The work programme of NERIS in post-accident recovery.

NERIS is the European platform on preparedness for nuclear and radiological emergency response and recovery. Created in 2010 with 57 organisations from 28 different countries, the objectives of the platform are to: improve the effectiveness and coherency of current approaches to preparedness; identify further development needs; improve 'know how' and technical expertise; and establish a forum for dialogue and methodological development. The NERIS Strategic Research Agenda is now structured with three main challenges: (i) radiological impact assessments during all phases of nuclear and radiological events; (ii) countermeasures and countermeasure strategies in emergency and recovery, decision support, and disaster informatics; and (iii) setting up a multi-faceted framework for preparedness for emergency response and recovery. The Fukushima accident has highlighted some key issues for further consideration in NERIS research activities, including: the importance of transparency of decision-making processes at local, regional, and national levels; the key role of access to environmental monitoring; the importance of dealing with uncertainties in assessment and management of the different phases of the accident; the use of modern social media in the exchange of information; the role of stakeholder involvement processes in both emergency and recovery situations; considerations of societal, ethical, and economic aspects; and the reinforcement of education and training for various actors. This paper emphasises the main issues at stake for NERIS for post-accident management.

Monitoring long-term evolution of engineered barrier systems using magnets: Magnetic response.

Remote and non-destructive monitoring of the stability and performance of Engineered Barrier Systems for Geological Disposal Facility of is gaining considerable importance in establishing the safety cases for Higher Activity Wastes disposal. This study offers an innovative use of mineral magnetism for monitoring groundwater saturation of the barrier. Four mixtures of permanent magnets (Nd-Fe-B, coated and uncoated; SmCo and AlNiCo) and bentonite were reacted for 4, 8 and 12 months with mildly-saline, high-pH leachates, representing the fluids saturating a time-evolved engineered barrier. Coupled hysteresis and thermomagnetic analyses demonstrate how Nd-Fe-B feature a time-dependent transition from square-like ferromagnetic to superparamagnetic loop via pot-bellied and wasp-waist loops, whereas SmCo and AlNiCo do not show so extensive corrosion-related variations of the intrinsic and extrinsic magnetic properties. This study allowed to identify magnetic materials suitable for shorter- (Nd-Fe-B) and longer-term (SmCo and AlNiCo) monitoring purposes.

Spin reorientation and magnetic structure of HoCo12B6 ferrimagnetic compound.

The magnetic phase diagram is determined by combining magnetization measurements, ac susceptibility and neutron diffraction. The crystal and magnetic structures are also investigated. The HoCo12B6 compound exhibits ferrimagnetic behavior below TC = 147 K. Two antiferromagnetically coupled sublattices cancel out at the compensation temperature TComp = 46 K. HoCo12B6 undergoes a spin reorientation transition at TSR = 76 K; the easy magnetization axis changes from axial to basal plane upon heating. The magnitude of the magnetic moments and their orientation are described and discussed. It is revealed that HoCo12B6 compound exhibits a commensurate magnetic structure below TSR and an incommensurate one slightly above TSR. Significantly different magnetic moments have been observed on the two Co crystal sites, a very low magnetic moment of 0.14 µB being refined on the Co 18 g position. In addition, the second order crystal electric-field parameter A2(0) at the rare-earth site is determined. This result is discussed and used to explain the observed spin reorientation transition by a competition between the Co and Ho sublattice anisotropy.

Large magnetovolume effects due to transition from the ferromagnetic to antiferromagnetic state in Hf0.825Ta0.175Fe2 intermetallic compound.

Intrinsic magnetic properties and magnetovolume effects have been investigated for the Hf0.825Ta0.175Fe2 itinerant-electron system, which exhibits a temperature-induced first-order transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) state. The spontaneous volume magnetostriction contraction due to this transition from the high-volume FM state to the low-volume AFM state is about 0.66%. Applying a magnetic field increases significantly the FM-AFM transition temperature T(FM-AFM), with a rate of 7.2 K T(-1). At temperatures T > T(FM-AFM) a first-order metamagnetic transition between the AFM and FM states has been observed from isothermal magnetization curves, a result attributed to the itinerant-electron character of the Fe magnetism. This AFM-FM transition is accompanied by a huge field-induced volume magnetostriction. The change in ΔV/V due to the AFM-FM transition is about 0.75%.

Ferrimagnetism in GdCo(12-x)Fe(x)B6.

The effects of iron substitution on the structural and magnetic properties of the GdCo(12-x)Fe(x)B6 (0 ≤ x ≤ 3) series of compounds have been studied. All of the compounds form in the rhombohedral SrNi12B6-type structure and exhibit ferrimagnetic behaviour below room temperature: T(C) decreases from 158 K for x = 0 to 93 K for x = 3. (155)Gd Mössbauer spectroscopy indicates that the easy magnetization axis changes from axial to basal-plane upon substitution of Fe for Co. This observation has been confirmed using neutron powder diffraction. The axial to basal-plane transition is remarkably sensitive to the Fe content and comparison with earlier (57)Fe-doping studies suggests that the boundary lies below x = 0.1.

Magnetic and magnetocaloric properties of the high-temperature modification of TbTiGe.

The high-temperature form (HT) of the ternary germanide TbTiGe was prepared by melting. The investigation of HT-TbTiGe by x-ray and neutron powder diffractions shows that the compound crystallizes in the tetragonal CeScSi-type structure (space group I4/mmm; a = 404.84(5) and c = 1530.10(9) pm as unit cell parameters). Magnetization and specific heat measurements as well as neutron powder diffraction performed on HT-TbTiGe reveal a ferromagnet having T(C) = 300(1) K as the Curie temperature; the Tb-moments are aligned along the c-axis. This magnetic ordering is associated with a modest magnetocaloric effect around room temperature. The isothermal magnetic entropy change ΔS(m) was determined from the magnetization data; ΔS(m) reaches, respectively, a maximum value of  - 4.3 and  - 2.0 J K(-1) kg(-1) for a magnetic field change of 5 and 2 T.

Importance of interplane coupling on the magnetic phases of quasi-two-dimensional tantalites.

We propose a three-dimensional model to describe magnetic interactions in a class of tantalite compounds of compositions A(x)A'(1-x)Ta(2)O(6), with A,A' = Fe, Co or Ni. Due to the quasi-two-dimensional nature of the magnetism in these compounds, experimental data have been previously interpreted using two-dimensional models. These are anisotropic Heisenberg models or Ising models and include competing exchange interactions from different neighbors. Taking into account all the relevant exchange terms, which include interplane interactions, we show that the latter allows us to understand the various low-temperature magnetic phases observed by neutron diffraction in this family of compounds. This is done by studying the eigenvalues of the exchange-interaction matrix in wavevector space for different sets of coupling parameters, of which those relative to in-plane interactions have been obtained from high-temperature series analysis of the magnetic susceptibility. This approach is rather general and the model presented here is directly applicable to isostructural compounds like ASb(2)O(6).

(Lu0.8Ce0.2)2Fe17 single crystal under hydrostatic and 'negative' pressure induced by hydrogenation.

An experimental study of correlations between the magnetovolume effects and the type of magnetic ordering in the Lu(2)Fe(17)-based intermetallics is shown for the example of variation of the interatomic distances by means of substitution of Ce for Lu, hydrogenation and subsequent high-pressure investigation of both the initial (Lu(0.8)Ce(0.2))(2)Fe(17) and a hydride (Lu(0.8)Ce(0.2))(2)Fe(17)H(0.4) single-crystalline compounds. The magnetization study was carried out under hydrostatic pressures of up to ∼ 0.8 GPa. Hydrogenation is found to totally suppress the antiferromagnetic order of (Lu(0.8)Ce(0.2))(2)Fe(17) by favoring domination of the ferromagnetic interactions which increases the magnetic ordering temperature from 247 to 300 K. The application of pressure decreases the Curie temperature and induces the non-collinearity of the magnetic moments in (Lu(0.8)Ce(0.2))(2)Fe(17)H(0.4), confirming the strong influence of hydrogenation on exchange coupling.

On the R 5d band polarization in rare-earth-transition metal compounds.

Magnetic measurements and band structure calculations were performed on RT(2) and RT(5) compounds, where R is a heavy rare-earth and T = Fe, Co, Ni, Al, as well as on pseudobinary compounds GdCo(2 - x)A(x) (A = Ni, Cu, Si), YFe(2 - x)V(x) and YCo(4 - x)Ni(x)B. The calculated moments per formula unit described well the experimentally determined magnetizations. By considering the 4f-5d-3d exchange interactions, we evaluate the contributions of local 4f-5d and short range 5d-3d interactions to R 5d and Y 4d band polarizations. The 4f-5d induced polarizations are proportional to the De Gennes factor and are the same for a given R and a similar type structure. The R 5d and Y 4d band polarizations induced by R 5d-T3d or Y 4d-T3d hybridizations are proportional to the number of neighbouring T atoms, to a given R, and their magnetic moments. Previous results on the matter are also discussed.

Specific heat measurements and structural investigation of CeCu(6-x)Sn(x) compounds.

The evolution of the crystal structure and some magnetic properties of the heavy-fermion material CeCu(6-x)Sn(x) (x = 0, 0.25, 0.65, 0.75, 0.85 and 1.0) has been studied by powder neutron diffraction and by specific heat measurements. The substitution of Cu by Sn suppresses the temperature induced orthorhombic to monoclinic transition, known to occur in the pure CeCu(6) phase. No structural phase transition has been observed in these samples as a function of x but the cell volume increases considerably in an anisotropic way. Sn occupies preferentially the special Cu crystallographic site which is next to each of the four Ce atoms in the unit cell. The transition to antiferromagnetic order, characterizing the samples with higher x, is sensitive to both x and magnetic field. The results are discussed in the context of the competition between Kondo and RKKY interactions in disordered or not heavy-fermion systems and reveal an interesting interplay between composition, structure and magnetism in CeCu(6-x)Sn(x).

Magnetic phases of the quasi-two-dimensional compounds FexCo1 - xTa2O6.

We report new results on the magnetic properties of the FexCo1 - xTa2O6 series of compounds. Essentially using neutron-diffraction and magnetic measurements we study, in more detail, the low-x limit of the temperature versus x phase diagram, where a new bicritical point is observed. The complete phase diagram shows three different magnetic phases at low temperature, for a high, intermediate and very low iron content. These phases consist of distinct antiferromagnetic orderings, characterized by different pairs of propagation vectors. We obtain information about the intraplane exchange interactions by fitting a high-temperature series of the magnetic susceptibility. Here we improve on a previously employed model, showing that two non-equivalent next-nearest-neighbor interactions must be taken into account in order to allow for in-plane magnetic orderings that are consistent with the neutron-diffraction results.

Crystallography of Chevrel phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and their cation mobility.

Chevrel phases (CPs), M(x)Mo(6)T(8) (M = metal, T = S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at room temperature. In spite of extensive studies of these materials, the origin of their high ionic mobility remained unclear. In a previous paper we presented for the first time a proper classification of the very complex transport behavior of different cations in the Mo(6)T(8) hosts: (i) apparent immobility of the large M cations such as Pb(2+), Sn(2+), Ag(+) in the ternary phases, MMo(6)T(8); (ii) coupled M+M' diffusion in the quaternary phases, M(x)M'(y)Mo(6)T(8), where both large and small cations can assist; (iii) cation trapping in the Mg-Mo(6)S(8), Cd-Mo(6)S(8), and Na-Mo(6)T(8) systems; (iv) a combination of low- and high-rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo(6)S(8), Mn-Mo(6)S(8), and Cd-Mo(6)Se(8) systems, and (v) a fast ionic transport for small cations such as Ni(2+), Zn(2+), and Li(+). It was shown that this behavior could be understood by a relatively simple crystallographic analysis (mapping of all the cation sites and estimations of their potential energy according to the distances of these sites from adjacent anions and cations) of the diffusion routes, which differ for different cations. For this analysis, it was necessary to complete our knowledge about the cation location in the crystal structure for several CPs with known ionic mobility. This article presents the results of a combined Rietveld analysis of powder X-ray and high-resolution neutron diffraction profiles for NaMo(6)T(8), ZnMo(6)T(8), CdMo(6)T(8), and MnMo(6)S(8). All seven compounds can be defined as classic CPs, where cation delocalization from the center of the largest cavity between the Mo(6)T(8) blocks decreases with the length of the ideal chemical bond, M-T. In addition, this work details the effect of the structural parameters on the ionic conductivity in CPs, namely, it shows how subtle changes in cation delocalization may be crucial for diffusion kinetics.

Magnetic properties of the iron sublattice in the YFe(12-x)M(x) compounds (M = Ti, Mo or V; x = 1-3.5).

The magnetic properties of the YFe(12-x)M(x) compounds (M = Ti, Mo or V; x = 1-3.5) have been determined in the ordered ferromagnetic state as well as in the paramagnetic state. The iron magnetic moment has been determined from 4 K up to the Curie temperature whereas the analysis of the paramagnetic region has led to the determination of the effective iron magnetic moment. The number of spins has been calculated below and above the Curie temperature in order to discuss the degree of itinerancy of the Fe magnetic behavior in the YFe(12-x)M(x) compounds. All the YFe(12-x)M(x) compounds (M = Ti, Mo or V; x = 1-3.5) have very similar crystalline properties: they crystallize in the same crystal structure and all the M elements used here are known to substitute for iron on the same crystal site. In contrast, they exhibit a wide range of magnetic behavior; the Curie temperature varies from 63 to 539 K and the mean magnetic moment per iron atom is also very dependent upon the M element used and its concentration. Furthermore the degree of itinerancy of the iron is not preserved along YFe(12-x)M(x) compounds but is found to depend significantly upon the nature of the substituting element M and its concentration. The results are discussed and compared to earlier published results obtained on binary R-Fe and ternary R-Fe-B compounds.

Phase diagram of Mg insertion into Chevrel phases, MgxMo6T8 (T = S, Se). 3. The crystal structure of triclinic Mg2Mo6Se8.

This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group P1, a = 6.868 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg(2+) ions in Mg2Mo6Se8.

On the mechanism of triclinic distortion in Chevrel Phase as probed by in-situ neutron diffraction.

This work presents, for the first time, a general mechanism of a rhombohedral (R)-triclinic (T) phase transition in Chevrel Phases (CPs) with small cations (radius<1 A), which was unclear in spite of intensive studies of these important materials in the past. In contrast to previous interpretation of the R<-->T transition in some CPs as cation ordering, T-distortion is regarded here as a particular case of general adaptation of the framework to cation insertion, which includes the deformations of the coordination polyhedra and their tilting. The research is based on a combination of experimental studies (in-situ neutron diffraction at different temperatures) for one model compound, MgMo6Se8, and structural analysis for a variety of known CPs. This analysis shows that the structure flexibility is fundamentally different for the R and T forms. As a result of the lower flexibility, in the R form, a strict correlation exists between the compression of the framework along the -3 symmetry axis and the cation position in the structure (the so-called 'delocalization'). The decreasing delocalization in the R-CPs, which occurs on cooling, leads to excessive repulsion within the cations pairs (R-Cu1.8Mo6S8 case) or undesirable asymmetry in the cation polyhedra (R-MgMo6Se8 case). The higher flexibility of the T framework allows for relaxation of these structural strains by increasing the cation-cation distances and forming a more symmetric cation environment, sometimes with higher coordination number (CN), like CN=5 in the T-Fe2Mo6S8 type. Thus, this work also proposes possible driving forces for T-distortion in CPs.

Bicriticality in FexCo1-xTa2O6.

X-ray and neutron-diffraction, dc magnetic susceptibility, magnetization, and specific-heat measurements are reported for FexCo(1-x)Ta2O6 mixed oxides. X-ray refinement indicates homogeneous samples for all the reported concentrations. The neutron-diffraction measurements reveal magnetic structures with double propagation vectors (+/-1/4,1/4,1/4) for CoTa2O6, and (1/2,0,1/2) and (0,1/2,1/2) for FeTa2O6. The latter remain unchanged in the Fe-rich samples, for 0.46< or =x<1.00, while the Co-rich samples show propagation vectors (+/-1/4,1/4,0) for 0.09< or =x<0.46. The temperature vs x phase diagram exhibits a bicritical point at about T=4.9 K and x=0.46. For this concentration, and at low temperatures, the system shows coexistence of both magnetic structures. This novel bicritical behavior is interpreted as induced by competition between the different magnetic and crystallographic structures.