A study of the influence of physical parameters on the drying of porous media

An experimental setup is developed to observe the mass transfer that occurs in the drying of saturated porous media due to bulk flow of gas. The analysis and equations are developed in a one-dimensional perspective, and focus on the convection and evaporation that results. Humidity readings are monitored at various locations and used in the analysis of the results. These procedures are used in test cases for Steel spheres and Ceramic beads both 4.5 mm in diameter at.25 L/min,.5 L/min,.75 L/min and 1 L/min flow rates; Drying times and humidity plots versus time for both steel and ceramic are witnessed to be similar in nature. A correlation for the drying time is developed through use of Sherwood number, Reynolds number, and Schmidt number. This is achieved by using the non-dimensional quantities and acquiring a linear regression and equation. The coefficients and exponent values of the general equation for the Sherwood number are then solved

Thermohydraulic and nuclear modeling of natural fission reactors

Experimental verification of proposed nuclear waste storage schemes in geologic repositories is not possible, however, a natural analog exists in the form of ancient natural reactors that existed in uranium-rich ores. Two billion years ago, the enrichment of natural uranium was high enough to allow a sustained chain reaction in the presence of water as a moderator. Several natural reactors occurred in Gabon, Africa and were discovered in the early 1970\u27s. These reactors operated at low power levels for hundreds of thousands of years. Heated water generated from the reactors also leached uranium from the surrounding rock strata and deposited it in the reactor cores. This increased the concentration of uranium in the core over time and served to refuel the reactor. This has strong implications in the design of modern geologic repositories for spent nuclear fuel. The possibility of accidental fission events in man-made repositories exists and the geologic evidence from Oklo suggests how those events may progress and enhance local concentrations of uranium; Based on a review of the literature, a comprehensive code was developed to model the thermohydraulic behavior and criticality conditions that may have existed in the Oklo reactor core. A two-dimensional numerical model that incorporates modeling of fluid flow, temperatures, and nuclear fission and subsequent heat generation was developed for the Oklo natural reactors; The operating temperatures ranged from about 456 K to about 721 K. Critical reactions were observed for a wide range of concentrations and porosity values (9 to 30 percent UO2 and 10 to 20 percent porosity). Periodic operation occurred in the computer model prediction with UO2 concentrations of 30 percent in the core and 5 percent in the surrounding material. For saturated conditions and 30 percent porosity, the model predicted temperature transients with a period of about 5 hours. Kuroda predicted 3 to 4 hour durations for temperature transients. The large instantaneous jumps in temperature could be an indication of the violent ejection of water that Kuroda predicted, resulting in ongoing geyser activity. The range of temperatures simulated by the computer model within the Oklo reactors agreed with evidence from the Oklo geology

Assessment of Criticality Safety for Cylindrical Containers to be Used In the Processing of Spent Fuel

The UREX process separates uranium from transuranic wastes (TRU) and fission products (FP). Nuclear reactors require fissile isotopes that will absorb neutrons and break apart into smaller nuclei while releasing a large amount of energy as well as multiple neutrons. Fissile isotopes in spent fuel include not only 235U, but also 239Pu, 241Pu, and several isotopes of americium (Am) and curium (Cm). TRU contains the actinides with atomic numbers greater than that of uranium. This includes Pu, Np, Am, and Cm. When TRU is separated from uranium, the TRU still poses a significant risk of sustaining a chain reaction. This is quantified through the effective neutron multiplication factor, keff. To prevent TRU from becoming critical (sustaining a chain reaction), keff must be maintained at a value of less than 1. The presence of neutron poisons (Sm, Xe, B, Hf, Cd, etc.) will decrease keff. Neutron poisons are found in fission products. The presence of neutron moderators (H, C, Be) or materials that reflect neutrons will enhance keff. To assess keff, Monte Carlo simulation codes are used. The concentration of TRU, process salts, and fission products along with the geometry of the mixture and surrounding reflective material are inputs to these codes

The Fission Properties of Curium Separated from Spent Nuclear Fuel

Curium poses special problems in the chemical preparation of spent nuclear fuel for transmutation. Once separated from the other minor actinides, the seven curium isotopes in spent fuel can lead to nuclear fission with the subsequent release of a large amount of radiation. Several isotopes of curium also generate a significant amount of heat by radioactive decay. Sustained fission can be avoided by preventing the accumulation by more that a critical mass of curium. The heat generation of curium presents even more restriction on the mass of curium that can safely be contained in one location. To analyze the nuclear and thermal properties of curium, the curium isotopes within spent fuel were quantified using RADDB, a light water reactor radiological database. The critical mass of curium was analyzed for shielded and unshielded cylindrical and spherical geometries. The criticality studies were completed using SCALE 4.4a, a Monte Carlo code approved by the Nuclear Regulatory Commission for the analysis of critical nuclear reactors. The results include recommendations on the maximum mass of curium that may be safely handled. Finally, a conservative case for the buildup of decay heat was analyzed to determine the equilibrium temperature of a curium-filled container. Both natural convection and radiation heat transfer were considered. The equilibrium temperature was used to recommend the maximum mass of curium that can be safely handled or stored before melting occurs

Status of the nevada Shocker At the University of nevada, Las Vegas

The Nevada shocker is a 540 kV, 7 /spl Omega/, 50 ns pulsed power device based on Marx bank and Blumlein technologies. The Marx bank is composed of nine 60 kV capacitors charged in series with a gamma high voltage source connected by means of Ross relays in an air environment. A trigatron switch energized with an isolated mini capacitor bank is used to erect the Marx bank. The trigatron switch and erecting electrodes are contained in a gas manifold pressurized to 20 /spl plusmn/ 1 psi with dry air. The energy is released sequentially through an inductor and a water filled charging transmission line to the Blumlein immersed in deionized water. The Blumlein shapes and compresses the energy into a 50 ns pulse upon discharge. A self-breaking water switch initiates the release of energy in the Blumlein. The energy flows through a water filled discharging transmission line to the diode end of the Nevada Shocker. The current diode end of the Blumlein supports vacuum pressures as low as 6.5 /spl times/ 10/sup -6/ Torr. The chamber is pumped with the aid of a roughing pump and a cryogenic vacuum pump. The vacuum section of the Nevada Shocker is currently being rebuilt to incorporate mechanical and thermal loading capabilities with sensors located at the experiment. A number of diagnostic developments are currently underway to support flashover studies on plastics. Resistive probe and differential B-dot diagnostics with the aid of a 6 GHz 20 GS/s TDS 6604 real time scope is documented demonstrating the capability of the machine