(Slide 1) Question 1. What are the differences in RPV steel chemical composition of three WWER generations?
Answer
In general there are 3 generations of WWER reactors:
- WWER-440/230
- WWER-440/213
- WWER-1000
All three generations have the same basic chemical composition of reactor pressure vessel steel. However there are substantial differences in the contents of several alloying and residual elements.
Reactor pressure vessels of old WWER-440/230 units were manufactured in the 60’s. The basic composition of the reactor vessel steel was chosen successfully and there were no essential changes in it until now. However, at that time there was a lack of knowledge concerning steel behavior under irradiation.
(Slide 2). It was not known, that a rather small content of impurities such as phosphorous and copper can essentially reduce the radiation stability of the material. The major part of the WWER-440/230 pressure vessels were made of steel with high phosphorous and copper content in the welds. The radiation embrittlement caused by the influence of these impurities shorted the safe operational lifetime below the design value.
At the end of the 70’s the industry had started the fabrication of WWER-440/213 reactor pressure vessels. These reactor vessels had been made of steel with low residual contents. Consequently the steel of this WWER generation has high radiation stability.
(Slide 3).The metal of WWER-1000 RPV differs from the WWER-440 material in the higher Ni content. . Nickel was added in order to increase the toughness, hardenability and weldability of steel.
(Slide 4).Whereas the nickel content in WWER-440 vessels is not more then 0.4 %, in WWER-1000 RPVs the nickel content is 1.1-1.4 % for the base metal and up to 1,9 % for the welds.Taking into account that the negative Cu and P influence was known by that time, the content of these elements was strictly limited for WWER-1000.