Test program confirms the viability of visual inspection for identifying boric acid leakage, rather than more expensive techniques such as ultrasonic testing.

Various Alloy 600 penetrations through low alloy steel pressure boundary components in pressurized water reactors are susceptible to primary water stress corrosion cracking, including the control rod drive mechanism and bottom mounted instrument nozzle penetrations. Through-wall cracks in Alloy 600 penetrations can expose the low alloy steel to borated primary coolant, potentially resulting in relatively rapid corrosion of the low alloy steel adjacent to the annular gap of the penetration. The speed of this corrosion dictates the frequency and extent of inspection to ensure that such degradation does not threaten the safe operation of the plant.

Boric acid corrosion associated with Alloy 600 penetrations has impacted several nuclear plants over the last 20 years. The first observed instance occurred at a control rod drive mechanism penetration in a French plant in 1991. In response to more significant industry events in 2002, the Nuclear Regulatory Commission (NRC) established formal inspection requirements in 2003 for primary pressure boundary components susceptible to boric acid corrosion.

In setting these requirements, the NRC recognized that minimal test data existed on boric acid corrosion of low alloy steel under conditions relevant to reactor pressure vessel nozzles; specifically, exposure to borated reactor coolant at low leakage rates into a tight heated annulus over an extended period. The limited field experience and available laboratory data resulted in conservative inspection requirements.

Boric acid deposits at low alloy steel/Alloy 600 annular gap after seven days of a 30-day test run

To develop a more robust technical basis for the requirements, EPRI’s Materials Reliability Program initiated a comprehensive test program in 2004 to generate data on corrosion rates associated with boric acid exposure. Initial bench-scale testing improved the fundamental understanding of the factors that control the corrosion rate and identified the primary corrosion mechanisms.

In 2008, engineering-scale mock-up testing began. EPRI developed full-scale mock-ups for both the control rod drive mechanism and bottom mounted nozzle configurations and performed testing at varying leakage rates and annular gap dimensions. Most test runs lasted 30 days, with one long-term bottom mounted nozzle test run of 164 days.

The test program results support the technical adequacy of the current inspection guidance based on two results: 1) the amount of wastage (corrosion attack) observed was less than that predicted by existing models, and 2) deposits were readily observable after just a short time even at low leakage rates (see figure). Confirmation of the viability of visual inspection is important because alternative inspection techniques, such as ultrasonic testing, are more expensive in terms of cost, schedule and radiation exposure. The most recent program results have been documented in EPRI reports 1019085 (CRDM Mock-Up Testing), 1021015 (BMN Mock-Up Testing) and 1021134 (Inverted CRDM Mock-up Testing). The program will be completed in 2011 with a revision to the Boric Acid Corrosion Guidebook (MRP-58).

For more information, please contact Rick Reid at 650.855.2237 or rreid@epri.com.