Entry Date:
September 12, 2014

The Development of CRUD-Resistant Materials

Principal Investigator Michael Short

Co-investigator Ju Li

The buildup of corrosion deposits known as CRUD (an acronym for Chalk River Unidentified Deposits) on reactor core fuel rods represents a major problem in the operation of light water reactors LWRs. In particular, the porous nature of CRUD in pressurized water reactors (PWRs) allows for boron, present in the coolant to control the power, to "hide out" in CRUD's pores and change the power shape of the core. In addition, CRUD can often impede heat transfer from the fuel cladding rods, resulting in CRUD-induced localized corrosion (CILC). Finally, the composition of CRUD is mainly iron and nickel oxides, which become more radioactive after sitting in the core. The dissolution of CRUD upon reactor shutdown means this radioactivity is released into the coolant, adding radiation dose to workers on the plant.

In addition to simply understanding or simulating CRUD's effects (which is not simple!), this project sets out to develop materials, coatings, and surface treatments to resist or slow CRUD formation. We are pursuing multiple, parallel strategies, including surface modifications to inhibit CRUD formation, and surface texturing to control how CRUD grows, in order to make it improve heat transfer. We pre-screen many materials in our pool boiling facility, and select materials will be tested in our up-and-coming pressurized water reactor (PWR) loop. This loop will expose a single rod of Zircaloy to high heat fluxes, 15MPa of pressure, 340C water, and the same precursors that form CRUD in real PWRs.

We use thin film techniques to grow candidate materials on silicon substrates for testing CRUD resistance. Materials are exposed to sub-cooled water with CRUD precursors for various times to grow synthetic CRUD. A scanning electron microscope (SEM) and a focused ion beam (FIB) are used to section the CRUD, and analyze the CRUD-surface bond. In addition, we use atomic force microscope (AFM) in force spectroscopy mode to directly measure the adhesion of CRUD particles to material surfaces. This is done by affixing a particle of CRUD to an AFM tip, bringing it into contact with a material surface, retracting it, and measuring the difference in applied vs. measured axial position. This technique helps us determine how the CRUD-surface bond forms, and screen which materials don't bond to CRUD.

Advances on this project should be highly applicable to other situations where the growth of porous deposits must be stopped or controlled. For example, the oil industry suffers from fouling in heat exchangers, which can clog the pipes and cause refinery shutdowns. Solving the CRUD issue could also lead to advances in non-fouling surfaces in the extreme environment of oil refineries.