Abstract
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.
