An Evaluation of the Departure from Nucleate Boiling in Bundles of Reactor Fuel Rods

Flow boiling crisis can be categorized into at least two types: Departure from Nucleate Boiling (DNB) in the subcooled and low-quality region and dry-out in the high-quality region. To analyze the DNB flux in a rod bundle, the flow conditions should be evaluated by a subchannel analysis and a single-channel DNB correlation can be used for predictions. Rod-bundle DNB data have been presented to verify the above statement. The uncertainties in the measured DNB heat fluxes are listed as: statistical nature of flow turbulences and surface conditions, ±3%; fabrication tolerances of test sections, ±5%; imperfectness of correlation in handling the parameter effects, ±5%; and random and systematic instrumentation errors and various loop system characteristics, ±10%. The probability of a rod bundle reaching DNB for a given DNB ratio predicted by W-3 correlation has been evaluated and demonstrated for its use. The effect of a DNB rod in a rod bundle is considered as not detrimental and not contagious. The above statement is demonstrated and evidenced by a photograph of the bird's-eye view of DNB and two sets of experimental data.