Np Zr Research Articles

TAF-ID: An international thermodynamic database for nuclear fuels applications

The Thermodynamics of Advanced Fuels – International Database (TAF-ID) was developed using the Calphad method to provide a computational tool to perform thermodynamic calculations on nuclear fuel materials under normal and off-normal conditions. Different kinds of fuels are considered: oxide, metallic, carbide and nitride fuels. Many fission products are introduced as well as structural materials (e.g., zirconium, steel, concrete, SiC) and absorbers (e.g., B4C), in order to investigate the thermochemistry of irradiated fuels and to predict their chemical interaction with the surrounding materials. The approach to develop the database and the models implemented in the database are described. Examples of models for key chemical systems are presented. Finally, a few examples of application calculations on severe accidents with UO2 fuels, irradiated fuel chemistry of MOX and metallic fuels and metallic fuel/cladding interaction show how this tool can be used. To validate the database, the calculations are compared to the available experimental data. A good agreement is obtained which gives confidence in the maturity degree and quality of the TAF-ID database. The working version is only accessible to the participants of the TAF-ID project (Canada, France, Japan, the Netherlands, Republic of Korea, United Kingdom, USA). A public version is accessible by all the NEA countries. The current version contains models on the Am–Fe, Am–Np, Am-O-Pu, Am–U, Am–Zr, C–O–U-Pu, Cr–U, Np–U, Np–Zr, O–U–Zr, Re–U, Ru–U, Si–U, Ti–U, U-Pu-Zr, U–W systems. It is progressively extended with our published assessments. Information on how to join the project is available on the website: https://www.oecd-nea.org/science/taf-id/.

Thermodynamic evaluation of the Np–Zr system using CALPHAD and ab initio methods

A new thermodynamic description of Np–Zr alloys is developed using the CALPHAD (CALculation of PHAse Diagrams) method based on available experimental information on phase equilibria, supplemented by ab initio energetics for end members that are not available in the standard CALPHAD unary database. The present thermodynamic description shows improvements compared to previous models in the predicted phase diagram when comparing to assessed reliable experimental data. Ab initio Density Functional Theory (DFT) calculations are also performed on all known stable solid phases of Np–Zr alloys and the end member Np and Zr metals. Comparing to the formation energetics predicted from the CALPHAD models of both this work and a previous study we find that DFT with the Generalized Gradient Approximation (GGA) to the exchange-correlation potential overestimates the formation enthalpies of Np and Np–Zr by about 0.15eV/atom, and the so-called DFT+U approach with a Ueff of near 0.65eV can reduce this error by about 0.07–0.10eV. Our comprehensive comparison between existing CALPHAD, ab initio and experimental results for Np–Zr indicates a need for further experiments on the phase equilibrium.

The U–Ti system: Strengths and weaknesses of the CALPHAD method

Input from Density Functional Theory (DFT) calculations is used to understand phase equilibria in a binary metallic alloy fuel system: U–Ti. The CALPHAD approach is employed to calculate a U–Ti phase diagram that is consistent not only with experimental data but also—more importantly—with thermodynamic data from DFT calculations: heat of formation of γ(bcc)-U–Ti alloys as a function of composition, and formation enthalpy of the δ-U 2Ti compound. Three DFT-based electronic structure methods are utilized: SR-KKR-ASA-CPA, SR-EMTO-CPA, and FPLMTO-SQS, and the use of derived ab initio data avoids the manifestation of unreasonable or inaccurate phase stabilities that result from an otherwise unconstrained Gibbs energy minimization within the CALPHAD approach. We also investigate phase formation of the δ-U 2Ti phase in the U–Ti system, that stabilizes in the same C32 structure as other binary metallic fuel alloys such as U–Zr and Np–Zr.

Thermodynamic study of the Np–Zr system

A thermodynamic model of the Np–Zr system is developed using the CALPHAD method, and a review of previous work performed on this system is presented here. In general, results obtained are in good agreement with those proposed from experimental observations. It is found that the nature of reactivity of Np with Zr, is different from that of U and Pu: an expected elevation of melting point of Np–Zr alloys was not seen and a miscibility gap existed between the high-temperature bcc phases of Np and Zr. Formation enthalpy of the bcc phase obtained from the model is compared with results from KKR–ASA–CPA calculations. Lattice stabilities of various phases in the system are compared to values obtained from first-principles LDA and GGA calculations. The δ-NpZr 2 phase is modeled as a non-stoichiometric phase with a C32 structure, similar to what has been determined for the δ-phase in the U–Zr system. This phase is analogous to ω-phase in pure Zr, which is stabilized at high pressures. Two different possibilities for stability of the δ and ω phases have been proposed in the present work. Finally, calculated changes in enthalpy versus temperature are plotted for two alloys to guide future experimental work in resolving important issues in this system.

Phase characteristics of a number of U–Pu–Am–Np–Zr metallic alloys for use as fast reactor fuels

Metallic fuel alloys consisting of uranium, plutonium, and zirconium with minor additions of americium and neptunium are under evaluation for potential use to transmute long-lived transuranic actinide isotopes in fast reactors. A series of test designs for the Advanced Fuel Cycle Initiative (AFCI) have been irradiated in the Advanced Test Reactor (ATR), designated as the AFC-1 and AFC-2 designs. Metal fuel compositions in these designs have included varying amounts of U, Pu, Zr, and minor actinides (Am, Np). Investigations into the phase behavior and relationships based on the alloy constituents have been conducted using X-ray diffraction and differential thermal analysis. Results of these investigations, along with proposed relationships between observed behavior and alloy composition, are provided. In general, observed behaviors can be predicted by a ternary U–Pu–Zr phase diagram, with transition temperatures being most dependent on U content. Furthermore, the enthalpy associated with transitions is strongly dependent on the as-cast microstructural characteristics.

Thermodynamic analysis on the Np–Zr–H system

The thermodynamic data base of the Np–Zr–H ternary system was developed based on the description of its binary constituents, coupling with the calculated enthalpies of formation obtained using the Miedema model and the Griessen–Driessen model. The provisional isothermal sections of the Np–Zr–H phase diagram were calculated using the data base. It was found that the Np–Zr–H ternary system at 972 K includes the liquid phase in the region of low hydrogen to metal ratio. It is pointed out that the composition of the Np–Zr–H hydride should be selected to avoid the hydride fuel melting in designing of a nuclear reactor fuel element.

Thermodynamic analysis of Np–Zr–H, Am–Zr–H, Pu–Zr–H systems

High-level radioactive wastes generated after reprocessing spent nuclear fuels from nuclear reactors include long-lived and highly toxic radioactive nuclides. Recently, the transmutation method using actinide hydride targets in fast reactor has been proposed to reduce the amount of actinide in nuclear wastes. The burning of Pu using Pu–Zr–H fuel in light water reactor has also been proposed. Given the potential for high-temperature state during routine or extraordinary operating circumstances, accurate knowledge or reliable predictions of materials behaviour under a variety of conditions is required in order to develop safe and effective nuclear handling and remediation technologies. Semi-empirical Miedema and Griessen–Driessen models on the enthalpy of formation coupling with the available experimental entropy information were utilized to estimate the thermodynamic stability of the selected actinide–zirconium hydrides. The calculations showed that at high temperature, stable hydrides might be expected to form for the Np–Zr–H, Am–Zr–H and Pu–Zr–H systems at selected compositions, which can be effectively used for the MA transmutation or plutonium burning.

Semi-empirical models of actinide alloying

Alloys of Np have been studied less than those of the neighboring elements, U and Pu; the higher actinides have received even less attention. Recent interest in 237Np, 241Am and other actinide isotopes as significant, long-lived and highly radiotoxic nuclear waste components, and particularly the roles of metallic materials in new handling/separations and remediation technologies, demands that this paucity of information concerning alloy behaviors be addressed. An additional interest in these materials arises from the possibility of revealing fundamental properties and bonding interactions, which would further characterize the unique electronic structures (e.g., 5f electrons) of the actinide elements. The small empirical knowledge basis presently available for understanding and modeling the alloying behavior of Np is summarized here, with emphasis on our recent results for the Np–Am, Np–Zr and Np–Fe phase diagrams. In view of the limited experimental data base for neptunium and the transplutonium metals, the value of semi-empirical intermetallic bonding models for predicting actinide alloy thermodynamics is evaluated.

The investigation of selected NpZr alloys by X-ray diffraction up to 700 °C

High temperature X-ray diffraction analysis of two NpZr (25 and 33 at.% Np) binary alloy specimens prepared by arc melting was performed to help elucidate important features of the NpZr phase diagram, several features of which have not been resolved. Temperatures in these studies ranged from 25 to 700 °C. The NpZr specimens were characterized as consisting of different combinations of Np, Zr and NpZr2 phases at the different temperatures. The results obtained for this Zr-rich portion of the phase diagram diagram suggest that there is not a continuous mutual solubility between γ-Np and β-Zr, as known to exist in the UZr binary system. The solubility of Zr in γ-Np was estimated to be ca. 4% at 600 °C. Only the α form of Zr was observed in these studies, indicating that there was insufficient Np dissolved in the Zr phase to stabilize its β phase below the normal α-β transition temperature of 850 °C. The hexagonal NpZr2 was detected up to 550 °C; above this temperature it appeared to decompose. Diffraction lines for the α, β and γ forms of Np were observed as a function of the temperature. Diffraction lines for Np disappeared above 650 °C, presumably because of its “localized” melting in micro-domains.

Investigation of the neptunium-zirconium system by X-ray diffraction

X-ray powder diffraction analysis at 25 °C has been used to further characterize the NpZr phase diagram. We have studied a variety of NpZr aggregate compositions obtained by different preparative methods, including arc melting of the components. For the materials at ambient temperature, obtained by either slow cooling or quenching from elevated temperatures, we observed one or more of the following product phases: α-Np, α-Zr, NpZr 2 and one new phase. The last was interpreted tentatively to be either Np 4Zr or Np 6Zr. Calculations based on the X-ray data suggested a limited solubility of Np in α-Zr and an even lower solubility of Zr in α-Np. The X-ray data for the NpZr 2 phase showed that it possesses hexagonal symmetry and is isostructural with UZr 2 and PuZr 2. Annealing of NpZr composites at selected temperatures did not enhance the formation of NpZr 2.

Alloying behavior in selected neptunium binary systems: the role of 5f bonding

We have undertaken to establish the phase relations for selected binary alloy systems of Np with another actinide or with a d-block transition metal. Empirical data obtained by differential thermal analysis and powder X-ray diffraction have been supplemented by elementary modeling procedures and correlations with similar alloy systems to propose the primary features of selected phase diagrams. The initial focus of these investigations has been on the NpAm, NpZr and, most recently, the NpFe systems. The findings for the former two systems are summarized, and preliminary results for the NpFe system are reported. The potential role of 5f bonding in determining alloying behavior is discussed in conjunction with regular-solution model predictions to address unique features evident in certain Np alloy systems.

Investigation of the NpZr and UZrNp systems

Dilatometry, electron probe microanalysis and micrographic techniques were used to determine the melting points, the phase transformations and the phase relationships of the binary NpZr and of the U-rich ternary U-Zr-Np alloys. The high temperature allotropes γ-Np, γ-U and β-Zr are mutually miscible. The mutual solubility of the allotropic forms α-Zr with α- and β-U is, on the contrary, relatively poor both in the binary and in the ternary alloys. No ternary compounds were detected. The addition of Zr leads to an increase in the melting points of the U-Np alloys. A tentative NpZr phase diagram and three isothermal sections of the U-rich corner of the UZrNp phase diagram are presented.