Gas coolants

Substantial heat-transfer intensification was also described for a special micro heat exchanger reactor [104]. By appropriate distribution of the gas-coolant stream, the axial temperature gradient can be decreased considerably, even under conditions corresponding to very large adiabatic temperature rises, e.g. of about 1400 °C. 

The major function of a bipolar plate, or simply called "plate," is to connect each cell electrically and to regulate the reactant gas (typically, hydrogen and air in a hydrogen fuel cell) or reactant liquid (typically, methanol in a DMFC) and liquid or gas coolant supply as well as reaction product removal in desired patterns. This plate must be at least electrically conductive and gas and/or liquid tightened. Considering these important functions and the larger fraction of volume, weight, and cost of the plate in a fuel cell, it is worthwhile to construct this chapter with emphasis on the current status and future trend in bipolar plate research and development, mainly for the plate materials and fabrication process. 

Cmoo is the specific heat at constant volume of the gas coolant (J/(kgK)),... 

Extensive experimental work was undertaken at Babcock and Wilcox to obtain detailed heat transfer and pressure drop information as well as operating experience. Summary articles by Rhode et al. [245] and Schluderberg et al. [246] elaborate on the conclusions of this work. Heat transfer coefficients for heating were improved by as much as a factor of 10 through the addition of graphite. The suspensions were also shown to be far superior to gas coolants on the basis of pumping power requirements, especially when twisted-tape inserts... 

Thus gas and liquid coolants may be contrasted by saying that gas coolants already have a poor heat... 

Although the density of the gas coolant is considerably smaller than that of the water or sodium coolant, the allowable temperature rise and coolant velocity can be substantially larger. In addition, of course, the coolant cross-sectional area can be made larger if necessary, with some sacrifice to the power density but without neutron absorption. The temperature rise for the water coolant is generally limited by the amount of boiling allowable, whereas the temperature rise for the sodium coolant is generally limited by thermal-stress problems that can arise in metal... 

Although helium gas was used in the previous illustration, similar arguments can be made for CO2 as a coolant. Carbon dioxide has been the preferred coolant for the magnox and AGR reactors helium is required for high-temperature reactors using graphite as a moderator and either helium or CO2 might be used in a fast-spectrum reactor. The choice of a gas coolant for a particular reactor can depend on heat-transfer considerations, chemical and radiation stabUity, and interaction effects, or perhaps other factors. 

It was pointed out in the section on gas coolants that the heat removal capacity of helium or CO2 could be made equivalent to that of sodium by choosing appropriate coolant pressure and flow rate conditions for the gas coolant. The studies on the GCFBR have shown that the heat transfer with oxide fuel elements is not limited by the coolant when the coolant pressure is 1000 psia. Under these conditions, it is found that a net plant efficiency of 40 % can be obtained in a large GCFBR having a fuel rating of 900 kW/kg of fissile fuel, a power density of 240 kW/liter, an overall conversion ratio 1.55, and a doubling time of 8 years (see Table I). 

G. Melese-d Hospital and P. Fortescue, Thermodynamic comparison of gas coolants for nuclear reactors. Paper presented at Inst. Mech. Engineers Conf, London, March 1967. 

The simplicity of the GT-MHR is reflected in its instrumentation systems Total visibility of plant conditions, and all control and protection actions are provided with a relatively low number of instrumentation channels. Clear and concise operator information is provided. The passive safety of the reactor, its slow response, and the neutronic transparency and the absence of phase changes in the gas coolant eliminate many of the human factors complications found in other reactors. 

The instrument should be thermally equilibrated and stable before calibration. This is accomplished by turning on the purge gas, coolant, and instrument 45 min before calibration. The analyzer should be equilibrated at load temperature, which is usually set to 25°C. 

All the high temperature concepts will require at least a decade and a half or two before commercialization thermal spectrum concepts using TRISO fuel and molten salt or gas coolant could reduce this time by drawing on a partial experience base from previous programmes conducted in the 1950s through the 1970s. 

Because the AHTR uses a liquid coolant, rather than a gas coolant, some differences in requirements for the fuel will exist. This paper describes the reactor concept and the potential differences in fuel requirements. 

The gas-cooled reactors were developed in Great Britain using CO2 as the gas coolant and graphite as the moderator with natural uranium metal as the fuel. The thermal efficiency is about 25%. With uranium enriched to 2.2% as the oxide (UO2) fuel, the thermal efficiency increased to 41% and is called the advanced gas reactor (AGR). Helium is also used in a high-temperature version. 

For Any Coolant Exit Temperature, the Average Temperature of Delivered Heat (the Product) Is Higher with Liquid Coolants than with Gas Coolants... 

Photodissociation regions (PDRs) are defined as regions where the chemistry is dominated by photons. Hence the chemistry of PDR tracers must be dominated by photo-induced processes, at least indirectly. Reactive species, rapidly destroyed by reactions with H2 or abundant neutrals are therefore good tracers of the illuminated outer layers of molecular clouds. The list includes the gas coolants [CII], [OI] and [Cl], radicals like HCO, CCH or C-C3H2, as well as reactive ions like CO, HOC or CF" ". The rotational and rovibrational lines of H2 are also bright in PDRs. 

Helium is the traditional high temperature, high pressure gas coolant. Liquid fluoride salts are a traditional high temperature, low pressure liquid coolant. The only other potential candidates are liquid metals, particularly molten lead or lead alloys for fast spectrum reactors. Because of their relatively low boiling points, traditional liquid metals such as sodium are not candidates for high temperature operations. 

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