Mixture Of Fission Products Research Articles

Effect of lanthanide fission product concentrations on the mechanical properties of UO2: A first principle based study

The fission products generated from UO2 during the nuclear fission process alters the mechanical properties of the nuclear fuel. The effect of fission atom substitution in UO2 is a requisite for the safe operation of nuclear reactors. The lanthanide atoms contribute significantly to the mixture of fission products generated. Therefore, in this work, we investigate the effect of lanthanum (La), promethium (Pm), and dysprosium (Dy) doping on mechanical properties of UO2. Further, the relation between doping concentration and different mechanical properties are derived by varying the La, Pm and Dy substitution in UO2. All the calculations are performed with density functional theory with Hubbard U corrections in the DFT + U formalism. Mechanical properties such as bulk, Young’s and shear modulus of UO2 change with the amount of dopant atom in the fuel. The results obtained from our calculations show good agreement with available experimental and theoretical findings. Thus, the present study validates the relationship between fission atom substitution and mechanical properties of UO2, which will help to understand the fuel performance at reactor conditions.

Evaporation of materials from the molten salt reactor fuel under elevated temperatures

The release of fission product and salt compounds from a molten salt reactor fuel under accident conditions was investigated with coupled computer simulations. The thermodynamic modeling of the salt and fission product mixture was performed in The Gibbs Energy Minimization Software GEMS and the obtained compound vapor pressures were exchanged with the severe accident code MELCOR, where the evaporation from a salt surface located at the bottom of a confinement building was simulated. The fuel salt considered in the simulations was LiF-ThF4-UF4 with fission products Cs and I. The composition of the fuel salt material was obtained from an equilibrium fuel cycle simulation of the salt using the EQL0D routine coupled to the Serpent 2 code. The results were compared to simulations using pure compound vapor pressures in the evaporation simulations. It was observed that by modeling the salt mixing the release of fission products and salt materials was reduced when compared to the pure compound simulations. The mixing effects in the salt, when compared to the pure compound simulation also affected evaporation temperatures and therefore the timing of the release of compounds. In an additional simulation in which the depressurization of the confinement was considered, the total evaporated mass of compounds increased due to increased mass transfer at the salt surface. The simulation process described in this paper can be used for a more comprehensive accident analysis of molten salt reactors once the detailed description of the reactor confinement and accident sequences are available and more fission product elements have been added to the analysis.

Comparison of the radial effects of burnup on fast reactor MOX fuel microstructure and solid fission products

This work presents a comparison between the microstructural evolution of three annular fast-reactor mixed-oxide (MOX) fuel pellets irradiated to varying burnups at the Fast Flux Test Facility (FFTF). Fuel pellets irradiated to 3.4%, 13.7%, and 21.3% fissions per initial metal atom (FIMA) were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. The cross-section of the low burnup pellet displayed minor structural changes, but the central annulus of the pellets at 13.7% and 21.3% FIMA shrank from their starting size. The high burnup fuel pellet featured streaking and porosity migration associated with columnar grain growth. The radial fission product distribution in each of the pellets had a higher number density of metallic particles >5 μm in diameter near the fuel centerline. Solid fission products in the fuel-cladding gap were observed in the low and intermediate burnup pellets. The low burnup sample showed minor accumulation of Ba in the gap, while the volatile Cs was primarily observed at the pellet surface. The intermediate burnup pellet displayed a porous mixture of fission products, consistent with the joint-oxide gain (JOG) that has been previously observed in fast-reactor MOX fuel pellets.

Ac, La, and Ce radioimpurities in 225Ac produced in 40–200 MeV proton irradiations of thorium

Abstract Accelerator production of 225Ac addresses the global supply deficiency currently inhibiting clinical trials from establishing 225Ac's therapeutic utility, provided that the accelerator product is of sufficient radionuclidic purity for patient use. Two proton activation experiments utilizing the stacked foil technique between 40 and 200 MeV were employed to study the likely co-formation of radionuclides expected to be especially challenging to separate from 225Ac. Foils were assayed by nondestructive γ-spectroscopy and by α-spectroscopy of chemically processed target material. Nuclear formation cross sections for the radionuclides 226Ac and 227Ac as well as lower lanthanide radioisotopes 139Ce, 141Ce, 143Ce, and 140La whose elemental ionic radii closely match that of actinium were measured and are reported. The predictions of the latest MCNP6 event generators are compared with measured data, as they permit estimation of the formation rates of other radionuclides whose decay emissions are not clearly discerned in the complex spectra collected from 232Th(p,x) fission product mixtures.

Long-term performances of the 95ZR/95Nb chronometer for nuclear events dating

The present work reports on a long-term analysis of the performances of the 95Zr/95Nb chronometer for dating a nuclear event. Taking benefit of a recent Profiency Test Exercise, a sample containing a standardized mixture of fission products has been measured repeatedly with a low background HPGe spectrometer during a period extending up to one year with the aim of assessing the accuracy of the various zero-time determinations. Evaluation of the uncertainties associated to these evaluations was performed using a Monte Carlo approach. Input parameters sensitivity has been investigated, especially the influence of the 95mNb decay branch. The 95Zr/95Nb chronometer was found to be accurate for zero-time determination within one day and one week for a decay of 3 months and 10 months respectively. Sub-day uncertainties are achievable for a two months old sample whereas sub-week uncertainties are reached after a decay of six months. Limitations of the technique for dating a real event are investigated.

A highly efficient supported liquid membrane system for selective strontium separation leading to radioactive waste remediation

The present paper deals with the selective separation of radio-strontium from acidic feeds using a highly efficient novel supported liquid membrane method suggesting possible applications in radioactive waste remediation. This method is a first ever report on near quantitative (∼99%) Sr transport with crown ether carrier from acidic feeds (∼3 M HNO 3) using a supported liquid membrane method. The supported liquid membrane consists of a solution of di- tert-butyl-dicyclohexano-18-crown-6 (DTBCH18C6) dissolved in a mixture of 2-nitrophenyl ether and n-dodecane absorbed into a microporous polytetrafluoroethylene film. A mixture of 80% 2-nitrophenyl ether and 20% n-dodecane was found to be the most suitable diluent system and was evaluated in the present studies. The transport rates were correlated with the carrier extractant concentration, strontium concentration as well as the feed acid concentration. Transport of Sr from Pressurized Heavy Water Reactor Simulated High Level Waste (SHLW) was also studied with encouraging results. Selective transport of Sr(II) was observed from a feed solution constituting a mixture of fission products spiked individually. Radiation stability of the carrier solvent with respect to the efficiency and selectivity was also investigated. The stability of the membrane was remarkably good when tested over 20 days of continuous operation.

NEW HUMAN DATA VERSUS ESTIMATES OF EFFECTS OF INHALING FISSION PRODUCT MIXTURES

Recently, data on exposures of humans as well as animals to fission products in plumes emitted by underground Soviet tests have been declassified by the Khazakhstan government and published in English. Similar human intakes of gross fission product mixtures that caused acute prodromal symptoms have not been previously reported. Animal experiments with such complex mixtures have not received sufficient support to provide data that could be reliably extrapolated with dose-response models to humans for use in triage of internally exposed persons. This commentary compares some of the acute prodromal effects on humans from the recently released Soviet data with the estimates of Cowan and Kuper, and later estimates by Brodsky and colleagues. The latter estimates are concluded to be safer, and more easily adaptable, for use in triage of persons exposed to internal deposition of fission products of various mixtures.

Quantum-Chemical Calculations of the Equilibrium Constants of a Fission Product Mixture in a Steam-Hydrogen Carrier Gas in Severe Accident Conditions

Behavior of iodine fission product is of prime importance for short-term radiological consequences in a severe accident occurring on a pressurized water nuclear reactor. Iodine speciation in the reactor coolant system is commonly predicted with severe accident simulation software devoted to the transport and deposition of fission products and structural materials, for instance, the SOPHAEROS module of ASTEC. In these calculation tools, chemical equilibrium is assumed to be reached instantaneously whatever the conditions are. However, some thermodynamic data are still uncertain because of lack of experimental data. Quantum-chemical calculations can be appropriate tools to estimate equilibrium constants in a first step and maybe later to determine some kinetic constants for further implementation in such codes to better assess iodine chemical behavior. This paper is an attempt to calculate some equilibrium reactions for relevant reactions that are susceptible to impact iodine chemistry. The accuracy obtained for such calculations depends on the basis set used. Moreover, relativistic effect has to be taken into account for heavy atoms like iodine or cesium to get reliable predictions.

Basic trends in practical use of ion-and electron-exchange fibres and materials made from them

The optimum conditions for use of synthesized redox and ion-exchange fibres of different types in biosynthesis of the antibiotic heptaene, selective extraction of gold and silver from hydrometallurgical plant solutions, and sorption of radioactive iodine from a mixture of fission products were established. The basic physicochemical indexes of ion-exchange fibrous-porous materials were comprehensively evaluated in comparison to nonwoven needle-punch fabric made of VION KN-1 fibre. It was shown that fibrous-porous material (FPM) is more stable in a large number of liquid treatment cycles and is better wet with aqueous solutions of acids and bases when the sorption activity indexes closely correspond, and this facilitates regeneration and increases their efficacy. The use of a mixed fibre-resin charge in a small filter module for local treatment of waste washing waters from electroplating plants to remove heavy metal ions increased output from 250 to 500 liters/h.

Preparation of calibrated reference sources containing "fresh" fission products

NPL was recently asked to provide reference sources containing g-ray emitting fission product radionuclides, with certification completed within 50 days of the irradiation. Furthermore, the reference sources were to contain fission products, but no fissile material. The samples in the first reference source were to contain a mixture of fission products with only the 235U removed, leaving the fission products as 'undisturbed' as possible. A number of radionuclides were reported in this sample and included: 91Y, 95Zr, 95mNb, 95gNb, 99Mo, 99mTc, 103Ru, 106Ru, 127Sb, 129mTe, 129gTe, 131I,132Te, 132I, 137Cs, 140Ba, 140La, 141Ce, 144Ce, and147Nd. Of these nuclides, NPL provided certified values for 91Y, 95Zr, 95mNb, 95gNb, 99Mo, 99mTc, 103Ru, 106Ru, 132Te, 132I, 137Cs, 140Ba, 140La, 141Ce, 144Ce, and 147Nd. The second reference source focused on a smaller subset of radionulides: 95Zr, 99Mo, 103Ru, 106Ru, 140Ba, and 155Eu, although 95gNb, 99mTc, 132I and 140La were present as daughter radionuclides and 91Y, 127Sb, 129mTe, 129gTe, 131I, 132Te, 137Cs, 152Eu and 154Eu were present as impurities. This paper describes the preparation of these sources, based on a combination of cation- and anion-exchange chromatography with selective precipitation. The separation techniques were used to (1) produce the 'fresh' fission product mixture from irradiated 235U, and (2) isolate the specific set of radionuclides in the second exercise from mixtures containing irradiated 235U and fission products. To enable accurate assay of parent-daughter systems, integrated ingrowth and decay equations were derived from original Bateman equations and tested by observing the 140Ba-140La system over time.

Reaction Kinetics of a Fission-Product Mixture in a Steam-Hydrogen Carrier Gas in the Phebus Primary Circuit

Radioiodine entering the containment from the postaccident primary circuit in vapor or gaseous form, as observed in the Phebus FPT0 and FPT1 tests, has a direct impact on the source term evaluation. State-of-the-art fission-product transport codes based on the assumption of thermochemical equilibrium failed to predict this phenomenon. In this work the standard approach of assuming the instantaneous establishment of thermochemical equilibrium is questioned and it will be argued that kinetic limitations may have existed under the severe-accident boundary conditions of the FPT0 and FPT1 tests. To this end a simple monodimensional transport model was developed in an attempt at introducing kinetic aspects within the primary circuit. A number of homogeneous gas-phase reactions between selected fission products and structural materials, complemented by condensation reactions, underlies the kinetic model. In the absence of experimental data, the kinetic constants were estimated using the transition-state theory or semi-empirical methods. The kinetic model was then applied to the analysis of Phebus FPT0 and FPT1 yielding a satisfactory agreement between experimental data and model predictions.

Separation of lanthanides using ion-interaction chromatography with HDEHP coated columns

A rapid and high resolution separation of lanthanides by HPLC technique has been developed using Di-(2-ethylhexyl) phosphoric acid (HDEHP) coated reverse phase column and a-hydroxy isobutyric acid as the complexing reagent for elution. A gradient elution technique has been developed for achieving the separation of the entire lanthanide series. Isocratic elution procedure has also been developed for the separation of lighter (La to Gd) as well heavier lanthanides (Lu to Tb). This paper describes the separation methods developed and their application for the determination of lanthanides in a fission product mixture.

A new method for separation of 131I, produced by irradiation of natural uranium

Separation of 131I from natural uranium fission product mixtures has been accomplished by sorbing the 131I on special platinum–charcoal sorbent and desorbing by buffer solution (NaHCO3+Na2S2O3). High radiochemical and chemical purity is obtained by this method. Important parameters such as temperature, distillation rate, sorbing and desorbing rates and 131I separation yields have been investigated.

Analysis of the surface technology of silicon detectors for imaging of low-energy beta tracers in biological material

Using silicon surface barrier detectors, the counting sensitivity of low-energy beta tracers is considerably influenced by surface technology applied in detector manufacturing. Original diagnostic procedure, using a mixture of uranium fission products, is described to trace the behaviors of different admixtures as in the etching bath as in the water used during development of the detector surface. In combination with some other described analyses, the detectors produced with the developed surface control are used in a PC – controlled scanning equipment reaching at room temperature an FWHM of 3.4 keV for 241Am. Such detectors make it possible to image distribution, of e.g., 3H, 125I, 3 H+ 14 C and other beta tracer combinations applied in life and environmental sciences.

Disarmament and disposal of fissile material

The disarmament agreements between the USA and Russia have resulted in huge quantities of fissile materials, plutonium and highly enriched uranium, becoming redundant. These materials are extremely attractive to terrorist organisations and rogue states who would like to add nuclear weapons to their armouries. Unfortunately, while the USA has done a great deal to assist Russia to protect its nuclear materials, European countries, including the UK, have not risen to the challenge of events. If chaos is to be avoided then Europe must provide Russia with far more assistance to help it deal with its nuclear inheritance. The problem highlights the importance of the activities of those who devote their lives to investigations of the best means of disposing of nuclear waste. The MOX route for disposing of plutonium is gaining support but it is still vitally important to have an alternative route available such as vitrifying a mixture of plutonium and fission products and burying it in a deep repository.

Quantitative portable gamma-spectroscopy sample analysis for non-standard sample geometries

Utilizing a portable spectroscopy system, a quantitative method for analysis of samples containing a mixture of fission and activation products in nonstandard geometries was developed. This method was not developed to replace other methods such as Monte Carlo or Discrete Ordinates but rather to offer an alternative rapid solution. The method can be used with various sample and shielding configurations where analysis on a laboratory based gamma-spectroscopy system is impractical. The portalle gamma-spectroscopy method involves calibration of the detector and modeling of the sample and shielding to identify and quantify the radionuclides present in the sample. The method utilizes the intrinsic efficiency of the detector and the unattenuated gamma fluence rate at the detector surface per unit activity from the sample to calculate the nuclide activity and Minimum Detectable Activity (MDA). For a complex geometry, a computer code written for shielding applications (MICROSHIELD) is utilized to determine the unattenuated gamma fluence rate per unit activity at the detector surface. Lastly, the method is only applicable to nuclides which emit gamma-rays and cannot be used for pure beta or alpha emitters. In addition, if sample self absorption and shielding is significant, the attenuation will result in high MDA's for nuclides which solely emit low energy gamma-rays. The following presents the analysis technique and presents verification results using actual experimental data, rather than comparisons to other approximations such as Monte Carlo techniques, to demonstrate the accuracy of the method given a known geometry and source term.

Determination of the half-life of 105mRh

Following a fast chemical separation of Ru isotopes from a fission-product mixture, 105mRh was periodically extracted from its precursor (4.44-h 105Ru) for measurements of its half-life. The new value for the T 1/2 of 105mRh of (43.0±0.3) s resolves the long-standing conflict in the literature between the two earlier measured values of 45 and 30 s.

A rapid and selective separation of palladium

Palladium was separated from Al, Ru, Rh, U, Pu and also from a mixture of fission products using α-benzoin-oxime as the extractant and Solvesso-100 as the diluent. The extraction is quantitative over a wide range of acidity from 0.1M to 4M HNO3. The method has been found to be equally effective to separate Pd at trace level (carrier-free form). The stoichiometry of the complex, the interference of foreign elements in Pd separation, etc., are reported. The applicability of the method to separate palladium formed in fission rapidly from all other fission products, especially the most strongly interfering molybdenum isotopes, and the recovery of this element from high level radioactive waste are also mentioned.

Foliary contamination in the area of Bratislava (Czecho-Slovakia) after the Chernobyl accident

The contamination of leaves of some ornamental and fruit-tree plants (18 species), herbs (6) and early leafy vegetable (2) were assessed in the region of Bratislava and its vicinity through the first months after the Chernobyl accident. The levels of contamination showed local and temporal dependence. In October compared to its levels five months earlier, foliar contamination showed a relative 12- to 200-fold decrease of radioactivity. The effective half-life of the mixture of fission products in cumulative fallout on leaves of vegetation changed depending on time after the accident from 4 days (on day 10) to 150 days (after 2 months). The soluble fraction of radioactive contaminants on plant foliage ranged from 0.12 to 0.64.

Evidence for intruder states in111Rh

Levels in111Rh have been investigated via the γ -rays following the β−-decay of 2.1 s111Ru. The Ru activity was produced in the fission of249Cf and separated chemically from the fission product mixture. The emitted γ-rays were studied by γ singles and γ(t) coincidence measurements. Evidence for intruder states in111Rh has been obtained. Their properties are discussed and compared with those in the lighter Rh isotopes.