Recent applications of PSA for managing nuclear power plant safety

The safety design and regulation of nuclear power plants has traditionally been based upon deterministic approaches that consider a set of challenges to safety, e.g. design basis accidents, and determine how those challenges should be handled. The approach has been very successful since no plant designed or regulated to United States standards has ever harmed a member of the public. The arbitrary nature of these safety criteria, the potential inconsistencies in the judgments on relative probabilities, and the lack of definition for ‘safety’ became increasingly evident during the 1960s. Probabilistic approaches to reactor safety were proposed ^1,2,3 but did not take off in the United States until publication of the Reactor Safety Study ⁴ in 1975. Even as the methodology matured, there remained a challenge to integrate it into the regulatory process. This article will describe this integration process. A probabilistic approach to regulation enhances and extends the traditional deterministic approach by introducing the concept of safety (risk) significance that allows the designer/operator to focus on important issues. Emphasis was initially placed on relative risk but now regulatory decision-making is employing both relative and absolute risk. Measures of importance will be defined. Risk information can be used to prioritize the allocation of resources and three examples will be described. Equipment configuration control systems are being installed and used at nuclear power plants to enhance safety and to reduce Operating and Maintenance costs; they will be described. Finally, the US Nuclear Regulatory Commission's introduction of risk-informed decision-making into the regulatory process will be discussed.     

Radioecological consequences of a radiation accident in a nuclear-powered submarine in Chazhma cove

Conclusions The results of field observations and numerous radioecological inspections show that the submarine accident in Chazhma Cove in 1985 had no measurable radiation effect on Vladivostok and its beaches or the town of Shkotovo-22. The residual long-lived radioactive contamination of the site and bottom deposits in the region of Chazhma Cove is strictly localized and will not lead to serious radioecological consequences. Studies of the radioecological consequences of the accident will have to be continued in the future, including in order to determine more accurately the boundaries of the radioactive deposits in the sea water and the rates at which the radioactive deposits spread over the bottom of the bay and the inlet. Institute of Nuclear Reactors, Russian Scientific Center "Kurchatovskii institut." Naval Fleet, Vladivostok. Translated from Atomnaya énergiya, Vol. 76, No. 2, pp. 158–160, February, 1994.     

Safety study of the coupling of a VHTR with a hydrogen production plant

The present paper deals with specific safety issues resulting from the coupling of a nuclear reactor (very high temperature reactor, VHTR) with a hydrogen production plant (HYPP). The first part is devoted to the safety approach consisting in taking into account the safety standards and rules dedicated to the nuclear facility as well as those dedicated to the process industry. This approach enabled two main families of events to be distinguished: the so-called internal events taking place in the coupling circuit (transients, breaks in pipes and in heat exchangers) and the external events able to threat the integrity of the various equipments (in particular the VHTR containment and emergency cooling system) that could result from accidents in the HYPP. By considering a hydrogen production by means of the iodine/sulfur (IS) process, the consequences of the both families of events aforementioned have been assessed in order to provide an order of magnitude of the effects of the incidents and accidents and also in order to propose safety provisions to mitigate these effects when it is necessary. The study of transients induced by a failure of a part of the HYPP has shown the possibility to keep the part of the HYPP unaffected by the transient under operation by means of an adapted regulation set. Moreover, the time to react in case of transfer of corrosive products in the VHTR containment has been assessed as well as the thermohydraulic loading that would experience the coupling pipes in case of very fast uncoupling of the facilities aiming at avoiding an excessive pressurization of the VHTR containment. Regarding the external events, by applying a method used in the process industries, the bounding representative scenarios have been identified on the basis of their consequences but also on the basis of their occurrence frequency. The consequences of the selected bounding scenarios, calculated taking into the source-term, the atmospheric dispersion and the pressure and toxic effects induced respectively by a hydrogen unconfined vapour cloud explosion (UVCE) and a sulfur dioxide release have been assessed. The resulting safety distance of about 100 m for the UVCE is fairly acceptable in terms of performance (head loss and thermal loss) of the coupling system. However, the longer safety distance (about 1.5 km) calculated for a SO₂ release implies to foresee a long distance to settle the control room of the site or to foresee provisions able to stop very fast the SO₂ leak.     

秦山二期扩建工程低温超压事故PSA分析

低温超压事故在电厂停堆期间发生频率较高,并有可能导致堆芯熔化,是停堆工况下一个重要的安全问题。本文对一回路发生低温超压事故进程进行研究和分析,参考相关资料建立事件树,进行定量化计算,得到低温超压事故导致的堆芯损坏频率,并进行简单的结果评价。     

Nuclear heat for hydrogen production: Coupling a very high/high temperature reactor to a hydrogen production plant

Hydrogen has been dubbed the fuel of the future. As fossil fuel reserves become depleted and greenhouse gas emissions are reduced inline with the Kyoto protocol, alternative energy sources and vectors, such as hydrogen, must be developed. Hydrogen produced from water splitting, as opposed to from hydrocarbons, has the potential to be a carbon neutral energy solution. There are several methods to extract hydrogen from water, three leading candidates being high temperature electrolysis, the SI thermochemical cycle and the HyS hybrid thermochemical cycle. All three of these processes involve a section requiring very high temperatures. The Very High Temperature Reactor (VHTR), a gas cooled Generation IV reactor, is ideally suited for providing this high temperature heat. Nuclear hydrogen production is being investigated around the world. The four leading consortiums are the Japan Atomic Energy Agency (JAEA), PBMR/Westinghouse, GA, and AREVA NP/CEA/EDF. There are also many smaller R&D efforts focussing on the development of particular materials and components and on process flowsheeting.A nuclear hydrogen plant involves four key pieces of equipment: the VHTR, the hydrogen production plant (HPP), the intermediate heat exchanger (IHX) and the power conversion system (PCS). The choice of all four items varies dramatically between programmes. Both pebble bed and prismatic fuel block VHTRs are being developed, which can be directly or indirectly coupled to a HPP and PCS placed either in series or parallel. Either a Rankine steam cycle or a Brayton gas turbine cycle can be employed in the PCS. This report details the choices made and research being carried out around the world.Predicted process efficiencies and plant costs are currently at a preliminary stage and are very similar, regardless of the options chosen. The cost of hydrogen produced from water splitting using nuclear technologies is around $2/kg H₂. This is competitive with hydrogen produced by other methods, particularly if carbon emissions are regulated and costed. The technological feasibility and testing of key components will be one of the determining factors in plant viability.     

Analysis of the national legislation governing the complete salvaging of nuclear-powered submarines

The normative-legal documents pertaining to the field investigated are analyzed. It is noted that they correspond to the generally acknowledged approaches to ensuring nuclear and radiation safety. The inconsistencies between different forms of regulation are investigated. The changes in the normative-legal regulation and ensuring safety made in connection with the introduction of the law “On technical regulation” are examined. The discrepancies between the different parts of the federal legislation are indicated, and the importance of constant monitoring of the normative legal base for complete salvaging of nuclear-powered submarines is noted. __________ Translated from Atomnaya énergiya, Vol. 101, No. 1, pp. 17–22, July, 2006.     

PSA study of the effect of extreme snowfall on a floating nuclear power plant: case study in the Bohai Sea

This study presents a probabilistic safety analysis(PSA) method for the external event of extreme snowfall on a floating nuclear power plant(FNPP) deployed in the Bohai Sea. We utilized the Weibull and Gumbel extreme value distributions to fit the collected meteorological data and obtained a hazard curve for the event of an extreme snowfall where the FNPP is located, providing a basis for the frequency of extreme snowfall-initiating events. Our analysis indicates that extreme snowfall primarily ...     

PSA as a tool for decision making

The question on “How safe is safe enough?” is being responded presently by deterministic criteria. Probabilistic criteria in support to more rational and less emotional decisions in regulatory and licensing issues, rationalization of resource allocation and research prioritization, among others, have a potential which is only marginally being explored.This paper discussed PSA limitations and proposes three areas for the use of PSA in decision making, namely:

1. (a) preventing accidents,

2. (b) mitigating accidents, and

3. (c) defining regulatory requirements.

Current activities of the International Atomic Energy Agency in these areas are mentioned.PSA studies depict clearly the uncertainties and this is viewed as a positive aspect, which is unique to the use of probabilistic methods.     

Radiation consequences of seawater contamination during floating storage of reactor-compartment units from salvaged nuclear-powered submarines

The storage time before salvaging of decommissioned nuclear-powered submarines and the floating storage time of reactor units from salvaged submarines can total decades. During this time, radioactive contamination of seawater occurs as a result of the corrosion of the pressure hull in the region of the reactor system and rupturing of the pressure hull - as a result of corrosion of the components of the reactor system. A computational analysis shows that under standard storage conditions the specific activity of water is 10⁵–10⁶ times lower than the intervention level. In the case when the hull and the reactor system are destroyed even with remaining fuel, the specific activity of the sea water near the point of sinking also does not exceed the intervention level because of the slow emission of fission products and transuranium nuclides.Translated from Atomnaya Énergiya, Vol. 97, No. 3, pp. 210–218, September, 2004.     

Engineering design elements of a two-phase thermosyphon for the purpose of transferring NGNP thermal energy to a hydrogen plant

Two hydrogen production processes, both powered by Next Generation Nuclear Plant (NGNP), are currently under investigation at the Idaho National Laboratory and University of Idaho. The first is high temperature steam electrolysis utilizing both heat and electricity, and the second is thermo-chemical production through the sulfur iodine process which primarily utilizes heat. Both processes require high temperature (>850 °C) for enhanced efficiency; temperatures indicative of NGNP. Safety and licensing mandates prudently dictate that the NGNP and the hydrogen production facility be physically isolated, perhaps requiring separation of over 100 m. There are several options to transferring multi-megawatt thermal power over such a distance. One of the options is two-phase heat transfer utilizing a high temperature thermosyphon. Heat transport occurs via evaporation and condensation, and the heat transport fluid is re-circulated by gravitational force. A thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. This paper addresses the engineering design elements of an industrial-scale (50 MW), high temperature controllable thermosyphon for NGNP process heat transfer. Although several different working fluids are under consideration, alkali metals are used herein as reference fluids to illustrate elements of design.     

Markov和积分相结合方法在PSA中应用研究

提出一种Markov方法与积分法相结合的方法,通过全厂断电后电源恢复的概率安全评价(PSA)详细阐述了该方法。分析表明该方法可以有效地处理时间相关性,消除PSA模型中不必要的保守,同时又可以考虑电厂维修策略等动态过程。     

Safety design consideration for HTGR coupling with hydrogen production plant

Safety requirements and design considerations are examined for a nuclear hydrogen production system that consists of High Temperature Gas-cooled Reactor (HTGR) and a hydrogen production plant by thermochemical water splitting iodine–sulfur process (IS process). Requirements in order to construct hydrogen production plants under conventional chemical plant regulation are identified in order to take into account a fundamental difference in safety philosophy between the nuclear facility and chemical plant and meet requests from the potential users of nuclear heat. In addition, safety requirements for the collocation of the nuclear facility and hydrogen production plant utilizing IS process (IS plant) are investigated. Furthermore, design considerations to comply with the requirements are suggested and the technical feasibility of the design considerations is evaluated. The evaluation results for a reference plant showed that safe distance determined by the chemical plant regulation against combustible gas and hazardous chemical leakages comply with the plant layout design. Furthermore, the results demonstrated the feasibility of IS plant construction under non-nuclear regulation by showing that the tritium concentration in IS plant can be maintained below the regulation limit and reactor normal operation can be achieved during abnormal conditions in the IS plant. These results clarified that design considerations suggested for coupling the IS plant to HTGR are reasonably practicable. The proposed criteria can be used not only for coupling hydrogen production plants but also for other chemical plants such as steam reforming plants, etc.     

A study of a hydrogen atom in a two-dimensional quantum well

A feature of a hydrogen atom confined in a two-dimensional quantum well in the presence of an electric field is investigated. The binding energy of the confined hydrogen atom is calculated as a function of the confined potential radius and as a function of the intensity of an applied electric field. It is shown that the binding energy and the oscillator strength of the confined hydrogen atom are highly dependent on the confined potential radius and the intensity of an applied electric field.     

Complete salvaging of decommissioned objects of the nuclear-powered fleet in the northwest and methodological approaches

The basic problems of complete salvaging of the decommissioned Russian nuclear-powered fleet in the northwest are examined: salvaging of nuclear-powered submarines, nuclear-powered surface ships, nuclear tenders, and ecological remediation of the objects of the shored infrastructure supporting the operation of the ships in the nuclear-powered nanval fleet of Russia. The basic methodological aproaches whose implementation will make it possible to determine rational ways to decrease the ecological threat and to increase national safety are formulated on the basis of an analysis of the scales and complexity of the problems which have accumulated concerning the complete salvaging and remediation of the objects of the nuclear-powered fleet. __________ Translated from Atomnaya énergiya, Vol. 101, No. 1, pp. 4–11, July, 2006.     

Validation of the priorities for complete salvaging and ecological rehabilitation of objects of the nuclear-powered fleet

An approach to validation of the priority problems and measures which must be taken first for complete salvaging of the nuclear-powered ice breakers in the northwest region of Russia is proposed and implemented. A list of the priority measures which must become the foundation for formulating projects financed by Russia or donor countries is presented. __________ Translated from Atomnaya énergiya, Vol. 101, No. 1, pp. 11–17, July, 2006.     

Control strategies for transients of hydrogen production plant in VHTR cogeneration systems

Operability of Very High Temperature Reactor (VHTR) hydrogen cogeneration systems in response to abnormal transients initiated by the hydrogen production plant is one of the important concerns from economical and safety points of views. The abnormal events in the hydrogen production plant could initiate load changes and induce temperature variations in a primary cooling system. Excessive temperature increase in the primary cooling system would cause reactor scrams since the temperature increase in the primary cooling system is restricted in order to prevent undue thermal stresses from reactor structures. Also, temperature decrease has a potential propagation path for reactor scrams by reactivity insertions as a consequence of the reactivity feedbacks. Since suspensions of reactor operation and electricity generation should be avoided even in case of abnormal events in the hydrogen production plant from an economical point of view, an establishment of a control scheme against abnormal transients of hydrogen production plant is required for plant system design.In the present study, basic controls and their integration for the GTHTR300C, a VHTR cogeneration system designed by JAEA with a direct Brayton cycle power conversion unit and thermochemical Iodine-Sulfur process hydrogen production plant (IS hydrogen production plant), against abnormal transients of IS hydrogen production plant are presented. Transient simulations for selected load change events in the IS hydrogen production plants are performed by an original system analysis code which enables to evaluate major phenomena assumed in process heat exchangers of the IS hydrogen production plant.It is shown that abnormal load change events are successfully simulated by the system analysis code developed. The results demonstrated the technical feasibility of proposed controls for continuous operation of the reactor and power conversion unit against load change events in the IS hydrogen production plant.