When departure from nucleate boiling (DNB) occurs, the critical heat flux (CHF) must be accurately determined for different conditions in order to maintain reasonable margin to CHF during the operation of a pressurized water reactor (PWR). Cladding surface texture may influence DNB by providing boiling nucleation sites and by introducing turbulence that inhibits film boiling. Textured surfaces can increase or decrease CHF, depending on their structure. The chemistry of the cladding surface controls wetting. Stronger wetting inhibits detachment of coolant from the surface and initiation of film boiling. Westinghouse has developed a new cladding product that consists of a metallic chromium layer on Optimized ZIRLO cladding. Since both the surface microstructure and chemistry of the Cr-coated cladding are different from the zirconium alloy surfaces that have been previously characterized, it is necessary to show that DNB margin is similar or greater than current cladding surface.

In this paper, the Westinghouse advanced loop tester (WALT) generated DNB test data at PWR thermal hydraulic and chemistry conditions to support the Westinghouse EnCore® accident tolerant fuel (ATF) products. With an Apple-to-Apple comparison DNB strategy, the Westinghouse WALT loop team has been performing DNB tests using heater rods with Cr-coated and Optimized ZIRLO uncoated surfaces. Meanwhile, the cladding local resistivity measurement is critical for an accurate calculation of the local heat flux. Under this defined DNB strategy, a new methodology has been developed for performing DNB tests and measuring/calculating the CHF values for different cases.

In this paper, the new methodology is described for local resistivity measurement and heat flux calculations. In addition to measuring the CHF for different coolant conditions, possible cladding failure modes were also investigated during the DNB tests. There has been concerns that the coating would spall off from cladding surface, or cracking of the cladding surface might be enhanced. The cladding section around the DNB location were examined to address such concerns.

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