Examensarbete - Corrosion of Ni-based alloys – study of the degradation of BWR spacer materials

4 days ago
Full time role
In-person · Uppsala, Uppsala County, SE... more

In BWRs, radiolysis of the cooling water generates  oxygen, hydrogen, hydrogen peroxide and hydrogen-oxygen radicals which have an impact on the corrosion of the Zr-based cladding and structural materials. Currently there is a significant increase in Co58 release in the cooling water of Forsmark units 1 and 2 which could be related to increase in corrosion rate or metal release rate from the Ni-based alloys. The release of activated corrosion products has radiological impact at the powerplant generating increased contamination levels and higher dose rates in the plant and for the personnel.

BWR fuel assemblies are equipped with Ni-based alloy spacers. There are different parameters that impact the corrosion behavior of Ni-based alloys in a BWR core that needs to be investigated for more specific exposure conditions that are relevant for the Forsmark NPPs.

This work is expected to shed light on some specific aspects regarding the corrosion of different Ni-based alloys and the impact of water chemistry, operating conditions, the alloys chemical composition and spacers final surface finish on the corrosion behavior.

The corrosion process of Ni-based alloys is quite a complex process in high temperature water and steam and is driven by an electrochemical process where the corrosion potential is the driving force for the corrosion process. The aggressive radiolysis products have an impact on the corrosion potential, but  other factors are also known to impact the corrosion of Ni-based alloys, such as the Fe-content in the reactor water but this behavior needs to be verified experimentally. There is a clear difference between the observations in Forsmark 1 (F1) and Forsmark 2 (F2) versus Forsmark 3 (F3) which needs to be investigated more in detail to understand the observed differences. It is of great value to better understand the in-reactor corrosion processes and the contributing parameters that have impact on the degradation process of Ni-based spacers to better understand the impact on the current Co58 trends in the RW of F1 & F2.

Assignment

This project should include:

  • Summary of the current knowledge regarding the corrosion of Ni-based alloys in BWR
  • Evaluate the impact of different Ni-based alloys material variability on the corrosion performance of the different Ni-based alloys used as spacer materials.
  • Evaluate the impact of water chemistry variability on the corrosion performance of the different Ni-based alloys used as spacer materials (including the formation and solubilities of the different corrosion products and metal release processes).
  • Summarize the fuel inspection results of the different fuel designs that have been operating in F1, F2 and F3 and correlated with the variations in the reactor water chemistry and operating conditions.
  • Study the radiological consequences due to the increased Co58  and other radionuclides in the reactor water.
  • Support the experimental work that will be performed in Studsvik: corrosion testing of spacer materials in relevant water chemistry for F1 & F2 reactors
  • Identify gaps in knowledge and make recommendations for future testing & inspections.