Assessment of material degradation in the CRD system of a NPP with the Software program COMSY

Topics :

Flow Accelerated Corrosion
FAC prediction tools

ABSTRACT :

Flow-induced corrosion (FIC), e.g. flow-accelerated corrosion, droplet impingement corrosion and cavitation corrosion is a degradation process resulting in wall thinning of piping, vessels, heat exchanger and further equipment made of carbon and low alloy steel. The FIC degradation mechanism occurs only locally under specific condition of flow, water chemistry, temperature and materials applied.

In some cases combined effects can occur. These are under single phase flow conditions flow-accelerated corrosion and cavitation corrosion or under two phase flow considerations the combined appearance of flow-accelerated corrosion and droplet impingement corrosion.

Localized wall thinning was found behind an orifice in the condensate water supply for control rod drive hydraulic control system (CRD) of a Japanese NPP. The condensate water is supplied to the hydraulic pump of the CRD through a valve and an orifice. Degradation caused a local reduction of the original wall thickness from 6 mm down to 3.4 mm at the location of the range of 100 mm to 290 mm downstream the orifice.

The operating parameters as well as the pictures on the inner surface lead to the conclusion, that FAC may not be the dominating degradation mechanism. As operating parameter (temperature, oxygen concentration) did not indicate a risk of FAC, a study was performed to reflect the influence of parameters which affect the wall thinning in this specific case.

In a first step a pressure drop calculation was applied with the intension to reliably calculate the local pressure ratio along the throttling location of CRD system. The results of the calculation showed that the local static pressure behind the orifice reaches a level below the vapor pressure of the liquid at the actual temperature. This lead to the situation that wall thinning is likely caused by cavitation erosion in combination with flow-accelerated corrosion. In such a case cavitation erosion damages the protective layer or increases the local turbulence in a way that wall thinning due to flow-accelerated corrosion can occur, even at temperature as low as 40°C.

In a second step a sensitivity study was performed in order to evaluate the influence on material degradation rate of the following parameters:

• mass flow,
• opening ratio of the orifice and
• distance between valve and orifice.

For each parameter threshold values were evaluated as counter measure against the occurrence of flow-induced corrosion.