Minimum fuel mass

Using the assumption of a minimum flame temperature needed for ignition of the mixture, determine the minimum fuel mass loss rate per unit surface area (m l) to cause flame propagation through the boundary layer. The heat of combustion that the volatile wood produces (Ahc) is 15 kJ/g. (Hint the adiabatic flame temperature at the lower flammable limit for the mixture in the boundary layer must be at least 1300 °C.)... 

The smoke point is another measure of the tendency of a fuel to produce smoke and this quantity, like SEA, is related to the chemical composition and structure of the fuel. The smoke point is defined as the minimum fuel mass flow rate at which smoke first escapes from the tip of a laminar diffusion flame (see Fig. 2), ie, the residence time of the smoke in the combustion zone becomes too short to effect complete oxidation. The results of the smoke point test are qualitatively similar to the SEA data in Table 15 with respect to chemical structure and smokeforming tendency. In particular, it is found that the smoke-forming tendency, as determined by smoke point measurements, is lowest for oxygenated fuels (alcohols, aldehydes, esters, ethers) and increases through the series alkanes, branched alkanes, alkenes, and aromatics. 

Frequently coupled with this problem is the determination of the optimum nuclear configuration which yields a minimum fuel mass. Reasonable estimates for preliminary studies can be made with relatively little effort, and many crude analytical models are available for this purpose. Accurate estimates require more elegant methods or the use of critical experiments. Although precise mass figures per se are only infrequently required in modern practice, this information is usually available in every reactor study as the by-product of solutions to more essential problems involving neutron-density distributions. As a practical matter, relatively large discrepancies in mass estimates can be readily accommodated with the increased availability of high-enrichment fuel samples. 

Better to demonstrate some of the suggestions on the use of the Fermi age model, let us consider the problem of determining the critical size and minimum fuel mass of a uniform bare reactor. For simplicity, assume that the basic constituents fuel, moderator, structure, and coolant have all been selected and an estimate of the relative proportions of these materials in the reactor is available. Our problem is to find the dimensions of the reactor with these basic characteristics which has the smallest fuel mass. The procedure is straightforward and involves the calculation of a number of systems each of different size. 

At minimum fuel and air input, the ingot top-to-bottom temperature differential was again about 40°F (22°C). This difference was caused by the heat losses of the pit bottom. The basic reasoning for this is that with a smaller mass of gas flowing, the temperature drop of the gas must be greater to supply the bottom heat loss. Example 6.6 below illustrates this. 

Particularly, the minimum critical mass for Pu-239 is less than 1.5 times the appropriate parameter for U-235, whilst the critical masses for dry dioxides of the above isotopes differ from each other more than 3 times. Moreover, the mixed uranium-plutonium fuel is a powerful source of neutron and gamma radiation. 

Minimum critical cylinder diameter, volume, nd slab thickness occur at a water-to-fuel volume ratio of 10 minimum critical mass and number of puis, at a watef-to-fuel volume ratio of 21.5. 

The results of this analysis are shown in Fig. 1 for a system with and without edge reflectors. The three temperatures shoym are assumed values for re-thermallzed neutrons at the core surface. Similar results were obtained using as fuel. The minimum critical masses obtained were 35 g for [Pg.297]

Small mass critical configura ns have always been of interest and importance in criticality safety. Olson and Robkin have previously calcuUted crHical masses for thin flat foils of U and immersed in Urge Dfi reflectors. This study was based on the mathematical observation that an infinite sUb of fissionable materUl with 1) > 1, immersed in an infinite nonabsorbing reflector, would approach zero thickness. The temperature of the fissile core and DjO reflector was lowered to 4 K in the calculations to rethermalize neutrons striking the core and to take advantage of the absorption characteristics of the fuel. Under these conditions a minimum critical mass of 35 g was obtained for U and 22 g for Pu. The method of calculation was a two-group diffusion analysU of a thin, centrally located core. 

The fuel pins were latticed in regular-spaced square configurations at lattices pitches of 7.606, 9.677, 12.446, 15.392, and 19,255 min. These centerrto-center fuel pin spacings correspond to water-to-fuel volume ratioB of i.69, 3.49,6.68, 10.96, and 18.14, respectively, and cover the neutron moderation range from near optimum with respect to minimum critical mass to undermoderated. 

As stored, the fuel mass will be 3I1O pounds per square foot of floor area idilch Is less than 2/3 of the 6IO pounds per square foot given in Thble IB as the minimum critical mass per unit area for the irradiated fuel element. 

After the leak rate test of the reactor containment vessel, which was the final system function test, a criticality approach test was performed as the first performance test. The minimum critical mass was measured, replacing dummy elements with core fuel stored in the... 

From the standpoint of commercialization of fuel ceU technologies, there are two challenges initial cost and reHable life. The initial selling price of the 200-kW PAFC power plant from IFC was about 3500/kW. A competitive price is projected to be about 1500/kW orless for the utiHty and commercial on-site markets. For transportation appHcations, cost is also a critical issue. The fuel ceU must compete with conventional mass-produced propulsion systems. Furthermore, it is not clear if the manufacturing cost per kilowatt of small fuel ceU systems can be lower than the cost of much larger units. The life of a fuel ceU stack must be five years minimum for utiHty appHcations, and reHable, maintenance-free operation must be achieved over this time period. The projection for the PAFC stack is a five year life, but reHable operation has yet to be demonstrated for this period. 

Process Fuel Major products Approximate core temperature (K) Minimum mass in solar masses... 

The critical burning mass flux at extinction is between 2 and 4 g/m2 s when burning in air, as seen in the theoretical plot of Figure 9.27. A curious minimum mass flux at extinction appears to occur for the oxygen associated with air for nearly all of the fuels shown. The critical mass flux at exinction is difficult to measure accurately, but experimental literature values confirm this theoretical order-of-magnitude for air. The asymptote at 0.12 has not been verified. These theoretical renditions should be taken as qualitative for now. 

---