Properties of the Optokinetic Motor Fibres in the Rock Lobster: Build-Up, Flipback, Afterdischarge and Memory, Shown by Their Firing Patterns

The properties of sets of motor fibres responding to both clockwise and anticlockwise rotation have been studied in the oculomotor nerve of the rock lobster. There are probably three, but perhaps four, units in each set. None of these fibres has statocyst input, but there is weak input onto the tonic fibres from the antennal joints such that the eye turns in the direction toward which the antenna points. Many preparations show bilateral visual input onto all fibres but the degree of coupling between the eyes is very variable, and at times can be nearly totally absent. Depending on the speed of rotation the fibres show a gradual build-up in frequency, during rotation in the preferred direction, interrupted by flipbacks. During the fast stage of the resulting nystagmic movements all agonistic fibres can be completely inhibited and all antagonistic ones can be activated, usually for a period of about 0.5 sec. Fibre activity is demonstrated which appears to underlie an ‘optokinetic memory’ of contrasting target position in the visual field. It consists of (a) very prolonged after-discharges for a stationary striped pattern (b) resumption of discharges at an appropriate frequency after dark periods up to 2 min, and (c) adjustment of such frequencies to changes in stripe position during the dark period. The fibres show habituation to repeated stripe movement but the response can be dishabituated by passive rotation of the animal. The largest visual responses were obtained to intermediate speeds of stripe rotation (about 2°/sec)

Input Sources and Properties of Position-Sensitive Oculomotor Fibres in the Rock Lobster, Panulirus Interruptus (Randall)

Sets of head-up, head-down, eye-up and eye-down motor fibres were studied in the oculomotor nerve of the rock lobster. An eye-withdrawal fibre was also investigated. Apart from the statocyst input, light distribution on the eyes has the strongest influence on the position-sensitive fibres. Weaker optokinetic input from moving targets is also present. Strongly habituating input is obtained from the antennal joints. This input causes orientation of the eye toward the direction in which the antenna points. The same antennule movement in the vertical plane can result in either excitation or inhibition of the head-down fibre, suggesting the presence of two opposing inputs, presumably from the statocysts and basal joint receptors of the antennule. The inputs on to the position-sensitive fibres which indicate body position are such as to stabilize the eye position in space during body movement. The optokinetic and antennal joint inputs are probably involved in tracking and antennal pointing reactions. The eye-withdrawal fibre is stimulated by touch of the head and around the eye, but is inhibited by the excited state

TREATMENT TANK CORROSION STUDIES FOR THE ENHANCED CHEMICAL CLEANING PROCESS

Radioactive waste is stored in high level waste tanks on the Savannah River Site (SRS). Savannah River Remediation (SRR) is aggressively seeking to close the non-compliant Type I and II waste tanks. The removal of sludge (i.e., metal oxide) heels from the tank is the final stage in the waste removal process. The Enhanced Chemical Cleaning (ECC) process is being developed and investigated by SRR to aid in Savannah River Site (SRS) High-Level Waste (HLW) as an option for sludge heel removal. Corrosion rate data for carbon steel exposed to the ECC treatment tank environment was obtained to evaluate the degree of corrosion that occurs. These tests were also designed to determine the effect of various environmental variables such as temperature, agitation and sludge slurry type on the corrosion behavior of carbon steel. Coupon tests were performed to estimate the corrosion rate during the ECC process, as well as determine any susceptibility to localized corrosion. Electrochemical studies were performed to develop a better understanding of the corrosion mechanism. The tests were performed in 1 wt.% and 2.5 wt.% oxalic acid with HM and PUREX sludge simulants. The following results and conclusions were made based on this testing: (1) In 1 wt.% oxalic acid with a sludge simulant, carbon steel corroded at a rate of less than 25 mpy within the temperature and agitation levels of the test. No susceptibility to localized corrosion was observed. (2) In 2.5 wt.% oxalic acid with a sludge simulant, the carbon steel corrosion rates ranged between 15 and 88 mpy. The most severe corrosion was observed at 75 C in the HM/2.5 wt.% oxalic acid simulant. Pitting and general corrosion increased with the agitation level at this condition. No pitting and lower general corrosion rates were observed with the PUREX/2.5 wt.% oxalic acid simulant. The electrochemical and coupon tests both indicated that carbon steel is more susceptible to localized corrosion in the HM/oxalic acid environment than in the PUREX/oxalic acid environment. (3) The corrosion rates for PUREX/8 wt.% oxalic acid were greater than or equal to those observed for the PUREX/2.5 wt.% oxalic acid. No localized corrosion was observed in the tests with the 8 wt.% oxalic acid. Testing with HM/8 wt.% oxalic acid simulant was not performed. Thus, a comparison with the results with 2.5 wt.% oxalic acid, where the corrosion rate was 88 mpy and localized corrosion was observed at 75 C, cannot be made. (4) The corrosion rates in 1 and 2.5 wt.% oxalic acid solutions were temperature dependent: (a) At 50 C, the corrosion rates ranged between 90 to 140 mpy over the 30 day test period. The corrosion rates were higher under stagnant conditions. (b) At 75 C, the initial corrosion rates were as high as 300 mpy during the first day of exposure. The corrosion rates increased with agitation. However, once the passive ferrous oxalate film formed, the corrosion rate decreased dramatically to less than 20 mpy over the 30 day test period. This rate was independent of agitation. (5) Electrochemical testing indicated that for oxalic acid/sludge simulant mixtures the cathodic reaction has transport controlled reaction kinetics. The literature suggests that the dissolution of the sludge produces a di-oxalatoferrate ion that is reduced at the cathodic sites. The cathodic reaction does not appear to involve hydrogen evolution. On the other hand, electrochemical tests demonstrated that the cathodic reaction for corrosion of carbon steel in pure oxalic acid involves hydrogen evolution. (6) Agitation of the oxalic acid/sludge simulant mixtures typically resulted in a higher corrosion rates for both acid concentrations. The transport of the ferrous ion away from the metal surface results in a less protective ferrous oxalate film. (7) A mercury containing species along with aluminum, silicon and iron oxides was observed on the interior of the pits formed in the HM/2.5 wt.% oxalic acid simulant at 75 C. The pitting rates in the agitated and non-agitated solution were 2 mils/day and 1 mil/day, respectively. A mechanism by which the mercury interacts with the aluminum and silicon oxides in this simulant to accelerate corrosion was proposed

MP733: Preliminary Database Assessment for International Long-Term Ecological Monitoring Efforts

Description of a database assessment of international ecological monitoring efforts.https://digitalcommons.library.umaine.edu/aes_miscpubs/1034/thumbnail.jp

Receptief, productief, en receptief+productief woorden leren: Wat maakt het uit?

De centrale vraag in dit artikel hoe woorden in een vreemde taal het best geleerd kunnen worden: als je woorden in een vreemde taal receptief moet kennen, van de vreemde taal naar de moedertaal, moet je ze dan alleen receptief leren, of heeft het zin om ze ook ze ook productief te leren omdat je ze dan wellicht beter onthoudt? Deze praktijkvraag vormde het uitgangspunt voor het leerexperiment waarvan Jan-Arjen Mondria en Boukje Wiersma in dit artikel verslag doen. Daarbij bleek, in afwijking van de verwachting, dat het zowel receptief als productief leren van woorden tot een vergelijkbare receptieve retentie leidde als receptief leren alleen. Eveneens onverwacht leidde het zowel productief als receptief leren tot een vergelijkbare productieve retentie als productief leren alleen

Spectroscopy of Chemically Linked Dimers:1,3-(1,1’-Dinaphthyl)propane in a Naphthalene Host

Results of absorption, fluorescence and selective-excitation experiments on the chemically linked dimer 1,3-(1,1'-dinaphthyl)propane in a naphthalene host at 1.8 K are presented. This system is shown to consist of two translationally inequivalent pairs of the naphthalene moiety, occupying non-substitutional sites in the host lattice. This gives rise to two sets of dimer absorptions, split by 40 and 97 cm -1

DETERMINATION OF CORROSION INHIBITOR CRITERIA FOR TYPE III IIIA TANKS DURING SALT DISSOLUTION OPERATIONS

Preparation of high level waste for vitrification involves in part the dissolution of salt cake from the carbon steel storage tanks. The salt crystals composing this cake are high in nitrate concentration with the interstitial liquid being high in hydroxide and nitrite concentration. During the salt dissolution process, a stage is reached in which the inhibitors, hydroxide and nitrite, are insufficient to prevent nitrate stress corrosion cracking (SCC) and fall outside the requirements of the corrosion control program. Additional inhibitors, which are necessary to meet the requirements, may be counterproductive to the efficiency of the process and waste minimization. Corrosion testing was initiated to better characterize the necessary inhibitor concentration for high nitrate waste during salt dissolution processing. A four-phase test program is being conducted: (1) electrochemical characterization, (2) accelerated or polarized U-bend testing, (3) long-term (non-polarized) U-bend testing and (4) vapor space U-bend tests. Electrochemical testing, which included cyclic potentiodynamic polarization (CPP), linear polarization resistance (LPR) and open-circuit potential (OCP) measurements, was performed to identify stress corrosion cracking susceptibility, to characterize pitting resistance and to determine the general corrosion rate. Polarized U-bend tests were utilized to assess the effect of minimum inhibitor concentrations and heat treatment on SCC and to determine test parameters for future long-term U-bend testing. Results from CPP, LPR and OCP tests demonstrated that carbon steel formed a protective oxide film and the potential became electropositive during exposure to the waste at all inhibitor concentrations. The tenacity of this film improved as the inhibitor concentration level was increased and the temperature was decreased. This passive film increased the resistance to localized corrosion significantly. Therefore if any of these inhibitor levels are selected for storage of dissolved salt solutions, no changes to the service life estimates that were based on general corrosion are necessary. The breakdown potential for SCC as well as the other electrochemical parameters were independent of nitrate concentration (4.5-8.5 M). The breakdown potential, however, was strongly affected by temperature (i.e., 25 and 50 C) and inhibitor concentration. These results indicate that for this nitrate concentration range a critical inhibitor level is necessary for minimizing the occurrence of SCC. The polarized U-bend tests were in good agreement with the electrochemical tests. The U-bend testing clearly demonstrated that the heat treating of the samples clearly improved the SCC resistance of A537 carbon steel even at the low inhibitor concentration (0.01 M hydroxide and 0.01 M nitrite). This concentration was insufficient to prevent cracking for any tested nitrate concentration (4.5-8.5 M). At a 7 M nitrate concentration, SCC was prevented for inhibitor concentrations as low as 0.3 M hydroxide and 0.1 M nitrite. The current inhibitor requirements for a waste containing 7 M nitrate are 0.6 M hydroxide and 1.1 M hydroxide and nitrite. Thus, a considerable reduction in the amount of inhibitor necessary may be attained. It will also be recommended that the temperature of the dissolved salt solution be maintained below 50 C

Probing the eigenfunction fractality with a stop watch

We study numerically the distribution of scattering phases ${\cal P}(\Phi)$ and of Wigner delay times ${\cal P}(\tau_W)$ for the power-law banded random matrix (PBRM) model at criticality with one channel attached to it. We find that ${\cal P}(\Phi)$ is insensitive to the position of the channel and undergoes a transition towards uniformity as the bandwidth $b$ of the PBRM model increases. The inverse moments of Wigner delay times scale as $\sim L^{- q D_{q+1}}$, where $D_q$ are the multifractal dimensions of the eigenfunctions of the corresponding closed system and $L$ is the system size. The latter scaling law is sensitive to the position of the channel.Comment: 5 pages, 4 figure

ASSESSMENT OF THE POTENTIAL FOR HYDROGEN GENERATION DURING GROUTING OPERATIONS IN THE R- AND P-REACTOR VESSELS

The R- and P-reactor buildings were retired from service and are now being prepared for deactivation and decommissioning (D&D). D&D activities will consist primarily of immobilizing contaminated components and structures in a grout-like formulation. Aluminum corrodes very rapidly when it comes in contact with the alkaline grout materials and as a result produces hydrogen gas. To address this potential deflagration/explosion hazard, the Materials Science and Technology Directorate (MS&T) of the Savannah River National Laboratory (SRNL) has been requested to review and evaluate existing experimental and analytical studies of this issue to determine if any process constraints on the chemistry of the fill material and the fill operation are necessary. Various options exist for the type of grout material that may be used for D&D of the reactor vessels. The grout formulation options include ceramicrete (pH 6-8), low pH portland cement + silica fume grout (pH 10.4), or Portland cement grout (pH 12.5). The assessment concluded that either ceramicrete or the silica fume grout may be used to safely grout the P-reactor vessel. The risk of accumulation of a flammable mixture of hydrogen between the grout-air interface and the top of the reactor is very low. Portland cement grout, on the other hand, for the same range of process parameters does not provide a significant margin of safety against the accumulation of flammable gas in the reactor vessel during grouting operations in the P-reactor vessel. It is recommended that this grout not be utilized for this task. The R-reactor vessel contains significantly less aluminum and thus a Portland cement grout may be considered as well. For example, if the grout fill rate is less than 1 inch/min and the grout temperature is maintained at 70 C or less, the risk of hydrogen accumulation in the R-reactor vessel is very low for the Portland cement. Alternatively, if the grout fill rate is less than 0.5 inch/min and the grout is maintained at a temperature of 80 C, the risk will again be very low. Although these calculations are conservative, there are some measures that may be taken to further minimize the potential for hydrogen evolution. (1) Minimize the temperature of the grout as much as practical. Lower temperatures will mean lower hydrogen generation rates. For P-reactor, grout temperatures less than 100 C should provide an adequate safety margin for the pH 8 and pH 10.4 grout formulations. For R-reactor, grout temperatures less than 70 C or 80 C will provide an adequate safety margin for the Portland cement. The other grout formulations are also viable options for R-reactor. (2) Minimize the grout fill rate as much as practical. Lowering the fill rate takes advantage of passivation of the aluminum components and hence lower hydrogen generation rates. For P-reactor, fill rates that are less than 2 inches/min for the ceramicrete and the silica fume grouts will reduce the chance of significant hydrogen accumulation. For R-reactor, fill rates less than 1 inch/min will again minimize the risk of hydrogen accumulation. (3) Ventilate the building as much as practical (e.g., leave doors open) to further disperse hydrogen. The volumetric hydrogen generation rates in the P-reactor vessel, however, are low for the pH 8 and pH 10.4 grout, (i.e., less than 0.32 ft{sup 3}/min). If further walk-down inspections of the reactor vessels suggest an increase in the actual areal density of aluminum, the calculations should be re-visited