On the Limitations of Positron Annihilation Spectroscopy in the Investigation of Ion-Implanted FeCr Samples

New materials for advanced fission/fusion nuclear facilities must inevitably demonstrate resistance to radiation embrittlement. Thermal and radiation ageing accompanied by stress corrosion cracking are dominant effects that limit the operational condition and safe lifetime of the newest nuclear facilities. To study these phenomena and improve the current understanding of various aspects of radiation embrittlement, ion bombardment experiments are widely used as a surrogate for neutron irradiation. While avoiding the induced activity, typical for neutron-irradiated samples, is a clear benefit of the ion implantation, the shallow near-surface region of the modified materials may be a complication to the post-irradiation examination (PIE). However, microstructural defects induced by ion implantation can be effectively investigated using various spectroscopic techniques, including slow-positron beam spectroscopy. This method, typically represented by techniques of positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy, enables a unique depth-profile characterisation of the near-surface region affected by ion bombardment or corrosion degradation. One of the best slow-positron beam facilities is available at the pulsed low-energy positron system (PLEPS), operated at FRM-II reactor in Munich (Germany). Bulk studies (such as high energy ion implantation or neutron irradiation experiments) can be, on the other hand, effectively performed using radioisotope positron sources. In this paper, we outline some basics of the two approaches and provide some recommendations to improve the validity of the positron annihilation spectroscopy (PAS) data obtained on ion-irradiated samples using a conventional 22Na positron source.

Fracture Mechanics Assessment of Reactor Pressure Vessel Irradiated Structural Steel for Short Column Type Supports and Neutron Shield Tank

As plants apply for 80 year licensure (subsequent license renewal), the United States Nuclear Regulatory Commission (U.S. NRC) has queried the nuclear power plant industry to investigate the impact of neutron embrittlement (radiation effects) on the reactor pressure vessel (RPV) structural steel supports due to extended plant operation past 60 years. The radiation effects on RPV supports were previously investigated and resolved as part of Generic Safety Issue No. 15 (GSI-15) in NUREG-0933 Revision 3 [1], NUREG-1509 [2] (published in May 1996), and NUREG/CR-5320 [3] (published in January 1989) for design life (40 years) and for first license renewal (20 additional years). The conclusions in NUREG-0933, Revision 3 stated that there were no structural integrity concerns for the RPV support structural steels; even if all the supports were totally removed (i.e. broken), the piping has acceptable margin to carry the load of the vessel. Nevertheless, for plants applying for 80 year life licensure, the U.S. NRC has requested an evaluation to show structural integrity of the RPV supports by accounting for radiation embrittlement (radiation damage) for continued operation into the second license renewal period (i.e. 80 years). The RPV support designs in light water reactors are grouped into one of five categories or types of supports: (1) skirt; (2) long-column; (3) shield-tank; (4) short column; and (5) suspension. In this paper, two of these RPV support configurations (short column supports and neutron shield tank) will be investigated using fracture mechanics to evaluate the effect of radiation embrittlement of the structural steel supports for long term operations (i.e. 80 years). The technical evaluation of other support configurations will be provided in a separate technical publication at a future date.

Regression Analysis of Transition Temperature Shift Database for the Core Region Beltline Welds of WWER-1000 RPVs

The paper describes the history of application in Ukraine the methods of critical temperature of brittleness (CTB is WWER’s analogue of PWRs ductile to brittle transition temperature) prediction. Also, a current state of chemical factor consideration in radiation embrittlement of PWR and WWER regulatory documentation is briefly presented. It is shown that a chemical factor for WWER RPV have to be defined. The present paper devoted to the identification of the radiation embrittlement chemical factor for welds only of RPV core region, since it is the most critical zone in terms of RPV resistance against fast fracture. For that, based on the results of WWER-1000 RPV surveillance program of all Ukrainian NPPs, the CTB shift database is created. And it is important to note that database of Ukrainian WWER-1000 reactors is the biggest among other WWER countries. With using the statistical treatment of CTB shift Database the degree of influence of each chemical element (C, Cr, Cu, Mn, Mo, Ni, P, S and Si) on WWER-1000 RPV weld radiation embrittlement is obtained. It is also found that Silicon and Nickel (in decreasing order) have the greatest impact on the CTB shift. At that, influence of other element separately – is negligible from practical point of view. Nevertheless, due to the literature data (in the radiation material science field) about impact of manganese, additional statistical investigations are performed, which showed us that Nickel and Manganese have synergetic effect which almost the same as level of impact from Silicon element alone. Similar treatments, as with pairs Silicon-Nickel and Silicon-Manganese, as well as Silicon or Nickel with any other element combinations, showed as absence of any other synergetic effect. Based on the statistical evaluation the general shape of CTB shift trend curve is proposed. Moreover, the recommendations related to the CTB shift trendline prediction are given, which, besides the chemical factor and shape of curve, include the way of considering the CTB shift data scatter. The obtained results are expected to be the basis in the modern regulatory method for radiation embrittlement assessment of the Ukrainian WWER-1000 RPVs.

Reactor Pressure Vessel Steel Smart Behavior as Cause of Instability in Kinetics of Radiation Embrittlement

Fast neutron intensity influence on reactor materials radiation damage is a critically important question in the problem of the correct use of the accelerated irradiation tests data for substantiation of the materials workability in real irradiation conditions that is low neutron intensity. Investigations of the fast neutron intensity (flux) influence on radiation damage and experimental data scattering reveal the existence of non-monotonous sections in kinetics of the reactor pressure vessels (RPV) steel damage. Discovery of the oscillations as indicator of the self-organization processes presence give reasons for new ways searching on reactor pressure vessel (RPV) steel radiation stability increasing and attempt of the self-restoring metal elaboration. Revealing of the wavelike process in the form of non monotonous parts of the kinetics of radiation embrittlement testifies that periodic transformation of the structure take place. This fact actualizes the problem of more precise definition of the RPV materials radiation embrittlement mechanisms and gives reasons for search of the ways to manage the radiation stability (nanostructuring and so on to stimulate the radiation defects annihilation), development of the means for creating of more stableness self recovering smart materials.

On the radiation embrittlement of materials of support structures for WWER RPV. Part 1. Experimental studies

The features of the radiation embrittlement of materials of support structures for WWER RPV are considered. These features are connected with low irradiation temperature no exceeding90°Cand also with a use of the steels which are usually applied for building of the metal structures and have not a high resistance to the radiation embrittlement. It has been shown that support structure (SS) of WWER-440 of V-179, V-230 types may cause the operation life limit. The experimental data on the standard mechanical properties and fracture toughness are presented for different steels and weld metals in the initial and irradiation conditions. SEM investigation of fracture surface of broken specimens and atomic tomography have been performed.

On the radiation embrittlement of materials of support structures for WWER RPV. Part 2. Analysis of the completed studies

The features of the radiation embrittlement of materials of support structures for WWER RPV are considered. These features are connected with low irradiation temperature no exceeding 90oC and also with a use of the steels which are usually applied for building of the metal structures and have not a high resistance to the radiation embrittlement. The model for prediction of material radiation embrittlement as function of the neutron fluence and impurity of Cu and P is proposed. An irradiation temperature effect on the different mechanisms resulting in the material radiation embrittlement is considered: materials hardening due to nucleation of point defects and formation of dislocation loops, copper precipitation and materials non-hardening due to phosphorus segregation.