Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

γH2AX, 53BP1 and Rad51 protein foci changes in mesenchymal stem cells during prolonged X-ray irradiation

Abstract: // Anastasia Tsvetkova 1 , Ivan V. Ozerov 2, 3 , Margarita Pustovalova 2, 4 , Anna Grekhova 2, 4, 5 , Petr Eremin 6 , Natalia Vorobyeva 2, 3 , Ilya Eremin 6 , Andrey Pulin 6 , Vadim Zorin 6, 7 , Pavel Kopnin 8 , Sergey Leonov 9 , Alex Zhavoronkov 3 , Dmitry Klokov 10 and Andreyan N. Osipov 2, 3, 4, 9 1 Lomonosov Moscow State University, Moscow 119991, Russia 2 State Research Center - Burnasyan Federal Medical Biophysical Center of Federal Medical Biological Agency (SRC-FMBC), Moscow 123098, Russia 3 Insilico Medicine, Inc, ETC, Johns Hopkins University, Baltimore, Maryland 21218, USA 4 Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 119991, Russia 5 Emanuel Institute for Biochemical Physics, Russian Academy of Sciences, Moscow 119991, Russia 6 Federal State Budgetary Institution “Central Clinical Hospital with Outpatient Health Center” of The Business Administration for The President of The Russian Federation, Moscow 121359, Russia 7 PJSC Human Stem Cells Institute, Moscow 119333, Russia 8 N.N. Blokhin Cancer Research Center, Moscow 115478, Russia 9 Moscow Institute of Physics and Technology, Moscow 141700, Russia 10 Canadian Nuclear Laboratories, Chalk River, Ontario K0J1P0, Canada Correspondence to: Andreyan N. Osipov, email: andreyan.osipov@gmail.com Keywords: DNA double strand breaks, DNA repair, mesenchymal stem cells, X-rays, continuous irradiation Received: December 02, 2016 Accepted: June 20, 2017 Published: July 12, 2017 ABSTRACT At high exposure levels ionizing radiation is a carcinogen. Little is known about how human stem cells, which are known to contribute to tumorigenesis, respond to prolonged radiation exposures. We studied formation of DNA double strand breaks, accessed as γH2AX and 53BP1 foci, in human mesenchymal stem cells (MSCs) exposed to either acute (5400 mGy/h) or prolonged (270 mGy/h) X-irradiation. We show a linear γH2AX and 53BP1 dose response for acute exposures. In contrast, prolonged exposure resulted in a dose-response curve that had an initial linear portion followed by a plateau. Analysis of Rad51 foci, as a marker of homologous recombination, in cells exposed to prolonged irradiation revealed a threshold in a dose response. Using Ki67 as a marker of proliferating cells, we show no difference in the γH2AX distribution in proliferating vs. quiescent cells. However, Rad51 foci were found almost exclusively in proliferating cells. Concurrent increases in the fraction of S/G2 cells were detected in cells exposed to prolonged irradiation by scoring CENPF-positive cells. Our data suggest that prolonged exposure of MSCs to ionizing radiation leads to cell cycle redistribution and associated activation of homologous recombination. Also, proliferation status may significantly affect the biological outcome, since homologous repair is not activated in resting MSCs.

Random Excitation Forces in Tube Bundles Subjected to Two-Phase Cross-Flow

Abstract: Data from two experimental programs have been analyzed to determine the characteristics of the random excitation forces associated with two-phase cross-flow in tube bundles. Large-scale air-water flow loops in France and Canada were used to generate the data. Tests were carried out on cantilevered, clamped-pinned, and clamped-clamped tubes in normal-square, parallel-triangular, and normal-triangular configurations. Either strain gages or force transducers were used to measure the vibration response of a centrally located tube as the tube array was subjected to a wide range of void fractions and flow rates. Power spectra were analyzed to determine the effect of parameters such as tube diameter, frequency, flow rate, void fraction, and flow regime on the random excitation forces. Normalized expressions for the excitation force power spectra were found to be flow-regime dependent. In the churn flow regime, flow rate and void fraction had very little effect on the magnitude of the excitation forces. In the bubble-plug flow regime, the excitation forces increased rapidly with flow rate and void fraction.

The Effect of Fluence and Irradiation Temperature on Delayed Hydride Cracking in Zr-2.5Nb

Abstract: Zirconium alloys are susceptible to a stable cracking process called delayed hydride cracking (DHC). DHC has two stages: (a) crack initiation that requires a minimum crack driving force (the threshold stress intensity factor, K{sub IH}) and (b) stable crack growth that is weakly dependent on K{sub l}. The value of K{sub lH} is an important element in determining the tolerance of components to sharp flaws. The rate of cracking is used in estimating the action time for detecting propagating cracks before they become unstable. Hence, it is important for reactor operators to know how these properties change during service in reactors where the components are exposed to neutron irradiation at elevated temperatures. DHC properties were measured on a number of components, made from the two-phase alloy Zr-2.5 Nb, irradiated at temperatures in the range of 250 to 290 C in fast neutron fluxes (E {>=} 1 MeV) between 1.6 {times} 10{sup 17} and 1.8 {times} 10{sup 18} n/m{sup 2} {center_dot} s to fluences between 0.01 {times} 10{sup 25} and 9.8 {times} 10{sup 25} n/m{sup 2}. The neutron irradiation reduced K{sub IH} by about 20% and increased the velocity of cracking by a factor of about five. The increase in crackmore » velocity was greatest with the lowest irradiation temperature. These changes in the rack velocity by neutron irradiation are explained in terms of the combined effects of irradiation hardening associated with increased -type dislocation density, and {beta}-phase decomposition. While the former process increases crack velocity, the latter process decrease it. The combined contribution is controlled by the irradiation temperature. X-ray diffraction analyses showed that the degree of {beta}-phase decomposition was highest with an irradiation temperature of 290 C while -type dislocation densities were highest with an irradiation temperature of 250 C.« less

Low moments in heavy-fermion systems

Abstract: A theoretical and experimental search is underway for hidden order parameters in the phase of weakly broken symmetry that precedes superconductivity in several heavy-fermion intermetallics. The apparent order parameter in URu2Si2 breaks time reversal symmetry, as would magnetic dipole order, but with a moment that is too small to account for the large specific heat jump at the transition. Polarized neutron scattering and high-field experiments have eliminated theories proposing that quadrupoles or exotic triple-spin states are directly responsible for the new diffraction peaks of the ordered phase. For the dynamic behaviour it is suggested that, if the weak peaks were extrinsic, then quadrupoles could account for the soft spin-wave mode, the gap formation and longitudinal symmetry of the spin spectrum.