Publications

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A selective literature review was conducted to assess whether currently available accelerated aging and original qualification data could be used to establish operational margins for the continued use of cable insulation and jacketing materials in nuclear power plant environments. The materials are subject to chemical and physical degradation under extended radiationthermal- oxidative conditions. Of particular interest were the circumstances under which existing aging data could be used to predict whether aged materials should pass loss of coolant accident (LOCA) performance requirements. Original LOCA qualification testing usually involved accelerated aging simulations of the 40-year expected ambient aging conditions followed by a LOCA simulation. The accelerated aging simulations were conducted under rapid accelerated aging conditions that did not account for many of the known limitations in accelerated polymer aging and therefore did not correctly simulate actual aging conditions. These highly accelerated aging conditions resulted in insulation materials with mostly inert aging processes as well as jacket materials where oxidative damage dropped quickly away from the air-exposed outside jacket surface. Therefore, for most LOCA performance predictions, testing appears to have relied upon heterogeneous aging behavior with oxidation often limited to the exterior of the cable cross-section a situation which is not comparable with the nearly homogenous oxidative aging that will occur over decades under low dose rate and low temperature plant conditions. The historical aging conditions are therefore insufficient to determine with reasonable confidence the remaining operational margins for these materials. This does not necessarily imply that the existing 40-year-old materials would fail if LOCA conditions occurred, but rather that unambiguous statements about the current aging state and anticipated LOCA performance cannot be provided based on original qualification testing data alone. The non-availability of conclusive predictions for the aging conditions of 40-year-old cables implies that the same levels of uncertainty will remain for any re-qualification or extended operation of these cables. The highly variable aging behavior of the range of materials employed also implies that simple, standardized aging tests are not sufficient to provide the required aging data and performance predictions for all materials. It is recommended that focused studies be conducted that would yield the material aging parameters needed to predict aging behaviors under low dose, low temperature plant equivalent conditions and that appropriately aged specimens be prepared that would mimic oxidatively-aged 40- to 60- year-old materials for confirmatory LOCA performance testing. This study concludes that it is not sufficient to expose materials to rapid, high radiation and high temperature levels with subsequent LOCA qualification testing in order to predictively quantify safety margins of existing infrastructure with regard to LOCA performance. We need to better understand how cable jacketing and insulation materials have degraded over decades of power plant operation and how this aging history relates to service life prediction and the performance of existing equipment to withstand a LOCA situation.

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Liquid foams are viscoelastic liquids, exhibiting a fast relaxation attributed to local bubble motions and a slow response due to structural evolution of the intrinsically unstable system. In this work, these processes are examined in unique organic foams that differ from the typically investigated aqueous systems in two major ways: the organic foams (1) posses a much higher continuous phase viscosity and (2) exhibit a coarsening response that involves coalescence of cells. The transient and dynamic relaxation responses of the organic foams are evaluated and discussed in relation to the response of aqueous foams. The change in the foam response with increasing gas fraction, from that of a Newtonian liquid to one that is strongly viscoelastic, is also presented. In addition, the temporal dependencies of the linear viscoelastic response are assessed in the context of the foam structural evolution. These foams and characterization techniques provide a basis for testing stabilization mechanisms in epoxy-based foams for encapsulation applications.

Polymer foams are used as encapsulants to provide mechanical, electrical, and thermal isolation for engineered systems. In fire environments, the incident heat flux to a system or structure can cause foams to decompose. Commonly used foams, such as polyurethanes, often liquefy and flow during decomposition, and evolved gases can cause pressurization and ultimately failure of sealed containers. In systems safety and hazard analyses, numerical models are used to predict heat transfer to encapsulated objects or through structures. The thermo-mechanical response of systems involving coupled foam decomposition, liquefaction, and flow can be difficult to predict. Predicting pressurization of sealed systems is particularly challenging. To mitigate the issues caused by liquefaction and flow, hybrid polyurethane cyanate ester foams have been developed that have good adhesion and mechanical properties similar to currently used polyurethane and epoxy foams. The hybrid foam decomposes predictably during decomposition. It forms approximately 50 percent by weight char during decomposition in nitrogen. The foam does not liquefy. The charring nature of the hybrid foam has several advantages with respect to modeling heat transfer and pressurization. Those advantages are illustrated by results from recent radiant heat transfer experiments involving encapsulated objects, as well as results from numerical simulations of those experiments.

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Both conventional and combinatorial approaches were used to study the pore formation process in epoxy based polymer systems. Sandia National Laboratories conducted the initial work and collaborated with North Dakota State University (NDSU) using a combinatorial research approach to produce a library of novel monomers and crosslinkers capable of forming porous polymers. The library was screened to determine the physical factors that control porosity, such as porogen loading, polymer-porogen interactions, and polymer crosslink density. We have identified the physical and chemical factors that control the average porosity, pore size, and pore size distribution within epoxy based systems.

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Many weapons components (e.g. firing sets) are encapsulated with blown foams. Foam is a strong lightweight material--good compromise between conflicting needs of structural stability and electronic function. Current foaming processes can lead to unacceptable voids, property variations, cracking, and slipped schedules which is a long-standing issue. Predicting the process is not currently possible because the material is polymerizing and multiphase with changing microstructure. The goals of this project is: (1) Produce uniform encapsulant consistently and improve processability; (2) Eliminate metering issues/voids; (3) Lower residual stresses, exotherm to protect electronics; and (4) Maintain desired properties--lightweight, strong, no delamination/cracking, and ease of removal. The summary of achievements in the first year are: (1) Developed patentable chemical foaming chemistry - TA; (2) Developed persistent non-curing foam for systematic evaluation of fundamental physics of foams--Initial testing of non-curing foam shows that surfactants very important; (3) Identified foam stability strategy using a stacked reaction scheme; (4) Developed foam rheology methodologies and shear apparatuses--Began testing candidates for shear stability; (5) Began development of computational model; and (6) Development of methodology and collection of property measurements/boundary conditions for input to computational model.

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The U.S. is addicted to petroleum--a dependency that periodically shocks the economy, compromises national security, and adversely affects the environment. If liquid fuels remain the main energy source for U.S. transportation for the foreseeable future, the system solution is the production of new liquid fuels that can directly displace diesel and gasoline. This study focuses on advanced concepts for biofuel factory production, describing three design concepts: biopetroleum, biodiesel, and higher alcohols. A general schematic is illustrated for each concept with technical description and analysis for each factory design. Looking beyond current biofuel pursuits by industry, this study explores unconventional feedstocks (e.g., extremophiles), out-of-favor reaction processes (e.g., radiation-induced catalytic cracking), and production of new fuel sources traditionally deemed undesirable (e.g., fusel oils). These concepts lay the foundation and path for future basic science and applied engineering to displace petroleum as a transportation energy source for good.