H. Blank

Oxide fuel transients

Abstract The behavior of UO 2 fuel from power reactors has been studied up to high burn-up (~ 60 GWd/tU), under both steady state and power transient conditions in the range of 35 to 48 kW/m. Careful post-irradiation examination involving electron probe microanalysis, transmission and scanning electron microscopy in addition to detailed hot cell examination have provided a large data base on the radial migration and release of volatile fission products. Theoretical evaluation with different computer codes and supporting laboratory experiments established the basis for understanding the kinetics and mechanisms operative during transients in high burn-up UO 2 . Typical examples are given to demonstrate the degree of understanding achieved

Dense fuels in Europe

Abstract After a reduction of the activities on dense fuels in Europe during the late seventies, the interest in this fuel type was revived at the beginning of the eighties, when the economy of the closed FBR fuel cycle was reassessed. Strategies were developed to bring its economy into equilibrium with that of the LWR fuel cycle. The most important step was the decision to develop the European Fast Reactor (EFR) and to continue using oxide fuel at first. However, for economical reasons in an optimised fuel cycle a better fuel than the oxide should be employed. This fuel must avoid the deficiencies of the oxide but retain its advantages. Mixed nitride would satisfy this condition if it can be demonstrated that it can attain a burnup of at least 15 at%. With the know-how available in Europe it should be possible to achieve this goal with a rather limited research programme during the coming 10 years.

Fuel research and basic aspects of fuel in-pile performance

Abstract The approach adopted in the European Institute for Transuranium Elements to study basic aspects of the in-pile performance of a wide range of nuclear fuels (e.g. oxides, carbides, nitrides etc. for LWRs and LMFBRs) is described. It consists of an “all in-house activity” of fuel fabrication, careful and very detailed characterization and post-irradiation examination of fuels at different burn-ups and following different irradiation conditions, parallel laboratory studies under “clean conditions” and supporting theoretical and modelling work. The approach is illustrated with typical examples of results obtained in the past 25 years. Finally, more recent results of basic experiments related to fuel in-pile performance are given.

Specification and characterization of dense fuels for liquid metal cooled fast breeder reactors

Abstract The specifications of a given nuclear fuel depend on the general requirements of the fuel cycle and especially on the pin design and the reactor operation conditions. For the dense LMFBR fuels these requirements have changed several times in the course of the development of the LMFBRs since 1950. By following the course of this development it is shown how one arrived at the fuel specifications and pin design which are in agreement with the current fuel cycle requirements.

Experimental results on properties of Pa revisited

The literature on experimental data of the properties of Pa is collected and inconsistencies in the data are discussed. In particular the origin and the stability of the fcc phase of Pa are analyzed. Data of interest for the role of α-Pa in the previous paper on Phase stabilization in the Light Actinides and Binary Alloys are compared with corresponding data of α-Th and α-U.

Fast diffusion in dilute binary alloys and its analysis based on Hägg's rule

Abstract Fast solute diffusion in certain dilute binary alloys can be understood by a modified version of Haggs rule. This method is substantiated and used to analyse published data on fast diffusion of (mainly) 3d-solutes in the anisotropic host phases β-U and α-Zr and to compare the results with those in bcc host phases. The activation enthalpy Q of fast diffusion consists of elastic contributions, Δ H el and Δ h el , related to the Hagg parameter λ and a chemical one, Δ H cov , due to the electronic solute-solvent interaction. For Fe, Co, Ni and O in α-Zr, Δ H cov can be inferred from relevant solute–host phase diagrams. The Hagg-approach defines the geometrical frame within which fast solute diffusion occurs. To a certain degree it is complementary to the Miedema model which, however, is bound to neglect the structural details of the host lattice essential for interstitial diffusion.

Phase stabilization in the light actinides and binary alloys

Abstract From a phenomenological analysis of phase stabilization in the binary actinide alloy systems, U–Pu, U–Np–Pu and U–transition metals, the existence of a relatively stable electronic complex with the configuration (5f nd) v is deduced. It must be formed by itinerant 5f and nd electron states with n =3, 4, 5, 6 and v should be a fractional number since only band states are involved. The stability of the complexes should depend on temperature. In α-Pa, equal numbers of 5f and 6d states in the valence electron system result: (i) in a (5f6d)* supercomplex which correlates with the unique crystal structure of α-Pa and (ii) with metal properties which rather resemble those of α-Th than that of α-U. The alleged properties of the complex are discussed and compared with the thermal behavior of α-Pa and of the α-U on approaching the α/β transition temperatures and Pa should play a key role for understanding the temperature effects in the light actinide series. In a following paper, the literature on fabrication and properties of Pa metal is critically reviewed for the comparison of relevant properties between α-Th, α-Pa and α-U.

Microstructure and fission gas bubbles in irradiated mixed carbide fuels at 2 to 11 a/o burnup

An analysis of the defect structure and of small fission gas bubbles has been performed on mixed carbide fuels with burn-ups between 1.8 and 11 a/o by transmission electron microscopy (TEM). A complex defect structure consisting of dislocations, loops and at least 3 types of solid fission product precipitates was observed. Na-bonded carbides develop predominantly a dislocation network increasing in density with burn-up whereas He-bonded carbides showed mainly a corresponding network of crystallographic needle precipitates. Locally the nucleation and growth of small fission gas bubbles with 1 to 20 nm diameters (bubble population P1) is closely related to their dislocation or needle environment, larger bubbles with diameters 30 to 50 nm appear to be mostly associated with platelike precipitates or dislocation boundaries. The local swelling contribution μ1 of bubble population P1 is ≲ 0.5% and its fission gas content G1 is 4 to 5% of the total amount of gas created over the whole burn-up range investigated.

Properties of 5f6d electron complexes in the light actinides

Recently temperature-sensitive 5f6d electron complexes were postulated to explain certain properties of the light actinide metals Pa, U, Np and Pu, but the sizes of these complexes remained undefined. Here it is shown that these complexes are localized in the short (covalent) bonds of α-U, govern the anomalous properties of α-Pa and explain the stabilization of certain binary alloy phases between light actinide metals. The systematic variation in certain properties in the light actinide series can be correlated with the presence of 5f6d complexes. In the first three metals (Th, Pa and U), typical 5f effects are more or less suppressed. In contrast these effects exist with clearly different strengths in Np and Pu. Experiments are proposed by which the properties of 5f6d complexes might be studied further.