Product Abstract

The steam generator performance degradation case studies in this report form the foundation for an industry thermal performance database. The database's plant-specific information can help utilities identify causes of thermal performance degradation and develop remedial measures.

Background

During the last several years, a significant number of pressurized water reactor (PWR) plants have exhibited decreases in secondary-side steam generator steam pressure. In some cases, the decreases have caused reduced high-pressure turbine pressure and, hence, reduced electrical generating capacity. A reduction of one percent in electrical generating capacity in a typical PWR is equivalent to roughly $2 million per year in replacement power costs, creating a significant impact on utility revenue. In one plant, secondary pressure loss of nearly 80 psi occurred; chemical cleaning of the steam generator restored most of this pressure loss. Secondary tube deposits were the primary cause of this plant's pressure loss. However, secondary deposits are not necessarily the primary cause of thermal performance degradation, as this study demonstrates.

Objective

To collect and evaluate plant performance history data from three selected plants; to perform a global fouling factor analysis; to quantitatively evaluate the best-estimate and bounding pressure decreases associated with all potential causes of steam pressure degradation for each plant in the study; to analyze available tube deposit data and calculate the thermal resistance of the deposit layers; and, to correlate changes observed in the thermal performance in one plant with historical Dimethylamine (DMA) addition.

Approach

Investigators applied a global fouling factor analysis method to quantify degradation in steam generator performance. In addition to the detailed study of fouling effects due to secondary deposits, this method accounts for changes in thermal power, primary temperatures, heat transfer area (for example, due to tube plugging), and their effect on performance degradation. Inputs to this analysis are thermal hydraulic design data and plant instrument measurements recorded over the operating life of the plants. These measurements include steam pressure, primary temperatures, feed water flow rate, and the number of plugged and sleeved tubes for each outage. In addition to the fouling factor analysis, investigators studied other potential factors that may contribute to steam pressure degradation.

Results

The report details relative contributions to thermal performance degradation (indicated by steam pressure decreases) in three selected plants due to secondary fouling and other sources. These additional sources include power uprates, primary temperature changes or errors, and tube plugging, each source acting alone or in combination. Fouling due to secondary deposits can be a significant cause and, in fact, may be the major cause of documented pressure loss at one plant.

EPRI Perspective

Steam generator thermal performance degradation -- manifested by steam pressure decreases -- can be a potential cause of lost generating capacity in PWRs. This pilot study identified factors that may contribute to performance degradation. The three plants in the study were a feed ring design plant (Union Electric's Model F plant, Callaway), a preheat design (Houston Lighting and Power's Model E plant, STP) and an older model 51 (TVA's Sequoyah). Analysts selected these plants based on the severity of their pressure loss and other factors. A fourth plant, TU Electric's Comanche Peak, was added later to evaluate potential effects of DMA soaks on performance.

The study showed that none of the plants suffered severe fouling due to secondary deposits, though it was a contributing factor. Major causes included power uprates, hot leg temperature streaming, and primary temperature decrease. Tracking the global fouling factor, installing more accurate instrumentation, and characterizing the secondary deposit properties can all aid in evaluating and remedying potential causes. The plant-specific information and analysis results add to the industry database of thermal performance information, thereby helping utilities identify causes of -- and prepare remedial measures for -- potential thermal performance degradation.