Session: Session 3. Integrity Management of Critical Systems

Paper Number: 120648

120648 - Darlington Steam Generator Life Extension Scope 

Darlington Nuclear Generating Station, located approximately 50 Km East of Toronto Ontario, is a 4-unit power plant with total generation capacity of approximate 3500 MWe output.  This accounts for 20% of the province’s power demand.  In October of 2016, Darlington Station started its refurbishment to provide safe, reliable, and low-cost power to the province of Ontario.  Since then, Units 2 and 3 have successfully completed their refurbishment and synchronized to the grid and Units 1 and 4 are currently being refurbished with target completion date of Q2 2026.

In 2009, a series of condition assessments were performed on the plant’s critical equipment to determine the scope of refurbishment project.  A rigorous process was implemented internally and verified by third party to evaluate the health of life limiting components and equipment.  As a result, it was determined that reactor pressure tubes and feeders would require replacement. However, steam generators (4 per unit) would remain fit for service providing a series of modification and maintenance .  The maintenance based projects are executed during refurbishment outages in accordance with requirements stipulated in Life Cycle Management Plan (LCMP).     

Steam generators play a crucial role in maintaining reactivity control, cooling the fuel and containing radiological hazards that make up the three elements of nuclear safety known as 3 C’s. Therefore, it is important to proactively mitigate and/or eliminate any plausible degradation mechanism that might exist to impair any of the 3 C’s functions.

Under deposit corrosion is one of the key areas of concerns when it comes to health of steam generators tubes. The sludge accumulated in between the operating period on the outer diameter of tube could cause fretting and wall thinning.    Not taking proactive measures will result in tube plugging.  Darlington has a very tight margin for tube plugging to sustain its full power operation. To alleviate this issue, a process known as water lancing is used.  Water lancing utilizes high pressure jets (~ 10,000 Psi) to break the accumulated sludge at tube sheet level and divert it to annulus region through sweeping motion.  Sludge is then flushed over to suction feet which are then vacuumed out of steam generator into setting tanks.

Prior to commencement of refurbishment in Darlington, steam generators were not equipped with access ports in upper support plates and pre-heater region.  These ports are essential for inspection and future water lancing as well as chemical clean.  Should the upper support plates lattice get blocked, flow restriction will impact the water levels and in turn control logic of the reactor. During refurbishment, 7 access ports per steam generators are being installed.  The gasket sealing surface is machined on the shell of steam generators without any welding or preheating requirements.

Candu reactors are subject to a phenomenon known as Flow Assisted Corrosion (FAC) which causes the magnetite from the feeders piping to get transported into steam generators inner diameter and adhere to the surface.  This causes reduction in thermal efficiency and increase in temperature of reactor inlet header.  Increase in temperature will reduce the thermal gradient between fuel and heat transport water (coolant) which lowers the safety margin of reactor at full power.  Deration of the unit has significant economic consequences due to reduction in megawatt output.  To gain back the required margin, a mechanical process utilizing pressurized air and stainless-steel shot is used to extract the deposited magnetite from the ID.  Unit 2 and 3, that have been brought back to service, have shown 5 to 6 degrees Celsius temperature reduction on the outlet of steam generator.

Presenting Author: Pejman Asgaripour Ontario Power Generation

Presenting Author Biography: Pejman has nearly 25 years of experience in the electricity industry. He joined Ontario Power Generation, as an engineer in 2000 and proceeded to hold progressively senior positions in the company’s Nuclear Division. Pejman has extensive experience in managing mega projects, including planning and site preparation of first fleet of on-grid Small Modular Reactors in Canada and Darlington Nuclear Generation Refurbishment.
Pejman has an extensive background on system/design engineering, as well as project and contract management. He has a Bachelor of Applied Science Degree in Chemical Engineering from Toronto Metropolitan University and is a licensed Professional Engineer in Ontario.