Showing papers by "Chalk River Laboratories published in 2007"

The 15-Country Collaborative Study of Cancer Risk among Radiation Workers in the Nuclear Industry: Estimates of Radiation-Related Cancer Risks

Abstract: Cardis, E., Vrijheid, M., Blettner, M., Gilbert, E., Hakama, M., Hill, C., Howe, G., Kaldor, J., Muirhead, C. R., Schubauer-Berigan, M., Yoshimura, T., Bermann, F., Cowper, G., Fix, J., Hacker, C., Heinmiller, B., Marshall, M., Thierry-Chef, I., Utterback, D., Ahn, Y-O., Amoros, E., Ashmore, P., Auvinen, A., Bae, J-M., Bernar, J. S., Biau, A., Combalot, E., Deboodt, P., Diez Sacristan, A., Eklof, M., Engels, H., Engholm, G., Gulis, G., Habib, R. R., Holan, K., Hyvonen, H., Kerekes, A., Kurtinaitis, J., Malker, H., Martuzzi, M., Mastauskas, A., Monnet, A., Moser, M., Pearce, M. S., Richardson, D. B., Rodriguez-Artalejo, F., Rogel, A., Tardy, H., Telle-Lamberton, M., Turai, I., Usel, M. and Veress, K. The 15-Country Collaborative Study of Cancer Risk among Radiation Workers in the Nuclear Industry: Estimates of Radiation-Related Cancer Risks. Radiat. Res. 167, 396– 416 (2007). A 15-Country collaborative cohort study was conducted to provide direct estimates of cancer risk following protracted low d...

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Abstract: Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Curvature Effect on the Structure of Phospholipid Bilayers

Abstract: High-resolution small-angle X-ray scattering (SAXS), complemented by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments, was used to study the effect of curvature ...

Influence of cholesterol on the bilayer properties of monounsaturated phosphatidylcholine unilamellar vesicles.

Abstract: The influence of cholesterol on the structure of unilamellar-vesicle (ULV) phospholipid bilayers is studied using small-angle neutron scattering. ULVs made up of short-, mid- and long-chain monounsaturated phospholipids (diCn :1PC, n = 14 , 18, 22, respectively) are examined over a range (0-45mol %) of cholesterol concentrations. Cholesterol's effect on bilayer structure is characterized through changes to the lipid's transmembrane thickness, lateral area and headgroup hydration. For all three lipids, analysis of the experimental data shows that the addition of cholesterol results in a monotonic increase of these parameters. In the case of the short- and mid-chain lipids, this is an expected result, however, such a finding was unexpected for the long-chain lipid. This implies that cholesterol has a pronounced effect on the lipid's hydrocarbon chain organization.

Inter-comparison of absorbed dose rates for non-human biota

Abstract: A number of approaches have been proposed to estimate the exposure of non-human biota to ionizing radiation. This paper reports an inter-comparison of the unweighted absorbed dose rates for the whole organism (compared as dose conversion coefficients, or DCCs) for both internal and external exposure, estimated by 11 of these approaches for selected organisms from the Reference Animals and Plants geometries as proposed by the International Commission on Radiological Protection. Inter-comparison results indicate that DCCs for internal exposure compare well between the different approaches, whereas variation is greater for external exposure DCCs. Where variation among internal DCCs is greatest, it is generally due to different daughter products being included in the DCC of the parent. In the case of external exposures, particularly to low-energy β-emitters, variations are most likely to be due to different media densities being assumed. On a radionuclide-by-radionuclide basis, the different approaches tend to compare least favourably for 3H, 14C and the α-emitters. This is consistent with models with different source/target geometry assumptions showing maximum variability in output for the types of radiation having the lowest range across matter. The intercomparison demonstrated that all participating approaches to biota dose calculation are reasonably comparable, despite a range of different assumptions being made.

The 15-Country Collaborative Study of Cancer Risk among Radiation Workers in the Nuclear Industry: Study of Errors in Dosimetry

Abstract: Thierry-Chef, I., Marshall, M., Fix, J. J., Bermann, F., Gilbert, E. S., Hacker, C., Heinmiller, B., Murray, W., Pearce, M. S., Utterback, D., Bernar, K., Deboodt, P., Eklof, M., Griciene, B., Holan, K., Hyvonen, H., Kerekes, A., Lee, M-C., Moser, M., Pernicka, F. and Cardis, E. The 15-Country Collaborative Study of Cancer Risk among Radiation Workers in the Nuclear Industry: Study of Errors in Dosimetry. Radiat. Res. 167, 380–395 (2007). To provide direct estimates of cancer risk after low-dose protracted exposure to ionizing radiation, a large-scale epidemiological study of nuclear industry workers was conducted in 15 countries. As part of this study, identification and quantification of errors in historical recorded doses was conducted based on a review of dosimetric practices and technologies in participating facilities. The main sources of errors on doses from “high-energy” photons (100–3000 keV) were identified as the response of dosimeters in workplace exposure conditions and historical ca...

Low Doses of Radiation Reduce Risk In Vivo

Abstract: The “Linear No Threshold” hypothesis, used in all radiation protection practices, assumes that all doses, no matter how low, increase the risk of cancer, birth defects and heritable mutations. In vitro cell based experiments show adaptive processes in response to low doses and dose rates of low LET radiation, and do not support the hypothesis. This talk will present cellular data and data from animal experiments that test the hypothesis in vivo for cancer risk. The data show that a single, low, whole body dose (less than about 100 mGy) of low LET radiation, given at low dose rate, increased cancer latency and consequently reduced both spontaneous and radiation-induced cancer risk in both genetically normal and cancer-prone mice. This adaptive response lasted for the entire lifespan of all the animals that developed these tumors, and effectively restored a portion of the life that would have been lost due to the cancer in the absence of the low dose. Overall, the results demonstrate that the assumption of a linear increase in risk with increasing dose in vivo is not warranted, and that low doses actually reduce risk.

Neutron diffraction study of Pseudomonas aeruginosa lipopolysaccharide bilayers.

Abstract: Lipopolysaccharides (LPSs) are a major class of macromolecules populating the surface of Gram-negative bacteria. They contribute significantly to the bacterium's surface properties and play a crucial role in regulating the permeability of its outer membrane. Here, we report on neutron diffraction studies performed on aligned, self-assembled bilayers of LPS isolated from Pseudomonas aeruginosa PAO1. This LPS system is comprised of a mixture of rough and smooth A-band and B-band LPS, similar to that naturally found in P. aeruginosa. Temperature scans were conducted at various levels of hydration, and the phases adopted by LPS, along with their corresponding transition temperatures, have been identified. Because of LPS's chemical heterogeneity, the gel-to-liquid-crystalline transition was continuous and not abrupt as commonly observed in single-component phospholipid systems. From the construction of one-dimensional scattering length density profiles, we find that water penetrates into the hydrocarbon region up to and including the center of liquid-crystalline LPS bilayers. This permeability to water also extends to bilayers in the continuous phase transition region and could have far-reaching implications as to how small molecules penetrate the outer membrane of Gram-negative bacteria.

Transformation Temperatures and Welding Residual Stresses in Ferritic Steels

Abstract: Residual stress in the vicinity of a weld can have a large influence on structural integrity. Here the extent to which the martensite-start temperature of the weld filler metal can be adjusted to mitigate residual stress distributions in ferritic steel welds has been investigated. Three single-pass groove welds were deposited by manual-metal-arc welding on 12mm thick steel plates using filler metals designed to have different martensite-start temperatures. Their residual stress distributions were then characterised by neutron diffraction. It was found that a lower transformation temperature leads to a potentially less harmful stress distribution in and near the fusion zone. The experimental method is reported and the results are interpreted in the context of designing better welding consumables.Copyright © 2007 by ASME and National Research Council Canada

A Mechanistic Study on Lead-Induced Passivity-Degradation of Nickel-Based Alloy

Abstract: The mechanism of lead-induced passivity-degradation in a nickel-based alloy (UNS N06690) was explored in a simulated stream generator alkaline crevice chemistry. The pitting induction test indicated that the detrimental impact of lead contamination on pitting resistance was significantly enhanced by the presence of calcium ions. An X-ray photoelectron spectroscopy analysis revealed the incorporation of lead into the passive film. Calcium ions in the alkaline chemistry entered the passive films with the aid of the lead species but the ingress of calcium did not occur in the solution free of lead contamination. The incorporation of lead reduced chromium and iron content in passive films and hindered the dehydration processes during the passivation. Mott-Schottky and photoelectrochemical measurements showed that passive films on the nickel-based alloy are p-type semiconductors and the incorporation of lead may reduce the acceptors in the passive films. A further analysis suggested that the lead-induced passivity degradation of nickel-based alloys may be related to the increases in M-O and M-OH 2 bonds, the electronic structural changes of spinel oxides in the passive film, and reduced ion-selectivity due to the surface adsorption of the lead species.

The study of liposomes, lamellae and membranes using neutrons and X-rays

Abstract: Advances in colloid and interface science have stimulated a renewed interest in the study of lipid–water systems. In recent years, much progress has been achieved in the domains of sample preparation and sample environments, offering the unique possibility of studying these systems under physiologically relevant conditions. In the case of neutron reflectometry, new experimental protocols allow for the unique structural determination of one-dimensional membrane profiles, while the advantages offered by synchrotron radiation (e.g., high flux and spatial resolution) make X-rays an excellent tool for addressing questions pertaining to membrane interactions. Most recently, holographic techniques are evolving so that one day they may be able to resolve, to atomic resolution, the structure of poorly crystallized membrane associated proteins.

Detrimental and Protective Bystander Effects: A Model Approach

Abstract: Schollnberger, H., Mitchel, R. E. J., Redpath, J. L., Crawford-Brown, D. J. and Hofmann, W. Detrimental and Protective Bystander Effects: A Model Approach. Radiat. Res. 168, 614–626 (2007). This work integrates two important cellular responses to low doses, detrimental bystander effects and apoptosis-mediated protective bystander effects, into a multistage model for chromosome aberrations and in vitro neoplastic transformation: the State-Vector Model. The new models were tested on representative data sets that show supralinear or U-shaped dose responses. The original model without the new low-dose features was also tested for consistency with LNT-shaped dose responses. Reductions of in vitro neoplastic transformation frequencies below the spontaneous level have been reported after exposure of cells to low doses of low-LET radiation. In the current study, this protective effect is explained with bystander-induced apoptosis. An important data set that shows a low-dose detrimental bystander effect f...

A militarily fielded thermal neutron activation sensor for landmine detection

Abstract: The Canadian Department of National Defence has developed a teleoperated, vehicle-mounted, multi-sensor system to detect anti-tank landmines on roads and tracks in peacekeeping operations. A key part of the system is a thermal neutron activation (TNA) sensor which is placed above a suspect location to within a 30 cm radius and confirms the presence of explosives via detection of the 10.835 MeV gamma ray associated with thermal neutron capture on 14N. The TNA uses a 100 μ g 252Cf neutron source surrounded by four 7.62 cm × 7.62 cm NaI(Tl) detectors. The system, consisting of the TNA sensor head, including source, detectors and shielding, the high-rate, fast pulse processing electronics and the data processing methodology are described. Results of experiments to characterize detection performance are also described. The experiments have shown that anti-tank mines buried 10 cm or less can be detected in roughly a minute or less, but deeper mines and mines significantly displaced horizontally take considerably longer time. Mines as deep as 30 cm can be detected for long count times (1000 s). Four TNA detectors are now in service with the Canadian Forces as part of the four multi-sensor systems, making it the first militarily fielded TNA sensor and the first militarily fielded confirmation sensor for landmines. The ability to function well in adverse climatic conditions has been demonstrated, both in trials and operations.

Characterization of protein resistant, grafted methacrylate polymer layers bearing oligo(ethylene glycol) and phosphorylcholine side chains by neutron reflectometry.

Abstract: Neutron reflectometry was used to investigate the structures of end-tethered protein resistant polymer layers based on poly(oligo(ethylene glycol) methyl ether methacrylate) [poly(OEGMA)] and poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)]. Layers having different graft densities were studied in both the dry and wet states. A stretched parabolic model was used to fit the neutron data, resulting in a one-dimensional scattering length density profile of the polymer volume fraction normal to the film. Measured in D2O, the cutoff thicknesses of OEGMA and MPC layers at high graft density (0.39 chains/nm2 for OEGMA and 0.30 chains/nm2 for MPC) and a chain length of 200 repeat units were 450 and 470 A, respectively, close to their contour length of 500 A, suggesting that the grafts become highly hydrated when exposed to water. It was also found that at similar graft density and chain length, the volume fraction profiles of poly(OEGMA) and poly(MPC) layers are similar, in line with the authors’ previous results showing that these surfaces have similar protein resistance [W. Feng et al., BioInterphases 1, 50 (2006)]. The possible correlation of protein resistance to water content as indicated by the average number of water molecules per ethylene oxide (Nw,EO) or phosphorylcholine (Nw,PC) moiety was investigated. Nw,EO and Nw,PC, estimated from the volume fraction data, increased with decreasing graft density, and when compared to the reported number of water molecules in the hydration layers of EO and PC residues, led to the conclusion that water content slightly greater than the water of hydration resulted in protein resistant surfaces, whereas water content either less than or greatly in excess of the water of hydration resulted in layers of reduced protein resistance.

Scattering from laterally heterogeneous vesicles. II. The form factor

Abstract: Despite growing interest in the formation of domains or `rafts' in cell and model membranes, there have been relatively few attempts to characterize such systems via scattering techniques. Previously [Pencer et al. (2006). J. Appl. Cryst. 39, 293–303], it was demonstrated that the Porod invariant, Q, could be used to detect lateral segregation. Here, the general theory for scattering from laterally heterogeneous vesicles is outlined and form factors are derived for vesicles containing either single circular or annular domains. These form factors are then applied to the analysis of neutron scattering data from heterogeneous vesicles. Potential advantages and limitations of this technique are also discussed.

Small-angle neutron scattering to detect rafts and lipid domains.

Abstract: The detection and characterization of lateral heterogeneities or domains in lipid mixtures has attracted considerable interest, because of the roles that such domains may play in biological function. Studies on both model and cell membranes demonstrate that domains can be formed over a wide range of length scales, as small as nanometers in diameter up to microns. However, although the size and shape of micron-sized domains are readily visualized in freely suspended vesicles, by techniques such as fluorescence microscopy, imaging of nanometer-sized domains has thus far been performed only on substrate-supported membranes (through, e.g., atomic force microscopy), whereas additional evidence for nanodomains has depended on indirect detection (through, e.g., nuclear magnetic resonance or fluorescence resonance energy transfer). Small-angle neutron scattering (SANS) is a technique able to characterize structural features on nanometer length scales and can be used to probe freely suspended membranes. As such, SANS shows promise to characterize nanometer-sized domains in model membranes. The authors have recently demonstrated the efficacy of SANS to detect and characterize nanodomains in freely suspended mixed lipid vesicles.

Portable spectroscopic neutron probe.

Abstract: Bubble Technology Industries (BTI) has built a revolutionary portable neutron scintillation spectrometer, N-Probe, designed to be used by non-specialists for measurement of low-intensity neutron doses in the mixed field environments often found in nuclear utilities, fuel storage areas, fuel and waste processing operations and military applications. It is compatible with the current generation of BTI MICROSPEC analysers and shares the philosophy of spectral dosimetry with other BTI spectroscopic probes, where the dosimetric quantities are computed from the spectrum using appropriate fluence-dose conversion functions.

Thermoelectric Properties of Re3Ge0.6As6.4 and Re3GeAs6 in Comparison to Mo3Sb5.4Te1.6

Abstract: Heavily doped narrow gap semiconductors with complex crystal structures are prime candidates for thermoelectric materials. The arsenides Re3(Ge,As)7 are new examples with promising thermoelectric properties, comparable with the isostructural Mo3Sb5.4Te1.6, a competitive high-temperature material. Various doping levels may be achieved by using different Ge/As ratios. Re3(Ge,As)7 was prepared by heating the elements in the desired ratios in evacuated silica tubes between 600 and 800 °C. Re3GeAs6 crystallizes in the cubic Ir3Ge7 type, space group Im3m, with a = 8.73202(8) A. It exhibits high Seebeck coefficient, high electrical conductivity, and reasonably low thermal conductivity. Moreover, the thermoelectric figure-of-merit ZT = TS2σ/κ increases rapidly with increasing temperature, as desired for high-temperature thermoelectrics.

Low-background single-crystal silicon sample holders for neutron powder diffraction

Abstract: Neutron diffraction measurements on weakly scattering or highly absorbing samples may demand custom mounting solutions. Two low-background sample holders based on inexpensive single-crystal silicon are described. One uses a conventional cylindrical geometry and is optimized for weakly scattering materials, while the other has a large-area flat-plate geometry and is designed for use with highly absorbing samples. Both holders yield much lower backgrounds than more conventional null-matrix or null-scattering materials and are essentially free from interfering Bragg peaks.

Mechanical Properties of Zr-2.5Nb Pressure Tubes Made from Electrolytic Powder

Abstract: Pressure tubes in CANDU and RBMK reactors are fabricated from Zr-2.5Nb alloy. This paper describes the mechanical properties of tubes used in power reactors made by four routes using electrolytic powder as the base material. The microstructures developed by each route are distinct: CW: cold-worked material consists of flattened α-Zr grains surrounded by a skin of β-Zr phase; used in CANDU 6 reactors. CW-A: material that was cold-worked and annealed at 540°C contains elongated α-Zr grains mixed with equiaxed α-Zr grains and particles of β-Nb phase; used in all RBMK 1000 reactors. TMT-1: material quenched from the (α+β)-Zr phase into water follow by cold-working consists of α′-phase and between 10 and 20 % of untransformed α-phase; used in RBMK 1500, Ignalina 1. TMT-2: material quenched from the (α+β)-phase into argon-helium gas mixture followed by cold-working consists of Widmanstatten α-phase and untransformed α-phase. This material is used in RBMK 1500, Ignalina 2. The CW and TMT-2 tubes have a higher proportion of grains with basal plane normals in the transverse direction, FT of 0.52 to 0.57, than in the radial direction, FR of 0.38, while quenched and annealed materials (TMT-1 and CW-A) have similar values of FT and FR, about 0.38 in quenched materials and 0.41 in annealed materials. Transverse tensile strength, crack growth resistance, dJ/da, and axial crack velocity, VH, of delayed hydride cracking (DHC) were evaluated, using standard techniques, between 250 and 300°C on as-fabricated materials. In-reactor creep deformation was evaluated from measurements of tube diameter in RBMK 1000s, RBMK 1500s and two CANDU 6 power reactors. Strength and crack growth resistance were measured on TMT-1 and TMT-2 tubes removed from Ignalina NPP Units 1 and 2 after 12–17 years of in-reactor service. As-received cold-worked material had the highest strength; the annealed material had the lowest strength while the quenched materials had intermediate strength. Irradiation increased the strength by about 200 MPa in all four materials. Although DHC is sensitive to texture and the distribution of the β-Zr phase, the dominating factor controlling crack velocity appears to be material strength: with an increase of strength by a factor of about two, VH increased by a factor of 30. Since harmful trace elements were well controlled during manufacturing, other factors affecting crack growth resistance could be assessed. Again, strength appeared important; dJ/da declined approximately linearly with increase in strength induced by irradiation, decreasing from about 350 to 100 MPa as the strength increased from about 250 to 850 MPa. The exception was TMT-2 material where crack growth resistance was maintained after irradiation. TMT-2 material also had good diametral creep resistance in-reactor, attributed to both its texture and grain structure. The other three materials had similar creep resistance controlled mostly by their texture.

THE ATOMIC STRUCTURE AND HYDROGEN BONDING OF DEUTERATED MELANTERITE, FeSO4·7D2O

Abstract: The atomic structure of synthetic, deuterated melanterite (FeSO4·7D2O), a 14.0774(9), b 6.5039(4), c 11.0506(7) A, β 105.604(1)°, space group P 21/ c , Z = 4, has been refined from the combined refinement of 2.3731(1) A and 1.3308 A neutron powder-diffraction data to a R wp (tot) = 3.01% and R p (tot) = 2.18%. Both the short- and long-wavelength data were required to obtain a satisfactory fit in the Rietveld refinement. The results of this study confirm the previously proposed H-bonding scheme for melanterite. Small but significant variations of the Fe–O bond lengths are attributed to details of the hydrogen bonds to the oxygen atoms of the Fe octahedra. We draw comparisons between the monoclinic and orthorhombic heptahydrate sulfate minerals associated with mine wastes and relate differences in the structure to strengths and weaknesses in their H-bond networks.

Neutron microdosimetric response of a gas electron multiplier.

Abstract: A new high-sensitivity tissue equivalent proportional counter (TEPC) on the basis of the gas electron multiplier (GEM) detector used in high-energy physics experiments has been designed, constructed and tested in a variety of neutron fields. The GEM-TEPC makes use of a lithographically produced strip readout system to achieve the equivalent of a large number of miniature TEPC detector elements. This new device could be used as the basis of an electronic personal dosemeter for gamma and neutron mixed radiation fields.

Evidence for decay of spin waves above the pseudogap of underdoped YBa2Cu3O6.35

Abstract: The magnetic spectrum at high energies in heavily underdoped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.35}$ $({T}_{c}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ has been determined throughout the Brillouin zone. At low energy, the scattering forms a cone of spin excitations emanating from the antiferromagnetic (0.5,0.5) wave vector with an acoustic velocity similar to that of insulating cuprates. At high-energy transfers, below the maximum energy of $270\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at (0.5,0), we observe zone-boundary dispersion much larger and spectral weight loss more extensive than in insulating antiferromagnets. Moreover, we report phenomena not found in insulators, an overall lowering of the zone-boundary energies and a large damping of $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ of the spin excitations at high energies. The energy above which the damping occurs coincides approximately with the gap determined from transport measurements. We propose that as the energy is raised, the spin excitations encounter an extra channel of decay into particle-hole pairs of a continuum that we associate with the pseudogap.

Screening for effects of maternal age and birth order in a register of handicapped children.

Abstract: Summary Studies of effects of maternal age and birth-order differences on the risk of handicapping conditions among children are described. Data were obtained by linking 4716 records of cases in the British Columbia Register of Handicapped Children and Adults born in the years 1953 to 1958, to birth registrations of the same children, using as controls all live births in the province over the same period. Only the broad categories of disease recognized by the International Classification have been considered in the present account. The Register was used to screen for examples of special risks to (1) offspring of very young mothers, (2) offspring of older mothers, (3) first-born offspring, and (4) offspring of higher birth orders. Appropriate statistical analyses were used to ensure the detection and measurement of maternal age effects independent of an influence of birth order, and vice versa. The methods, both of obtaining data systematically and of analysis, serve to illustrate the kind of work that will be needed in the future if information on parental age and parity are to be used effectively to detect mutation in humans. I am indebted to Mrs Olwyn Tavendale who carried out all of the statistical calculations reported here (first by hand and again using the G20 computer), to Miss Martha Smith who wrote the computer program and did the record linkages by which information from the birth registration and handicap registration files was integrated, to Mrs Pamela Attree who wrote the ‘101 Simulator’ program which was used to produce the tabulations, and to Dr J. M. Kennedy who wrote the ‘Newstat’ program used for analysing the data. The project of which this study is a part has depended much upon the help and collaboration of A.E.C.L.'s Computation Centre, the British Columbia Vital Statistics Division, and the Health and Welfare Division of the Dominion Bureau of Statistics. Permission to use the handicap records in this study was obtained from the Health Branch, Department of Health and Welfare, Province of British Columbia. The permission was conditional upon strict observance of the oath of secrecy respecting the non-statistical information contained in the records.

On the fracture of cold-worked Zr-2.5Nb pressure tubes fabricated from 100% recycled material

Abstract: A study was completed on irradiated and unirradiated (offcut) material from Pickering Nuclear Generating Station (NGS) A Unit 3 pressure tubes which were fabricated from ingots of 100% recycled material (quadruple-vacuum-arc-melted). The toughness of the material is shown to be generally higher than that of previous material fabricated from < 100% recycled material (double-vacuum-arc-melted). Fractographic and microstructural studies demonstrate that variations in toughness of the low chlorine (< 1 ppm) material are predominantly due to preferential void nucleation at particles, in particular aligned zirconium phosphides. In the absence of such particles, very high toughness levels can be maintained after irradiation.