Nuclear Systems Analysis

ISL supports domestic and international clients in nuclear reactor safety, security, regulatory and environmental assessment with a focus on thermal-hydraulic analysis and probabilistic risk assessment. ISL’s experienced, highly educated, and motivated staff of engineers and scientists provide technical services to the U.S. Nuclear Regulatory Commission (NRC), National Aeronautics and Space Administration (NASA), U.S. Department of Energy (DOE), and foreign clients. Its staff assesses the behavior of complex nuclear systems for current, new, and advanced reactor designs including the Advanced Passive 1000 (AP-1000), the Evolutionary Power Reactor (EPR), the U.S. Advanced Pressurized-Water Reactor (USAPWR), the Pebble Bed Modular Reactor (PBMR), and the Economic Simplified Boiling-Water Reactor (ESBWR). ISL staff is knowledgeable in domestic and international regulatory practices and has supported the development and assessment of regulatory guidance and requirements for various clients. ISL is ready to provide a full range of technical services through its experienced in-house staff and its many teaming partners.

Thermal Hydraulic Analysis

ISL’s staff of experienced analysts use open source and proprietary techniques to perform fluid systems and thermal hydraulic analyses. Where existing software lacks sufficient capabilities to perform the required analysis, ISL’s software developers can provide the tools needed to do the job. ISL’s analysis and code development expertise covers the full range of fluid-thermal systems, computational fluid dynamics, nuclear thermal-hydraulics and thermal analysis. Services range from expert consulting to the performance of complete analyses using software developed specifically for the work assignment. ISL is currently developing computer software and analysis systems to simulate thermal-hydraulic and neutronic responses of complex nuclear power systems. The work includes updating and maintaining Reactor Excursion and Leak Analysis Program 5 (RELAP5) and related computer codes, as well as participating in the development of next generation advanced analytical techniques, including TRAC/RELAP Advanced Computational Engine (TRACE). ISL covers the complete range of analysis methodology development activities including preparation of software requirements specifications, code design, programming, testing and assessment, code distribution, help desk, training, and user support. acrobatThermal-Hydraulics Capability Overview (285KB)

Simulation of Complex Systems Behavior

ISL applies large computer programs to simulate the physical behavior of commercial nuclear power plant systems during accident conditions. These activities address the phenomenological aspects of thermal-hydraulics such as:

  • boiling and condensation
  • fluid flow and heat transfer, including details of convective heat transfer
  • nucleate boiling, transition boiling/quench front advance, and dispersed flow film boiling
  • detailed coupling of the flow regimes and associated heat transfer mechanisms
  • multi-surface models for radiation heat transfer
  • multi-dimensional drift-flux and two-fluid models of two-phase flow for bubbly, slug annular, and droplet/mist flow
  • condensation in the presence of non-condensable gases

Typical problems addressed by ISL include: thermal design analysis, nuclear plant accident simulation, fluid system simulation to support design, nuclear plant performance analysis, analysis of operational transients, test program reviews, simulator benchmarking, containment analysis, severe accident analysis, hydrodynamic force calculations, control system studies, spent fuel cooling analysis, and safety analysis.

Computational Fluid Dynamics

Recognizing the tremendous potential of Computational Fluid Dynamics (CFD) methods in support of complex systems analysis, ISL has developed techniques and quality standards for CFD applications that significantly extend the capabilities of traditional methods in the area of thermal-hydraulics. ISL’s research staff has CFD expertise in the following areas:

  • Generation of structured, unstructured and hybrid grids.
  • Modeling of laminar and turbulent single-phase flow.
  • Computational Multiphase Flow Dynamics (CMFD).
  • Coupling of CFD methods with neutronic/thermal-hydraulic system codes for safety analysis of nuclear power plants.
  • Implementation of specific physical models in commercial CFD software through external software routines.
  • Parallel computing.
  • Validation and Verification (V&V) of CFD analyses (Quality Assurance).
  • Effective data post-processing and visualization.
acrobat Computational Fluid Dynamics Capability Overview (280KB)

Probabilistic Risk Assessment Review

Probabilistic risk assessments (PRAs) are used by regulatory agencies and operating organizations to make better safety decisions and focus resources. ISL’s PRA experts are continuing to assist both NASA and the NRC with their safety missions by providing risk assessment analysis support. ISL is also supporting the NRC in evaluating licensee programs for minimizing the probability of severe accidents by analyzing options for plant modifications for license renewal applications—the agency’s Severe Accident Mitigation Alternatives (SAMA) program. ISL has also been called on to evaluate the costs and benefits of alternatives that could be used to tackle important safety issues. Past projects include support for the development of new regulatory requirements by the NRC for operator fatigue and additional electric power supplies for plant safety equipment.

Risk-Informed Regulation

Risk analysis is being used to improve the focus of regulatory requirements on safety-significant areas. ISL has supported the NRC in the application of risk analysis to regulatory decision making for many years. This support includes the development of plant risk models (scenario development, including fault tree/event tree analysis of systems, and the assessment of mission success criteria), quantification (analysis of performance data and application of these data to model quantification), and consequence assessment (evaluation of the physical consequences of selected event sequences). These models are applied to determine the adequacy of design features and operational practices, or to evaluate the emphasis or level of conservatism of proposed or existing regulatory requirements.

Accident Sequence Precursor Analysis

As the agency responsible for ensuring adequate protection of public health and safety, the NRC conducts programs for monitoring the safety performance for all nuclear plants in the United States (U.S.). The Accident Sequence Precursor (ASP) program is a key part of this agency performance assessment program. ISL is the principal contractor supporting the NRC ASP program. The primary objective of the ASP Program is to systematically evaluate nuclear plant operating experience to identify, document, and rank operating events most likely to lead to inadequate core cooling and severe core damage (precursors), if additional failures occur.

Assessment of Safety Issues

Nuclear power safety analysis and assessment is an essential part of the NRC’s safety review process. ISL risk analysts have supported many of these assessments, including the analysis of power uprates, the evaluation of licensee programs for minimizing the probability of severe accidents by analyzing options for plant modifications, the analysis of safety issues for operating reactors, and analysis of safety issues associated with design certification for advanced reactors such as GE’s ESBWR and the Canadian reactor ACR-700.

Advanced Reactors

ISL supported the NRC’s development of the regulatory framework for advanced reactors. This risk informed performance-based framework was developed by the NRC using a technology-neutral structure to enable an efficient, stable and predictable regulatory process. ISL tested this framework using modified risk models associated with current light water reactors. This approach allowed for comparison of the current licensing framework with the proposed technology-neutral framework. ISL also participated in the framework’s development. This framework was published in December 2007 as NUREG-1860, “Feasibility Study for a Risk-Informed and Performance-Based Regulatory Structure for Future Plant Licensing.”

Risk Performance Metrics

ISL has supported the NRC in the development of the “Mitigating Systems Performance Indices” (MSPI). The MSPI is a simplified linear approximation of the change in core damage frequency (CDF) attributable to changes in the reliability and availability of risk-significant elements of the monitored system. These indices provide an indication of the risk significance of declining equipment performance. ISL contributed to the formulation and verification of these indices, and provided technical support and analysis tools in support of the review of licensee MSPI bases documents and their underlying PRA.


Security Assessment

ISL is supporting the NRC in the development of guidance and the review of new reactor applicants’ security programs. This work is focused on the process of developing and identifying vital equipment and target sets, including methodologies used for determining and grouping the target set equipment, and methodologies used for performing the assessment. Target sets are the combination of equipment or operator actions, which, if all are prevented from performing their intended safety functions or prevented from being accomplished, would likely result in significant core damage barring extraordinary action by plant operators.

Nuclear Reactor License Renewal

“License renewal” refers to the process and granting (renewal) of a new reactor operating license by the NRC for an additional 20 years beyond the original 40 year license term. The NRC has established a regulatory process focusing on equipment and potentially significant aging issues over the life of the plant. Regulatory requirements ensure that important systems, structures and components will continue to perform their intended functions during the 20-year period of extended operation. ISL provides technical support to the NRC in reviewing utility applications for a renewed operating license. ISL experts in nuclear systems operation, design, and risk assessment work with the NRC to ensure that the utility application is sound and necessary programs have been put into place at the plant. Detailed system reviews are performed to confirm that required plant safety equipment is included in maintenance programs. Additional review, including audits at the plant site, are performed to ensure that the utility has appropriately implemented the NRC’s requirements for identifying aging issues that can be expected over the extended operating life.