Thermal Stability and Safe Venting of the Tri-N-Butyl Phosphate-Nitric Acid-Water ("Red Oil") System - II: Experimental Data on Reaction Self-Heat Rates and Gas Production and Their Correlation

Adiabatic calorimetry testing was performed to determine the Arrhenius relations for the chemical self-heat rates generated by the oxidation of tri-n-butyl phosphate saturated with nitric acid ("organic phase"). The adiabatic calorimetry tests showed that the runaway reaction is tempered at ~109°C when the organic phase rests on top of a layer of aqueous nitric acid ("aqueous phase"). It is believed that tempering in the laboratory-scale two-layer organic/aqueous system is mainly due to the upward transport of dissolved water from the aqueous phase to the organic phase where the water evaporates into rising reaction product gas bubbles. The rate of water transport depends strongly on the location and rate of product gas bubble generation. Isothermal tests were performed that clearly reveal that the reaction product gas bubbles originate in the underlying aqueous layer and that their rate of generation is bubbling enhanced reactant mass transfer controlled. A semiempirical expression for the rate of gas generation was developed from the measurements and from available correlations on enhanced mass transfer in bubbling pools. The empirical and semiempirical relations reported here for chemical self-heat rates and reaction product gas production are necessary to determine the thermal stability boundaries of single-layer and two-layer systems, predictions of which appear in the companion paper, "Thermal Stability and Safe Venting of the Tri-N-Butyl Phosphate-Nitric Acid-Water ("Red Oil") System - III: Predictions of Thermal Stability Boundaries and Required Vent Size," Nuclear Technology, Vol. 163, p. 307 (2008).