Life Extension Of Nuclear Power Plants Research Articles

Role and importance of NDE in nuclear power plant life extension

Nuclear power plants are ageing but there is a need for their further use. Ageing may lead to the gradual degradation of the safety margins of structures and components. Service life extension of nuclear power plants became a world tendency, the key condition of which is the assurance of components’ structural integrity until the end of the extended lifetime. Non-destructive evaluation is a solid pillar of structural integrity assessment, and in the case of nuclear power plants, the general objectives (social and commercial) of non-destructive evaluation are highly relevant.The paper will show examples on the substantial role and importance of non-destructive evaluation in ensuring nuclear component’s long-term integrity. The examples are selected from the technical justification of service life extension of Paks Nuclear Power Plant, Hungary. They will cover ultrasonic and eddy current examinations of critical areas and sub-components of the reactor pressure vessel, the reactor internals, the steam generator and some pipeline systems. The results contributed to completion of key time-limited ageing analyses which were absolutely necessary for getting the operating license beyond the design life.

Radiation-induced bcc-fcc phase transformation in a Fe[sbnd]3%Ni alloy

The issue of neutron irradiation embrittlement of Reactor Pressure Vessel steels must be considered for Nuclear Power Plant life extension. This phenomenon partly arises from the existing interactions between dislocations and nanometric clusters composed of Cu, P, Si, Mn and Ni. The latter alloying element, playing a key role in the evolution of solute enriched clusters under irradiation, is the focus of this publication. To assess the effect of Ni on microstructure evolution under irradiation, particle accelerator based experiments were conducted. An under-saturated Fe3at.%Ni alloy was irradiated with self-ions, at 673 K, up to ∼1.2 dpa. Then, the microstructural damage was characterized, at the atomic scale, by conventional Transmission Electron Microscopy, Scanning Transmission Electron Microscopy coupled to Energy Dispersive X-ray Spectroscopy and Electron Energy Loss Spectroscopy, while chemical features were investigated by Atom Probe Tomography. Informations obtained by combining these coupled techniques provide evidence for the formation of a FCC phase, containing 25 at.%Ni, which can be either the disordered γ phase or the ordered L12 type Fe3Ni phase. The metastable or stable state of this FCC phase is discussed in the light of what is known from the literature. It is the first time that this BCC-FCC phase transformation is observed in an under-saturated α-FeNi alloy and this likely occurred via a Radiation Induced Precipitation (RIP) mechanism. Ni atom segregation is observed on cavities, dislocation lines and dislocation loops. The latter constitute nuclei for precipitates, leading to the formation of an additional segregation site for Ni: the precipitate FCC - matrix BCC nearly coherent interface. Similar mechanisms are argued to be operating also in high Ni RPV steels under neutron irradiation.

Pipe/duct system design for tornado missile impact loads

For nuclear power plant life extension projects, it may be convenient and in some instances necessary to locate safety-related steel ducts and pipes outside of the main structures, exposing them to extreme environmental loads such as tornado missile impact. Examples of this application include emergency firewater lines and Control Room vent ducts. A typical exposed commodity run could be comprised of a rectangular or circular cross-section with horizontal and vertical segments supported at variable spans off of roof and wall panels, respectively. Efficient and economical design of such a tornado-impacted duct or pipe system, consisting of the commodity and its supports, must exploit all of the system's capability to absorb the impact energy by deforming plastically to the fullest extent allowable. Energy can be absorbed locally in the vicinity of impact on the commodity, globally through rotation at flexural plastic hinges, and through yielding of the supports. In this paper a simplified NDOF lumped parameter nonlinear analysis methodology is presented and applied to the coupled commodity/support system subjected to tornado impulse loading. The analysis methodology is confirmed using a detailed ANSYS nonlinear finite element model. Optimization of the initial trial design is achieved by progressively decreasing the support resistances, while monitoring the response ductilities throughout the system. Evaluation methodologies are provided for the four types of plastic deformation responses which occur in the system: local response in the immediate vicinity of impact, flexural and membrane response of the sidewall out to one or two times the commodity depth beyond the point of impact, global response of the commodity as a beam spanning between supports, and the shear and flexural response of support. The inelastic responses are evaluated against AISC N690 acceptance criteria (ANSI, 2006), supplemented as appropriate by triaxiality considerations for inelastic strain in the pipe material.

Irradiation-Assisted Stress Corrosion Cracking and Impact on Life Extension

The paper aims to give a present status of knowledge on irradiation-assisted stress corrosion cracking (IASCC) from the point of view of nuclear power plant life extension. Field experience as well...

Aging and Life Management System of Reactor Pressure Vessel

Reactor pressure vessel (RPV), the only key component that can not be replaced in nuclear power plants (NPPs), is the main barrier against the radioactive leakage. The lifetime of NPPs is dependent heavily on the life of RPV, and thus, the aging and life research on a RPV is a key factor in determining the life extension of NPPs. The purpose of this paper is to introduce an aging and life management system for an operating RPV which can be used as a reference of the lifetime extension. In order to realize the objective, an aging and life management system was developed. It is an comprehensive knowledge management system that integrates decentralized information and serves as a valuable data center. Based on the storage and management of RPV state information and operation data, this system provides real-time monitoring of important operating parameters, evaluation of irradiation embrittlement, and RPV aging assessment. Therefore, it is anticipated that the developed system can be used as an efficient tool for aging and life estimation of RPV.

A COST/BENEFIT PERSPECTIVE OF EXTENDED UNIT SERVICE AS A DECOMMISSIONING ALTERNATIVE

Some people consider life extension (and its cousin, license renewal) an alternative to decommissioning. The reasons for the popularity of such alternatives include presumed cost effectiveness, retention of scarce power plant sites, and the continued ability to pass on waste storage expenses as a cost of service. In this chapter, James Hewlett addresses nuclear power plant life extension—which he calls NUPLEX--in its economic garb, starting with a look at the common utility presumption that life extension of a nuclear plant will allow it to produce electricity at a lower rate than new coal generation. This presumption, he argues, may not be supportable by analysis. He concludes that the deferral of constructing new replacement capacity would result in cost savings only if both the level and escalation rate of the operating costs for the refurbished unit fall substantially from 1986 levels. Therefore, it is unclear whether the deferral of the construction of new capacity would result in the cost savings, although it definitely shifts the financial burden into the future.

Evaluation of fatigue of reactor components by on-line monitoring of transients

The components of nuclear power plants are generally designed for a lifetime of 40 years. The basis for this is a catalogue of transients representing the expected loadings. Incidence and nature of loadings actually sustained in operation depend, however, on the way in which individual plants are operated. The fatigue monitoring system described in the present paper considers loadings which actually occur in operation. It permits evaluation of plant operation from the point of view of the material fatigue experienced by components and consequently enhances understanding of operating performance. In many cases the design transients are conservative simulations of operating transients. The fatigue monitoring system enables these design reserves to be quantified and this permits component service life to be extended. On-line fatigue monitoring is an important element in nuclear power plant life extension, a topic which is currently much debated in the USA. When fatigue progresses more quickly than postulated in design because of the way the plant is operated, such monitoring makes it possible to target inservice inspections and to carry out upgrading operations during scheduled inspection outages to combat the problem. In Germany the first system for continuous recording of operational loadings was installed in the 1300 MWe plant Isar 2. This system is described and initial results are presented.