Publications

The author reports experimental measurements for the argon and oxygen permeability coefficients for the new EPDM material (SR793B-80) used for the environmental o-ring seals of the W88. The results allow the author to refine the argon gas analysis modeling predictions for W88 surveillance units. By comparing early surveillance results (up to four years in the field) with the modeling, the author shows that (1) up to this point in time, leakage past the seals is insignificant and (2) the argon approach should be able to inexpensively and easily monitor both integrated lifetime water leakage and the onset of any aging problems. Finally, the author provides a number of pieces of evidence indicating that aging of the SR793B-80 material will not be significant during the expected lifetime of the W88.

This paper describes the connection between mechanical degradation of common cable materials in radiation and elevated temperature environments and density increases caused by the oxidation which leads to this degradation. Two techniques based on density changes are suggested as potential non-destructive evaluation (NDE) procedures which may be applicable to monitoring the mechanical condition of cable materials in power plant environments. The first technique is direct measurement of density changes, via a density gradient column, using small shavings removed from the surface of cable jackets at selected locations. The second technique is computed X-ray tomography, utilizing a portable scanning device.

To allow more reliable estimates to be made of the amount of water that permeates through weapon environmental seals, we have generated extensive water permeability coefficient data for numerous o-ring materials, including, weapon-specific formulations of EPDM, butyl, fluorosilicone and silicone. For each material, data were obtained at several temperatures, ranging typically from 21[degrees]C to 80[degrees]C; for selected materials, the effect of relative humidity was monitored. Two different experimental techniques were used for most of the measurements, a permeability cup method and a weight gain/loss approach using, a sensitive microbalance. Good agreement was found between the results from the two methods, adding confidence to the reliability of the measurements. Since neither of the above methods was sufficiently sensitive to measure the water permeability of the butyl material at low temperatures, a third method, based on the use of a commercial instrument which employs a water-sensitive infrared sensor, was applied under these conditions.

In two earlier reports, we derived a time-temperature-dose rate superposition methodology, which, when applicable, can be used to predict cable degradation versus dose rate, temperature and exposure time. This methodology results in long-term predictive capabilities at the low dose rates appropriate to ambient nuclear power plant aging environments. The methodology was successfully applied to numerous important cable materials used in nuclear applications and the extrapolated predictions were verified by comparisons with long-term (7 to 12 year) results for similar or identical materials aged in nuclear environments. In this report, we test the methodology on three crosslinked polyolefin (CLPO) and two ethylene propylene rubber (EPR) cable insulation materials. The methodology applies to one of the CLPO materials and one of the EPR materials, allowing predictions to be made for these materials under low dose-rate, low temperature conditions. For the other materials, it is determined that, at low temperatures, a decrease in temperature at a constant radiation dose rate leads to an increase in the degradation rate for the mechanical properties. Since these results contradict the fundamental assumption underlying time-temperature-dose rate superposition, this methodology cannot be applied to such data. As indicated in the earlier reports, such anomalous results might be expected when attempting to model data taken across the crystalline melting region of semicrystalline materials. Nonetheless, the existing experimental evidence suggests that these CLPO and EPR materials have substantial aging endurance for typical reactor conditions. 28 refs., 26 figs., 3 tabs.

In this report, we provide a detailed discussion of methodology of predicting cable degradation versus dose rate, temperature, and exposure time and its application to data obtained on a number of additional nuclear power plant cable insulation (a hypalon, a silicon rubber and two ethylenetetrafluoroethylenes) and jacket (a hypalon) materials. We then show that the predicted, low-dose-rate results for our materials are in excellent agreement with long-term (7 to 9 years), low dose-rate results recently obtained for the same material types actually aged under nuclear power plant conditions. Based on a combination of the modelling and long-term results, we find indications of reasonably similar degradation responses among several different commercial formulations for each of the following generic'' materials: hypalon, ethylenetetrafluoroethylene, silicone rubber and PVC. If such generic'' behavior can be further substantiated through modelling and long-term results on additional formulations, predictions of cable life for other commercial materials of the same generic types would be greatly facilitated. Finally, to aid utilities in their cable life extension decisions, we utilize our modelling results to generate lifetime prediction curves for the materials modelled to data. These curves plot expected material lifetime versus dose rate and temperature down to the levels of interest to nuclear power plant aging. 18 refs., 30 figs., 3 tabs.

Analyses of the internal argon gas concentrations monitored on surveillance units of the W84 indicates that field aging of this weapon for times up to {approximately}4 years does not lead to important increases in the rate at which water leaks into the interior of the weapon. This implies that the EPDM environmental seals used on the W84 do not age significantly over this time period. By comparing the percentages of oxygen and argon in the internal atmosphere, an estimate of the oxygen consumption rate is made for a typical W84 unit. The argon gas analysis approach is then applied to the W88, which is sealed with a new EPDM material. Predictive expressions are derived which relate the anticipated argon gas concentrations of future, field-returned units to their water leakage rates. The predictions are summarized in convenient plots, which can be immediately and easily applied to surveillance data as reported. Since the argon approach is sensitive enough to be useful over the entire lifetime of the W88, it can be used to point out leaking units and to determine whether long-term aging has any significant effect on the new EPDM material. 11 refs., 10 figs., 3 tabs.

Exposure of polymers to air during aging (radiation, thermal, UV) often results in inhomogeneously oxidized samples, a complication which impacts attempts both to understand the oxidation process and to extrapolate accelerated exposures to long-term conditions. The most important such complication involves diffusion-limited oxidation which can occur if the rate of oxygen consumption in a material is greater than the rate at which oxygen can be resupplied by diffusion processes from the surrounding air atmosphere. Thus, in order to confidently extrapolate accelerated simulations to long-term, air-aging conditions, one must be able to monitor and quantitatively understand diffusion-limited oxidation effects. Experimental techniques for monitoring diffusion-limited oxidation profiles have recently been reviewed. By comparing experimental profiles with theoretically derived profiles, the theories can be verified and then confidently used to predict the importance of diffusion effects prior to the initiation of aging tests. This paper summarizes what we believe is the first quantitative experimental confirmation of diffusion-limited oxidation theories.

Whenever a new elastomer is formulated or an old formulation is modified, it is important to estimate the material's anticipated lifetime in various use environments. For extended lifetimes (years) this often requires the application of accelerated aging techniques which typically involve the modelling of results obtained at higher-than-ambient environmental stress levels. For many practical applications of elastomers, air is present during environmental exposures -- this usually implies that important oxidation effects underly the degradation of the material. Unfortunately, exposure of elastomers to air during aging often results in inhomogeneously oxidized samples, a complication which impacts attempts both to understand the oxidation process and to extrapolate accelerated exposures to long-term conditions. As has been clear for many years, in order to confidently extrapolate shorter-term accelerated simulations to long-term, air-aging conditions, one must be able to monitor and quantitatively understand diffusion-limited oxidation effects. In this review we will highlight some of the recent developments in both experimental techniques and theoretical modelling of relevance to this goal. 28 refs., 12 figs.