Load Follow

Load follow for a nuclear power plant refers to the plant's ability to change its power output, up or down, in response to variations in electricity demand on the grid or environmental factors, rather than operating continuously at a constant power level.

It is true that electricity demands vary. Power output may vary whether it's a weekday, weekend, daytime, nighttime, or serious weather condition. Load follow is accomplished by adjusting power through control rod movements, changing coolant chemistry (ex. boric acid), or turbine and steam controls. If a plant experiences load follow regularly or intermittently, it is important that the plant operator understands the impact it may have to their current licensing basis.

Impacts to a Current Licensing Basis (CLB)

1. Reactor Power History and EFPY Projections

Neutron fluence for reactor licensing are tied to EFPY. More often than not, load follow can be beneficial from a material degradation standpoint due to the overall decrease in average thermal rated power. However, if a plant that experiences load follow regularly is licensed using EFPY projections that accounted for potential load follow scenarios in the neutron fluence calculations, there is concern over whether the plant exceeds the approved fluence/EFPY outlined in the CLB. In some cases, the NRC may require confirmation that conservative assumptions are retained.

2. Reactor Vessel Embrittlement and P-T Limits

P-T limit curves are typically approved for a specific EFPY interval using a bounding fluence projection. Load follow may actually improve the embrittlement margin that the plant has to work with, however, this must be explicitly approved. A license amendment would be required if it is found that the conservative P-T curve is found to not be bounding.

3. Fatigue

Critically, load follow has been found to accelerate fatigue usage of nozzles, spray lines, and pressurizer and reactor coolant system (RCS) components by increasing thermal cycling. This has the potential of plants exceeding their fatigue design assumptions leading to a re-analysis of fatigue, UFSAR updates, and/or aging management program revisions which could prove costly.

While other impacts exist, the bottom line is that load follow may help extend the calendar life for EFPY-limited components but have negative impacts to fatigue damage and previous assumptions and projections. Feel free to contact us for a free consultation if you have concerns about your EFPY fluence projections.