Corrosion & Materials

EPRI RESEARCH PROGRAM: Following discovery of boric acid corrosion of a reactor vessel head, DEI led a multi-year EPRI research program to establish and justify inspection protocols for control rod drive mechanism (CRDM) nozzles to ensure safe plant operation and protection of the public and environment. DEI’s support of this effort included:

• Stress analysis, fracture mechanics and crack growth rate modeling for asymmetric welds in CRDM nozzles
• Modeling and simulation of boric acid leakage through tight cracks, followed by evaporation and concentration in crevices and subsequent corrosion of pressure boundary materials
• Experimental testing to validate modeling results and corrosion predictions

DEI’s modeling methods were reviewed and accepted by the US NRC, and this body of work culminated in numerous technical reports and industry guidance documents, including EPRI’s Boric Acid Corrosion Guidebook.

UTILITY CONSULTING: For more than 15 years, DEI has performed assessments of feeder pipe and fuel channel degradation at CANDU plants, including cracking, flow accelerated corrosion (FAC), and other integrity issues. Some examples of our work include:

• Performing probabilistic safety evaluations to assess the potential for feeder pipe rupture based on finite-element analysis of feeder bends and welds, fracture mechanics calculations, statistical analyses of crack initiation and growth, calculations of critical flaw sizes, leak rate calculations, and ultrasonic testing probability of detection assessments
• Development of a probabilistic model to predict the risk of fuel channel pressure tube (PT) rupture  as a result of radiation-assisted creep of the PT, axial movement of the annular helicoil spacers between the PT and the calandria tube (CT), contact between the PT and the CT, hydrogen blister formation, and subsequent blister cracking

DEI has presented its results to the Canadian regulator on behalf of several CANDU utilities, and these results have been accepted as the technical basis for establishing appropriate inspection intervals.

UTILITY CONSULTING: PWR and CANDU steam generator (SG) tubing experiences degradation by a number of mechanisms including stress corrosion cracking, tubesheet denting, and tube support plate blockage. DEI assists utilities in the development and implementation of programs to optimize the economics of SG inspection and maintenance and ensure long-term SG reliability. In a recent example project, DEI provided the following services:

• Prediction of SG tubing degradation for multiple plants with different SG designs, tubing materials (Alloy 600, Alloy 690, Alloy 800), and operational conditions
• Economic analysis of candidate maintenance strategies to identify the most cost effective way to ensure steam generator reliability, including: chemical and mechanical cleaning, dispersants addition, plugging, sleeving and SG replacement
• Identification of the optimal timing and frequency for inspection and maintenance activities

DEI technical support assisted the utility in performing corrective maintenance to arrest active degradation in some units, and to perform proactive maintenance to avoid SG replacement at other units, saving the utility on the order of $100 MUSD.

ROOT CAUSE INVESTIGATION: Prior to its retirement in 2018, Canada’s National Research Universal (NRU) reactor produced essential medical isotopes used to treat cancer and diagnose other illnesses. In 2009, the NRU reactor was temporarily shutdown due to corrosion of the aluminum reactor vessel. At that time, only one of the other four worldwide medical isotope sourcing reactors was in operation, leading to a global shortage in essential medical isotopes. Thus, safe and efficient return to service of the NRU reactor was a top priority. DEI provided significant material expertise and consulting in support of this objective, including:

• Root cause investigation of the observed corrosion
• Development of probabilistic models for predicting future corrosion rates, acceptable inspection intervals, assessing candidate repair strategies, and estimating life expectancy of repaired vessels, and
• Experimental testing to validate material performance predictions and modeling results

DEI’s technical insights were beneficial in helping to return the NRU reactor to service and ensuring continued reliable operation until this research reactor was later retired in 2018.