FAC Study in feeder pipes of 220 MWe Indian PHWR

Authors : P. Goyal, Vishnu Verma, Rohit Rastogi,
                Vivek Bhasin, A.K. Ghosh
                

Company : Reactor Safety Division,
                   Bhabha Atomic Research Centre
    
Country : India

Topic :

Flow Acclerated Corrosion
FAC prediction tools

ABSTRACT :

The Indian Pressurized Heavy Water Reactor (PHWR) core consists of a number of horizontal channels containing nuclear fuel bundles. The parallel coolant channels are connected to headers at the inlet and outlet through feeder pipes. Coolant from Reactor Inlet Header is distributed into the coolant channel through feeders and after removing heat from core coolant combines at Reactor Outlet header through outlet feeders. Due to space constraints the feeder pipe are joined to the channel with one or two elbows close to the end fittings of the coolant channels. The carbon steel outlet feeders carry high temperature water to the headers and from headers, high temperature water goes to steam generator where steam is produced. Due to high velocity and high pressure and temperature, feeders are liable to undergo flow accelerated corrosion (FAC).

Recent inspections of the outlet feeder pipe showed measurable wall thinning due to FAC at locations close to the end fitting. It has been found that feeder having double elbows are more susceptible to FAC. Maximum thinning was observed at the intrados of the second elbow. CFD analysis has been carried out for some feeders and thinning rate due to FAC has been obtained empirically from the computed value of the wall shear stress. Effect of weld protrusion on wear rate has been studied for one of the feeder.

CFD analysis was also carried out for feed water suction line at Surry-2 with the details available in the literature. Standard k-ε turbulence model with wall function has been used. Mesh size specially close to the wall modeled carefully based on y+ requirements for wall function model for the turbulence model.  

Two different approaches are possible for predicting the FAC rate from calculated value of wall shear stress by CFD. One method is based on evaluating of wear rate using Colburn analogy and the other using an empirical equation between wear rate and shear stress. The latter method due to is also dependent on pH. In Colburn analogy, mass transfer coefficient is evaluated by knowing shear stress and equilibrium concentration by Sweeton and Bses method. Diffusion coefficient of Iron species in water is evaluated as a function of temperature and viscosity.

Detailed numerical results are presented in the paper.