Steam generator and circulator model for the HELAP code

An outline is presented of the work carried out in the 1974 fiscal year on the GCFBR safety research project consisting of the development of improved steam generator and circulator (steam turbine driven helium compressor) models which will eventually be inserted in the HELAP (1) code. Furthermore, a code was developed which will be used to generate steady state input for the primary and secondary sides of the steam generator. The following conclusions and suggestions for further work are made: (1) The steam-generator and circulator model are consistent with the volume and junction layout used in HELAP, (2) with minor changes these models, when incorporated in HELAP, could be used to simulate a direct cycle plant, (3) an explicit control valve model is still to be developed and would be very desirable to control the flow to the turbine during a transient (initially this flow will be controlled by using the existing check valve model); (4) the friction factor in the laminar flow region is computed inaccurately, this might cause significant errors in loss-of-flow accidents; and (5) it is felt that HELAP will still use a large amount of computer time and will thus be limited to design basis accidents without scram or loss of flow transients with and without scram. Finally it may also be used as a test bed for the development of prototype component models which would be incorporated in a more sophisticated system code, developed specifically for GCFBR's. (auth

Bouncing trimer: a random self-propelled particle, chaos and periodical motions

A trimer is an object composed of three centimetrical stainless steel beads equally distant and is predestined to show richer behaviours than the bouncing ball or the bouncing dimer. The rigid trimer has been placed on a plate of a electromagnetic shaker and has been vertically vibrated according to a sinusoidal signal. The horizontal translational and rotational motions of the trimer have been recorded for a range of frequencies between 25 and 100 Hz while the amplitude of the forcing vibration was tuned for obtaining maximal acceleration of the plate up to 10 times the gravity. Several modes have been detected like e.g. rotational and pure translational motions. These modes are found at determined accelerations of the plate and do not depend on the frequency. By recording the time delays between two successive contacts when the frequency and the amplitude are fixed, a mapping of the bouncing regime has been constructed and compared to that of the dimer and the bouncing ball. Period-2 and period-3 orbits have been experimentally observed. In these modes, according to observations, the contact between the trimer and the plate is persistent between two successive jumps. This persistence erases the memory of the jump preceding the contact. A model is proposed and allows to explain the values of the particular accelerations for which period-2 and period-3 modes are observed. Finally, numerical simulations allow to reproduce the experimental results. That allows to conclude that the friction between the beads and the plate is the major dissipative process.Comment: 22 pages, 10 figure

Ten microsatellite loci for the strawberry poison frog (Oophaga pumilio)

We describe primers and PCR conditions to amplify nine new tetranucleotide loci and one new dinucleotide locus isolated from the strawberry poison frog (Oophaga pumilio). In 21 individuals from Costa Rica, the number of alleles ranged from 4 to 16, observed heterozygosities from 40 to 100%, and polymorphic information content ranged from 0.60 to 0.90 per locus. Evidence for linkage disequilibrium was found only between two loci, but this pattern was not found in other populations tested. All primer pairs cross-amplified in Oophaga vicentei from Panama

Understanding and manipulating non-templated peptide bond formation by macrocyclase enzymes

Peptide macrocycles are attractive molecules because they are drug-like, protease resistant, cell permeable, and possess a rigid structure. They have been shown to possess various biological activities and to be able to inhibit protein-protein interactions and other complex targets. Although several macrocyclases have been characterized to date, only two can catalyze the formation of cyclic peptides containing less than 9 amino acids in their core. PatGmac, from the biosynthesis of cyanobactins, is a versatile catalyst with very broad substrate specificity. It can utilize varied peptide sequences, incorporate unnatural amino acids, including substrates that are peptide “chimeras” containing triazoles, peg linkers and sugars (Figure 1A, bottom). Despite its remarkable substrate promiscuity, PatGmac is extremely slow, with turnover rates in the vicinity of once per day. In search for a more efficient macrocyclase we studied GmPOPB, a prolyl oligopeptidase from the mushroom Galerina marginata. GmPOPB (fast macrocyclase) participates in the biosynthesis of the toxic amanitins, catalyzing both peptide bond hydrolysis and peptide bond formation with equal efficiency (Figure 1A, top). We determined crystal structures of apoGmPOPB and GmPOPB mutants bound to a peptidase and a macrocyclase substrate unveiling a mechanism by which the enzyme controls which reaction will be catalyzed. We have also performed an extensive kinetic analysis of this enzyme in comparison to the slow PatGmac. Crucial differences exist between the fast and the slow macrocyclases. Substrate positioning plays an important role towards catalytic efficiency. For the fast macrocyclase GmPOPB there is product inhibition and the rate-limiting step for the reaction is product release. For the slow macrocyclase PatGmac product release is not rate determining for the majority of the substrates tested, and the rate-limiting step is coupled to chemistry. Guided by our kinetic studies, we have designed modified peptide substrates, which eliminate the requirement for a long peptide substrate from 25 amino acids to 13 amino acids for the fast macrocyclase. We are currently designing enzyme variants to improve the catalytic efficiency of the slow macrocyclase and to broaden the substrate scope of the fast macrocyclase. We hope our findings will result in a better, more efficient and substrate permissible macrocyclase that can be used for the biocatalytic generation of cyclic peptide libraries to be tested for biological function. Please click Additional Files below to see the full abstract

Characterization of a dual function macrocyclase enables design and use of efficient macrocyclization substrates

H.L. is funded by the George & Stella Lee Scholarship and Criticat EPSRC. This project was also funded by the European Research Council project 339367 NCB-TNT and by the BBSRC (K015508/1). JHN is 1000 talent scholar of the Chinese Academy of Sciences at the University of Sichuan.Peptide macrocycles are promising therapeutic molecules because they are protease resistant, structurally rigid, membrane permeable and capable of modulating protein-protein interactions. Here, we report the characterization of the dual function macrocyclase-peptidase enzyme involved in the biosynthesis of the highly toxic Amanitin toxin family of macrocycles. The enzyme first removes 10 residues from the N-terminus of a 35-residue substrate. Conformational trapping of the amino acid peptide forces the enzyme to release this intermediate rather than proceed to macrocyclization. The enzyme rebinds the 25 amino acid peptide in a different conformation and catalyzes macrocyclization of the N-terminal 8 residues. Structures of the enzyme bound to both substrates and biophysical analysis characterize the different binding modes rationalizing the mechanism. Using these insights simpler substrates with only five C-terminal residues were designed, allowing the enzyme to be more effectively exploited in biotechnology.Publisher PDFPeer reviewe

Combined Active/Passive Decay Heat Removal Approach for the 24 Mwt Gas-Cooled Fast Reactor

Decay heat removal at depressurized shutdown conditions has been regarded as one of the key areas where significant improvement in passive response was targeted for the GEN IV GFR over the GCFR designs of thirty years ago. It has been recognized that the poor heat transfer characteristics of gas coolant at lower pressures needed to be accommodated in the GEN IV design. The design envelope has therefore been extended to include a station blackout sequence simultaneous with a small break/leak. After an exploratory phase of scoping analysis in this project, together with CEA of France, it was decided that natural convection would be selected as the passive decay heat removal approach of preference. Furthermore, a double vessel/containment option, similar to the double vessel/guard vessel approach of the SFR, was selected as the means of design implementation to reduce the PRA risks of the depressurization accident. However additional calculations in conjunction with CEA showed that there was an economic penalty in terms of decay heat removal system heat exchanger size, elevation heights for thermal centers, and most of all in guard containment back pressure for complete reliance on natural convection only. The back pressure ranges complicated the design requirements for the guard containment. Recognizing that the definition of a loss-of-coolant-accident in the GFR is a misnomer, since gas coolant will always be present, and the availability of some driven blower would reduce fuel temperature transients significantly; it was decided instead to aim for a hybrid active/passive combination approach to the selected BDBA. Complete natural convection only would still be relied on for decay heat removal but only after the first twenty four hours after the initiation of the accident. During the first twenty four hour period an actively powered blower would be relied on to provide the emergency decay power removal. However the power requirements of the active blower/circulators would be kept low by maintaining a pressurized system coolant back pressure of {approx}7-8 bars through the design of the guard containment for such a design pressure. This approach is termed the medium pressure approach by both CEA and the US. Such a containment design pressure is in the range of the LWR experience, both PWRs and BWRs. Both metal containments and concrete guard containments are possible in this pressure range. This approach is then a time-at-risk approach as the power requirements should be low enough that battery/fuel cell banks without diesel generator start-up failure rate issues should be capable of providing the necessary power. Compressed gas sources are another possibility. A companion PRA study is being conducted to survey the reliability of such systems

2400mwt Gas-Cooled Fast Reactor Dhr Studies Status Update.

A topical report on demonstrating the efficacy of a proposed hybrid active/passive combination approach to the decay heat removal for an advanced 2400MWt GEN-IV gas-cooled fast reactor was published in March 2006. The analysis was performed with the system code RELAP5-3D (version 2.4.1.1a) and the model included the full complement of the power conversion unit (PCU): heat exchange components (recuperator, precooler, intercooler) and rotating machines (turbine, compressor). A re-analysis of the success case in Ref is presented in this report. The case was redone to correct unexpected changes in core heat structure temperatures when the PCU model was first integrated with the reactor model as documented in Ref [1]. Additional information on the modeling of the power conversion unit and the layout of the heat exchange components is provided in Appendix A