Computer simulation was used to determine the pressure drop resulting from a new flow restrictor design in a replacement steam generator to be installed at Duke Energy’s Oconee nuclear plant in South Carolina. The new flow restrictor design, which is integral with the steam outlet nozzle, was installed to improve safety margins and reduce forces on the tubes and broached plates during a postulated main line steam break accident.

Engineers from Babcock & Wilcox Canada, the company that designed and are fabricating the replacement steam generators, were concerned that the assumed flow restrictor pressure drop may be unconservatively low, which might make it difficult to meet performance guarantees, primarily the guaranteed minimum superheated steam temperature and pressure. In that case, a significant financial bonus tied to performance would be jeopardised. To put everyone’s mind at rest, computational fluid dynamics (CFD) was used to model the complex geometry involved and calculate the pressure drop. The CFD simulation with software from AEA Technology improved the accuracy of the initial prediction and confirmed that the flow restrictor pressure drop is low enough to ensure that the guaranteed thermal performance requirements will be satisfied with adequate margin.

In PWRs such as at Oconee, heated water is carried out of the reactor core by the primary loop to the steam generator, where the heat is transferred to the secondary loop. Tubes containing primary-loop water, which has passed through the reactor and hence may contain radioactive elements, heats up the secondary-loop water and converts it into steam. The primary-loop water is then pumped through the reactor again, reheating the water and starting the primary side cycle again.

Meanwhile in the secondary loop, steam exits at the top of the steam generator and is piped to a turbine generator. The steam leaving the turbine, which is now lower in pressure than when it leaves the steam generator, is converted back into water in the condenser and returned to the steam generator as feedwater to begin the secondary cycle again. The Once-Through steam generators used at Oconee are a unique design developed by Babcock & Wilcox, in which the secondary loop water is converted into dry superheated steam, eliminating the need for moisture separation equipment normally used to protect the steam turbine from degradation due to water droplet erosion.

Stress corrosion cracking has been a problem in many steam generators, including the existing Oconee units. This problem is primarily related to the use of mill-annealed Inconel 600, which has proven to be susceptible to many forms of stress corrosion cracking and other types of degradation.

As a result of tube degradation at Oconee, the existing steam generators need to be replaced. The replacement steam generators will use thermally treated Inconel 690. The replacement steam generators for unit 1 will be installed during a scheduled refuelling outage next year, while steam generators for units 2 and 3 will be relaced the following year.

Babcock & Wilcox is guaranteeing the superheated steam temperature provided by the replacement steam generators. Superheated steam temperature is potentially reduced by the flow restrictor due to increased steam pressure and temperature within the boiling region of the tube bundle which reduces the effective heat transfer rate. Despite its detrimental impact on performance, a flow restrictor is required to maximise safety margins on steam generator internal components during a hypothetical full guillotine rupture of the main steam line leading from the steam generator to the turbine generator. In addition, the flow restrictor limits the rate at which steam would be released inside the containment building.

The design objectives of the Oconee replacement steam generator design team are to achieve a required minimum flow area within the steam outlet nozzle to limit choked flow under accident conditions while minimising normal operation pressure drop. These objectives are met using seven venturi openings in each of the two steam outlet nozzles. It is important accurately to predict pressure drop, since measuring the pressure drop and reworking the steam flow restrictor after installation is not an option, as this would be prohibitively expensive, if possible. Physical testing is not an option, since the fluid of interest is superheated steam at 6.44MPa and a temperature of 314°C, which is not readily available. Babcock & Wilcox engineers decided to perform a CFD simulation because it provides detailed values of fluid velocity, pressure, temperature and other relevant variables throughout the solution domain for problems with complex geometry and boundary conditions. CFD is also an excellent tool for design optimisation if design improvements are needed. As part of the analysis, a researcher may change the geometry of the system or the operating conditions, and view the effect on fluid flow patterns or other variable distributions.

Due to the geometric complexity of the problem, Babcock & Wilcox engineers decided to engage CFX consultants to model the normal full power operation of the flow restrictor. These consultants, led by Mihajlo Ivanovic, used CFX-TASCflow to model the flow of superheated steam from the top of the annulus at the exit of the tube bundle to the outlet of the venturis. This was done in order to examine fluid flow features in detail and accurately predict pressure drop. The model domain included the annulus, commencing at the bundle outlet; seven flow restrictor venturis machined into the steam outlet nozzle and a portion of the steam pipe attached to the nozzle exit. The consultants created grids with three levels of refinement: coarse, medium and fine. The basic idea was that solving each of the three grids would make it possible to determine the sensitivity of the results to grid size and extrapolate a grid- independent solution. The flow was modelled as turbulent, compressible, single-phase flow of superheated steam. The inflow boundary was a pressure boundary equal to 6.44MPa. The outflow boundary was a defined mass flow rate of 341kg/s. All walls of the annulus, nozzle and pipe were considered to be thermally inactive and hydraulically smooth. Based on the estimated flow rate, the Reynolds number for the flow along the flow path was determined to be on the order of 1.7E+09. For that reason, the flow was modelled as turbulent and the standard model with standard wall functions was used. Superheated steam was assumed to be a single-phase, compressible, Newtonian fluid whose properties were determined according to the Virial Equations of State which are comparable to the NIST steam tables.

Ivanovic noted that since this problem was completed, Babcock & Wilcox have frequently worked with the CFX consultants on many proposed design improvements. Most recently, CFX-5.5, the latest addition to the company’s CFD software, has been used to perform other steam generator and nuclear reactor analyses for Babcock & Wilcox Canada. A key advantage of this software package is that it offers automated unstructured meshing capability that can quickly mesh the most complicated geometries with minimal intervention on the part of the analyst or design engineer. Automatic grid adaptation dynamically redistributes and refines the grid so that it is concentrated in those regions where it is most needed. CFX-5.5 can adapt all types of mesh elements, including tetrahedral, hexahedral, pyramids and prisms, while maintaining the original underlying surface descriptions. The CFX-5.5 solver provides high parallel efficiency, innovative smoothing and coarsening algorithms. For large meshes which are increasingly required in engineering simulations, this translates into significant and predictable reductions in run times and faster project turnaround.

The results of the computation of the flow field were provided in a series of plots of velocity vectors and contour plots of velocity, total pressure and Mach number at various planes in the domain of computation. In particular, the consultants provided Babcock & Wilcox engineers with total pressure drop values calculated as a difference between the total pressure at the annulus inlet and at several locations downstream along the flow path. The overall pressure drop, calculated with a fine grid, as a difference between the mass-averaged total pressure at the top of the annulus and the nozzle exit was 0.038MPa. The total pressure drop to the inlet was 0.071MPa. Extrapolation of the results from the three different models indicated that a grid-independent total pressure drop would be 0.030MPa. The analysis also helped to rule out several other potential problems by indicating that the flow is well distributed in all seven venturis. The flow is subsonic with a maximum fluid velocity of 173m/s (Mach 0.32) in the throat. These results demonstrated that the pressure drop provided by the flow restrictor was low enough to meet pressure and temperature guarantees with an adequate margin of safety. Working with CFX consulting to perform the analysis during the design phase confirmed the adequacy of the flow restrictor design. In addition, CFX provided a more thorough understanding of the flow field in the restrictor than could have been achieved with physical testing.