The nuclear industry continues to rise above challenges resulting from major plant events around the world. It is important to study the significant events, and find solutions to overcome the vulnerabilities identified, and retain the lessons while technology evolves to the next generation of SMRs. The plant events discussed are from Light Water reactors (LWRs) but he derived lessons could apply to LWR–SMRs and it could be extrapolated for other designs. The historical lessons on Station-Black-Out needs to be re-examined further in a new dimension in the light of “Fukushima type” events where AC & DC power was lost to the entire station from a common cause and the recovery in a reasonable period was not practical. The SMRs are specially considered for smaller grid size and for remote locations where a large reactor is unsuitable. The specific advantages for SMRs are its smaller foot print, modular type construction and flexibility for incremental units to meet rising demand in power. These features are very attractive to countries with small grids and for countries that are in the early stages of nuclear power technology development. In anticipation of these opportunities, when we launch into new technology of evolutionary and revolutionary advancements, the reactors have to be more rugged with ample diversity in critical reactor safety areas through both passive and active means to account for a conditions that may inhibit electrical energy as a source of motive power. An electrical event in Forsmark plant that propagated from an electrical switchyard resulted in two core cooling divisions disabled and consequently exacerbating the plant condition by opening the steam relief system for reactor coolant system and that significantly increased the probability for core damage. A minor spark in an electronic control system card in a US plant caused inadvertent actuation of emergency core cooling and disabled the capability of control room operators to intervene and prevent the primary cooling system from completely going water solid in the primary loop of Reactor Coolant System. A renewed assessment is needed to address the following questions for advancing reactor safety in the new evolving generation of LWR-SMR plants and to other types as applicable to preserve the historic lessons in safety. (1) Evaluate the diversity in emergency core cooling systems following loss of all AC power onsite; (2) Ensure diversity in reactor trip & depressurization; (3) Evaluate the suitability of logic/control system failure mode resulting from power supply failures in instrument channels and/or divisions, i.e. by conducting failure mode and effects analysis (FMEA) for system, power supplies and component level; (4) Confirm independence in reactor trip and emergency core cooling systems for sensors, power supplies and actuation systems.

Small modular reactors (SMRs) offer potential for addressing the nation’s long-term energy needs. However, the project design cycle for new reactor concepts is lengthy. As part of the Department of Energy’s Advanced SMR research and development program, Oak Ridge National Laboratory (ORNL) is developing a Dynamic System Modeling Tool (MoDSIM) to facilitate rapid instrumentation and controls studies of SMR concepts. Traditional nuclear reactor design makes use of verified and validated codes to meet the strict quality assurance requirements of the licensing process for the Nuclear Regulatory Commission. However, there are significant engineering analyses and high-level decisions required prior to the rigorous design phase. These analyses typically do not require high-fidelity codes. Different organizations and researchers may examine various plant configuration options prior to formal design activities. Engineers and managers must continuously make down-selection decisions regarding potential reactor architectures and subsystems. Traditionally, the modeling of these complex systems has been based on legacy models. Considerable time and effort are necessary to understand and manipulate these legacy models. For trade-space studies, two developments in the model-based systems engineering space represent a significant advancement in the ability of engineering tools to meet these demands. The first is Modelica: a nonproprietary, equation-based, object-oriented modeling language for cyber-physical systems. The second is the Functional Mockup Interface: a standardized, open interface for model exchange, simulation, and deployment. ORNL’s MoDSIM tool makes use of these developments and is intended to provide a flexible and robust dynamic system-modeling environment for SMRs. This includes single or multiple reactors, perhaps sharing common resources, or producing both electricity and process heat for local consumption or feeding a regional grid. MoDSIM uses the open-source modeling language (Modelica) and incorporates a user interface, coupled dynamic models, and analysis capabilities that will enable non-expert modelers to perform sophisticated end-to-end system simulations of both neutronic and thermal-hydraulic models. This approach enables open-source and crowd-source-type collaborations for model development of SMRs in an approach similar to open-source and open-design techniques currently used for software production and complex system design. As part of the tool development, an example SMR was chosen (advanced liquid metal reactor [ALMR]) and the ALMR models developed and interface tools demonstrated. For initial verification purposes, the results from these Modelica simulations are compared with the results documented for the earlier ALMR power-reactor innovative small-module concept. These results, as well as initial demonstrations of the tool for different control strategies, are presented in this paper.

Small Modular Reactors (SMRs) under design and development today are working to crystallize the measurements that must be made to control the reactor and monitor its safety. Traditionally, temperature, pressure, level, flow, and neutron flux are measured in conventional nuclear reactors for operation and control and to protect against equipment and process deviations that can affect safety. In most current SMR designs, essentially the same process variables may have to be measured; especially primary coolant flow depending on whether the core cooling and heat transfer results from natural circulation or forced flow. The flow can be measured directly or inferred from other measurements or estimated through empirical or physical modeling. The conventional sensors that are qualified for nuclear services and are currently used in nuclear power plants may or may not be suitable for SMRs. It all depends on the size and qualification requirements, installation details, static and dynamic performance specifications, wiring details, and sensor life expectancy. This paper will explore the possibilities that exist for SMRs to use today’s sensors and any need for new sensor designs. In addition, the paper will identify new means for automated monitoring of instrumentation and control (I&C) sensor performance in SMRs. In particular, the existing array of online calibration monitoring techniques and in-situ response time measurement methods will be evaluated for implementation in SMRs. This is important at this early stage as SMRs can easily build provisions in their mechanical, electrical, and I&C designs to accommodate online and automated I&C maintenance. For example, it is envisioned that SMRs will not be performing periodic sensor calibrations using classical hands-on procedures. Rather, SMRs are expected to be equipped with new technologies to verify the I&C performance automatically and flag the sensors and systems to be calibrated, response time tested, repaired, or replaced. The paper will explore these possibilities and will report on a current R&D project that is underway at AMS with funding from the U.S. Department of Energy (DOE) with the goal to adapt the existing online monitoring (OLM) technologies for implementation in SMRs. The existing OLM technologies have been used by AMS in commercial nuclear power plants and research reactors for monitoring of I&C equipment performance including calibration, response time, detection of sensing line blockages, and to distinguish whether a signal anomaly is due to cables/connectors, electromagnetic interference, an end device being a sensor or a pump, other rotating equipment, etc.

One of the advantages of small modular reactors (SMRs) is their possible deployment in remote locations and continued long-term operation with minimum downtime. In order to achieve this operational goal, the SMRs may require remote and continuous monitoring of process parameters. This feature is also important in monitoring critical parameters during severe accidents and for post-accident recovery. Small integral light water reactors have in-vessel space constraints and many of the traditional instrumentation are not practical in actual implementation. In order to resolve this issue, experiments were carried out on a flow test loop to characterize the relationship among process variables (flow rate, pressure, water level) and pump motor signatures. The paper presents the findings of this research with implications in relating electrical signatures to pump parameters.

Temperature is a key process variable at any nuclear power plant (NPP). The harsh reactor environment causes all sensor properties to drift over time. At the higher temperatures of advanced NPPs the drift occurs more rapidly. Johnson noise is a fundamental expression of temperature and as such is immune to drift in a sensor’s physical condition. In and near the core, only Johnson noise thermometry (JNT) and radiation pyrometry offer the possibility for long-term, high-accuracy temperature measurement due to their fundamental natures. Small Modular Reactors (SMRs) place a higher value on long-term stability in their temperature measurements in that they produce less power per reactor core and thus cannot afford as much instrument recalibration labor as their larger brethren. The purpose of the current ORNL-led project, conducted under the Instrumentation, Controls, and Human-Machine Interface (ICHMI) research pathway of the U.S. Department of Energy (DOE) Advanced SMR Research and Development (R&D) program, is to develop and demonstrate a drift free Johnson noise-based thermometer suitable for deployment near core in advanced SMR plants.

Advanced small modular reactors (AdvSMRs) will use advanced digital instrumentation and control systems, and make greater use of automation. These advances not only pose technical and operational challenges, but will inevitably have an effect on the operating and maintenance cost of new plants. However, there is much uncertainty about the impact of AdvSMR designs on operational and human factors considerations, such as workload, situation awareness, human reliability, staffing levels, and the appropriate allocation of functions between the crew and various automated plant systems. Existing human factors and systems engineering design standards and methodologies are not current in terms of human interaction requirements for dynamic automated systems and are no longer suitable for the analysis of evolving operational concepts. New models and guidance for operational concepts for complex socio-technical systems need to adopt a state-of-the-art approach such as Cognitive Systems Engineering (CSE) that gives due consideration to the role of personnel. The approach described here helps to identify and evaluate human challenges related to non-traditional operational concepts. A framework for defining operational strategies was developed based on an analysis of the Experimental Breeder Reactor-II (EBR-II), a small (20MWe) sodium-cooled reactor that was successfully operated for thirty years. Insights from the application of the systematic application of the methodology and its utility are reviewed and arguments for the formal adoption of CSE as a value-added part of the Systems Engineering process are presented.