Clean nuclear weapons tests worked

ABOVE: a relatively dirty nuclear fallout situation: the limited reality of fallout gamma doses and dose rates outdoors on an ideal smooth infinite surface, 2 Mt land surface burst with 1 Mt fission yield and a 24 km/hour wind speed. Curves are from Glasstone and Dolan, 1977 which is based on DELFIC, DEfence Land Fallout Interpretative Code; for a discussion of DEFLIC's cloud rise model see Daniel E. Zalewski's report AFIT/GNE/ENP/01M-06, Vincent J. Jodoin's ADA265587, and Karson A. Sandman's report AFIT/GNE/ENP/05-11, and for an analysis of the particle size distribution details, their effect on fallout pattern predictions, and how DELFIC calculates rainout and other weather phenomena effects on fallout, see Eric T. Skaar's report AFIT/GNE/ENP/05-13. For a detailed comparison of DELFIC predictions against measured fallout at six Nevada tests, including subsurface bursts and low air bursts (George, Ess, Zucchini, Priscilla, Smoky and Johnie Boy) see Richard W. Chancellor's report AFIT/GNE/ENP/05-02. You can escape lethal doses the fallout by walking crosswind. You don't need a radiation meter because fallout is visible (see the full discussion in an earlier post, here). For a history of fallout prediction see Jay C. Willis' report ADA079560.

ABOVE: doses and dose rates from a clean bomb (made by simply replacing the U238 pusher around the lithium deuteride secondary unit with one made from lead or tungsten). The total yield here is 2 megatons, but it is 95 % clean (only 100 kt comes from fission). Dose rates and integrated doses are both 10 times smaller than for the 50 % fission standard weapon fallout shown at the top of the page. Notice that even after 18 hours outside in the open at 30 miles directly downwind, you would not get more than 100 roentgens of gamma exposure, which corresponds to a skin dose equivalent of about 0.93 Sievert.

In the previous post we discussed the effects on 64 people without any shielding on Rongelap, who received 175 roentgens over a two day period before evacuation after the Bravo test. The chief problem they had were beta radiation burns due to prolonged skin contamination while they were out of doors in the 'snow-like' fallout. It is easy to deal with this since even a single layer of clothing prevented skin burns (most of the damaging skin dose is delivered by 'soft' or low energy beta particles). Washing or staying under cover also prevent skin burns.

What evidence is there that clean bombs are possible? The 1962 Sedan test of 104 kt total yield only derived 30 kt from fission. The Sedan fallout patterns have been published (DASA-1251-1-EX). Similarly, the Zuni test of 3.53 Mt total yield and 15 % fission and the Navajo test of 4.5 Mt total yield and 5 % fission have been published. Dr Bethe explains the humane case for a clean nuclear weapon to be deployed in another on-line report, while yet another report expands on the political case for clean nuclear deterrence in detail.

America has declassified the 1958 clean nuclear weapon Handbook for United Nation Observers, Pinion Test, Eniwetok, UCRL-5367. After President Eisenhower publically announced the success of clean bomb test in July 1956, a lot of doubt and suspicion was raised in the media. Test Pinion (which became Poplar) was the idea to prove the practicality of clean weapons to the world, inviting United Nations scientific observers to watch a clean test at Eniwetok Atoll, measuring the total yield by filming the fireball expansion rate and applying Sir G. I. Taylor's shock wave law, and measuring the fission yield from doing a radiochemical analysis of the contents of the radioactive fallout.

Unfortunately, Dr Mark M. Mills who came up with the idea was killed in a helicopter accident at Eniwetok on 6 April 1958. On 25 April 1958, the United States at the United Nations did actually offer the U. N. Scientific Committee on the Effects of Atomic Radiation the opportunity to evaluate a clean nuclear test. But with the enthusiastic project leader lost, the energy in the project fizzled out and it was cancelled on 26 July 1958. The weapon to have been tested as Pinion was instead secretly detonated at Bikini Atoll on 12 July 1958 under the codename Poplar (9.3 megatons total yield, only 450 kt fission yield, hence just 4.8 % fission).

In reviewing the fireball/shock wave mathematical model that report gives, it is interesting to see what was known in 1958 above that of Taylor's 1950 report (G. I. Taylor, Proc. Roy. Soc. v. 2001A, p. 159). I have already given a proof on this blog that Taylor's numerican integration and approximate solution can be replaced entirely by an analytical proof which gives an exact solution for the situation of interest: R = {[75E(g - 1)t²]/(8pr_o)}^1/5.

Page 4 of the 1958 report expresses this new analytically proved energy formula E = 8pr_oR⁵/[75(g - 1)t²] as: E = K r_oR⁵/t², where K - which in my proof is simply K = 8p/[75(g - 1)] - is a complicated, numerically computed function from Taylor's long-winded paper, or calculated empirically from nuclear test data: '... K is a dimensionless parameter dependent upon the gamma of the medium inside the fireball. This relation was published by its originator, Sir Geoffrey I. Taylor ... expressed in CGS units [cm, grams, seconds, and ergs for energy] the first determination based on about six shots was K = 1.740. After a few years it appeared that the value K = 1.709 gave a better fit with the radiochemical [yield determination] data. This value continues to be good to this day, when applied to the prescribed portion of the hydrodynamic-growth curve.'

The handbook then on page 5 points out errors on pages 161 and 162 of Taylor's mathematics, particularly his error of expressing the velocity of sound in air using variable gamma instead of ambient gamma. Taylor also falsely assumed that the ambient air density at the New Mexico Trinity test was 1.25 kg per cubic metre, when it was (according to the handbook) 1.006. (The value of air density is not directly measured before an explosion, but is reliably calculated from the measured temperature, pressure and humidity of the ambient air.) The exact value of gamma is dependent not only upon well-known dissociation of molecules of gases as a result of the high shock wave and fireball temperature, but also due to the effects of nuclear radiation on the air in the fireball region.

ABOVE: the empirically determined fireball sizes using nuclear test data.

ABOVE: the previous data plotted to show how the gradual temperature/nuclear radiation induced variation in gamma affects the expansion rate slightly; the assumption of constant K applies to the parts of the curves which are approximately horizontal.

Fusion efficiency depends on compression of the fusion (lithium deuteride) capsule via the recoil of a heavy metal pusher (surrounding the lithium deuteride) by the recoil from ablation caused by x-rays from the fission primary component. The radiating temperature of a fission bomb (the primary stage) is lower for smaller yields, because the yield variation is greater than the corresponding variation in bomb masses, so a smaller fraction of the energy is initially radiated as x-rays in a very low yield weapon. Although it is possible to partially focus a shock wave (hydrodynamic lensing) to make use of the energy that is in the bomb case shock and not in x-rays, that is more difficuly than focussing x-rays.

The simplicity of using x-rays is that they exert little pressure directly. Intense pressures are only generated indirectly when x-rays are absorbed sharply. The inside of the outer casing of a nuclear weapon is lined with a layer of plastic foam, to reduce ablative recoil and delay expansion. But the x-rays absorbed in a thin layer of the dense lead, tungsten, or U238 jacket around the lithium deuteride capsule cause ablation which, by way of 'action-produces-an-equal-and-opposite-reaction' physics of Newton's 3rd law, compresses the capsule. The heavy metal layer is called the 'pusher'. The way to combine x-ray and shock wave compression of the fusion charge is called boosting, and is completely different. A mixture of tritium and deuterium gas is placed inside a hollow in the core of a fission device. (However this is no use for a neutron bomb, because most of the neutrons produced by fusion then get shielded by the surrounding layers of material.)