Donald G. Schweitzer

Design of particle bed reactors for the space nuclear thermal propulsion program

Abstract This paper describes the design for the Particle Bed Reactor (PBR) that we considered for the Space Nuclear Thermal Propulsion (SNTP) Program. The methods of analysis and their validation are outlined first. Monte Carlo methods were used for the physics analysis, several new algorithms were developed for the fluid dynamics, heat transfer and transient analysis; and commercial codes were used for the stress analysis. We carried out a critical experiment, prototypic of the PBR to validate the reactor physics; blowdown experiments with beds of prototypic dimensions were undertaken to validate the power-extraction capabilities from particle beds. In addition, materials and mechanical design concepts for the fuel elements were experimentally validated. Four PBR rocket reactor designs were investigated parametrically. They varied in power from 400 MW to 2000 MW, depending on the missions goals. These designs all were characterized by a negative prompt coefficient, due to Doppler feedback, and a moderator feedback coefficient which varied from slightly positive to slightly negative. In all practical designs, we found that the coolant worth was positive, and the thrust/weight ratio was greater than 20.

Oxidation and Heat Transfer Studies in Graphite Channels: III. The Chemical Reactivity of BNL Graphite and Its Effect on the Length of Channel Cooled by Air

Thermal equilibria between the heat produced by graphite oxidation and the heat removed by coolant air streams were investigated in the temperature range from 650 to 735 deg C. The studies were made on graphite channels whose reactivities differed by a factor of eight. Equilibrium occurs in channels shorter than 10 ft if the numerical value of the reactivity (cal/cm/sup 2/-sec) is 100-fold greater than the heat transfer coefficient (cal/cm/sup 2/-sec- deg C). The length of channel cooled depends on the heat transfer coefficient and is insensitive to the reactivity when the heat transfer coefficient is numerically equal to or greater than the reactivity of the graphite. (auth)

Threshold effects and large Tc reductions in neutron‐irradiated A‐15 superconductors

A‐15 superconductors show very large decreases in critical temperature after 60 °C neutron irradiations exceeding about 1018 neutrons/cm2 (E>1 MeV). These effects are not observed in superconductors such as Nb, NbN, or Nb–Ti.

OXIDATION AND HEAT TRANSFER STUDIES IN GRAPHITE CHANNELS*: I. THE EFFECT OF AIR FLOW RATE ON THE C–O2 AND CO–O2 REACTIONS

When air is passed through a hot graphite channel, the heat produced by the chemical reactions is due to both the C-O/sub 2/ and CO-O/sub 2/ reactions. The data show that the largest and most rapid temperature rises are due to the CO- O/sub 2/ gas phase reaction. Serious instability (where the heat generated by the reactions is greater than the heat removed by the air stream) does not occur below 650 deg C and is confined to flow rates where the Reynolds numbers lie between 2000 and 8000. Although the experiments were designed to provide information for operation of the BNL Reactor, the results were found to be general in nature. (auth)

Inconsistencies in Thermodynamic Analyses of Long-Term Isolation of High-Level Waste

Important thermodynamic analyses of possible reactions affecting the long-term performance of the engineered materials being considered for isolation of high-level waste are reviewed. Analysis of the literature on the possible failure mechanisms of copper waste packages in granite and basalt environments shows that many of the conclusions from postulated thermodynamic equilibria reactions are inconsistent with the original assumptions and with observations. The absence of evidence for the existence of reactions calculated to have negative free energy changes is classically explained by kinetic inhibitions.

Anomalous stored energy and c-axis changes in alternately irradiated and annealed graphite

Stored energy releases in graphites exhibiting normal and enhanced dimensional changes indicate that the change in rate of dimensional expansion after large neutron irradiations is due to a change in apparent defect volume and is not due to a change in the rate of accumulation of defects. Reverse stored energy annealing experiments are correlated with the reverse c-axis annealing experiments. The assumption that interstitials anneal to sinks rather than to vacancies can be used to explain these data. The nature of the annealing reactions producing stored energy releases is discussed.

OXIDATION AND HEAT TRANSFER STUDIES IN GRAPHITE CHANNELS. IV. COMBINED EFFECTS OF TEMPERATURE, FLOW RATE, DIAMETER, AND CHEMICAL REACTIVITY ON THE LENGTH OF CHANNEL COOLED BY AIR

The activation energy for graphite oxidation was obtained from the change in the stable length of channel with temperature. The maximum temperature at which thermal equilibrium (between the heat generated by graphite oxidation and the heat removed by the air stream) will occur in a channel can be predicted from the heat transfer coefficient, the activation energy, and a single value of the graphite reactivity at any temperature. Above this maximum temperature, the total length of channel is thermally unstable. An equation is given expressing the length of channel that can be cooled as a function of temperature, flow rate, diameter, and reactivity. (auth)

OXIDATION AND HEAT TRANSFER STUDIES IN GRAPHITE CHANNELS. II. THERMAL CHANGES WITH TIME AND DISTANCE IN AIR-COOLED GRAPHITE CHANNELS

In an air-cooled graphite channel in which heat is being generated by oxidation, the length of channel cooled is a function of the heat transfer coefficient. No diameter effect other than that accounted for by the heat transfer coefficient was observed. Cooling rates and temperature differences (between the graphite and air) vary very slowly in the cooled section of the channel. Temperature changes vary rapidly with time in the section of channel in which more heat is generated than is removed by the air stream. These changes do not affect the cooled section of channel. (auth)

Behavior of solenoids in type-I superconducting shield tubes

A model is described which accounts for the observed properties of solenoids in superconducting shield tubes.

Determination of the single interstitial migration energy from stored energy and thermal resistivity changes in irradiated graphite

Abstract The model used to evaluate the single interstitial migration energy from property changes due to interstitials is extended to account for vacancy contributions. The annealing function obtained can be used to determine the relative contributions of the defects and is sufficiently sensitive to distinguish vacancy effects that are an order of magnitude less than interstitial effects. Application of the model to stored energy and thermal resistivity data yields the same values of the activation energy and temperature independent term obtained from c -axis and macroscopic length expansion rates. The results indicate that the stored energy associated with the di-interstitial is at least ten times greater than the stored energy associated with the vacancy. The minor role of vacancies in phonon scattering is discussed. The annealing function obtained from electrical resistivity changes in irradiated graphite is explained by assuming that the ratio of charge-carriers to scattering centers varies with irradiation temperature below 55°C. Above this temperature the changes are attributed to equal contributions from vacancies and interstitials.