The electrical output of the nuclear power plant fleet can be increased either by constructing new plants or by requesting license modifications to enable operations at a higher power level, which is called ‘uprating’ in the industry.
The definition of power uprates is the operation beyond the power level licensed by the regulatory body.
One way of increasing the thermal power from a reactor is to increase the amount of fissile material in use, either by increasing the degree of enrichment or by increasing the density of the fuel. It is also possible to increase the core power by increasing the performance of the high power bundles.
Optimization of the fuel reload focuses on increasing the output from the fuel bundles using less power without affecting the high power bundles.
Extended uprates, up to 20%, may be limited by critical reactor components like reactor vessel, pressurizer, primary heat transport systems, piping etc., or secondary components like turbine or main generator.
Since the first requests in 1977, the NRC has approved 144 uprates, totaling more than 6,500 megawatts-electric (MWe) having been approved, and most of these have already been implemented.
All but six of the 104 reactors have applied for an uprate, and only two reactors, Clinton (near Clinton, Illinois) and Vermont Yankee (near Brattleboro, Vermont), applied and was approved for a full 20% extended uprate.
Currently, the NRC is reviewing applications for seven extended and nine measurement uncertainty recapture (MUR) uprates, which would add about 1,140 MWe of nuclear capacity.
The total 7,640 MWe is roughly the equivalent of seven reactors the size of each of the Vogtle Units 3 and 4 reactors, which just received their combined construction and operation licenses in February 2012.
Admittedly, problems have arisen in BWR steam dryers due to the increased flows, pressures and temperatures causing vibrations that have led to fatigue cracking.
Quad Cities Unit 1 BWR NPP had been operating with new power uprates for about 1 year when significant cracking was discovered in the steam dryer hood. Furthermore, a piece of the outer vertical plate was missing. At the same time, a pilot vent line on a main steam relief valve (SRV) was found to be sheared off and the solenoid actuator for the SRV was damaged. Some collateral damage occurred to the main steamline supports.
After just 90 days of extended power uprate (EPU) operation of the NPP Quad Cities Unit 2, the steam dryer cover plate disintegrated and pieces of it were found on the steam separators and in the main steam-line. Approximately a year later, the steam dryer hood, internal braces and tie bars all failed.
Dresden Unit 3 was shut down for inspection after 10 months of EPU operation. Through-cracks, 10 cm long, were present in the steam dryer hood. Additionally, 2 feed-water sampling probes were found in the feed-water sparger.
In PWRs, the increased power output requires an increase either in the steam flow or in the coolant temperature difference across the core, or both. In all cases, the steam production to the main turbine increases with increased electrical output being achieved by the turbogenerators.
Increased flow will have an impact on flow-induced vibration in the steam/feedwater path; non-linear effects might occur. Vibration monitoring systems should be installed on the steam/feedwater path to check vibration. In addition, more frequent inspections may be required of components in the path with increased flow.
The US nuclear power industry has experienced over 60 events related to power uprates since 1997. From the Institute of Nuclear Power Operations (INPO) Significant Event Report (SER) SER-05-02: “Significant aspects of these events include the following:
—Some units have operated beyond their licensed power levels for extended periods because of errors in reactor thermal power calculations following uprates that changed secondary plant operating characteristics.
—An extended, unplanned shutdown was required to retrieve several loose parts as a result of a flow-induced, high-cycle fatigue failure of a steam dryer cover plate.
—Operational transients and equipment damage have occurred as a result of weaknesses in identifying, communicating, and training the plant staff on expected changes to secondary plant operating characteristics.
—Unanticipated operating challenges and degraded equipment performance have resulted from reductions in operating and design margins.”