The drama of plutonium

Sixty years ago the Manhattan Project carried out its first test of a secret weapon, forged from a metal first detected in sub-microgram amounts fewer than five years before. By David Fishlock

On Thursday 12 July 1945 a US Army sedan drove Philip Morrison the 210 miles from Los Alamos to Alamagordo with the plutonium core of the world’s first nuclear weapon on his lap. At dawn four days later the priceless hemispheres the physicist had helped forge, then assembled, vanished in the highly successful Trinity nuclear test. The scientists who witnessed the test estimated the energy released equivalent to 18,600t of TNT.

Morrison, like many intimately involved in the debut of this new metal, lived to a ripe old age. He died earlier this year, aged 89. Hans Bethe, who led the physicists who had conceived the new weapon, died in March, aged 98. Glenn Seaborg, the radiochemist who discovered plutonium in 1941 and wrote the rules for working with it, lived to 87. Edward Teller, who used plutonium to trigger a thermonuclear reaction for his H-bomb, died aged 94.

Almost always the toxicity of a substance comes to light when people drop dead, but the radiotoxicity of plutonium was known to Seaborg before he discovered it. He used its alpha emissions to prove that he’d found it. He planned the protection that would safeguard his chemists as they unravelled the idiosyncracies of this complex and infuriating new element; the “ornery element”, as he called it. It was all quite different from the mythology created by nuclear energy’s opponents in the 1970s, which spawned texts with such titles as Poisoned Power and The Deadly Element. So prevalent was this mythology by 1977 that Mr Justice Parker, inspector at the Windscale Inquiry into an expansion of plutonium separation in the UK, listed seven “misunderstandings” in his report. Some prevail to this day.

As the late John Fremlin, professor of radioactivity at Birmingham University, famously advised that public inquiry, plutonium can be sat upon safely by someone wearing only a stout pair of jeans. At Harwell in the 1950s the newly-crowned Queen Elizabeth was handed a lump of plutonium in a plastic bag and invited to feel how warm it was. Morrison had been protected from alpha rays from his hemispheres by nickel plating. The Aldermaston scientists used gold foil.

Plutonium is the most complex and perplexing element in the periodic table, say scientists with the Lawrence Livermore National Laboratory near San Francisco. This is one of the trio of US nuclear weapons research and development centres. At the Los Alamos, Sandia and Lawrence Livermore laboratories the USA has been spending about $100 million a year on characterising plute and the 13 other radioactive elements known as the actinides; mostly on plute. This is the painstaking study of a material’s properties and peculiarities; behaviour in all manner of circumstances. The engineer needs these details to design with confidence. The storekeeper needs them to know how a weapon will change over the decades it may spend in the stockpile. Chemical engineers need them to understand better the separation of plute from fission products. The reactor designer needs the data if he’s to devise ever-safer, more economic systems. Custodians of plutonium’s waste products need the data to plan repositories secure for thousands of years.

Plutonium has a host of peculiarities. It’s very heavy, the heaviest metal used industrially, nearly twice the density of lead. It has six allotropic forms or crystal structures; more than any other element. One is so brittle it shatters like glass. Worse, it has a perplexing tendency to switch from one to another with significantly different properties, as the temperature changes. Finely divided, as swarf or filings, it can catch fire spontaneously. No-one seems to know the colour of the flame, but magenta is a good guess. All this makes it infuriating to work with. Too much in one place can ‘go critical’, a weak but deadly kind of nuclear explosion that releases gamma rays.

PLUTONIUM'S ALLOTROPES

Let’s look more closely at the phase changes. As it warms up it flips through six different crystal structures, each with significantly different properties, before melting at 640°C into an intensely corrosive liquid. Sometimes it expands, sometimes it contracts with the phase change. Such unruly behaviour infuriates the fellow who wants to fashion it into precision engineering parts

At room temperature plute is in its alpha phase, strong but very brittle, more like a ceramic than a metal, with a density of 19.8g/cm³. Warm it to 112°C, and it flips to beta phase, 10% bulkier with a density of only 17.8g/cm³. At 185°C it changes to gamma phase, expanding another 3.5%. At 310°C it becomes delta phase, expanding another 7% to become ductile.

Then at 450°C it changes to a variant of the delta phase, delta prime, and shrinks 0.5%. Slightly hotter, 475°C, it changes again, to the epsilon phase, shrinking more dramatically by 3%.

Each of these six phase changes has its own distinct mechanical and electrical properties. An apparently simple question such as plutonium’s electrical resistance presents the metallurgist with a bafflingly difficult problem.

Fortuitously for weapon designers it was discovered at Los Alamos that small amounts of certain elements such as gallium added to molten plute would retain the ductile delta phase as it solidified and cooled, all the way down to room temperature. For example, the 5% gallium alloy can be rolled into sheet metal and machined by conventional metalworking methods. Gallium is an expensive metal, bluish-white, which softens like butter on a hot summer’s day. The plutonium-gallium alloy needs heat treatment to stabilise the crystal structure. The alloy is denser when molten – like water – so casting defects like bubbles are fewer. Less fortunately, for Los Alamos, this discovery was swiftly passed to the Soviet Union by the spy Klaus Fuchs.

SELF-IRRADIATION

Then there’s plute’s interaction with its own radioactivity, the activity that causes it to feel warm, “like a live rabbit” as Lenona Marshall Libby, Enrico Fermi’s assistant in the Manhattan Project, records in her memoires. Obviously this radioactivity is a complication for any investigator or machinist, who must be carefully shielded and will usually handle the metal in a glove box. The plute itself suffers from these emanations. Plutonium-239 emits 5MeV alpha particles; that is, fast-moving helium nuclei. The gas builds up in interstices in the crystal structure. After 10 years every plutonium atom will have been displaced by helium at least once, although most will eventually return. This nano-scale damage can change the material’s behaviour over long periods. Weapons are being kept in the stockpile for much longer than their designers had intended and accumulate other actinides such as americium, which slowly change the chemistry as well as the radiology of the weapon.

Some of the world’s most powerful scientific tools participate nowadays in the study of plute. The European Synchrotron Radiation Facility near Grenoble, France, is a good example. This accelerator, funded by more than a dozen European nations, generates an exceptionally brilliant beam of X-rays to illuminate crystal structures. US scientists are using the accelerator to study plutonium’s phonons; the crystal lattice vibrations caused by atoms becoming displaced by self-irradiation. How these atoms move around is believed to hold the key to better understanding plute’s bizarre physical and structural properties. Usually, such studies would be made with neutron beams using large single crystals as targets but such samples cannot be made of plutonium. The synchrotron can go to work on much smaller specimens. It’s been adapted to focus a microbeam of radiation on a single grain of polycrystalline alloy of plutonium-gallium alloy.

Another powerful instrument is Jasper, the Joint Actinide Shock Physics Experimental Research facility at the US Department of Energy’s Nevada Test Site near Las Vegas. JASPER first fired in 2003, is a 30m gas gun that shoots small projectiles at over 5km/s to gauge the effect of shock on materials. Shock physics is important in geophysics and planetary science; how planets were formed. When Jasper strikes a plutonium target, a shock wave passes through it in a microsecond, exerting pressure exceeding 600GPa – six million times atmospheric pressure – and raising the temperature thousands of degrees. Its density becomes several times that of the original target. Lawrence Livermore reports that 15 successful shots were fired by JASPER last year. The facility’s chief scientist, Neil Holmes says: “specifically, JASPER’s main goal is to measure plutonium’s equation of state”: the relationship between pressure, density and temperature in the metal under extreme conditions. It’s a crucial requirement in weapon stockpile stewardship, says Holmes.

CRITICALITY

Criticality occurs when there’s too much plutonium in one place. Two of plute’s 18 isotopes, Pu-239 and Pu-241 are fissile – fissioned by slow neutrons. Too much and a stray neutron can trigger a chain reaction spontaneously. There’s a blue flash and lethal amounts of gamma rays and neutrons shoot out; an excursion, the scientists call it. Key factors include the state of the plute – solid, liquid or gaseous, or its concentration in solution; the shape of its container; and the presence of other fissile substances or of neutron absorbers or reflectors. Some of the fancy shapes of equipment designed to store plutonium safely, such as harp-shaped vessels for plutonium nitrate solution, are a dead giveaway for clandestine attempts to engage in plutonium technology. They feature large on the “trigger lists” held by customs.

Criticality occurs only when about 200 grams or more of plute are present in solution, or when about a kilogram is present as metal or alloy. Such technical facts – nuclear constants – underpin the design of all equipment used to purify, fashion and store such products as MOX fuel. For example, no vessel will ever have a capacity greater than 4.8L, or will hold a concentration greater than 7g of plute per litre.

Aldermaston scientists had a nasty scare with their first criticality experiments at Easter 1952. They were melting a 500g billet of plutonium in a cerium sulphide crucible when, to quote Margaret Gowling, official historian of Britain’s nuclear programme, “a ghostly blue flame appeared. The team feared that the criticality calculations were wrong – one member of the team said: “Well, boys, it’s too late to run”. But the flame died and the scare was ascribed to an impurity.

TOXICITY

Plutonium never was “the most toxic substance known to man”, as has so often been asserted by its detractors. It is indisputably very toxic but in a different way from more familiar poisons such as cyanide or botulin. In the worst imaginable circumstances plutonium lodged in the body might cause cancer 20 years later. Cyanide can kill in minutes.

What was perhaps the world’s most exclusive club comprised a handful of Americans who became contaminated in accidents with plutonium in the scramble to make the first plutonium weapons. All were young white males who had been working under laboratory conditions acknowledged to have been “extraordinarily crude” in 1944-5, on one of four chemical processes: purification, fluorination, metal reduction and recovery. The kinds of accident they suffered included chemical burns by plutonium salt solutions. Members were enrolled by medics at Los Alamos because they were judged to have experienced the highest exposures to plutonium of all people engaged in the Manhattan Project. The chosen 26 were excreting the highest levels of plutonium in their urine. In 1952, when the club was formed, each was estimated to be contaminated with between 0.1-1.2µg of plutonium.

Most of the men left Los Alamos soon after the war ended and scattered throughout the USA. Three of them continued to work with plutonium. Four had been involved with three or more accidents with the stuff. The medics traced all 26 in 1952-3 and carried out their first follow-up of medical studies. Thereafter they were given a complete medical examination about every five years. Two decades later, in 1971-2, 22 of them returned to Los Alamos for a more complete study of their plutonium body burden, with two more opting for their own doctors instead of Los Alamos’s. One had died.

By 1979, when George L Voelz and his colleagues published their 32-year medical follow-up of club members, two had died: the first from a heart attack in 1959, aged 36; and another from a road accident in 1975, aged 52. The surviving 24 had suffered no cancers other than two skin cancers “that have no history or basis that relate them to plutonium exposure”, they reported. They found the diseases and physical changes in club members were “characteristic of a male population in their 50s and 60s”. The mortality rate of the club was about 50% of the expected deaths among white American males at that time.

The moral of this story is not, of course, that plutonium is good for you, but that it’s nowhere near as deadly as it’s been cracked up to be. Admittedly, the club members were above-average intelligence – college students or chemical engineering graduates in their early-20s who had been called up for the US Army and drafted to Los Alamos. Many returned to college after the war. Within a few years almost all were in supervisory, administrative or professional positions where they were no longer exposed significantly to any toxic chemicals or radioactive materials. Nine never smoked. Four had reached their sixties, one 69.

Voelz, speaking in 1999 after his retirement, recalled that he’d arrived in Los Alamos in 1952 for a year of in-plant training in industrial medicine and was intrigued with all the concern for protecting and following people exposed to plutonium. “I had never heard of plutonium until I got to Los Alamos”. The club had already been started. Describing the exposures of the 26, Voelz noted: “The work during World War II was done in ordinary wood frame buildings with openfaced chemical hoods”. Some work, such as weighing and centrifuging, was actually done outside the hoods”. Club members expressed no serious fears or concerns about their exposures to plute. “They are interested in hearing the results of our studies and have been fully cooperative through these many years”. He stressed the importance of a close rapport and kept in touch personally with letters and presentations, encouraging them to call if they had any questions – as any good club might do. None ever filed claims for compensation.

Today there are over 1200 plute-contaminated people under constant medical observation, with no detectable effects so far, Eric Voice, a British scientist who worked with plutonium at Harwell and Dounreay, told me in the summer of 2004. In retirement in 1992 Voice participated in several experiments, in one of which plutonium citrate solution was deliberately injected into several volunteers, for biomedical researchers to follow the patterns of plute excretion and movement of plute in blood, tissues, liver and bones. These metabolic experiments used short-lived plutonium isotopes. Twelve years later he’d reached the age of 80 and accumulated no fewer than 15 reports of results and deductions about these experiments published in the professional press. Is getting plutonium inside the body more dangerous than any radioactivity we already have inside us? No, Voice asserted, the radium in the world around us is twenty times more dangerous than the same mass of plutonium. “And there is no evidence that any human on Earth has ever died or suffered any health consequences whatever from plutonium radioactivity”.

Eric Voice died in September 2004 from motor neurone disease. An obituary in the Daily Telegraph recounted how in one experiment “Voice was one of a dozen guinea pigs who inhaled trace amounts of plutonium isotopes of the sort found in nuclear reactors. Measurements were then made tracking the progress of the substances through the body. The study was designed to find out how to treat people in the event of a nuclear accident”. He had lived for another five years after the UKAEA declared in 1999 that all of its guinea pigs were still alive and healthy.
Author Info:

David Fishlock, Traveller’s Joy, Copse Lane, Jordans, Buckinghamshire HP9 2TA, UK

hmmm... why didn't it roll off? I drop a brick on a ball, it just sits there on top?

He was stacking WC blocks, by hand, around a spherical piece of Pu 239 weighing about 6 kg, and measuring the neutron flux as he did so. He wanted to see how the configuration of WC blocks changed the reactivity of the system. This was about nine months before the Slotin incident, on August 21, 1945.

Keep in mind that, at that time, these guys were deep into a heavyweight tech-out in pursuit of the atomic bomb. They had essentially no practical knowledge of what they were doing, especially as related to handling large amounts of Pu, which (if you recall) had never existed... anywhere before it was first manufactured in bulk in 1945 in Hanford WA. The guys at Los Alamos, including Daghlian and Slotin, were getting it "hot out of the reactor," literally (in more ways than one...).

You know how, when you were a kid, and someone told you "be careful of that fan, it can cut your finger off," and you had this almost irresistable urge to put your hand near the blades to see what it would be like? I've always wondered if, maybe, those guys at Los Alamos weren't subject, just a little bit, to that same mentality. I know I would have been. You have all these theoretical guys saying "be very, very careful with this stuff, and whatever you do, don't put more than "X" amount together in any one place at any one time," but no one's ever done it, and you find the question of what would happen if you did put "X" amount together almost irresistable.

Anyway, I doubt very much this is what happened, but I've wondered.

(steely)

No, it's not that easy with plutonium; the design of plutonium device is not trivial. The nuclear characteristics of plutonium make it prone to pre-detonation (the chain reaction gets going before the optimum instant, with a decrease in the device's yield). The Manhattan Project had a great deal of difficulty in solving how to design the plutonium devices, which were used at Alomogordo and Nagasaki.

ohh i think i get the picture. the ball is encased in bricks except for the top. then he lays a brick over the ball, and zap!

Yeah, maybe. Or maybe, he gets the bricks stacked up around the core, and he's got a little "fort" of bricks stacked up (like a kid would make a "fort" out of toy blocks), and the geiger counters are really starting to sing, and he thinks "I'll just lower this piece by hand down through the middle of the 'fort' and see what happens with the radiation count." And then there's a sudden blue flash (just like the glow you see in those pictures of nuclear fuel elements under 25 feet of ultra-pure water in storage at nuclear power plants around the country). And maybe he hears something too... an alarm klaxon suddenly goes off, or the reaction itself makes a sound. And he jumps in alarm, and drops the brick (which is damn heavy, WC is really dense). The blue glow gets brighter, and he reaches in and grabs that sucker and pulls it the hell out of there.

Then he called his boss. He was worried, but not really scared.

(steely)

A bomb could conceiveably be made by surrounding the separated masses with a conventional explosive. Using them for an atomic explosion rather than a thermonuclear one. no?....

Yes, you have described a plutonium implosion device. Although I have never designed such a device (I am not a nuclear engineer, nor do I play one on TV), I believe that one would also need a neutron source to initiate the chain reaction, a neutron reflector to keep it going, and a perhaps some kind of containment increase the explosive yield.

I am told that the high-explosive shaped charges are not easy to design or manufacture. Proper timing of the chemical implosion and the neutron initiation can be very difficult to achieve. My guess is that it wold be much easier to create a fairly large bomb that would fit in a tractor-trailer rig than a device that would fit in the trunk of a car. I doubt that a terrorist group could make a small plutonium explosive on its own.

More accurate to say the trinity "blasting cap" was carried on his lap. The actual "device" was highly enriched uranium that weighed in at about 10,000 pounds (as big a load as a B-29 could carry).

No, the trinity device was Plutonium. The core was two nickle-coated hemispheres of Pu. At the center of the Pu core was a tiny assembly containing Polonium and some other materials to start the reaction; this was the "blasting cap," to use your analogy.

Surrounding the Pu core was roughly 10,000 lb of high explosives in a sphere about five feet in diameter.

This device was so complex and experimental that they had to test it before use; that is what happened at Trinity. It's first military use was at Nagasaki, Japan, on August 9, 1945.

The Hiroshima bomb used U 235, which, because it is not subject to pre-detonation, could be configured in a supercritical assembly in milliseconds, instead of microseconds (as required by Pu). Thus, it was determined that they could test the U 235 device in actual use over a target city.

(steely)

Was pre-detonation the reason that plutonium was deemed unsuitable for gun-type weapons?

A bomb could conceiveably be made by surrounding the separated masses with a conventional explosive. Using them for an atomic explosion rather than a thermonuclear one. no?....

I think you have the terms a bit confused. Weapons that contain only fissionable material, uranium or plutonium are generally referred to as "atomic bombs". Weapons that obtain most of their power through the fusing of atoms rather than the splitting of atoms are the ones classified as thermonuclear weapons or hydrogen bombs.

Secondly, simply strapping some TNT or C-4 to a piece of plutonium will not cause it to go nuclear. First you need an initiator (the Trinity device used gold foil wrapped Beryllium) which is surrounded by a sphere of plutonium. The plutonium is then encased in a set of high explosive, shaped charges which must detonate in an exact sequence (all within a few milliseconds of each other) in order for the plutonium to be imploded correctly. If it works correctly then you will have nuclear yield. If it doesn't you simply have a loud boom. (This is why the emergency way to dispose of a nuclear weapon is to either burn it or blow it up. It will cause the high explosives to go off incorrectly and not produce nuclear yield.)

As a side note, all thermonuclear weapons contain a standard atomic bomb. It is used as the initiator for the thermonuclear detonation.

so, it would be easier for a low budget nuker to make a uranium bomb?

Yes, and far simpler in terms of design. (This was one reason the scientists on the Manhattan Project didn't feel the need to test the design. The other was they didn't have enough uranium for more than one bomb.) However, one needs highly enriched uranium (HEU) to make a nuclear weapon and that is very hard to come by. The uranium used in nuclear power plants cannot be used as fuel in a nuclear weapon. It does not contain enough Uranium-235.

so, it would be easier for a low budget nuker to make a uranium bomb?

Not really. The thing about U 235 is that it has to be separated from its far-more-plentiful cousin isotope U 238, which which makes up more than 99% of naturally occuring U. This separation cannot be accomplished by means of chemistry, because the two isotopes are the same chemical. Instead, the isotope separation has to be done atomically, essentially by weighing mass quantities of the two isotopes and using the difference in their weights (which is very, very small) to sort them into separate bins. There are several ways to do this (in the Manhattan project they tried three or four, I'm not sure which) but all require a lot of space and a lot of equipment and a lot of energy and a hell of a lot of money.

The thing about Pu is that it can be manufactured by irradiating naturally occuring U with neutrons in the core of a nuclear reactor fueled with much-less-enriched U 235. Then, once the Pu has been formed by nuclear chemistry, it can be separated from the U by conventional chemistry, because it is chemically different from U. This makes the production of Pu a lot less costly on a "per bomb" basis than is the production of U 235.

Of course, there's a bit more to it than that. I think the only way a "low budget nuker" can get hold of a bomb is to steal one or buy one from a corrupt government source.

(steely)

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