Troublesome Issues for Nuclear Weapons Owners
Plutonium aging is going to have some serious implications. Since the government has been so worried about this, it is important to try to understand why.
First of all, spallation due to PU aging and corrosion effects will ruin the implosion symmetry. But, the central issue with boosting is to keep the boost gas cool as long as possible. This is very difficult to do inside an exploding nuclear bomb.
Spalled flakes due to PU aging will fly off the inner pit wall at around 5000 m/s. This conceivably could generate entropy in the boost gas which would force you to compress it at a higher adiabat.
Then, as the neutrons start to build up, the Spalding flakes are going to fission, evaporate, and generate all kinds of entropy in the gas. This is potentially a big effect on weapons failure.
A larger boost gas volume will also end up heating up more due to recoil from fission neutrons.
More heat and more volume mean that further compression of the boost gas cavity after the neutrons build-up will be much less effective at increasing both temperature and density.
The greater volume means that it will take more energy in photons to heat it up so the boost doesn’t really go in that event.
If you are not using the most stable of Pu alloys, then the phase transition properties may be important in the hydrodynamics. If this is true, that would help explain why the government has been so worried about this. Anyway, it’s nuts to design bombs so close to the margin of error that this would become an issue….. (redacted)
Both uranium and plutonium are very susceptible to corrosion. A number of the problem-plagued W47UGM-27 Polaris warheads had to be replaced after corrosion of the fissile material was discovered during routine maintenance. The W58 pits also suffered corrosion problems. The W45 pit was prone to corrosion that could alter its geometry.
The Green Grass pit was also corrosion-prone. The radioactivity of the materials used can also cause radiation corrosion in the surrounding materials. Plutonium is highly susceptible to humidity; moist air increases the corrosion rate about 200 times. Hydrogen has a strong catalytic effect on corrosion; its presence can accelerate corrosion rate by 13 orders of magnitude. Hydrogen can be generated from moisture and nearby organic materials (e.g. plastics) by radiolysis. These factors cause issues with the storage of plutonium. The volume increase during oxidation can cause a rupture of storage containers or deformation of pits.
Contamination of the pit with deuterium and tritium, whether accidental or if filled by design, can cause hydride corrosion, which manifests as pitting corrosion and growth of a surface coating of pyrophoric plutonium hydride. It also greatly accelerates the corrosion rates by atmospheric oxygen. Deuterium and tritium also cause hydrogen embrittlement in many materials.
Improper storage can promote corrosion of the pits. The AL-R8 containers used in the Pantex facility for storage of the pits are said to promote instead of hinder corrosion and tend to corrode themselves. The decay heat released by the pits is also a concern; some pits in storage can reach temperatures as high as 150°C, and the storage facilities for larger numbers of pits may require active cooling. Humidity control can also present problems for pit storage.
Beryllium cladding can be corroded by some solvents used for cleaning of the pits. Research has shown that trichloroethylene (TCE) causes beryllium corrosion, while trichloroethane (TCA) does not. Pitting corrosion of beryllium cladding is a significant concern during the prolonged storage of pits in the Pantex facility.
Isotopic composition issues
The presence of plutonium-240 in the pit material causes increased production of heat and neutrons, impairs fission efficiency, and increases the risk of detonation and fizzle. Weapon-grade plutonium, therefore, has plutonium-240 content limited to less than 7%. Supergrade plutonium has less than 4% of the 240 isotopes, and is used in systems where the radioactivity is a concern, e.g. in the US Navyweapons which have to share confined spaces on ships and submarines with the crews.
Plutonium-241, commonly comprising about 0.5% of weapon-grade plutonium, decays to americium-241, which is a powerful gamma radiation emitter. After several years, americium builds up in the plutonium metal, leading to increased gamma activity that poses an occupational hazard for workers. Americium should therefore be separated, usually chemically, from newly produced and reprocessed plutonium.
However, around 1967 the Rocky Flats Plant stopped this separation, blending up to 80% of old americium-containing pits directly to the foundry instead, in order to reduce costs and increase productivity; this led to higher exposure of workers to gamma radiation.
Metallic plutonium, notably in the form of the plutonium-gallium alloy, degrades chiefly by two mechanisms: corrosion, and self-irradiation.
In very dry air, plutonium, despite its high chemical reactivity, forms a passivation layer of plutonium(IV) oxide that slows down the corrosion to about 200 nanometers per year. In moist air, however, this passivation layer is disrupted and the corrosion proceeds at 200 times this rate (0.04 mm/year) at room temperature, and 100,000 times faster (20 mm/year) at 100°C. Plutonium strips oxygen from water absorbs the liberated hydrogen and forms plutonium hydride.
The hydride layer can grow at up to 20 cm/hour, for thinner shells its formation can be considered almost instant. In the presence of water, the plutonium dioxide becomes hyperstoichiometric, up to PuO2.26. Plutonium chips can spontaneously ignite; the mechanism involves the formation of the Pu2O3 layer, which then rapidly oxidizes to PuO2, and the liberated heat is sufficient to bring the small particles with low thermal mass to autoignition temperature (about 500 °C).
The self-irradiation occurs as the plutonium undergoes alpha-decay. The decaying atom of plutonium-239 liberates an alpha particle and a uranium-235 nucleus. The alpha particle has an energy of more than 5 MeV and in the metal, the lattice has a range of about 10 micrometers; then it stops, acquires two electrons from nearby atoms, and becomes a helium atom. The contaminant plutonium-241 beta-decays to americium-241, which then alpha-decays to neptunium-237.
The alpha-particles lose most of their energy to electrons, which manifests as heating the material. The heavier uranium nucleus has about 85 keV energy and about three-quarters of it deposit as a cascade of atomic displacements; the uranium nucleus itself has the range of about 12 nanometers in the lattice.
Each such decay event influences about 20,000 other atoms, 90% of which stay in their lattice site and only are thermally excited, the rest being displaced, resulting in the formation of about 2500 Frenkel pairs and a local thermal spike lasting few picoseconds, during which the newly formed defects recombine or migrate. In a typical weapons-grade bulk material, each atom gets displaced on average once per 10 years.
At cryogenic temperatures, where next to no annealing occurs, the α-phase of plutonium expands (swells) during self-irradiation, the δ-phase contrasts markedly, and the β-phase contracts slightly. The electrical resistance increases, which indicates the increase of defects in the lattice.
All three phases, with sufficient time, converge to an amorphous-like state with density averaging at 18.4 g/cm3. At normal temperature, however, most of the damage is annealed away; above 200K vacancies become mobile and at around 400K the clusters of interstitials and vacancies recombine, healing the damage. Plutonium stored at non-cryogenic temperatures does not show signs of major macroscopic structural changes after more than 40 years.
After 50 years of storage, a typical sample contains 2000 ppm of helium, 3700 ppm americium, 1700 ppm uranium, and 300 ppm neptunium. One kilogram of material contains 200 cm3 of helium, which equals three atmospheres of pressure in the same empty volume. Helium migrates through the lattice similarly to the vacancies and can be trapped in them. The helium-occupied vacancies can coalesce, forming bubbles and causing swelling. Void-swelling is however more likely than bubble-swelling.
Production and inspections
The Radiation Identification System is among a number of methods developed for nuclear weapons inspections. It allows the fingerprinting of nuclear weapons so that their identity and status can be verified. Various physics methods are used, including gamma spectroscopy with high-resolution germanium detectors. The 870.7 keV line in the spectrum, corresponding to the first excited state of oxygen-17, indicates the presence of plutonium(IV) oxide in the sample.
The age of the plutonium can be established by measuring the ratio of plutonium-241 and its decay product, americium-241. However, even passive measurements of gamma spectrums may be a contentious issue in international weapon inspections, as it allows characterization of materials used e.g. the isotopic composition of plutonium, which can be considered a secret.
W78 Nuke Warhead Pit Corrosion Problems Confirmed
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- The W88 is the replacement warhead for our aging inventory. It is essentially a rehash of what we had on the shelf when we stopped making them in the 80s. After a ‘competition’, this design won. The W78 is in service, and its ‘pit’ isn’t a happy camper. The problem is, after 30 years we’ve really forgotten how to do this. People grow old and retire. Processes aren’t always documented… I believe NNSA Deputy Administrator for Defense Programs Don Cook has provided the first open-source confirmation of rumors that the W78 warhead has a pit corrosion problem.
For some time, I have wondered why the Obama Administration wants to replace both the W88 and the W78 with a common warhead, utilizing newly manufactured W88 pits. The statement of work for the W78 LEP isn’t all that helpful.
Stephen Young has previously noted rumors of pit corrosion problems with the W78. The Jasons themselves noted that the 100-year estimate was only for “most” or “predominant” pit types. There were other pit types for which NNSA was undertaking or considering “mitigation paths.”
Cook, in answer to a question by Senator Feinstein about why the Administration wants to produce between 50-80 pits per year appeared to confirm those rumors by describing how the decay of plastics and other materials may corrode the pit and then confirming “we’re seeing those kinds of problems” in some pit types.
MR. COOK: Again, I think this is a very good question. Let me try to give a quick technical answer.
JASON [An elite advisory committee of our scientific overlords -ed.] determined that the lifetime of the plutonium parts in pits are good for 100 years or 80 was their conclusion. Due to plutonium decay which is by alpha — that’s helium that interstitially causes a potential problem. The actual problems that we have to go well beyond that.
We have the plutonium pits in the midst of the chemistry of high explosives with binders that decompose just like plastics in cars exposed to the sun. The plutonium is radioactive. The decay goes on. That degrades all of the plastics, all of the cushions, all of the things that are around the pit. And it also causes corrosion in the pit.
So on the one hand, JASON is absolutely correct about what they said. But the difficulty is that, as weapons get older, much of the chemistry in a radiolytic environment starts to take over. And that has been the problem, and we’ve invested many of the people and time in surveillance to actually pin down in which weapons systems we’re seeing those kinds of problems. And we can predict how long they’re good for.
Those are not good for 100 years.
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Template measurement is an important method in nuclear disarmament. The gamma-ray spectrum of the Plutonium pit shows unique property due to the age, abundance, amounts, and thickness of the Plutonium pit; that is, the same designed pits yield similar gamma-ray spectra while different designs give distinct spectra.
Useful information is extracted from the gamma-ray spectrum generated by the reliable Plutonium pit radiation as a ‘template’. A comparison of the data from inspected objects with the template can give a conclusion whether they are of the same type. Because of the strong self-absorption of Plutonium, some characteristics of a Plutonium pit can only be identified by the gamma spectrum. MCNP simulation was employed to prove that in some cases, a template depending on the gamma-ray spectrum from the reliable Plutonium pit alone can’t effectively distinguish the spurious objects.
And a further approach indicates that enhancing the neutron counting rate of spontaneous fission of Plutonium can improve the problem. Neutron counting rate can be indirectly acquired by spontaneous fissile neutrons bombarding a 10B target. 478 keV γ rays are concomitant with the nuclear reaction 10B(n,α)7Li* from 7Li* nuclei’s deexcitation.
Neutron information is gathered by detecting 478 keV γ photons. Using the HPGe γ detector can both detect the γ-ray spectrum and acquire neutron counting rate. This method efficiently increases the confidence of template measurement and also ensures the dismantling process without revealing sensitive nuclear warhead design information.