Plutonium-238 metal as a multiphase system

Previous investigations at the Research Institute of Atomic Reactors of plutonium metal containing more than 80 percent plutonium-238 show a number of physical properties in the metal that are considerably different from those found in ordinary "low-radioactivity" plutonium-239. For example, the hydrostatic densities of plutonium-238 samples are always substantially lower than those of similarly treated samples of plutonium-239 (by 1.1-2.3 grams per cubic centimeter [g/cm3] and more).

When a special thermomechanical treatment is used on plutonium-238 samples, the densities increase only for a short time. The densities decrease quickly after pressure is released and the shapes of the samples become distorted.

Other differences between the two isotopes can be seen in the thermograms of plutonium-238 as the temperature is increased. The intensities of peaks of the alpha to beta, beta to gamma, and gamma to delta (a arrow b, b arrow g, g arrow d) allotropic transformations are always substantially lower than those of the corresponding plutonium-239 material. Plutonium-238 also has a temperature coefficient of electrical resistivity that is lower in the region of the alpha-phase field as well as some other differences in the temperature dependence of electrical resistance.

A combination of the effects observed in plutonium-238 metal can be explained by its continuous and intensive self-irradiation, which results in co-existence of the known plutonium crystal modifications in this metal at room temperature. To understand the nature of these differences, we studied samples of various ages using x-ray diffraction to reveal the crystal structure of the plutonium-238 metal and to discern how specific alpha radioactivity and self-irradiation (aging) affect this structure.

Results

The metal samples each had different plutonium-238 content and were not subjected to compression loads. They were composed of micron-thick layers of plutonium-238 condensed at high temperature and high vacuum onto flat tantalum substrates. Moreover, these samples did not contain radiogenic helium in the initial state.

The table below lists the properties of these samples. Samples 1Ð3 were considered high-radioactivity metals, sample 4 was labeled as medium radioactivity, and sample 5 was low radioactivity.

The samples were prepared, kept, and investigated under similar conditions. All of them had a comparable purity and, according to the spectral analysis results, did not contain certain impurities in the quantities sufficient to stabilize the "temperature" modifications of plutonium. The x-ray analysis was performed within about a one-year aging timeframe of the samples at room temperature.Only the alpha-phase diffraction peaks were observed in the "low-radioactivity" plutonium x-ray patterns (sample 5) during the whole period of the investigation (see table below). Individual diffraction peaks of the "high-temperature" modifications (beta, gamma, delta, and, probably, delta-prime) were recorded in the x-ray patterns of the "medium" specific alpha-radioactivity plutonium (sample 4) along with the strong alpha-plutonium peaks.

A unique phenomenon of the dynamic co-existence of four, five, and even six allotropic modifications of plutonium was observed in samples 1, 2 (see table above), and 3 of high-radioactivity metal.

During the long-term aging of highradioactivity samples 1, 2, and 3 and "mid-radioactivity" sample 4, a slow process of changes in their phase compositions was recorded: a relative decrease in the content of the "high-temperature" low-density modifications and an increase in the content of the high-density alpha-phase of plutonium (see table above).

In samples 1, 2, and 3, a ratio of the total intensity of the (020) and (211) alpha-plutonium diffraction peaks to the total intensity of the (111) delta-plutonium, (111) gamma-plutonium, and (101) delta-prime-plutonium peaks was designated as criterion C of this process. A number of transformation stages can be found on the curves resulting from plotting the value of criterion C versus the age of the sample since fabrication. Figure 1 presents the curve for sample 3 as an example. A similar curve was also obtained for "mid-radioactivity" sample 4.

The peak-by-peak analysis of the x-ray patterns of the high-radioactivity samples shows that the delta phase in them disappears more slowly than the other "high-temperature" plutonium modifications (see table at left). Nevertheless, at a certain stage of aging these modifications can recur, apparently, as intermediate phases in the (delta to alpha) transformation. So, in samples 1 and 2 within 40- to 60-day aging, the maximum number of the allotropic modifications is recorded (five and even six). At the end of long-term aging (about one year), only the alpha and delta phases are observed by x-ray analysis of the high-radioactivity samples, and no obvious signs of the "high-temperature" phases were revealed in the "mid-radioactivity" sample.

Some peculiarities were revealed on the curves showing the atomic volume changes of the delta-plutonium lattice in sample 3 and of the alpha-plutonium lattice in sample 4. A volume increase at the beginning of self-irradiation results in a "hole" (an abrupt decrease and subsequent recovery of this parameter) after some period of aging. The atomic volumes of the indicated lattices reduce slowly in the course of further aging of the samples. Ê

Discussion

In these experiments with thin layers of plutonium-238 metal deposited on a metal substrate, we have considerably increased the radiation-induced self-damage rate of the metal, but we cannot similarly increase the annealing rate of radiation defects, therefore, these experiments do not cover the accelerated aging of bulk plutonium metal. A quantitative change in the radiation-defects concentration leads to observable qualitative changes in the metal properties, but this does not change the nature of plutonium. An increase in the specific alpha radioactivity allows a deeper insight into the fundamental properties of plutonium.

The analysis of the experimental results shows that the co-existence of the different allotropic modifications in high-radioactivity plutonium at room temperature is caused both by intensive self-irradiation and the complex phase diagram of plutonium metal. Radiation-induced vacancies and clusters of vacancies appear to play a large role in the formation of the "high-temperature" modifications.

The bonding 5f-orbitals in plutonium have a small overlap; therefore, a considerable relaxation of the lattice can take place near the vacancy and an area with an increased atomic volume (fluctuation of atomic volume) appears. The 5f-electrons belonging to the atoms located near the vacancy decreases, the electrons localize (they transfer to the fluctuation states), and "fluctuons" (bound states of electrons and fluctuation) appear.

The bonds typical of one of the "high-temperature" modifications arise inevitably between these atoms with localized 5f-electrons. Thus, vacancies in plutonium apparently are nonlocal; they represent an ensemble of atoms with a different bonding pattern forming in the lattice as an entity. The behavior of ÒfluctuonsÓ was investigated theoretically by Russian theorist M.A. Krivoglaz. A comparison of properties of the fluctuons with those of plutonium-238 metal allows the so-called "unusual" behavior of this system to be explained.

An increase in the vacancy concentration due to intensive plutonium self-damage leads to an appearance of the "high-temperature" modifications already at room temperature. The crystal lattice around the large vacancy clusters arising in the deceleration regions of recoils is under tension. It provides relative stability of the "high-temperature" phases.

To explain the slow change in the phase composition of high- and "mid-radioactivity" plutonium during long-term aging as well as the complex multi-stage nature of this process, researchers proposed another mechanism. This was related to accumulation of radiogenic helium and evolution of its forms in the metal, especially in the intragrain cavities. Helium behavior also contributes to the changes in the atomic volumes of plutonium lattices under self-irradiation.

This article was contributed by Sergey I. Gorbunov of the Federal State Unitary Enterprise "State Scientific Centre of the Russian FederationÐResearch Institute of Atomic Reactors" (FSUE "SSC RF RIAR"), Russia.

NMT | LANL | DOE
Phone Book | Search | Help/Info

L O S  A L A M O S  N A T I O N A L   L A B O R A T O R Y
Operated by the University of California for the US Department of Energy