238Pu Content Research Articles

Plutonium Denaturing by 238Pu

This paper analyzes whether reactor plutonium after denaturing by increasing its isotopic content of 238Pu to 6 to 8% can be regarded as proliferation resistant. In this case the utilization of such denatured reactor plutonium would become unsuitable for a nuclear explosive device (NED) because the high-explosive lenses surrounding the plutonium would melt or their elevated temperature would lead to self-ignition. Eight different plutonium isotopic mixtures with increasing 238Pu content are analyzed, and their critical masses if surrounded by a 5-cm-thick reflector of natural uranium are determined. This allows calculation of the alpha-particle heat power generated in the plutonium sphere by 238Pu and other plutonium isotopes. Then, three levels of technology with regard to the size of such hypothetical NEDs (HNEDs) and the technological level of high explosives are defined. On the basis of material data available in the open scientific literature, the radial temperature profiles in such HNEDs of an assumed configuration are calculated, and it is found that for low-technology HNEDs the limiting temperatures are exceeded for a 238Pu content of 1.6%. For high-technology HNEDs these limiting temperatures are exceeded for a 238Pu content above ˜6% or somewhat more. Such denatured plutonium can be considered as proliferation resistant, similarly as uranium with <20% 235U or <12% 233U.

Self-Irradiation of Monazite Ceramics: Contrasting Behavior of PuPO4 and (La,Pu)PO4 Doped with Pu-238

AbstractTo investigate the behavior of monazite during accelerated radiation damage, which simulates effects of long term storage, 238Pu-doped polycrystalline samples of (La,Pu)PO4 and PuPO4 were synthesized for the first time ever and studied using powder X-ray diffraction (XRD) analysis and optical microscopy. The starting precursor materials were obtained by precipitation of La and (or) Pu from their aqueous nitrate solutions followed by calcination in air at 700°C for 1 hour, cold pressing, and sintering in air at 1200-1250°C for 2 hours. The 238Pu contents in ceramic samples measured using gamma spectrometry were (in wt.% el.): 8.1 for (La,Pu)PO4 and 7.2 for PuPO4. The (La,Pu)PO4 monazite remained crystalline at ambient temperature up to a cumulative dose of 1.19 × 1025 alpha decays/m3. In contrast, the PuPO4 monazite became nearly completely amorphous at a relatively low dose of 4.2 × 1024 alpha decays/m3. Swelling and crack formation due to the alpha decay damage was observed in the PuPO4 ceramic. Also, under self-irradiation this sample completely changed color from initial deep blue to black. The (La,Pu)PO4 monazite was characterized by a similar change in color from initial light blue to gray, however, no swelling or crack formation have so far been observed. The results of this study allow us to conclude that the radiation damage behavior of monazite strictly depends on the chemical composition. The justification of monazite-based ceramics as actinide waste forms requires additional investigation.

Behavior of 238Pu-Doped Cubic Zirconia under Self-Irradiation

ABSTRACTTo investigate the resistance of cubic zirconia to accelerated radiation damage, which simulates effects of long term storage, 238Pu-doped polycrystalline samples of cubic zirconia, (Zr,Gd,Pu)O2, were obtained and studied using X-ray diffraction analysis (XRD), electron probe microanalysis (EPMA), optical and scanning electron microscopy (SEM), and modified MCC-1 static leach test. The ceramic material was characterized by the following chemical composition (from EPMA in wt.% element): Zr = 50.2, Gd = 15.4, Pu = 12.2. This corresponds to the estimated formula, Zr0.79Gd0.14Pu0.07O1.99. The content of 238Pu estimated was approximately 9.9 wt.%. The XRD measurements were carried out after the following cumulative doses (in alpha decay/m3 × 1023): 3, 27, 62, 110, 134, 188, 234, and 277. Even after extremely high self-irradiation, cubic zirconia retained its crystalline structure. All XRD analyses showed no phases other than a cubic fluorite-type structure. The following results of normalized Pu mass loss (NL, in g/m2, without correction for ceramic porosity) were obtained from static leach tests (in deionized water at 90°C for 28 days) for 4 cumulative doses (in alpha decay/m3 × 1023):The results obtained confirm the high resistance of cubic zirconia to self-irradiation. This allows us to consider zirconia-based ceramic as the universal material that is suitable for actinide transmutation and geological disposal.

Plutonium in Lake Sediments of the Taimyr Peninsula

A radiochemical study is made of five cores of lake sediments collected from the Taimyr Peninsula. For the first time quantitative data are reported on the content and vertical distribution of 239,240Pu and 238Pu in lake sediments of this region. The plutonium content was found to range from 0.89 Bq/kg in the surface layers to 0.02 Bq/kg in the lower horizons (5-10 cm) of the sediment cores studied. The observed variations in the Pu vertical distribution are interpreted in terms of bioturbation and specific features of water catchment for the lakes studied. The 239,240Pu and 238Pu contents and the 238Pu/239,240Pu activity ratios obtained in this work (0.02-0.10) correspond to the global level for the North hemisphere, although the region studied is not far from the principal nuclear weapons test site of the former Soviet Union in the Novaya Zemlya Archipelago.

Optimising the removal of inhaled plutonium and americium from the rat by administration of ZnDTPA in drinking water.

1. The efficacy of ZnDTPA administered in drinking water has been investigated for removing 238Pu and 241Am from the rat after their simultaneous inhalation as nitrates. 2. The continual administration of ZnDTPA 95 mumol kg-1 d-1 over a 21 d interval commencing 1 h after exposure reduced the 238Pu content of the lungs and total body to 2% and 8% of those in untreated animals; the corresponding values for 241Am were 3% and 5%. 3. The continual intakes of 950 mumol kg-1 d-1, intermittent intakes of 3600 mumol kg-1 d-1 and the repeated injection of 30 mumol kg-1 body weight were considered no more effective. 4. All orally administered concentrations of ZnDTPA, commencing 7 d after exposure, reduced the total body contents of 238Pu and 241Am to 17% and 20% of controls by 28 d. 5. Histopathological examination of the kidneys, liver and gastrointestinal tract showed no apparent effects of these treatment protocols. 6. It is concluded that the oral administration of ZnDTPA could be an effective treatment for the removal of inhaled transportable forms of Pu and Am after human exposure.

Radiometric investigations of Kara Sea sediments and preliminary radiological assessment related to dumping of radioactive wastes in the Arctic Seas

There has been world-wide concern over the possible health and environmental impacts of the dumping of radioactive wastes in shallow waters of the Kara and Barents Seas. During 1992, IAEA-MEL participated in a joint Russian-Norwegian expedition to the region and collected a suite of sediment cores for characterization of downcore radioactivity distribution patterns and inventories with the aim of assessing past and present inputs of radionuclides. The 137Cs, 238Pu, 239,240Pu, and 241Am contents of the 0–1 cm sections range from 17 to 32, 0·01 to 0·06, 0·4 to 1·3 and 0·2 to 0·5 Bq kg−1 (dry wt), respectively. The 238Pu/239,240Pu and 241Am/239,240Pu activity ratios are consistent with those reported for global fallout. Detailed information on downcore radionuclide distribution patterns and inventories, as well as an account of sedimentation parameters, are presented.A preliminary assessment of the long-term radiological impact of radioactive waste disosal in the Arctic Seas, on global and regional scales, has been made. The modelling results suggest that the global radiological impact of the disposals will be comparable to or less than those resulting from other anthropogenic and natural sources of radioactivity. However, radiological effects on the regional scale may be of importance.IAEA is carrying out a 4 year International Arctic Seas Assessment Project (IASAP) that will comprehensively cover all aspects of radioactive dumping in the Arctic Seas and it will be reported to the London Convention in 1996. The IAEA-MEL study described here represents a preliminary contribution to IASAP.

Mass loading of soil particles on plant surfaces.

Radionuclide-bearing soil particles on plant surfaces can be ingested and contribute to human dose, but evaluating the potential dose is limited by the relatively few data available on the masses of soil particles present on plant surfaces. This report summarizes mass loading data (i.e., mass of soil per unit of vegetation) for crops in the southeastern United States and compares these data to 1) those from other regions and 2) the mass loadings used in radionuclide transfer models to predict soil contamination of plant surfaces. Mass loadings were estimated using the 238Pu content of crops as an indicator of soil on plant surfaces. Crops were grown in two soils: a sandy clay loam soil and a loamy sand soil. Concentrations of soil on southeastern crops (i.e., mg soil g-1 plant) differed by more than a factor of 100 due to differences in crop growth form and biomass. Mean concentrations ranged from 1.7 mg g-1 for corn to 260 mg g-1 for lettuce. Differences in mass loadings between soils were less than those among crops. Concentrations differed by less than a factor of two between the two soil types. Because of 1) the differences among crops and 2) the limited data available from other systems, it is difficult to draw conclusions regarding regional or climatic variation in mass loadings. There is, however, little evidence to suggest large differences among regions. The mass loadings used to predict soil contamination in current radionuclide transfer models appear to be less than those observed for most crops.

Contaminant Transport in Agroecosystems through Retention of Soil Particles on Plant Surfaces

AbstractThe contamination of plant surfaces with soil particles is a potentially important process in the transport of insoluble contaminants such as radionuclides, heavy metals, and hydrophobic organics in agroecosystems, but few data are available to assess the significance of this mechanism for different crop species. The mass of soil particles retained on the surfaces of corn (Zea mays L.) and sunflower (Helianthus annuus L.) grown under field conditions were measured using the 238Pu content of the plants to indicate retention of soil. The crops demonstrated similar quantities and height distributions of soil retained on leaf and stem surfaces. Mean retention was 0.86 g soil retained on corn vegetation per square meter of land surface and 0.79 g m−2 retained on sunflower. Most of the soil was on the lower 1 m of the vegetation. The height distributions of retained soil can explain the larger concentrations of soil observed in the mechanically harvested grains of short stature crops such as wheat (Triticum aestivum L.) (120 mg soil per kg grain) and soybean [Glycine max (L.) Merr.] (82 mg kg−1) than that observed in taller crops such as corn (2 mg kg−1). The significance of soil retention in determining the accumulation of contaminants in grains is evaluated for several important agricultural crops.

Preparation of a 238Pu standard source : I. Isotope dilution alpha-ray spectrometry used for standardization

A 238Pu source was prepared by electrodeposition method. For the standardization of the source, isotope dilution alpha-ray spectrometry (IDAS) was investigated: The content of 238Pu in a sample solution was determined both by IDAS and by isotope dilution mass spectrometry (IDMS) using a standard reference material of 239Pu as a spike. The estimated uncertainty in the determination of 238Pu is 0.13% for IDAS, and the factor of IDAS/IDMS is calculated to be 1.0000±0.0015.

Partitioning of 238Pu, 239Pu and 241Am in skeleton and liver of U.S. Transuranium Registry autopsy cases.

The content of 238Pu, 239Pu and 241Am in the liver and skeleton was estimated from radiochemical analysis of human liver and bone samples obtained at autopsy from former actinide workers whose occupational histories were suggestive of chronic inhalation exposures, with minor skin contamination and wounds documented in a few individuals. For times estimated to be several years to a few decades post intake, 75.8 +/- 15.3% of the total 241Am in the skeleton and liver was found in the skeleton (25 cases) as compared with 63.4 +/- 24.1% for 238Pu (36 cases) and 53.2 +/- 18.2% for 239Pu (43 cases). These differences are significant at the 95% confidence level. Of these cases, 34 included data on both 238Pu and 239Pu and were divided into high and low activity subgroups. The difference in the fractionation of the two Pu isotopes was apparent only in the low activity subgroup, suggesting that the difference observed between the Pu isotopes may be an artifact of the data. The different partitioning of these three nuclides suggests that the ALIs for 238Pu and 241Am may be high by about 25-50% if only the dose to bone is considered and may be high by 12-13%, based on the weighted committed dose equivalent in target organs or tissues.

Analysis of uranium and plutonium materials by radiometric methods

The radiometric methods used in the IAEA Safeguards Analytical Laboratory for the analysis of uranium-and plutonium-containing materials sampled in nuclear facilities under IAEA Safeguards are presented. Uranium-235 concentrations are determined in solutions by intensity measurements of the 185.7-keV γ rays with a scintillation detector. The factors affecting the accuracy of results, e.g. dead time and self-absorption corrections, background subtraction, and calibration procedure are discussed. When all these factors are properly taken into account the standard deviation of measurement approaches the value resulting from counting statistics. The 238Pu content in freshly separated plutonium solutions is measured by α-particle spectrometry in combination with mass spectrometry. The alpha sources are prepared by evaporation of analyte solution with tetraethyleneglycol on glazed porcelain discs and subsequent ignition at 850°C. The alpha-spectrometric measurement is performed using the spectrometer with a surface-barrier detector. The alpha-activity ratio of 238Pu to 239 Pu+ 240 Pu is calculated taking into account the tailing correction for 238Pu under the 239 Pu+ 240 Pu peaks. The two methods of tailing correction are described: (i) exponential fitting, and (ii) geometric progression. The atomic ratio of 238Pu/ 239Pu peaks is derived from the alpha activity, and finally, the 238Pu abundance is calculated using the values for 239Pu and 240Pu contents from mass-spectrometric measurement. Plutonium isotopic composition, elemental concentration of Pu, 241Am, 237Np and 235U/Pu ratio are assayed using a γ-ray spectrometer with a high resolution planar germanium detector. A set of computer programs is used for spectral evaluation and calculation of respective concentrations. The results obtained on a number of samples using this method are compared with those of other methods.

Retention of 238Pu-bearing particles by corn plants.

To measure the retention of 238Pu-bearing particulates on plant surfaces, corn plants were exposed to 238Pu deposition in the vicinity of the H-Area nuclear fuel chemical separations facility on the U.S. Department of Energy's Savannah River Plant. Following exposure, plants were removed to an area of lower 238Pu deposition rate and sampled every seven days for 28 days to determine 238Pu retention. To investigate the effects of exposure to wind and precipitation on 238Pu retention, some plants were maintained out of doors while others were maintained in a greenhouse. Plants maintained in the greenhouse showed no statistically significant reduction in 238Pu contents. Plants maintained outside showed 238Pu loss rates consistent with the 14-day retention half-times typically used in radiological assessments. The data analysis of outside plants was complicated by the occurrence of samples with nondetectable 238Pu contents.

Fuel Cycle Utilizing Plutonium-238 as a“Heat Spike” for Proliferation Resistance

The use of a “heat spike” in plutonium product is examined as a possible technical approach to improving the proliferation resistance of the light water reactor (LWR) fuel cycle. The heat spike is achieved by increasing the 238Pu content in reactor-generated plutonium above the usual levels. Because of the high heat generation rate of 238Pu, elevated material temperatures would result when significant concentrations of 238Pu are present. The high temperatures encountered during the fabrication, assembly, and storage of a nuclear device are expected to complicate weapon production. Although the concept would not render the reactor-grade plutonium useless for weapons purposes, it is expected to reduce the attractiveness of such material for this purpose. An important feature of the heat spike concept, as compared to other spiking concepts, is that the spikant (i.e., 238Pu) cannot be removed by chemical techniques. Among the subjects considered are: reasons why a heat spike may contribute to proliferation resistancemeans to enhance the production of 238Pu in reactor-generated plutoniumimpact of potential future reactor design modifications on the heat spike conceptimplementation timecertain fuel cycle economic penalties resulting from the conceptreprocessing considerations.Results show that the buildup of 238Pu in LWR-generated plutonium can be greatly enhanced by the recovery from spent fuel and recycle of 237Np and 236U (in recycle uranium). The penalty in resource utilization introduced by the concept is small, in comparison to that of the “once-through” fuel cycle. The penalty in separative work requirements varies significantly depending on the method selected for recycling spent uranium. Potential future reactor design modifications incorporating higher fuel burn-ups are shown to be advantageous to the concept. Reprocessing flowsheets have been developed to achieve recovery and purification of neptunium from LWR fuels and the subsequent conversion to the oxide. These operations appear to be technically feasible.

Dosage du237Np en trances dans l'uranium et le plutonium

The237Np content of238Pu or239Pu samples were determined by the gammaspectrometry of238Np formed by thermal neutron activation. The measurements were carried out on irradiated238Pu samples directly, and after the chemical separation of239Pu samples. The237Np content of natural uranium was determined from the ratio of the alpha-activities of238Pu and239Pu isotopes formed from the decay of neptunium isotopes produced by the activation of237Np and238U isotopes, respectively.