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Hunt R.D.,Oak Ridge National Laboratory | Silva C.M.,Oak Ridge National Laboratory | Lindemer T.B.,Harbach Engineering and Solutions | Johnson J.A.,Oak Ridge National Laboratory | Collins J.L.,Oak Ridge National Laboratory
Journal of Nuclear Materials | Year: 2014

The US Department of Energy is considering a new nuclear fuel that would be less susceptible to ruptures during a loss-of-coolant accident. The fuel would consist of tristructural isotropic coated particles with dense uranium nitride (UN) kernels with diameters of 650 or 800 μm. The objectives of this effort are to make uranium oxide microspheres with adequately dispersed carbon nanoparticles and to convert these microspheres into UN spheres, which could be then sintered into kernels. Recent improvements to the internal gelation process were successfully applied to the production of uranium gel spheres with different concentrations of carbon black. After the spheres were washed and dried, a simple two-step heat profile was used to produce porous microspheres with a chemical composition of UC0.07-0.10N0.90-0.93. The first step involved heating the microspheres to 2023 K in a vacuum, and in the second step, the microspheres were held at 1873 K for 6 h in flowing nitrogen. © 2013 Elsevier B.V. All rights reserved. Source


Lindemer T.B.,Harbach Engineering and Solutions | Voit S.L.,Oak Ridge National Laboratory | Silva C.M.,Oak Ridge National Laboratory | Besmann T.M.,Oak Ridge National Laboratory | Hunt R.D.,Oak Ridge National Laboratory
Journal of Nuclear Materials | Year: 2014

The US Department of Energy is developing a new nuclear fuel that would be less susceptible to ruptures during a loss-of-coolant accident. The fuel would consist of tristructural isotropic coated particles with uranium nitride (UN) kernels with diameters near 825 μm. This effort explores factors involved in the conversion of uranium oxide-carbon microspheres into UN kernels. An analysis of previous studies with sufficient experimental details is provided. Thermodynamic calculations were made to predict pressures of carbon monoxide and other relevant gases for several reactions that can be involved in the conversion of uranium oxides and carbides into UN. Uranium oxide-carbon microspheres were heated in a microbalance with an attached mass spectrometer to determine details of calcining and carbothermic conversion in argon, nitrogen, and vacuum. A model was derived from experiments on the vacuum conversion to uranium oxide-carbide kernels. UN-containing kernels were fabricated using this vacuum conversion as part of the overall process. Carbonitride kernels of ∼89% of theoretical density were produced along with several observations concerning the different stages of the process. © 2013 Elsevier B.V. All rights reserved. Source


De Almeida V.F.,Oak Ridge National Laboratory | Hunt R.D.,Oak Ridge National Laboratory | Collins J.L.,Harbach Engineering and Solutions
Journal of Nuclear Materials | Year: 2010

Drop-on-demand generation is an alternative approach to the traditional vibrating nozzle used for the production of nuclear fuel microspheres via the internal gelation method. We integrated a low-cost pneumatic setup and demonstrated that the drop-on-demand approach has some advantages, such as low inventory of feed solution (attractive for laboratory-scale research), improved drop diameter control, reproducibility, scale-up to desired throughput by simple multiplication of the number of dispensing units, and simple remote operation. However, limitations on reproducibility and drop diameter control still exist due to the intrinsic variation of physical properties, viscosity, and dispensing-tip wettability during the internal gelation process. These adverse effects can be mitigated, to a certain extent, by carefully controlling the temperature of the feed as uniformly as possible. We validated the drop-on-demand generation method by producing solid kernels of yttrium-stabilized zirconia and soft gel microspheres of iron hydroxide. In addition, we have measured the diameter change at each principal process stage. Based on the observed gas entrainment/absorption in the gel spheres, we conjectured that aging and washing are likely the critical stages determining the final precision to which microspheres can be made. Finally, we comment on potential improvements that add robustness to the method for handling other metal precursors in aqueous solutions. © 2010 Elsevier B.V. Source


Hunt R.D.,Oak Ridge National Laboratory | Montgomery F.C.,Oak Ridge National Laboratory | Collins J.L.,Harbach Engineering and Solutions
Journal of Nuclear Materials | Year: 2010

The internal gelation process has been used to make plutonium gel spheres and zirconium gel spheres with stabilized yttrium. However, attempts to convert these amorphous gel spheres into kernels have failed due to cracking during the subsequent heat treatments. The porosity of the amorphous microspheres is typically not sufficient to permit the gases that are formed during subsequent heat treatments to escape. The microspheres will crack when the internal pressure becomes too great. In this study, several treatment techniques were applied to zirconium microspheres stabilized with yttrium in an effort to reduce or eliminate cracking. A combination of water washes, a pressurized water treatment at 473 K for 3 h, and a Dowanol PM treatment was shown to eliminate the cracking problem with the zirconium microspheres, which were heated to 1438 K. © 2010 Elsevier B.V. All rights reserved. Source


Hunt R.D.,Oak Ridge National Laboratory | Silva G.W.C.M.,Oak Ridge National Laboratory | Lindemer T.B.,Harbach Engineering and Solutions | Anderson K.K.,AREVA | Collins J.L.,Oak Ridge National Laboratory
Journal of Nuclear Materials | Year: 2012

The US Department of Energy continues to use the internal gelation process in its preparation of tristructural isotropic coated fuel particles. The focus of this work is to develop uranium fuel kernels with adequately dispersed silicon carbide (SiC) nanoparticles, high crush strengths, uniform particle diameter, and good sphericity. During irradiation to high burnup, the SiC in the uranium kernels will serve as getters for excess oxygen and help control the oxygen potential in order to minimize the potential for kernel migration. The hardness of SiC required modifications to the gelation system that was used to make uranium kernels. Suitable processing conditions and potential equipment changes were identified so that the SiC could be homogeneously dispersed in gel spheres. Finally, dilute hydrogen rather than argon should be used to sinter the uranium kernels with SiC. © 2012 Elsevier B.V. All rights reserved. Source

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