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Somatostatin-expressing, low threshold-spiking (LTS) cells and fast-spiking (FS) cells are two common subtypes of inhibitory neocortical interneuron. Excitatory synapses from regular-spiking (RS) pyramidal neurons to LTS cells strongly facilitate when activated repetitively, whereas RS-to-FS synapses depress. This suggests that LTS neurons may be especially relevant at high rate regimes and protect cortical circuits against over-excitation and seizures. However, the inhibitory synapses from LTS cells usually depress, which may reduce their effectiveness at high rates. We ask: by which mechanisms and at what firing rates do LTS neurons control the activity of cortical circuits responding to thalamic input, and how is control by LTS neurons different from that of FS neurons? We study rate models of circuits that include RS cells and LTS and FS inhibitory cells with short-term synaptic plasticity. LTS neurons shift the RS firing-rate vs. current curve to the right at high rates and reduce its slope at low rates; the LTS effect is delayed and prolonged. FS neurons always shift the curve to the right and affect RS firing transiently. In an RS-LTS-FS network, FS neurons reach a quiescent state if they receive weak input, LTS neurons are quiescent if RS neurons receive weak input, and both FS and RS populations are active if they both receive large inputs. In general, FS neurons tend to follow the spiking of RS neurons much more closely than LTS neurons. A novel type of facilitation-induced slow oscillations is observed above the LTS firing threshold with a frequency determined by the time scale of recovery from facilitation. To conclude, contrary to earlier proposals, LTS neurons affect the transient and steady state responses of cortical circuits over a range of firing rates, not only during the high rate regime; LTS neurons protect against over-activation about as well as FS neurons. © 2011 Hayut et al.

A steam line break accident is one of the possible severe abnormal transients in a pressurized water reactor. It is required to present an analysis of a steam line break accident in the Final Safety Analysis Report (FSAR) of a nuclear power reactor. Only simple one-dimensional core simulations are currently applied to such accident analysis. However, the asymmetric characteristics of a steam break accident may require more detailed local information in order to determine the potential fuel damage resulting from the transient. TITAN, a coupled (neutronics and thermal-hydraulics) code with state-of-the-art neutronics and thermal-hydraulics models, is therefore modified and applied to steam line break accident simulations., The capabilities that are added to the code for a steam line break analysis include multiregion core inlet temperature forcing function, total inlet coolant flow rate boundary condition, total inlet coolant flow rate transient simulation capability, boron tracking equations, flow/coolant temperature transient plus control rod transient option, and one-dimensional, fully implicit numerical scheme.for thermal-hydraulics calculations. The modifications to TITAN are tested with a ten-channel PWR model. For inlet coolant temperature transients (one of the transients involved in a steam line break accident) test calculations lead to the conclusion that there is no significant difference between the results of closed- and open-channel calculations until boiling occurs., A ten-channel model with two partially inserted control rods is employed for the transient simulations. Steady state conditions are obtained first by both open- and closed-channel calculations. Results show that cross flow between channels is insignificant. Thus, the onedimensional, fully implicit numerical scheme for the thermal-hydraulics equations is useful to speed up the calculations. More than half of the computational effort is saved by using this scheme compared to the semi-implicit numerical scheme., Two extreme situations relevant to a steam line break accident are investigated: (1) Significant boiling due to severe depressurization when no return to power exists. (2) Return to power with no boiling because of high coolant temperature feedback coefficients. It is concluded that even after boiling occurs, the global parameters, such as total power and assembly power, still show no significant difference between the closed- and open-channel calcualtions. However, the local parameters, such as nodal power, void fraction and MDNBR, reveal differences between the two calculations. Results show that the open-channel calculation predicts lower MDNBR values as compared with the results of closed-channel calculation in a vapor generation process, since the coolant is driven away from of the hot spots. On the other hand, during a vapor condensation process, closed-channel calculations predict lower MDNBR results, since no cross flow is allowed to accelerated the condensation process., A closed-channel, uniform inlet coolant temperature transient calculation is performed. The results verify the necessity for a three-dimensional calculation of the accident simulations, since no boiling was predicted by-the one-dimensional calculation throughout the simulation period.

An analytical model of long distance transport of air pollutants (Fay and Rosenzweig, 1980) has been adapted for the estimation of long term (e.g. annual) wet sulfate deposition in eastern N. America. The model parameters have been optimized for best agreement with 1980- 1982 measurements at 109 monitoring sites in this region. The mean residual of the model and measurement comparison is 4 kg ha[superscript -l]y[superscript -1] (17% of the mean measured value). Transfer coefficients were found to decrease exponentially with source-receptor distance, having length scales between 1100 and 400 km depending upon whether the source is upwind or downwind of the receptor. Source apportionment calculated for four sites from this model shows that about half of the deposition is due to 7-8 of the largest source contributors to each site (aggregated to the state and sub-province level). A 17-year record of precipitation sulfate measured at Hubbard Brook, New Hampshire, compares favorably with the model calculation. Calculated U.S.-Canada transboundary fluxes agree with previous estimates. Isopleths of 1980-82 yearly depositions were determined, and the deposition effects of a typical U.S. emissions reduction proposal were evaluated.

This report contains the climatological, technical and economic factors for episodic and seasonal control of emissions in existing power plants. Analyzing a large data set of acid deposition for the years 1982-85, we find that the bulk of acid deposition comes down in a few precipitation episodes per year, mostly concentrated in the summer months. However, the episodes do not occur over wide areas, and are difficult to predict. About 75% of the annual acid deposition occurs during the summer half-year, April through September. Therefore, it would be effective to reduce acid precursor emissions, SO[subscript x] and NO[subscript x], during that period. One method to accomplish the summer precursor emission reduction is substituting natural gas (NG) for oil and coal in large electric utility and industrial boilers., Gas contains no sulfur and emits less NO[subscript x] than oil or coal. The cost of a summer fuel substitution is primarily dependent on the delivered fuel price differential and only to a small extent on the retrofit cost for dual fuel use. For example, with a delivered fuel price differential of $1.5/MMBtu, the annualized incremenmtal cost of electricity would be about 8 mills/kWh. For the same fuel price differential, the sulfur removal costs range from $400 - $1750 ton SO[subscript 2] depending on sulfur content of the fuel. If credit were given for the greater effect of summer emission reductions, the "effective" removal costs would be considerably less., Research performed with the support of the Electric Utility Program.

The policy and regulatory ramifications of U.S. acid rain control programs are examined; particularly, the alternative of a receptor-oriented strategy as constrasted to emission-oriented proposals (e.g., the Mitchell bill) which set sulfur emission reductions to uniform national levels. In receptor strategies, goals for deposition reductions in ecologically threatened areas are determined and the emission reductions are apportioned primarily to sources that cause the bulk of acid deposition in those areas. It is very likely that a receptor-oriented strategy would be less costly (on a national basis) than a uniform emission reduction strategy, and certainly more beneficial to the endangered areas. For a receptor-oriented strategy, a detailed economic analysis needs to be performed to select the least-cost emission control method for the individual sources. Such methods may include scrubbers, combustion modification, total or seasonal fuel substitution, and electricity import (i.e., emission export). An emission control scheme tailored for northern New York and New England would also benefit sensitive areas in southeastern Canada, and thereby help to defuse the present U.S./Canadian impass over acid rain control agreements.

Ozone (Os), a natural component of the troposphere, is augmented by photochemical processes involving manmade emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). Sufficiently high concentrations of ozone are detrimental to the respiratory system. Ozone exposure also reduces crop yields and damages forests. This study attempts to explain the underlying factors which contribute to observed ozone levels., Long range transport models of three species - NOx, VOCs and ozone - are developed for eastern North America. The seasonally averaged models include the essential physical and chemical processes in a relatively simple framework. NOx and VOCs are treated as primary species, i.e., they are modeled from their introduction into the atmosphere to their point of removal. Detailed emission inventories serve as input to the precursor models. Ozone is considered a secondary species because it is not directly emitted. Rather, its production is assumed to be a function of ambient NO, and VOCs levels., Measured concentrations, available for NO 2 and ozone, are compared with model predictions and aid in determining adjustable model parameters. Predicted NOx concentrations are consistent with rural observations but underestimate sites influenced by nearby sources at which the long range assumptions break down. Local models which properly treat proximate sources account for the discrepancy. The VOCs model, having no measurements for verification, adopts parameters consistent with the NOx model and known chemical properties. Both biogenic and manmade emissions contribute to ambient VOCs levels. Biogenic emissions are found to be more important over most of ENA; anthropogenic sources of VOCs are dominant only in urban areas., Consistent with empirical patterns, the ozone model predicts small regional gradients and hence a limited dependence on NOx and VOCs precursors. The natural background component is determined to be two-thirds of average ozone levels. Regional transport is significant; ozone lifetimes are estimated to be of the order of a day. The high background level and insensitivity to precursors suggests that significant reductions of average ozone concentrations will be difficult to achieve.

This study was undertaken to aid in the evaluation of Environmental Monitoring Plans (EMP) submitted to the U.S. Synfuel Corporation by prospective operators of large-scale synfuel plants who are applying for financial assistance. In accordance with the Energy Security Act of 1980, such applicants must provide an acceptable EMP in order to qualify for the assistance. The EMP is reviewed in consultation with the U.S. Environmental Protection Agency (EPA) and the Department of Energy. This study was sponsored by EPA to aid in their consultative role., The study is limited to evaluating the ambient monitoring plan of the EMP. Ambient monitoring we interpret as pertaining to the surveillance and measurement of pollutants that may emanate from the synfuel complex but are found outside the perimeter of the plant buildings and facilities. The perimeter usually will coincide with the fence or property lines of the complex. The ambient so-interpreted includes the airspace, and surface- and ground-waters, beyond, above and below the perimeter ("outside-the-fence") of the complex., Ambient air monitoring. The major emphasis of ambient air monitoring should be in the measurement of unregulated air pollutants such as volatile and condensible organic compounds, trace metals, and radionuclides for which emission standards are not yet defined, but are potentially hazardous to health and biota. As the identity, release rate and release height of the individual pollutants are not known a priori, tentative criteria are given for selecting the number and location of the monitors which can be modified as experience is gained on collected pollutant quantities and instrument sensitivities., It is expected that most pollutants will emanate from low level (height) sources such as retorts, gasifier columns, sulfur recovery systems, liquid and solid waste disposal sites, and coal storage and handling systems. Ground-level pollutant concentrations will decline exponentially with distance because of horizontal and vertical diffusion. Multihour exposure may be necessary to collect a sufficient quantity of a pollutant for analysis. The meteorological factor having the greatest effect on the duration and frequency of sampling, and the location of the monitors, is the prevailing wind direction. Historical wind persistence analysis must be performed at each plant location to determine the wind sectors where the probability is greatest that multihour exposure will occur repeatedly over the monitoring period, say 1 year. It is proposed that monitors be placed in wind sectors where there is at least a 25% chance (100 days/year) that a collector be exposed to 6 or more hours per day (25%) of steady wind. If no single wind sector satisfies this requirement, monitors should be placed in several wind sectors such that at least 100 samples will have an exposure of 6 hours or more. The radial distance of the monitor should subtend an angle of 22.5 (one wind sector) with a cross section of the synfuel complex perpendicular to the monitor., Ambient water monitoring. The study involved an analysis of the behavior of organic effluents from a generic synthetic fuel plant in several representative environments. A generic approach was used because of the wide range of technologies used in synfuel development, and the variability of the aquatic environments which will receive their discharges. The approach was also necessary because there are no existing large-size synthetic fuel plants from which to obtain data. The analysis consisted of modeling the behavior of the effluents in generic environments. Specific compounds were chosen to represent broad classes of organic effluents. Also described are the schematizations of environmental processes used in the modeling., Modeling results indicate that there is not a unique monitoring site or frequency for all compounds in all environments. The optimal monitoring site is determined mainly by effluent chemistry, and the optimal frequency by environmental characteristics. The results presented here will be useful once an effluent stream and environment are specified. Environmental monitoring plans may then be compared for the appropriateness of the planned monitoring sites and adequacy of sampling frequency.

While methane is chemically quite inert to reactions with atmospheric molecular species, it does react with atomic species and molecular radicals. Because of its relatively large abundance in the global troposphere and because of the apparent annual increase of its concentration, it is worthwhile exploring the role of methane in tropospheric chemistry, especialy the role in air pollution., From a detailed analysis of the reactions of methane with atmospheric species we conclude that while methane does play a role in generating the general background of ozone that permeates the global troposphere, it plays only a minor role in urban and rural ozone formation. In urban areas, most of which fail to meet the ozone standards established by the Clean Air Act, the prime facilitators of ozone formation are various higher hydrocarbon emissions. In rural and forested areas, biogenic emissions of terpenes, pinenes and isoprenes take a dominant role in creating ozone via their peroxy radical. Thus, the omission of methane in urban and rural ozone modeling is justified. The models should use a general ozone background level which in most areas of the U.S. amounts to 10-30 ppb. This background ozone may be due in part to global methane concentrations. Further, for ozone control purposes in exceedance areas, little would be gained by monitoring methane at the emission sources or in the ambient., Annually between 400-800 million tons (MTy-1) of CH4 are emitted, of which 100-200 MTy come from enteric fermentation, 280-500 MTy-l from wetlands and rice paddies, 16-50 MTy-1 from fossil fuel leakage and combustion and the rest from other minor sources. These emissions produce in the northern hemisphere an annual average concentration of methane in air of about 1600-1700 parts per billion by volume, abbreviated henceforth ppb. The concentration appears to increase by about 1% y-l. For comparison, the tropospheric abundance of other hydrocarbons ranges from 0.1-10 ppb. In urban, polluted areas both methane and non-methane-hydrocarbon (NMHC) concentrations could be larger. Unfortunately, few measurements are available that include the whole spectrum of hydrocarbons., The most important tropospheric reaction of methane is with the hydroxyl radical OH. This reaction initiates a chain that leads to the formation of the methyl peroxy radical CH302, and to a lesser extent to formaldehyde HCHO and methyl nitrate CH3 ONO 2 . The CH302 radical may be an important oxidant that converts atmospheric nitric oxide NO into nitrogen dioxide NO2 . Nitrogen dioxide is the major precursor of ozone 03. However, peroxy radical formation is much more rapid from higher alkanes, unsaturated hydrocarbons and aromatic hydrocarbons. The NMHC's react 25-830 times faster with hydroxyl than methane. The extent to which CH4 contributes to 03 concentrations is very much dependent on the ratio of ambient concentrations of NMHC/CH4. In our estimation, methane is a contributor to the general tropospheric ozone concentration or "background" ozone, which amounts to 10-30 ppb in the US. In rural and forested areas, where biogenic emissions of terpenes, pinenes and isoprenes are large, these hydrocarbons form rapidly peroxy radicals contributing to excess ozone formation over that of the general background. In urban areas, there are large emissions of higher hydrocarbons due to evaporation of fuels, solvents and paints, industrial (e.g. refinery) emissions, and incomplete combustion of fuels. In urban areas, we expect that these emissions are the prime facilitators of ozone formation, as well as the precursors of other "smog" ingredients, e.g. aldehydes, ketones, acids, organic nitrates and peroxyacetyl nitrate (PAN)., Supported in part by the American Gas Association.

High concentrations of sulfuric and nitric acid in raTn fn the northeastern USA are caused by the large scale combustion of fossil fuels within this region. Average precipitation acidity is pH 4.2, but spatial and temporal fluctuations of *1 pH unit have been observed. Amelioration of rain acidity requires significant reduction of precursor emissions--the oxides of sulfur and nitrogen. Such reduction of emissions from existing sources will be difficult and expensive. A pending legislativp proposal to reduce eastern U.S. emissions of SO2 by 10 Mty-' below the 1980 level of 22.5 Mty - 1 would reduce the acid sulfate deposition rate in the Adirondack mountains, an environmentally sensitive area, from 30 to 21.4 kg ha-ly-l. Based upon source/receptor modeling, an equal reduction of Adirondack sulfate deposition rate could be achieved by a 7 Mty- SO2 emission reduction if the reduction is allocated according to source proximity to the sensitive area. The cost of reducing 10 Mty - 1 SO2 emissions is estimated at $5 to $8 billion per year. Considerably lower costs could be realized in an emission control scheme that is the more stringent the nearer the sources are to the environmentally sensitive areas, and if novel approaches are implemented, such as seasonal or episodic emission reduction, NOx vs. SO2 emission control, emission redistribution, and least emission electricity dispatch. Substantial emission reductions will probably be achieved in the more distant future by employing new combustion technology, such as lime injected multistage combustion, fluidized bed combustion, coal gasifier combined cycle, magnetohydrodynamics, and- possibly others.

Includes bibliographical references., Supported in part by the American Gas Association., James A. Fay, Dan S. Golomb, Savvakis C. Zachariades.

Includes bibliographical references., Partial support by the New England Power Company under the Electric Utility Program at the Energy Laboratory, Massachusetts Institute of Technology., J.A. Fay, D. Golomb, S. Zemba.

The Commonwealth of Massachusetts promulgated an Act limiting S2 emissions from large sources that burn fuel at a rate greater than or equal to 100 million Btu (MBtu) of fuel input per hour. The Act requires that by 1995 the average emission rate at such facilities be less than or equal to 1.2 lb SO2 per MBtu fuel input. Because of their size, almost all power plants in Massachusetts could be subject to emission reductions. Since the average 1980-1982 annual emission rate of Massachusetts power plants was 1.84 lb S02/MBtu ("base case"), the Act requires the annual average emission rate of power plants to diminish by 35%. We use a source apportionment model to estimate the wet sulfate deposition to typical sensitive Massachusetts receptors from Massachusetts power plants, separately for the summer (April-September) and winter (October-March) half-years. We find that the summer wet deposition is about twice the winter deposition, although summer and winter SO2 emissions are approximately equal. Therefore, to reduce sulfate deposition, t is more effective to reduce emissions in the summer months rather than in winter. Using the seasonal source apportionment model we find that an annual wet deposition reduction equal to that resulting from the Act could be accomplished if only summer emission rates were reduced to 0.86 lb SO /MBtu, with winter emission rates remaining at 1.84 lb S02/MBtu. The resulging annual average emission rate is 1.35 lb SO /MBtu, 27% less than the base value. As 1980-1982 average annual emissions rom power plants amounted to 270,000 tons of SO annually, a summer emission control program would save about 21,000 tons of S emission reduction without sacrificing wet deposition protection. Te summer emission reduction could be acomplished by substituting lower sulfur content fuels, including natural gas, for higher sulfur content fuels., New England Power Company under the Electric Utility Program at the Energy Laboratory

Includes bibliographical references., Supported in part by the American Gas Association., James A. Fay and Dan S. Golomb.