Quantification of Nickel Leaching Rates from Nickel-Bound Tungsten Carbide (WC) Rotors used in Primary Heat Transport System and Reactor Cooling System Mechanical Pump Seals

Abstract

Mechanical seals used in Primary Heat Transport System (PHTS) pumps in CANDU reactors, or Reactor Cooling System (RCS) pumps in boiling water reactors (BWRs) and pressurized water reactors (PWRs) are typically designed as soft vs. hard seal face combinations. A common soft vs. hard seal face combination consists of a nickel-bound tungsten carbide (WC) rotor and a carbon graphite stator.

A primary mode of failure for this rotor seal face has been described as changes to the seal face profile due to erosive wear. The erosive wear can be triggered by the selective dissolution of Ni from the rotor material, which results in a mechanically weakened carbide “skeleton”. The dissolution of Ni binder from the rotor material is pH dependent and, as such, there is a concern that the stagnant wetted environment in the sealing face crevice, which occurs during outage periods and during exposure to post-installation chemistry (prior to start-up), impacts the subsequent performance of the seal during operation. This is particularly relevant now as nuclear stations are aging and the planned outages are tending to become longer and more frequent.

CNL has developed two methods to quantify the Ni leached from pump seal rotors in a representative environment. The first method consists of immersion testing of rotor sections. During exposure, dissolved Ni is collected and concentrated on ion-exchange (IX) columns containing cation-exchange resin, which are periodically removed from the test apparatus and replaced with columns containing virgin resin. Ni dissolution rates are then determined by quantitative measurements of Ni, after being chemically eluted from the resin, using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The second method uses rotor-stator pairs arranged in a creviced geometry. The assemblies are placed in an environmental chamber. After a determined amount of time, assemblies are extracted from the environmental chamber for Ni leachant analysis. Following extraction, the rotor and stator components undergo an acid rinsing procedure in order to collect the Ni leachant from the crevice. Both components are submerged in an HCl solution and cleaned in an ultrasonic bath at ambient temperatures. Following this, the samples are removed from the acid, rinsed with deionized water, and dried for potential further analyses. The acid solution is collected and analyzed using ICP-MS for Ni content. Depending on the extent of Ni loss as determined by ICP-MS, some samples can be sent for cross sectioning in order to examine the Ni depleted region.

Ongoing testing seeks to characterize the Ni leaching rate through prolonged periods of time and to evaluate how Ni leaching affects seal performance during operation and seal life expectancy.