Reactor Pressure Vessel Integrity Assessment
SIB-96-140

The Issue: RPV Fracture Toughness
Reactor pressure vessels are manufactured of ferritic carbon steels selected for their medium strength, high toughness and good weldability. However, irradiation alters these properties as the vessel ages. An increase in hardness and tensile strength is accompanied by a reduction of fracture toughness and an increase in brittle fracture behavior. The key parameter which characterizes the fracture toughness of a material is the reference temperature of nil-ductility transition (RTNDT), the temperature below which failure occurs in a non-ductile, brittle manner. In most cases, normal reactor operating conditions lie well above the RTNDT; however, various transient conditions challenge the fracture toughness capabilities of the vessel, as shown below:

The Problem: P-T Curve Violations
Appropriate limits are set on reactor pressure and temperature through the use of pressure-temperature (P-T) curves. These curves present allowable pressure versus reactor temperature for both hydrotest and normal operating conditions, and they are documented in plant Technical Specifications. They are established to the requirements of 10CFR50, Appendix G to assure that brittle fracture of the reactor vessel is prevented. Periodically, P-T curves are modified to account for irradiation embrittlement effects based on the methods of Regulatory Guide 1.99, Revision 2. This has the effect of shifting the P-T curve to the right, thereby narrowing the allowable operating window. This can cause significant operational limitations as the reactor ages, and P-T curve violations typically occur during inadvertent transient events as depicted below:

When P-T curve limits are violated, evaluation must be performed immediately to assess fracture toughness margins. Such evaluation can be costly to utilities since it is typically performed on an urgent basis. In some cases, P-T curve violations can lead to plant shutdown pending further investigation.

The Solution: RPVIMS
Based on the difficulties encountered in this area on a repeated basis by utilities, Structural Integrity Associates (SI) has developed the Reactor Pressure Vessel Integrity Monitoring System (RPVIMS), which is an automated P-T monitoring software system that simplifies the entire P-T curve process. RPVIMS performs the following functions:

• Monitors actual plant operation against established Technical Specification P-T limits.
• Computes and adjusts P-T curves based on current reactor conditions (e.g., actual fluence and heatup/cooldown rates), to remove conservatism from established limits and maximize fracture toughness margins.
• Performs an immediate, on-line fracture toughness assessment to demonstrate acceptable fracture toughness margins in the event of a P-T curve violation.

The process used by RPVIMS is shown below:

SI has installed RPVIMS at two PWR units and four BWR units. In addition, an off-line EXCEL spreadsheet version of RPVIMS has been developed. Work is currently underway to develop alternate versions of RPVIMS operating under the Windows environment and as a modular add-in for the FatiguePro software. All documentation for using the program and understanding the methodology is included.

RPVIMS has been developed for generic plant use, is inexpensive to plant owners, and provides a very useful tool that will minimize costly evaluation in the future associated with periodic P-T curve assessments.

If you would like more information on RPVIMS, or are interested in RPV integrity assessments, please contact SI.