Symposium on Advanced Sensors and Modeling Techniques for Nuclear Reactor Safety

Coupled Modeling of NPPs: Evolution, Current Trends and Future Prospects

The foundation pillar in the peaceful utilization of fission nuclear power has always been the strong emphasis on nuclear safety. Safety has been accomplished through in-depth review of design and operations, incorporation of findings from safety R&D worldwide and improvement in methods of evaluation. In the entire chain of safety process, safety analysis has been a vital link which provides an estimation of the capability of the nuclear systems to control or accommodate departures from normal operation or postulated malfunctions or failures. Safety analysis is performed using numerical simulation tools (computer codes) which have evolved gradually in terms of accuracy and complexity over the past five decades. As a conventional practice, problem specific models addressing limited physical phenomenon were developed and used during design and analysis of present generation nuclear reactors. These code were based on strong emphasize on single physic phenomenon and they were improved and refined thoroughly in that dimension. Since these individual physical models were developed independently to pursue limited objectives, they had very little or almost no connections. These types of single physics models though helped appreciably in design and analysis of present generation reactors; they could not address finer aspects of many inter physics interactions, leading to significant over conservatism in nuclear design. The recent availability of strong computing resources and improved computational techniques has provided an opportunity to develop more realistic models of complex interacting phenomena in NPPs with more precise consideration of reactor multiphysics effects through coupling of different physics codes. Such coupled modeling of NPPs provides a basis to undertake a more in-depth evaluation of the safety margins found in previous analytical solution for which limited conservative models were used. Today, the coupled modeling of NPPs practice has become a necessary requirement of new generation large power reactors which are being designed to meet more stringent safety requirement. Comprehensive efforts are now being placed at combining knowledge of all involved diverse physical disciplines in high- performance multipurpose simulation tools to improve the reliability of computational simulations to the highest level and avoiding all possible sources of analytical uncertainties and ultimately eliminating unnecessary conservatism. The talk will present an overview of present generation best estimate coupled computing tools for NPPs which include mostly neutronics/thermal-hydraulics coupling into reactor core on assembly/channel basis. Ongoing efforts related to high-fidelity coupling on pinlevel of several physics phenomena in reactor core such as neutronics (reactor physics), thermal-hydraulics, fuel behavioretc. will be discussed. Relevant developments at AERB, India, for safety analysis of Indian NPPs will also be covered.