The MDTA-based method for assessing diagnosis failures and their risk impacts in nuclear power plants

In the emergency situations of nuclear power plants (NPPs), a diagnosis of the occurring events along an accident progression or as initiating events is crucial for managing or controlling a plant to a safe and stable condition. If the operators fail to diagnose the occurring event(s), their responses to a given event can eventually become inappropriate or inadequate. This paper presents an analytical method for assessing the potential for a diagnosis failure (or misdiagnosis) and its consequences for human behaviour and plant safety. The method largely comprises of three steps as follows: (1) Analysis of the potential for a diagnosis failure, (2) Identification of the human failure events (HFEs) that might be induced due to a diagnosis failure, and (3) Quantification of the HFEs and their modeling into a PSA model. The paper also presents a pilot application of the proposed method to the small loss of coolant accident of a Korean NPP.     

Investigation of fatigue-induced socket-welded joint failures for small-bore piping used in power plants

Nuclear power plants typically experience two or three high-cycle fatigue failures of stainless steel socketwelded connections in small bore piping during each plant-year of operation. This paper discusses fatigue-induced failure in socket-welded joints and the strategy Texas Utilities Electric Company (TU Electric) has implemented in response to these failures. High-cycle fatigue is invisible to proven commercial nondestructive evaluation (NDE) methods during crack initiation and the initial phases of crack growth. Under a constant applied stress, cracks grow at accelerating rates, which means cracks extend from a detectable size to a through-wall crack in a relatively short time. When fatigue cracks grow large enough to be visible to NDE, it is likely that the component is near the end of its useful life. TU Electric has determined that an inspection program designed to detect a crack prior to the component leaking would involve frequent inspections at a given location and that the cost of the inspection program would far exceed the benefits of avoiding a leak. Instead, TU Electric locates these cracks by visually monitoring for leaks. Field experience with fatigue-induced cracks in socket-welded joints has confirmed that visual monitoring does detect cracks in a timely manner, that these cracks do not result in catastrophic failures, and that the plant can be safely shut down in spite of a leaking socket-welded joint in a small bore pipe. Historical data from TU Electric and Southwest Research Institute are presented regarding the frequency of failures, failure locations, and the potential causes. The topics addressed include 1) metallurgical and fractographic features of fatigue cracks at the weld toe and weld root; 2) factors that are associated with fatigue, such as mechanical vibration, internal pulsation, joint design, and welding workmanship; and 3) implications of a leaking crack on plant safety. TU Electric has implemented the use of modified welding techniques for the fabrication of socket-welded joints that are expected to improve their ability to tolerate fatigue.     

A method for evaluating fault coverage using simulated fault injection for digitalized systems in nuclear power plants

The fault coverage for digital system in nuclear power plants is evaluated using a simulated fault injection method. Digital systems have numerous advantages, such as hardware elements share and hardware replication of the needed number of independent channels. However, the application of digital systems to safety-critical systems in nuclear power plants has been limited due to reliability concerns. In the reliability issues, fault coverage is one of the most important factors. In this study, we propose an evaluation method of the fault coverage for safety-critical digital systems in nuclear power plants. The system under assessment is a local coincidence logic processor for a digital plant protection system at Ulchin nuclear power plant units 5 and 6. The assessed system is simplified and then a simulated fault injection method is applied to evaluate the fault coverage of two fault detection mechanisms. From the simulated fault injection experiment, the fault detection coverage of the watchdog timer is 44.2% and that of the read only memory (ROM) checksum is 50.5%. Our experiments show that the fault coverage of a safety-critical digital system is effectively quantified using the simulated fault injection method.     

Selection of the working medium for 6- to 12-kW closed organic-fuel-powered gas-turbine plants

Results of evaluation of the cycle parameters of a low-power recuperative closed gas-turbine plant powered by organic fuel are reported. Analysis of the effect of the molar mass of the working medium on the plant’s basic characteristics and its features is carried out.     

A finite element method for contact using a third medium

The numerical simulation of contact problems is nowadays a standard procedure in many engineering applications. The contact constraints are usually formulated using either the Lagrange multiplier, the penalty approach or variants of both methodologies. The aim of this paper is to introduce a new scheme that is based on a space filling mesh in which the contacting bodies can move and interact. To be able to account for the contact constraints, the property of the medium, that imbeds the bodies coming into contact, has to change with respect to the movements of the bodies. Within this approach the medium will be formulated as an isotropic/anisotropic material with changing characteristics and directions. In this paper we will derive a new finite element formulation that is based on the above mentioned ideas. The formulation is presented for large deformation analysis and frictionless contact.     

A dynamic method for the production lot sizing with machine failures

This paper presents a dynamic method to deal with two economic lot sizing models with exponential machine failures. Its main difference from the static method is that the machine failure is separately treated from the perspective of failure realisation. The optimal lot size is derived through the equation between the incurred average cost and the marginal cost, which has to be performed for each lot due to the random failures. In the first no-resumption model, the simulation shows that two methods have the similar performance as the lot number increases. For the resumable model, we propose a zero-inventory resumption policy that always resumes the production after each machine failure but delays the resumption until the on-hand inventory is depleted. The simulation result indicates that the new policy by the dynamic method outperforms the initial abort/resume policy, and it also shows the convergence as the production continues.     

A new method to evaluate alternate AC power source effects in multi-unit nuclear power plants

In order to evaluate accurately a station blackout (SBO) event frequency of a multi-unit nuclear power plant that has a shared alternate AC (AAC) power source, an approach has been developed which accommodates the complex inter-unit behavior of the shared AAC power source under multi-unit loss of offsite power conditions. The SBO frequency at a target unit of probabilistic safety assessment could be underestimated if the inter-unit dependency of the shared AAC power source is not properly modeled.The approach is illustrated for two cases, 2 units and 4 units at a single site, and generalized for a multi-unit site. Furthermore, the SBO frequency of the first unit of the 2-unit site is quantified. The methodology suggested in the present paper is believed to be very useful in evaluating the SBO frequency and the core damage frequency resulting from the SBO event. This approach is also applicable to the probabilistic evaluation of the other shared systems in a multi-unit nuclear power plant.     

Thermocycling treatment of titanium alloys using hydrogen phase cold working

Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 1, pp. 109–110, January–February, 1991.     

Seismic PSA method for multiple nuclear power plants in a site

The maximum number of nuclear power plants in a site is eight and about 50% of power plants are built in sites with three or more plants in the world. Such nuclear sites have potential risks of simultaneous multiple plant damages especially at external events. Seismic probabilistic safety assessment method (Level-1 PSA) for multi-unit sites with up to 9 units has been developed. The models include Fault-tree linked Monte Carlo computation, taking into consideration multivariate correlations of components and systems from partial to complete, inside and across units. The models were programmed as a computer program CORAL reef. Sample analysis and sensitivity studies were performed to verify the models and algorithms and to understand some of risk insights and risk metrics, such as site core damage frequency (CDF per site-year) for multiple reactor plants. This study will contribute to realistic state of art seismic PSA, taking consideration of multiple reactor power plants, and to enhancement of seismic safety.     

Use of microwave working power standards as voltage standards

The results of experimental investigations of the parameters of the indirectly heated thermoelectric transducers in microwave wattmeters are presented and their errors are estimated. It is shown that they are suitable for use as voltmeters to measure the active voltage in matched lines up to 2 GHz and can be used to calibrate voltage calibrators at rf and microwave frequencies. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 48–53, May, 1997.     

A review on diffusion modelling in hydrogen related failures of metals

Modelling of hydrogen embrittlement in order to prevent engineering failures requires a characterization of transport phenomena in the bulk metallic as a first step. Interstitial solid state diffusion can be described as a random phenomenon, however there will be also some drift forces biasing this behaviour so a modification in Fick's laws is needed. The potential energy landscape of the metal lattice characterizes the influence of imposed fields and microstructural defects in transport kinetics. Thus, considering the chemical potential as a driving force, the physical basis of diffusion will be translated into continuum equations. Finally the two-level models that take into account lattice and trapping sites for hydrogen will be reviewed.     

Theoretical investigation of Ti-adsorbed graphene for hydrogen storage using the ab-initio method

Based on first-principles plane wave calculations, it was shown that boron substituted graphene with Ti metal atom adsorption can be used as a high capacity hydrogen storage material. Boron substitution in graphene enhances the Ti metal adsorption energy, which is much larger than that in the case of pure graphene, and than the Ti cohesive energy. The Ti metal atom can be well dispersed on boron-substituted graphene and can form a 2 x 2 pattern because the clustering of the Ti atoms is hindered by the repulsive Coulomb interaction between them. The H2 adsorption behavior on Ti metal atoms was investigated, along with the H2 bonding characteristics and the open-metal states of Ti. It was found that one Ti adatom dispersed on the double sides of graphene can absorb up to eight H2 molecules, corresponding to a 7.9% hydrogen storage capacity. In addition, the adsorption behaviors of non-H2 atoms like C and B were calculated to determine if Ti atoms can remain in an open-metal state in boron-substituted graphene.     

A method for identifying instrument faults in nuclear power plants possibly leading to wrong situation assessment

In this paper, we propose a method for identifying instrument faults that could potentially affect an operators’ situation assessment capability in nuclear power plants (NPPs), an issue which has received a lot of attention recently. In the proposed method, patterns of selected plant parameter trends in selected plant states and NPP operators’ patterns of selected plant parameter trends in selected plant states are analyzed, and a comparison between the two kinds of patterns is performed to identify instrument faults which could potentially affect a NPP operators’ capability to correctly assess a plant's conditions. An example application is presented to demonstrate how the proposed method can be used to identify the possibilities of operators’ developing a wrong situation assessment because of instrument faults and to identify the corresponding safety concerns. We conclude that in order to get more accurate results, an analysis with a full-scope NPP simulator and interviews with NPP operators will be necessary.     

A methodology for the prioritization of operating experience in nuclear power plants

The processing of operating experience at nuclear power plants is critically important to safe and reliable operations because it represents the process by which important external information is incorporated into the organization. Nuclear power plants typically receive between 800 and 1000 pieces of operating experience every year, of which 70–80 meet the criteria for a more extensive review. This paper deals with the prioritization of these items. Specifically, a prioritization methodology utilizing multiattribute utility theory has been developed. What sets this methodology apart from other techniques employing multiattribute methods is its emphasis on deliberations to achieve consensus among objectives and preferences among those objectives. Along with an explanation of this methodology, the results of its application to the prioritization of operating experience at a nuclear power plant are presented. Lastly, the results of a workshop that was held at MIT are presented. The workshop demonstrated the feasibility of the prioritization methodology and the validity of the case study results.     

A hydrogen plasmatron of 1 MW power

A plasmatron design is analyzed for producing hydrogen plasma at a mean-mass temperature above 4000K with a rather high efficiency.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 1, pp. 82–87, July, 1972.     

A model-based framework for the analysis of team communication in nuclear power plants

Advanced human-machine interfaces are rapidly changing the interaction between humans and systems, with the level of abstraction of the presented information, the human task characteristics, and the modes of communication all affected. To accommodate the changes in the human/system co-working environment, an extended communication analysis framework is needed that can describe and relate the tasks, verbal exchanges, and information interface. This paper proposes an extended analytic framework, referred to as the H-H-S (human-human-system) communication analysis framework, which can model the changes in team communication that are emerging in these new working environments. The stage-specific decision-making model and analysis tool of the proposed framework make the analysis of team communication easier by providing visual clues. The usefulness of the proposed framework is demonstrated with an in-depth comparison of the characteristics of communication in the conventional and advanced main control rooms of nuclear power plants.     

A method for comparison of absorption heat pump working pairs

Screening analyses based on thermodynamic and heat transfer principles are conducted for a single-effect absorption heat pump to enable the comparison of working pairs on the basis of performance and component size. After a broad survey of working pairs in the literature, in which 83 are discovered, forty-two are analyzed for cooling mode operation, and ten are analyzed for heating mode operation. Working pairs with water as the refrigerant are the most favorable thermodynamically in cooling mode, although some of them require more heat transfer area due to higher viscosity. Working pairs with alcohol refrigerants are most favorable thermodynamically in heating mode, although working pairs with ammonia as the refrigerant yield much more compact systems. The analysis methods developed here can also be applied to newly-characterized absorption working pairs to allow comparison with those previously investigated in the literature.