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Advanced reactors are often claimed to be passively safe against unprotected upset events. In common practice, these events are not considered in the context of the plant control system, i.e., the reactor is subjected to classes of unprotected upset events while the normally programmed response of the control system is assumed not to be present. However, this approach constitutes an oversimplification since, depending on the upset involving the control system, an actuator does not necessarily go in the same direction as needed for safety. In this work, dynamic simulations are performed to assess the degree to which the inherent self-regulating plant response is safe from active control system override. The simulations are meant to characterize the resilience of the plant to unprotected initiators. The initiators were represented and modeled as an actuator going to a hard limit. Consideration of failure is further limited to individual controllers as there is no cross-connect of signals between these controllers. The potential for passive safety override by the control system is then relegated to the single-input single-output controllers. The results show that when the plant control system is designed by taking into account and quantifying the impact of the plant control system on accidental scenarios there is very limited opportunity for the preprogrammed response of the control system to override passive safety protection in the event of an unprotected initiator. [ABSTRACT FROM PUBLISHER]

The interaction of the active control system with passive safety behavior is investigated for sodium-cooled fast reactors. A claim often made of advanced reactors is that they are passively safe against unprotected upset events. In practice, such upset events are not analyzed in the context of the plant control system, but rather the analyses are performed without considering the normally programmed response of the control system (open-loop approach). This represents an oversimplification of the safety case. The issue of passive safety override arises since the control system commands actuators whose motions have safety consequences. Depending on the upset involving the control system (operator error, active control system failure, or inadvertent control system override), an actuator does not necessarily go in the same direction as needed for safety. So neglecting to account for control system action during an unprotected upset is nonconservative from a safety standpoint. It is important then, during the design of the plant, to consider the potential for the control system to work against the inherent and safe regulating effects of purposefully engineered temperature feedbacks. [ABSTRACT FROM PUBLISHER]

Simulation results are presented for a design strategy that seeks to achieve inherent control and passive safety for liquid-metal advanced small modular reactors. The approach places an increased reliance on passive feedbacks to regulate plant operation. A reference liquid-metal reactor design is defined to serve as a baseline against which innovative design concepts can be compared with respect to operational performance. The definition assigns values to key plant parameters related to materials type, component data, system configuration (loop versus pool type), fuel cycle (burner versus breakeven versus breeder), and balance of plant. The reference design represents the state of the art of conventional fast reactor technology in terms of economics of electricity production, use of active control systems, and standard operation (e.g., refueling every 2 to 3 years). Innovative design features and associated control strategies are then investigated for reducing the size of upset imitators and for improving also the safety of the inherent response to the initiator. Initiators include failures of active systems and operator errors. At the same time the ability of the modified plant to meet normal grid demands subject to constraints on temperature rates of change is assessed. Results presented indicate that operational performance can be maintained while active system initiator size is reduced resulting in improved safety. Essentially, the innovations introduce inherent feedback mechanisms that serve to reduce the magnitude of the control action of the active control systems. [ABSTRACT FROM AUTHOR]

The recent interest in the Small Modular Reactor (SMR) for its potential increased economic competitiveness has focused attention in part on reducing operational costs to offset those plant costs that do not benefit from the economies of scale of large traditional units. Plant operation and maintenance economics are significantly driven by plant availability, which can be enhanced by means of innovative control strategies by avoiding unnecessary plant or unit trips. In this context, an effective strategy for achieving fast runback of a sodium-cooled SMR has been developed. In this work, after having defined and modeled a suitable control strategy by adopting the Petri nets formalism, a Model-based Predictive Control regulator has been developed in order to reduce as promptly as possible the power level, without scramming the reactor (fast runback) and possibly limiting the control rod contribution. Such flexibility could lead to significant savings in the operational costs of the reactor while also improving the system availability. The proposed procedure has been characterized by simulating the operational transients on both an oxide-fueled reactor and on a metal-fueled reactor, comparing the responses of the two different configurations and the respectively needed control rod contribution. [ABSTRACT FROM AUTHOR]