TRACE simulation of sodium boiling in pin bundle experiments under loss-of-flow conditions

In the framework of PSI's FAST code system, the thermal–hydraulic code TRACE is being extended for representation of sodium two-phase flow. As the currently available version (v.5) is limited to the simulation of only single-phase sodium flow, its applicability range is not enough to study the behavior of a Generation IV sodium-cooled fast reactor (SFR) during transients in which boiling is anticipated. The work reported here concerns the extension of the non-homogeneous, non-equilibrium two-fluid models, which are available in TRACE for steam-water, to sodium two-phase flow simulation. The conventional correlations for ordinary gas–liquid flows are used as basis, with optional correlations specific to liquid metal where necessary. A number of new models for representation of the constitutive equations specific to sodium, with a particular emphasis on the interfacial transfer mechanisms, have been implemented and compared with the original closure models.A first assessment of the extended TRACE version has been carried out, by using the code to model experiments that simulate a loss-of-flow (LOF) accident in a SFR. One- and two-dimensional representations of the test section have been considered. Comparison of the 1D model predictions, with both experiment and SIMMER-III code predictions, confirm the ability of the extended TRACE code to predict the principal sodium boiling phenomena. Two-dimensional representation of the test section, however, has been found necessary for providing more detailed comparisons with the experimental data and thereby studying, in greater detail, the influence of the physical models on the calculated results.The paper thus presents a first-of-its-kind application of TRACE to two-phase sodium flow. It shows the capability of the extended code to predict sodium boiling onset, flow regimes, pressure evolution, dryout, etc. Although the numerical results are in good agreement with the experimental data, the physical models should be further improved. Other integral experiments are planned to be simulated, in order to further develop and validate the two-phase sodium flow modeling.     

Void reactivity coefficient benchmark results for a 10 × 10 BWR assembly in the full 0–100% void fraction range

A boiling water reactor SVEA-96+ fresh fuel lattice has been used as the basis for a benchmark study of the void reactivity coefficient at assembly level in the full voidage range. Results have been obtained using the deterministic codes CASMO-4, HELIOS, PHOENIX, BOXER and the probabilistic code MCNP4C, combined for almost all cases with different cross section libraries. A statistical analysis of the results obtained showed that the void reactivity coefficient tends to become less negative beyond 80% void and that the discrepancies between codes tend to increase from less than 15% at voidages lower than 40% to more than 25% at voidages higher than 70%. The void reactivity coefficient results and the corresponding differences between codes were isotopically decomposed to interpret discrepancies. The isotopic decomposition shows that the minimum observed in the void reactivity coefficient between 80% and 90% void is largely due to the decrease in the relative importance of the ¹⁵⁷Gd(n, γ) rate with increasing voidage, and that the fundamental discrepancies between codes or libraries are mainly governed by the different predictions of the ²³⁸U(n, γ) variation with voidage.     

Development and benchmarking of a mechanical model for GFR plate-type fuel

The reference fuel design currently being considered within the Generation-IV Gas-cooled Fast Reactor (GFR) project is a ceramic plate matrix with a honeycomb inner structure containing small fuel cylinders. The fuel is mixed plutonium–uranium carbide, while the matrix material is silicon carbide. The present paper describes the mechanical part of a thermal–mechanical model being developed for studying the transient behavior of this highly heterogeneous fuel type. Benchmarking has been carried out against detailed finite-elements modeling (FEM).The resultant thermal–mechanical model can provide reliable fuel and cladding (matrix) stress/strain conditions to evaluate temperatures and neutronic feedbacks. As such, it has been integrated into PSI’s coupled code system “FAST”, which aims at the comprehensive safety analysis of advanced fast reactor systems.The detailed FEM analysis of the GFR fuel has been useful not only for benchmarking the new model, but also for obtaining an in-depth understanding of fuel stress/strain characteristics, which cannot be reproduced with simplified models. Thereby, the range of applicability of the new model has clearly been defined. In particular, the 3D FEM analysis has revealed a concentration of stresses at the pellet corners during pellet/matrix contact, which could lead to fuel element failure. This effect is found to be mitigated considerably, if the fuel pellets are shaped in a manner which enhances the contact area.     

Oxidation Behavior of Rare-Earth-Containing Pb-Free Solders

We have previously shown that small additions of the rare-earth (RE) element La to Sn-Ag-Cu alloys significantly increases their ductility, without significant loss in the overall strength. However, due to the high reactivity of La with oxygen, oxidation of the La-containing phases can affect the mechanical performance of the solder. In this work, we have investigated the effect of the addition of 2 wt.% Ce, La and Y on the oxidation behavior of Sn-3.9Ag-0.7Cu. Oxidation kinetics were established by heating samples in ambient air to 60°C, 95°C or 130°C for up to 250 h. Microstructural characterization of the samples, before and after oxidation, was conducted in order to determine the influence of RE-containing phases on the oxidation kinetics. The oxidation mechanism, including the phenomenon of Sn whiskering during oxidation, is also discussed.     

Effect of Rare-Earth (La,Ce, and Y) Additions on the Microstructure and Mechanical Behavior of Sn-3.9Ag-0.7Cu Solder Alloy

In this article, we report on the microstructure and mechanical properties of Ce- and Y-containing Sn-3.9Ag-0.7Cu solders. The microstructures of both as-processed solder and solder joints containing rare-earth (RE) elements (up to 0.5 wt pct) are more refined compared to conventional Sn-3.9Ag-0.7Cu, with decreases in secondary Sn dendrite size and spacing and a thinner Cu₆Sn₅ intermetallic layer at the Cu/solder interface. These results agree well with similar observations seen in La-containing solders reported previously. The monotonic shear behavior of reflowed Sn-3.9Ag-0.7Cu-X(Ce, Y)/Cu lap shear joints was studied as well as the creep behavior at 368 K (95 °C). The data were compared with results obtained for Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-XLa alloys. All RE-containing alloys exhibited creep behavior similar to Sn-3.9Ag-0.7Cu. Alloys with Ce additions exhibited a small decrease in ultimate shear strength but higher elongations compared with Sn-Ag-Cu. Similar observations were seen in La-containing solders. The influence of the RE-containing intermetallics (CeSn₃ and YSn₃) that form in these alloys on the microstructural refinement, solidification behavior, and mechanical performance of these novel materials is discussed.     

Integral measurements with a plutonium inert matrix fuel rod in a heterogeneous light water reactor lattice

As part of PSI's validatory efforts for neutronics calculations pertaining to inert matrix fuel (IMF) deployment in light water reactors, first-of-their-kind integral measurements have been carried out at the PROTEUS facility employing a specially fabricated Pu-Er-Zr IMF rod in a heterogeneous (boiling water reactor) test lattice. Analogous experiments have also been conducted with MOX and “dummy” IMF rods, providing the basis for a systematic comparison of experimental and calculational results.     

Megavoltage imaging with low Z targets: implementation and characterization of an investigational system

The poor quality of stereotactic radiotherapy portal images is a limiting factor in precise image registration. To alleviate this problem, a low atomic number (Z) target was implemented on our Siemens MXE linear accelerator. This investigational system was used to assess the performance of various target materials by filming an aluminum contrast object. Beryllium, carbon and conventional target materials were studied. The bremsstrahlung spectra of these materials were simulated using Monte Carlo techniques. These spectra were used to calculate the dependence of narrow beam contrast on phantom thickness for verification of the data measured from film. A Monte Carlo simulation of the beryllium spectrum in a wide beam geometry was used to evaluate the effect of phantom-to-film distance on contrast. Although the same degree of contrast improvement with distance was not realized in practice, the improvement in image quality rivaled that achieved using a scatter reduction grid. A comparison of conventional localization images of the head and neck of an anthropomorphic phantom with images produced with a beryllium or carbon target and a mammography film and screen system supports earlier suggestions that the technique is clinically useful.     

A two-term MEDLINE search strategy for identifying randomized trials in obstetrics and gynecology

In this study, we examined whether endothelin (ET) plays a role in the short-term increase in mean arterial pressure (MAP) after nitric oxide synthase (NOS) inhibition with N(omega)-nitro-L-arginine methyl ester (L-NAME) in stroke-prone spontaneously hypertensive rats (SHRSPs). Experiments were performed by using Inactin-anesthetized male SHRSPs that were pretreated with chlorisondamine to block reflex autonomic cardiovascular effects. Injection of L-NAME (10 mg/kg, i.v.), but not D-NAME, produced rapid and marked increases (74 +/- 3 mm Hg) in MAP that were sustained for >1 h. In SHRSPs that were treated with the ET(A/B) receptor antagonist, L-754,142 (15 mg/kg + 15 mg/kg/h), L-NAME increased MAP by 45 +/- 4 mm Hg (p < 0.0001 compared with L-NAME alone). L-754,142 blocked pressor responses to big ET-1 by >90% but was without effect on pressor responses to norepinephrine. Plasma levels of ET-1 averaged 5 +/- 1 pg/ml in animals given vehicle and were slightly increased in animals given either L-NAME alone (7 +/- 2 pg/ml) or L-754,142 alone (7 +/- 2 pg/ml) but increased markedly when L-NAME and L-754,142 were given together (114 +/- 18 pg/ml). This may relate to an effect of L-754,142 to block ET-receptor-mediated clearance of ET-1. We conclude that ET plays a role in the short-term pressor response after NOS inhibition in SHRSPs.     

Long-term potentiation in vivo in layers II/III of rat barrel cortex

Long-term potentiation was studied in vivo in the rat barrel cortex. It was found that LTP lasting several hours could be induced in layer II/III by tetanic stimuli applied in layer IV. The probability of inducing LTP at a given site was high (86%) provided that the electrodes were not displaced too far horizontally. LTP was not observed if the stimulating electrode was located on the far side of the neighbouring barrel-column from the recording electrode. The strongest LTP was induced by stimulating layer IV septal locations or the edge of the barrel and recording in the near half of the neighbouring barrel. However, examples were found of LTP from layer IV to II/III within the same barrel, within the same septum and from barrel to adjacent septum. The probability of inducing LTP on a particular occasion was greatly increased by iontophoresis of bicuculline at the recording site during the tetanus (from 20 to 55% judged by a change in peak amplitude). The average increase in the peak amplitude was 29 +/- 3.2% for protocol 1 (urethane anesthesia, monopolar stimulation) and 23 +/- 7% for protocol 2 (barbiturate anesthesia, bipolar stimulation). The probability of inducing LTP was greater if the first tetanus was accompanied by BMI application (67%) than for any subsequent attempts (39%). These results suggest it should be possible to study the effect of LTP on sensory processing in defined positions within the barrel field.