Nathan Almirall

Profile Information
Dr. Nathan Almirall
General Electric Global Research
Research Scientist - Metallurgy
University of California Santa Barbara
Atom Probe Tomography, Radiation Damage, Reactor Pressure Vessel
"Dose rate dependence of Cr precipitation in an ion-irradiated Fe18Cr alloy" Elaina Reese, Nathan Almirall, Takuya Yamamoto, Scott Tumey, G. Robert Odette, Emmanuelle Marquis, Scripta Materialia Vol. 146 2018 213-217 Link
"Infrastructure development for radioactive materials at the NSLS-II" Eric Dooryhee, Lynne Ecker, G. Robert Odette, David Sprouster, Peter Wells, Randy Weidner, Sanjit Ghose, Theodore Novakowski, Tiberiu Stan, Nathan Almirall, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment Vol. 880 2018 40-45 Link
The X-ray Powder Diffraction (XPD) Beamline at the National Synchrotron Light Source-II is a multipurpose instrument designed for high-resolution, high-energy X-ray scattering techniques. In this article, the capabilities, opportunities and recent developments in the characterization of radioactive materials at XPD are described. The overarching goal of this work is to provide researchers access to advanced synchrotron techniques suited to the structural characterization of materials for advanced nuclear energy systems. XPD is a new beamline providing high photon flux for X-ray Diffraction, Pair Distribution Function analysis and Small Angle X-ray Scattering. The infrastructure and software described here extend the existing capabilities at XPD to accommodate radioactive materials. Such techniques will contribute crucial information to the characterization and quantification of advanced materials for nuclear energy applications. We describe the automated radioactive sample collection capabilities and recent X-ray Diffraction and Small Angle X-ray Scattering results from neutron irradiated reactor pressure vessel steels and oxide dispersion strengthened steels.
"On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions" Nathan Almirall, Peter Wells, Takuya Yamamoto, G. Robert Odette, Journal of Nuclear Materials Vol. 536 2020 Link
Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (>3 wt%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs
"Precipitation and hardening in irradiated low alloy steels with a wide range of Ni and Mn compositions" G. Robert Odette, Nathan Almirall, Peter Wells, Takuya Yamamoto, Acta Materialia Vol. 2019 119-128 Link
Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPVtype steels, with an unusually wide range of dissolved Mn (0.06e1.34 at.%) and Ni (0.19e3.50 at.%) contents, were irradiated at z 290 C to z 1.4 1020 n/cm2 at an accelerated test reactor flux of z3.6 1012 n/cm2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with z Ni1.6Mn0.8. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed
"Precipitation in reactor pressure vessel steels under ion and neutron irradiation: On the role of segregated network dislocations" G. Robert Odette, Nathan Almirall, Takuya Yamamoto, Acta Materialia Vol. 212 2021 Link
"Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels" David Sprouster, Eric Dooryhee, John Sinsheimer, Sanjit Ghose, Peter Wells, Nathan Almirall, G. Robert Odette, Lynne Ecker, Tiberiu Stan, Scripta Materialia Vol. 113 2016 18-22 Link
Massive, thick-walled pressure vessels are permanent nuclear reactor structures that are exposed to a damaging flux of neutrons from the adjacent core. The neutrons cause embrittlement of the vessel steel that grows with dose (fluence), as manifested by an increasing ductile-to-brittle fracture transition temperature. Extending reactor life requires demonstrating that large safety margins against brittle fracture are maintained at the higher neutron fluence associated with beyond 60 years of service. Here synchrotron-based x-ray diffraction and small angle x-ray scattering measurements are used to characterize highly embrittling nm-scale Mn–Ni–Si precipitates that develop in the irradiated steels at high fluence. These precipitates lead to severe embrittlement that is not accounted for in current regulatory models. Application of the complementary techniques has, for the very first time, successfully identified the crystal structures of the nanoprecipitates, while also yielding self-consistent compositions, volume fractions and size distributions.
"The effect of composition variations on the response of steels subjected to high fluence neutron irradiation" Ben Jenkins, Nathan Almirall, G. Robert Odette, Materialia Vol. 11 2020 Link
A set of low alloy model reactor pressure vessel steels, with systematic variations in their Mn, Ni, and Si contents, were neutron-irradiated to high fluence (1.4 × 1020 n/cm2) in the Advanced Test Reactor at Idaho at 290°C and a flux of 3.6 × 1012 n/cm2s. The alloys were analysed using atom probe tomography and solute clusters were observed in each alloy, including in one alloy that contained low nominal levels of Mn (0.04 at. %) and Si (0.06 at. %). Changes in the mechanical properties of the alloys were correlated with cluster volume fractions. Whilst the effect of nominal composition was observed to influence cluster composition, cluster nucleation site was not observed to affect composition. Several grain boundaries were also analysed and the segregation behaviour of certain elements is discussed.
"The effect of phosphorus on precipitation in irradiated reactor pressure vessel (RPV) steels" Mukesh Bachhav, Nathan Almirall, Takuya Yamamoto, Emmanuelle Marquis, G. Robert Odette, Journal of Nuclear Materials Vol. 585 2023 Link
Embrittlement of light water reactor pressure vessel (RPV) steels by fast neutron irradiation may limit extended nuclear plant life. Embrittlement, which is manifested as increases in various indexes of a ductile to brittle transition temperatures (ΔT), is primarily due to hardening by nanoscale precipitates containing Cu, Ni, Mn, and Si, which form under irradiation. In addition to these elements, P has also been found to play a role in embrittlement. While only slightly enriched in the precipitates, hardening and embrittlement increase with trace P concentrations in low-Cu steels. Here, we characterize the individual and synergistic irradiation precipitation and hardening mechanisms in a series of RPV steels containing no to low-Cu and with systematic variations in Ni and P. The steels were irradiated to a fluence of ∼ 1.38×1020 n/cm2 at ∼ 292 °C in the UCSB ATR-2 experiment. In nominally Cu-free medium and high-Ni RPV steels, atom probe tomography shows that P and Ni promote precipitation of P-Mn-Si-Ni and Mn-Si-Ni precipitates, respectively. The precipitate microstructure correlates with the observed irradiation hardening.
"Thermodynamic and kinetic modeling of Mn-Ni-Si precipitates in low-Cu reactor pressure vessel steels" Nathan Almirall, Philip Edmondson, G. Robert Odette, Peter Wells, Huibin Ke, Leland Barnard, Dane Morgan, Acta Materialia Vol. 138 2017 10-26 Link
Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causes excess irradiation embrittlement of reactor pressure vessel (RPV) steels at high, extended-life fluences. Thus, a new and unique, semi-empirical cluster dynamics model was developed to study the evolution of MNSPs in low-Cu RPV steels. The model is based on CALPHAD thermodynamics and radiation enhanced diffusion kinetics. The thermodynamics dictates the compositional and temperature dependence of the free energy reductions that drive precipitation. The model treats both homogeneous and heterogeneous nucleation, where the latter occurs on cascade damage, like dislocation loops. The model has only four adjustable parameters that were fit to an atom probe tomography (APT) database. The model predictions are in semi-quantitative agreement with systematic Mn, Ni and Si composition variations in alloys characterized by APT, including a sensitivity to local tip-to-tip variations even in the same steel. The model predicts that heterogeneous nucleation plays a critical role in MNSP formation in lower alloy Ni contents. Single variable assessments of compositional effects show that Ni plays a dominant role, while even small variations in irradiation temperature can have a large effect on the MNSP evolution. Within typical RPV steel ranges, Mn and Si have smaller effects. The delayed but then rapid growth of MNSPs to large volume fractions at high fluence is well predicted by the model. For purposes of illustration, the effect of MNSPs on transition temperature shifts are presented based on well-established microstructure-property and property-property models.
"Thermodynamics and kinetics of core-shell versus appendage co-precipitation morphologies: An example in the Fe-Cu-Mn-Ni-Si system" Shipeng Shu, Peter Wells, Nathan Almirall, G. Robert Odette, DD Morgan, Acta Materialia Vol. 157 2018 298-306 Link
"Effect of Composition on Response of Model Alloys To Neutron Irradiation" Ben Jenkins, G. Robert Odette, Nathan Almirall, IGRDM May 19-23, (2019)
"Grain Boundary Analysis of Neutron-Irradiated Reactor Pressure Vessel Model Steels Using Correlative Transmission Kikuchi Diffraction And Atom Probe Tomography" Ben Jenkins, G. Robert Odette, Nathan Almirall, IGRDM May 19-23, (2019)
"Influence of Irradiation Conditions on Precipitation Behavior in Fe-Cr and Ni Alloys" Emmanuelle Marquis, E Reese, LJ Yu, Nathan Almirall, Takuya Yamamoto, G. Robert Odette, Grace Burke, Annual TMS meeting March 10-14, (2019)
"SOME USEFUL MECHANICAL PROPERTY CORRELATIONS FOR NUCLEAR REACTOR PRESSURE VESSEL STEELS" Randy Nanstad, G. Robert Odette, William Server, Mikhail Sokolov, Nathan Almirall, ASME 2018 July 15-20, (2018)
NSUF Articles:
U.S. DOE Nuclear Science User Facilities Awards 35 Rapid Turnaround Experiment Research Proposals - Awards total approximately $1.3 million These projects will continue to advance the understanding of irradiation effects in nuclear fuels and materials in support of the mission of the DOE Office of Nuclear Energy. Wednesday, September 20, 2017 - Calls and Awards
RTE 1st Call Awards Announced - Projects total approximately $1.4 million These projects will continue to advance the understanding of irradiation effects in nuclear fuels and materials in support of the mission of the DOE-NE. Friday, February 8, 2019 - Calls and Awards
DOE awards 39 RTE Projects - Projects total approximately $1.3 million Thursday, February 1, 2018 - Calls and Awards
NSUF awards 24 Rapid Turnaround Experiment proposals - Approximately $1.42M has been awarded. Wednesday, February 8, 2023 - Calls and Awards