Gyanender Singh - Research Areas

Advanced Nuclear Reactors: Making Reactors Safer

Importance: Safer operation is the major design criteria for the fourth generation of nuclear reactors. During a typical loss of coolant accident substantial amount of heat continues to generate even after the reactor scram. If emergency core cooling system is not operational then the temperature of the reactor core rises quickly leading to core meltdown and ultimately to a beyond-design-basis accident. The Fukushima Dai-ichi nuclear power plant accident that occurred in 2011 strongly stressed the need for implementation of passively safe reactor designs that resist severe degradation under beyond-design-basis accident conditions. During a beyond design basis accident, significant physical and chemical degradation of the reactor core components begins when the temperature approaches 800ºC. At temperatures greater than 1200ºC, zirconium reacts with steam in an exothermic reaction to produce zirconium dioxide and hydrogen. The heat generated due to the reaction between steam and the many tons of zirconium in the core can be as large as ten times the decay heat, leading to a further temperature increase. The high temperatures may lead to partial to complete core meltdown, and the generated hydrogen can compromise the containment through pressurization and explosive recombination with oxygen. Nuclear energy community is seeking alternate materials for fuel, cladding and other core components which have higher tolerance in accident scenarios.

Challenges:In addition to being stable under neutron irradiation, new materials should be resistant to oxidation, maintain structural integrity at elevated temperatures, and be able to contain the fission products in case of an accident. Understanding these effects of radiation, oxidation, steam and high temperature is essential for assessing potential materials. Once the potential material qualifies as a candidate material for reactor components further studies are needed to evaluate the performance of these core components made of new materials in reactor environment.

Goal: Our goal is to understand the behavior and performance of candidate materials for reactor core systems under normal operating conditions as well as in accident scenarios such as loss of coolant accident. Candidate concepts for fuel, cladding, channel boxes and other core components are studied through computational studies. These computational studies incorporate latest available material properties, multiphysics and multiscale capabilities which enable understanding the effects of microscale evolution of material under irradiation and temperature on the macroscale phenomena of deformation, stress, temperature and failure. This work is complemented by material characterization which informs about the properties of these materials for different material design parameters. External investigators collaborate on computational and experimental research fronts to assess candidate materials ultimately leading to an improved understanding about the new concepts for improving the performance and safety of nuclear reactors.

Non-Destructive Evaluation of Novel Materials

Importance: Development of a novel material for any engineering application requires extensive evaluation and testing of its mechanical properties at different stages in the operational environment. It is always desirable and often necessary to preserve the specimen for further tests. Multiple tests on a specimen enable to establish correlation between the differet properties of the material. Thus, non-destructive techniques play an important role in evaluating new materials.

Challenges: However, one of the major limitations of several commonly used non-destructive techniques is lack of sufficient accuracy. For before-exposure and after-exposure type of studies the changes in the properties are small enough to go undetected by these non-destructive techniques. Some of the novel materials have complex architecture and composition which adds further difficulties in accurately determing the properties of the materials.

Instrument

Our goal: We seek to develop non-destructive capabilites to overcome these limitations. Resonant Ultrasound Spectroscopy (RUS) has been found to be a promising technique capable of evaluating elastic properties with good accuracy. Non-linear RUS can provide valuable information about the defects in a material. Although these RUS techniques have been around for more than couple of decades they have not been extensively used. One of the reasons for their limited application is that the equations, which relate the resonance frequencies to the elastic properties, can be solved analytically for only few selected geometries. With the advancement of computing power and development of sophisticated codes the ability to solve equations for complex domains has greatly increased. When RUS is combined with the numerical modeling the domain of RUS' applicability increases vastly. We work on applying RUS in conjunction with finite element modeling to evaluate the properties of novel complex materials which are otherwise difficult to evaluate by other existing techniques.

Mechanical Characterization & Development of Test Standards

Importance: The deployment and commercialization of components for engineering applications manufactured using a new material, require several considerations including critical feasibility. A “qualified” database of properties of the new material is needed for performing rigorous experimental and numerical studies to determine the viability of the material for specific applications. In general, a “qualified” database is one which is generated using procedures that comply with standards associated with the design code of the component of interest. In the absence of a qualified database, studies for assessment of the material either cannot be carried out or the results of the studies cannot be interpreted and applied with confidence, leading to designs with higher safety margins. In addition, the designer cannot use a material without a database directly in new designs but has to (i) provide evidence that the material complies with the code requirement and (ii) obtain permission to use that material in design. Thus, the lack of a comprehensive property database and standard test procedures to test the material can significantly hamper the development of the technology and can negatively affect the material development.

Challenges: The existing test standards are either not appropriate or not complete for testing many novel materials. For example, the existing ASTM test standards for axial tensile test (ASTM C1773) and hoop tensile test (ASTM C1819) of continuous-fiber reinforced ceramic composite tubes lack the precision and bias statement. These statements convey important information to the users of the standard about the quality of their test results. The ISO 2032317, which is another standard for axial tensile testing of ceramic composites at ambient temperature, is also under development. The lack of completeness of the standard test procedure makes it difficult to deploy these novel materials for industrial applications.

Tests

Our goal: We aim to fill these gaps in the development process of new materials through collaborative experimental studies. The studies are conducted in collaboration with several laboratories from government institutions, academia and industry. The first step of these studies involve determining and employing the best techniques for dimensions and strain measurement, developing the optimum design of specimen, fixtures and gripping system, utilizing the state of the art techniques for damage assessment, development of test protocol to be followed by the operators and so forth. At the second step a 'lead' test is conducted by the leading institution. Once the results from the lead test are found to be satisfactory, further tests are conducted by other institutions. The generated data are compiled and analyzed to evaluate the properties of the material. Based on the results of the round robin study, precision and bias statement for the test procedure are established.