in clams, fish and birds which ate the fish after nuclear tests at Bikini and Eniwetok Atolls, during the 1950s. However, these nuclides are not concentrated in land vegetation, where the fission products cesium (which is similar to potassium) and strontium (which is similar to calcium) are of major importance. This is caused by the difference between the chemical composition of sea water and land. (Where necessary chemical elements are abundant, uptake of the chemically similar radioactive nuclide is greatly reduced by dilution.)
Fish caught at Eniwetok Atoll, a month after the 1.69 Mt Nectar shot in 1954, had undetectably low levels of fission products, but high levels of Fe-55 (95% of activity), Zn-65 (3.1%), and cobalt isotopes. In terns (sea birds) at Bikini Atoll, Zn-65 contributed almost all of the radioactivity after both the 1954 and 1956 tests. Fe-55 gave off 73.5% of the radioactivity of a clam kidney collected in 1956 at Eniwetok, 74 days after the 1.85 Mt Apache shot; cobalt-57, -58, and –60 contributed 9.6, 9.2, and 1.8%, while all of the fission products only contributed 3.5%.
Fish collected at Bikini Atoll two months after the 1956 Redwing series which included Zuni, Navajo and Tewa, had undetectably low levels of fission products, but Zn-65 contributed 35-58% of the activity, Fe-55 contributed 15-56%, and cobalt gave the remainder. (Frank G. Lowman, et al., U.S. Atomic Energy Commission report UWFL-51, 1957.)
In 1958, W.J. Heiman of the U.S. Naval Radiological Defense Laboratory released data on the sodium-24 activity induced in sea water after an underwater nuclear explosion in which 50 % of the gamma radiation at 4 days after burst is due to Np-239. He found that Na-24 contributed a maximum of 7.11 % of the gamma radiation, at about 24 hours after burst (Journal of Colloid Science, Vol. 13, 1958, pp. 329-36).
Hence even in a water burst, Np-239 radiation is far more important than Na-24.
Perhaps the most important modification in the April 1962 edition of The Effects of Nuclear Weapons was the disclosure that the radioactive fallout from nuclear weapons contains substantial amounts of radioactive nuclides from neutron capture in U-238. This had been pointed out by scientist George Stanbury (who worked with data from nuclear tests, and had attended British nuclear tests to study the effects) of the British Home Office Scientific Advisory Branch in report A12/SA/RM 75, The Contribution of U239 and Np239 to the Radiation from Fallout, November 1959, Confidential (declassified only in June 1988). Both Mr Stanbury and The Effects of Nuclear Weapons 1962 found 40% of the gamma radiation dose rate from fallout is the typical peak contribution due to Neptunium-239 and other capture nuclides (e.g., U-237, which is formed by an important reaction whereby 1 neutron capture in U-238 is followed by 2 neutrons being released), which all emit very low energy gamma radiation, and are important between a few hours and a few weeks after burst, i.e., in the critical period for fallout sheltering.
Because of the low energy of the gamma rays from such neutron-capture elements, which are present in large quantities in both Trinity-type fission bombs (with U-238 tampers) and thermonuclear bombs like Mike and Bravo, the fallout is much easier to protect against than pure fission products (average gamma energy 0.7 MeV). However, The Effects of Nuclear Weapons, while admitting that up to 40% of the gamma radiation is from such nuclides, did not point out the effect on the gamma energy and radiation shielding issue, unlike Stanbury’s Confidential civil defence report. This discovery greatly stimulated the “Protect and Survive” civil defence advice given out in Britain for many years, although it was kept secret because the exact abundances of these bomb nuclides in fallout were dependent on the precise bomb designs, which were Top Secret for decades.
NEUTRON CAPTURE-INDUCED NUCLIDES IN FALLOUT
Scroll down for the table. There is an error with this blog system changing html tables by prefixing them with large unwanted empty spaces. I'll fix this issue when I have time.
Nuclides formed by neutron capture in the thermonuclear bomb, 189 metric tons steel barge (NAVAJO AND TEWA TESTS), and the surrounding sea water
Measured Bikini Atoll test data for thermonuclear weapon designs of various fission yields, and two types of fusion charge ‘pusher’*
Exposure rate at 1 hour after detonation, (R/hr)/(kt/mi2) per capture atom/fission. 3 ft height, ideal theory, Triffet 61.
4.50 Mt, 5% fission
Lead (Pb) pusher
Bomb mass = 6.80 metric tons
3.53 Mt, 15% fission
Lead (Pb) pusher
Bomb mass = 5.51 metric tons
5.01 Mt, 87% fission
Bomb mass = 7.14 metric tons
Abundance of neutron induced nuclides in total fallout, atoms per fission:
0 (no gamma rays)
Total amount of neutron induced activity (capture atoms per fission):
* Compiled from the data in: Dr Terry Triffet and Philip D. LaRiviere, Operation Redwing, Project 2.63, Characterization of Fallout, U.S. Naval Radiological Defense Laboratory, 1961, originally Secret – Restricted Data (now unclassified), weapon test report WT-1317, Table B.22 and Dr Carl F. Miller, U.S. Naval Radiological Defense Laboratory report USNRDL-466, 1961, Table 11 on page 41, originally Secret – Restricted Data (now unclassified). The ‘pusher’ absorbs initial x-ray energy and implodes, compressing the fusion charge. Data for Fe-55 is based on the ratios of Fe-55 to Fe-59 reported by Frank G. Lowman, et al., U.S. Atomic Energy Commission report UWFL-51 (1957), and H.G. Hicks, Lawrence Livermore National Laboratory report UCRL-53505 (1984), assuming that the neutron capture ratios in iron were similar for shots Apache and Tewa. Data for Zn-65 is based on the ratios of Zn-65 to Mn-54 reported by F.D. Jennings, Operation Redwing, Project 2.62a, Fallout Studies by Oceanographic Methods, report WT-1316, Secret – Restricted Data, 1961, pages 115 and 120.
** The Zuni device contained antimony (Sb), which boils at 1750 C and was fractionated in the fallout. This is the only fractionated neutron capture nuclide. The data shown are for unfractionated cloud samples: for the close-in fallout at Bikini Lagoon the abundances for Sb-122 and Sb-124 are 8.7 times smaller.
***Note that this is not the maximum exposure rate from Np-239 (at 1 hour after detonation it is still increasing because it is the decay product of U-239).
“The first objection to battlefield ER weapons is that they potentially lower the nuclear threshold because of their tactical utility. In the kind of potential strategic use suggested where these warheads would be held back as an ultimate countervalue weapon only to be employed when exchange had degenerated to the general level, this argument loses its force: the threshold would long since have been crossed before use of ER weapons is even contemplated. In the strategic context, it is rather possible to argue that such weapons raise the threshold by reinforcing the awful human consequences of nuclear exchange: the hostages recognize they are still (or once again) prisoners and, thus, certain victims.”
- Dr Donald M. Snow (Associate Professor of Political Science and Director of International Studies, University of Alabama), “Strategic Implications of Enhanced Radiation Weapons”, Air University Review, July-August 1979 issue (online version linked here).
“You published an article ‘Armour defuses the neutron bomb’ by John Harris and Andre Gsponer (13 March, p 44). To support their contention that the neutron bomb is of no military value against tanks, the authors make a number of statements about the effects of nuclear weapons. Most of these statements are false ... Do the authors not realise that at 280 metres the thermal fluence is about 20 calories per square centimetre – a level which would leave a good proportion of infantrymen, dressed for NBC conditions, fit to fight on? ... Perhaps they are unaware of the fact that a tank exposed to a nuclear burst with 30 times the blast output of their weapon, and at a range about 30 per cent greater than their 280 metres, was only moderately damaged, and was usable straight afterwards. ... we find that Harris and Gsponer’s conclusion that the ‘special effectiveness of the neutron bomb against tanks is illusory’ does not even stand up to this rather cursory scrutiny. They appear to be ignorant of the nature and effects of the blast and heat outputs of nuclear weapons, and unaware of the constraints under which the tank designer must operate.”
- C. S. Grace, Royal Military College of Science, Shrivenham, Wiltshire, New Scientist, 12 June 1986, p. 62.