Operational Nuclear Plants Research Articles

Transactive Energy Systems: The Market-Based Coordination of Distributed Energy Resources

Due to pressing environmental concerns, there is a global consensus to commit to a sustainable energy future. Germany has embraced Energiewende, a bold sustainable energy policy of no operational nuclear plants by 2022. California has set an ambitious goal that mandates 50% renewable penetration by 2025, 60% by 2030, and 100% by 2045 [1]. The vast integration of renewable energy into the power grid imposes daunting challenges on the conventional supply-side control paradigm. First, renewable energy is intermittent and uncertain. Integrating renewable energy will reduce the system inertia, inject undesirable variability, and substantially increase the need for system reserves. These reserves are typically provided by conventional generators. However, conventional generators have limited capacity and ramping rate, and they produce carbon emissions that defeat the carbon benefit of renewable integration. On the other hand, the increasing penetration of renewable energy also squeezes the response time needed to balance the power grid. That is, the time required to make critical operating decisions is decreasing from minutes to seconds and, in some cases, even subseconds due to the increasing variability of supply and demand. The shorter response time for decision making places significant challenges on the conventional power grid, which requires human interaction.

Nonlinear vibrations of a nuclear fuel rod supported by spacer grids

The internal components of Pressurized Water Reactors (PWRs), particularly nuclear fuel assemblies, must be able to withstand Flow Induced Vibrations (FIVs) during operating conditions and during extreme accident conditions, such as earthquakes. Nuclear fuel assemblies are composed of long slender tubes, filled with uranium pellets that are bundled together by periodic support provided by spacer grids. Spacer grids are square structures used to increase thermal mixing in the core and provide support to the fuel rods and guide tubes allowing for the installation and removal of nuclear fuel rods. Nevertheless, spacer grids constitute a nonlinear flexible boundary condition experiencing friction forces and impacts complicating the dynamics of the fuel rod-spacer grid system. In order to improve safety margins in the design of nuclear fuel assemblies, it is of great interest to understand the nonlinear behavior at the interface of the spacer grids with the fuel rods, investigating the complexity due to the nonlinear evolution of the system stiffness and damping properties. In particular, the effect of the constant evolution of the vibration amplitude as a function of the change of the excitation forces on the dynamics of the fuel rod response is still undetermined. Experiments were carried out in quiescent water and in air to understand the nonlinear vibration response of a single zirconium fuel rod supported by spacer grids. The vibration response under a step-sine harmonic excitation at different force amplitude levels in the frequency neighborhood of the fundamental mode was measured. The response of the rod displayed nonlinear phenomena such as the shift of the resonant frequencies, multiple solutions with some instabilities (jumps) and hysteresis, and a weak one-to-one internal resonance. Tests were performed on an empty rod and on a rod filled with tungsten pellets representative of nuclear fuel. The pellets were let free to move and were subsequently blocked axially to reproduce the effect of the beginning-of-life constraint in operational nuclear plants. The experimental data were processed by means of a simplified identification procedure to extract the damping parameters of the vibrating system. The equivalent viscous damping is found to increase and to be a function of the level of excitation and of the peak vibration amplitude.

Homogeneity and Realism in Critical Group Assessments for Routine Radiological Discharges

The 'critical group' concept provides a robust means of demonstrating compliance with radiological dose limits set for routine effluent discharges arising from operational nuclear plants. Calculating the average dose to members of the critical group based on reasonable 'maximising assumptions' offers a defensible degree of conservatism in demonstrating compliance with the dose limits and, in principle, the identification of a 'relatively homogeneous' group, representative of those individuals in the population expected to receive the highest radiation dose, appears to offer no difficulties. Over the past twenty years, since publication of ICRP 26 guidance, however, there have been a number of changes to dose limits and interpretation of critical groups. In practice this has resulted in seeking more realistic dose estimates. Where a number of different habits are relevant to exposure (e.g. consumption of marine foods, fishing, occupancy of intertidal areas and components from terrestrial pathways) the requirements for homogeneity and realism may conflict. Radiological assessments based on critical groups, and associated environmental monitoring programmes, need to be sensitive to changes in individual pathways without necessarily switching the focus of presentation in a way which may appear haphazard to members of the public. A balanced approach is thus required in defining group characteristics, and presentation of ranges on estimated dose may be appropriate. These issues are considered in the context of assessment of doses to marine critical groups in West Cumbria.

Testing to determine relay seismic ruggedness

The seismic qualification of equipment in operating nuclear plants has been identified as a potential safety concern in U.S. Nuclear Regulatory Commission (USNRC) Unresolved Safety Issue (USI) A-46, “Seismic Qualification of Equipment in Operating Nuclear Power Plants”. In response to this concern, the Seismic Qualification Utility Group (SQUG), with support from the Electric Power Research Institute (EPRI), has undertaken a program to demonstrate the seismic adequacy of essential equipment by the use of actual experience with such equipment in plants which have undergone significant earthquakes and by the use of available test data for similar equipment. An important part of this program is the development of the methodology and test data for verifying the functionality of electrical relays used in essential circuits needed for plant shutdown during a seismic event. This paper describes the EPRI supported relay testing program to supplement existing relay test data. Many old relays which are used in safe shutdown systems of SQUG plants and for which seismic test data do not exist have been shake-table tested. The testing performed on these relays and the test results for two groups of relays are summarized in this paper.

Configuration management and load monitoring procedures for nuclear plant structures

Nuclear power plant design and modification require significant engineering effort to demonstrate that each structural member has the capacity to resist the actual loads applied to it. Operating nuclear plants undergo frequent modification, many of which affect loadings to the structure. Each change that impacts loads must be reassessed to maintain the design baseline. Tracking the structural framing configuration along with all loads supported by the structure is an essential part of configuration management. Typically , the design of structural steel floor framing requires the calculation of thousands of loads due to the numerous equipment supports and mechanical and electrical component supports involved. Manually tracking the load changes for thousands of components and determining their impact on the design adequacy of structural steel beams and connections is man-hour intensive and can have a potentially significant impact on schedules. This paper describes a computer-aided engineering tool called the Load Monitoring System (LMS) that was proven effective for monitoring floor framing, loads, and structural integrity. The system links structural analysis, design investigation, and reporting and automated drafting programs with a data base. It provides design engineers with a powerful tool for quickly incorporating, tracking, and assessing load revisions and determining effects on steel floor framing members and connections, thereby helping to reduce design man-hours, minimize the impact of structural modifications, and maintain and document the design baseline. The major benefit to utilities are the reduction in engineering costs, assistance with plant configuration management, and assurance of structural safety throughout the operating life of a nuclear plant and at evaluation for license renewal.

Seismic functionality of essential relays in operating nuclear plants

The regulatory criteria for licensing of nuclear power plants require that certain safety-related equipment and systems be designed to function during and following a postulated, design basis earthquake. Demonstration of seismic adequacy must be performed and formally documented by shake-table testing, analysis or other specified methods. Since many older, operating nuclear power plants were designed and constructed prior to the issuance of the current seismic qualification criteria, the NRC has questioned whether the seismic adequacy of the essential equipment has been adequately demonstrated and documented. This concern is identified in Unresolved Safety Issue A-46, “Seismic Qualification of Equipment in Operating Nuclear Power Plants”. In response to this concern, a group of affected plant owners, the Seismic Qualification Utility Group (SQUG), with support from the Electric Power Research Institute (EPRI), has undertaken a program to demonstrate the seismic adequacy of essential equipment by the use of actual experience with such equipment in plants which have undergone significant earthquakes and by the use of available seismic qualification data for similar equipment. An important part of this program is the development of data and the methodology for verifying the functionality of electrical relays used in essential circuits needed for plant shutdown during a seismic event. This paper describes this part of the Seismic Qualification Utility Group program. The relay functionality evaluation methodology is being developed under EPRI Project No. RP2849-1.