Transactions of the Korean Nuclear Society Fall Meeting Jeju, Korea, May. 16-18, 2018 Role of Aluminum Oxide Thin Film on Crud Deposition of the Fuel Cladding in a Primary Water of PWRs Hee-Sang Shim * , Moon-Sic Park, Seung Heon Baek, Do Haeng Hur Nuclear Materials Research Division, KAERI, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Korea * Corresponding author: hshim@kaeri.re.kr 1. Introduction As pressurized water reactors (PWRs) have been recently driven to power uprate, lifetime extension and higher burnup for enhancing economics of power generation, some reactors have experienced an increase of deposits on the fuel assemblies [1,2]. These deposits are called as ‘crud’ and arise from corrosion products released from the surfaces of the reactor coolant system (RCS). It is well known that crud has a porous structure because crud deposition is stimulated in a condition where sub-cooled nucleate boiling (SNB) occurs [3]. Various chemical species included in a primary coolant can be accumulated in the porous crud. Among them, concentrated boron-containing compounds can induce local power shift owing to neutron capture by boron, resulting in power output de-rating [4,5]. Lithium can also be concentrated inside the pores, elevate pH there, and increases the corrosion rate of fuel claddings. In addition, the crud on fuel assemblies can increase fuel cladding temperature due to increased thermal resistance, resulting in accelerated fuel cladding corrosion [6,7]. A part of crud activated on fuel assemblies is released again into the primary coolant, transported out of core, and deposited on ex-core surfaces, resulting in increased occupational radiation exposure. Thus, several methods have been implemented to plants to mitigate the above-mentioned problems caused by crud. Elevated pHT operation of the reactor coolant chemistry from 6.9 to 7.2-7.4 shows a reduction in crud deposition in PWRs [8,9]. Ultrasonic fuel cleaning technology has been also used to remove crud from the reloaded fuel assemblies during outage [10]. In addition, various mitigation methods such as passivation of steam generator (SG) tubes, electropolishing SG channel head, and reducing roughness of SG tubes have also been suggested for reducing radiation source terms in PWRs [11-13]. After the accident at Fukushima Daiichi in 2011, development of accident tolerance fuel (ATF) claddings have become an important research topic worldwide. One of the ATF options is to use a coated zirconium alloy cladding that can provide the necessary protection during an off-normal high temperature or loss of coolant accident (LOCA) conditions. The coating materials and their deposition technologies considered were extensively reviewed in the literature [14]. Similarly, Dumnernchanvanit et al. have recently reported on the initial experimental results of crud-resistant materials as fuel cladding coatings to reduce crud deposition [15]. In this work, we coated aluminum oxide (Al2O3) thin layer on a ZILRO TM fuel cladding tube by using the atomic layer deposition (ALD) technique. Crud deposition tests were performed to quantify the relative crud mass under a sub-cooled nucleate flow boiling condition in a simulated primary water at 328 o C. The obtained results are discussed in the view point of electrostatic forces between magnetite particle and cladding surfaces, and wettability of the cladding surfaces. 2. Experimental Al2O3 was chosen as a coating material, based on its thermal compatibility with Zr-based cladding alloys in neutron cross section and thermal property. A commercial ZIRLO TM cladding tube was used as the substrate for the coating. The cladding tube has a dimension of a 9.5 mm outer diameter (OD) and an 8.3 mm inner diameter. This tube was cut into tubular segments with a length of 6 mm and then the rings were ultrasonically cleaned in acetone and ethanol for 5 min. A part of the cladding tube was also segmented in small rectangle pieces and their OD sides were ground with SiC papers to have a flat surface for measurement of the wettability and surface zeta potential. At this time, the roughness of the flat surface was controlled to be the same as that of the as-received cladding tube. Al2O3 layer was deposited using ALD technique on the surface of the tubular and flat segments. ALD of Al2O3 layer was conducted at 250°C with trimethyl- aluminum (Al(CH3)3, TMA) and de-ionized water (DIW) as reactants. One cycle of thermal ALD Al2O3 growth consisted of a pulse of TMA, followed by a pulse of DIW. Each step was carried by a flow of nitrogen gas and separated by purge time. Two processes were performed alternately 200 times. The ring specimens of as-received (or uncoated) and Al2O3-coated segments were put on a cartridge heater. The diameter of the heater was designed to provide tight thermal contact with the ring specimens. Test solution was prepared by dissolving chemicals of LiOH and H3BO3 into high purity DIW. The prepared solution chemistry was 3.5 ppm Li and 1500 ppm B in weight, which was used to simulate a primary water in PWRs. This solution of 200 liters was stored in the solution tank. The sources of Ni and Fe ions for crud deposition were prepared using Ni- and Fe-ethylenediamine tetraacetic acid (EDTA), respectively. The mixed source solution of 1000 ppm Fe + 40 ppm Ni in weight was stored in the injection tank for injection to the test section. Crud deposition tests were performed in a 316 stainless steel autoclave connected to a primary water recirculating loop as schematically shown in Fig. 1. The pressure of the test section was regulated at 130 bars.