Effective full power years (EFPY)

Effective Full Power Years (EFPY) plays a direct and formal role in nuclear power plant licensing because U.S. and international regulators license plants not just by calendar time, but by accumulated irradiation and thermal exposure.

Regulatory aging concerns like neutron embrittlement, fatigue, and irradiation damage depend on how much power was actually produced, not by how much time has passed. As a simple example, a plant operating for one year at 100% power causes roughly twice the neutron damage of operating one year at 50% power. Therefore, using calendar years alone would misrepresent the degradation of materials within the reactor over time. Therefore, using EFPY to measure radiation exposure has been widely adopted by the industry.

Most RPV licensing analyses (P-T limit curves, Pressurized Thermal Shock, surveillance capsule withdrawal schedules) are specifically tied to EFPY. Submissions for License Renewal (LR) of Subsequent License Renewal (SLR) for extended operation use EFPY as a projection in their neutron fluence analyses to demonstrate that the reactor pressure vessel structure and components will maintain safe operating condition and remain within the bounds of the licensing basis during the license renewal time period.

In Regulatory Guide 1.190, it is explicitly stated that end-of-life (EOL) predictions (based on EFPY) "...should be made with a best-estimate or conservative generic power distribution. If a best estimate is used, the power distribution must be updated if changes in core loadings, surveillance measurements, or other information indicate a significant change in projected fluence values." [1]

While RAMA and TRANSFX, developed and used by TransWare, produce best-estimate neutron fluence calculations, other methodologies will provide grossly conservative EFPY fluence projections. While this may ensure that a plant has a "cushion" for any planned changes in fuel loading or operation, it may put them at risk for more frequent inspections and undue concerns surrounding specific components and licensing regulatory compliance. The amount of money that could be saved by using advanced, modern technology like RAMA and TRANSFX could be significant. If your fluence is calculated by a vendor other than TransWare, contact us to learn more about how we may be able to help you feel more confident in your neutron fluence projections.

1.   U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research. Regulatory Guide 1.190: Calculational and Dosimetry Methods for Determining Pressure Vessel Neutron Fluence. Washington, D.C.: Office of Nuclear Regulatory Research: 2001