Recent studies on terphenyl coolants

One of the principal sources of information in the United States of America since 1958 on the effect of reactor irradiation and heat on terphenyl coolants for nuclear reactors has been an USAEC-sponsored programme carried out at MIT. The effects of varying coolant composition, temperature, and neutron-to-gamma dose ratio on the rate of coolant degradation, physical properties, composition, and forced convection heat transfer have been studied. A circulating in-pile loop located in the core of the MIT reactor has been the principal experimental facility; the programme has also involved the development of methods for measurement of neutron and gamma-ray dose rates by adiabatic calorimetry and neutron activation of foils and of various methods for analysis of gas and liquid samples of irradiated coolant. An important phase of the work has been the correlation and comparison of the results obtained with those reported by other laboratories in an attempt to develop a consistent model for prediction of the performance in organic-cooled reactors. Terphenyl degradation. An empirical model which assumes that the degradation process can be considered to consist of two independent processes, radiolysis and radiopyrolysis, has been quite successful in correlating the results of most of the degradation studies reported to date. The best agreement between the various experimental results suggests second-order kinetics (approximately) and a ratio of fast-neutron-to-gamma-ray induced degradation (GN/Gγ) of between 4 to 5 for irradiation carried out below about 350 deg C with fast-neutron dose fractions ranging from 0 (electron and gamma irradiations) to about 95%. Above about 350 deg C the rate of degradation increases rapidly with increasing temperature. Using first-order kinetics for radiopyrolysis (pyrolysis of irradiated coolant) in the model mentioned above, the rate constants obtained from loop experiments at MIT and EURATOM are in good agreement. The rate constants for radiopyrolysis increase with increasing concentration of decomposition products and are significantly greater than the rate constants for pyrolysis of unirradiated coolants; for example, at 400 deg. C and 30 wt.% distillation bottoms (a measure of the concentration of high boilers) the rate of radiopyrolysis of a terphenyl mixture was more than a factor of 10 greater than the rate of pyrolysis of the unirradiated coolant. Other models are being investigated for improved correlation, and the experimental programme designed to measure directly the relative damage caused by neutrons and gamma rays, the reaction kinetics, and the relative effects of radiation and temperature on irradiated coolants is continuing. The rate at which decomposition gases are produced and their composition have also been measured. Physical properties and composition of irradiated coolants. Measurements of the density and viscosity of terphenyl coolant irradiated under various conditions of coolant composition and temperature have been made. The density of irradiated Santowax WR is given by the empirical correlation ρ = 1.152 + 0.600 x 10-3(B) - [4.87 x 10-4 - 1.768 x 10-6(B)] T ± 1% where ρ = sample density, g/cm3; B = wt. % bottoms; and T temperature, deg F. The viscosity of irradiated Santowax OMP and WR are correlated by an equation of the form: μ = μ1 exp [ΔE/R (1/T - 1.163 x 10-3)] where μ = viscosity, cP; μ1 = a proportionality factor varying between 1.0 and 2.0 cP depending on the bottoms concentration; ΔE = an 'activation energy', kcal/gmole varying between 4.5 and 4.9 depending on the bottoms concentration; R is the gas constant; ' and T = temperature, deg. R. Number average molecular weights of coolant and of distillation bottoms, initial and final melting points of irradiated terphenyl coolants, and limited thermal conductivity measurements have been determined. Gas chromatography has been employed to analyse decomposition gases and irradiated coolant and distillation bottoms for low, intermediate and high boiling decomposition products. Heat transfer. An in-line electrically-heated tube located in the out-of-pile section of the circulating MIT loop has been employed to measure the coefficients of heat transfer for forced convection for the coolants irradiated. Measurements of pressure drop across the tube also were made to determine friction factors and provide a check on the heat-transfer measurements by using heat- and momentum-transfer analogies. The purpose of these studies was to determine the best methods of using the measured physical properties to predict the coefficients of heat transfer and pressure drop in organic-cooled reactor systems. The results of the heat-transfer and friction experiments are correlated to within ±10% when the measured physical properties of the irradiated coolants are employed in the conventional engineering equations for forced convection. Furthermore, no significant fouling of the heat-transfer surfaces in this all-stainless-steel system has been observed although the various heaters have been operated for periods of time greater than one year at surface temperatures as high as 510 deg. C for test periods and generally over 425 deg. C while supplying heat to maintain the coolant temperature during irradiations. (author)