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.     

The product of the ade1: gene in Schizosaccharomyces pombe: a bifunctional enzyme catalysing two distinct steps in purine biosynthesis

The assignment of the known ade genes to steps in purine biosynthesis in Schizosaccharomyces pombe has been completed with the demonstration that an ade3 mutants lacks FGAR amidotransferase, ade1A mutants lack GAR synthetase and ade1B mutants lack AIR synthetase. A comparison of enzyme activity with map position for ade1 mutants shows that (1) complementing ade1A mutants lack GAR synthetase but posses wild type amounts of AIR synthetase, (2) complementing ade1B mutants lack AIR synthetase but posses variable amounts of GAR synthetase, (3) non-complementing mutants lack both activities. In wild type strains the two activities fractionate together throughout a hundred-fold purification. Hence the ade1 gene appears to code for a bifunctional enzyme catalysing two distinct steps in purine biosynthesis. The two activities are catalysed by two different regions of the polypeptide chain which can be altered independently by mutation. Gel filtration studies on partially purified enzymes from wild type and various complementing mutant strains, indicate that the bifunctional enzyme is a multimer consisting of between four and six sub-units of 40,000 daltons each. GAR synthetase activity is associated with both the monomeric and multimeric forms but AIR synthetase is only associated with the multimer. A comparison of enzyme levels between diploids and their original complementing haploid strains suggests that complementation is due to hybrid enzyme formation.     

Characterization of silicon carbide joints fabricated using SiC particulate-reinforced Ag–Cu–Ti alloys

CVD silicon carbide was brazed to itself using two Ag–Cu–Ti braze alloys reinforced with SiC particulates to control braze thermal expansion and enhance joint strength. Powders of the braze alloys, Ticusil (composition in wt%: Ag–26.7Cu–4.5Ti, T_L: 900 °C) and Cusil-ABA (Ag–35.3Cu–1.75Ti, T_L: 815 °C) were pre-mixed with 5, 10 and 15 wt% SiC particulates (～20–30 μm) using glycerin to create braze pastes that were applied to the surfaces to be joined. Joints were vacuum brazed and examined using optical microscopy (OM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and the Knoop hardness test. The SiC particles were randomly distributed in the braze matrix and bonded to it via reaction with the titanium from the braze alloy. Titanium together with Si and C segregated at the particle/braze interface, and promoted nucleation and precipitation of the Cu-rich secondary phase on particle surfaces. The Si–Ti–C-rich reaction layers also formed at the interface between CVD SiC substrate and the braze alloy. The loss of Ti in the reaction with SiC particulates did not impair either the bond quality or the thickness of the reaction layer on the CVD SiC substrate. Microhardness measurements showed that the dispersed SiC particulates lowered the braze hardness by depleting the braze matrix of Ti. Theoretical calculations indicated the CTE of the braze to decrease by nearly 45–60% with the incorporation of about 45 vol% SiC.     

Prediction of drinking outcomes for male alcoholics after 10 to 14 years

This study reports on the long-term outcomes of 360 men who were hospitalized for alcoholism during 1980 to 1984 and followed at 12 months and again 10 to 14 years later. At the 10/14-year follow-up, 96 (26.7%) men were confirmed as deceased; 255 (70.8%) men participated in the assessment/interview battery completed during baseline hospitalization. The battery consisted of psychosocial, alcohol-related, and psychiatric measures. Two distinct but highly correlated outcome measures were selected: a clinical rating scale and a factor score. Overall, predictors from baseline and 12-month follow-up included age at intake hospitalization, alcoholism severity, social stability, drinking days, and antisocial personality disorder. Approximately 37% of the assessed survivors were either totally abstinent or drinking nonabusively throughout the 10/14-year follow-up, whereas another 37% continued to drink abusively. Men who abstained or reduced alcohol intake reported better physical health at follow-up than those who continued to drink. Although our findings did not directly link alcoholism to death, they strongly indicate that chronic alcohol abuse may lead to premature death.     

DISCWorld: an environment for service-based matacomputing

We describe our DISCWorld system for wide-area, high-performance metacomputing in which we adopt a high-level, service-based approach. Users’ client programs request combinations of services from a set of server nodes which communicate at a peer-based level. DISCWorld is a constrained metacomputing system, running only the service operations its participating resource administrators have chosen to provide and advertise, and provides a common integration environment for clients to access these services and developers to make them available. We discuss our software architecture and experiences building DISCWorld using Java and CORBA components, and the associated research issues for metacomputing that we are addressing.     

Transient experiments from the thermal-hydraulic ADS lead bismuth loop (TALL) and comparative TRAC/AAA analysis

TALL is a medium-size experimental facility constructed at KTH, to study the steady-state and transient thermal-hydraulics performance of LBE-cooled reactors, with the primary purpose of supporting the European transmutation demonstration (ETD) using LBE-cooled accelerator-driven systems (ADS). This paper presents the results of transient experiments performed on the TALL test facility, whose aim is to provide a data base for validation of computer codes which may be used for the analysis of the safety of those systems. This paper also presents the results of the post-test calculations, carried out at PSI, using the TRAC/AAA code. The transient experiments performed include the loss of heat sink, the loss of pump, the loss of both primary and secondary flows, overpower, overcooling, heater trip, and the operational transients of start-up and shut-down. The experimental results show the excellent natural circulation performance of a LBE-cooled system which should contribute to a good safety performance. The TRAC/AAA calculations provide results which agree well with the experimental data.