High Resolution Measurements Reveal Transient Boiling Phenomena under Exponential Heat Inputs

Journal Article · Fri Jul 01 04:00:00 EDT 2016 · Transactions of the American Nuclear Society

OSTI ID:23042947

Su, Guan-Yu; Bucci, Matteo; McKrell, Thomas; Buongiorno, Jacopo ^[1]

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1. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02138 (United States)

Transient boiling heat transfer is important to the safety of nuclear reactors. Reactivity insertion accidents (RIA) in a nuclear reactor might result in a power excursion in which the heat generation in the fuel rises exponentially with time. The period of the exponential power excursion depends upon the magnitude of the reactivity insertion. The heat generated within the fuel is transferred to the water coolant which then starts to boil. The reactivity feedbacks caused by the heating (Doppler in the fuel and void in the coolant) represent an important mitigation mechanism for such accidents. The time delay between heat generation in the fuel and its transfer to the coolant is key to determining the outcome of the accident, in particular for experimental reactors using highly enriched fissile fuel with a very low Doppler effect. This time delay depends on conduction heat transfer within the fuel, single-phase convective heat transfer and eventually transient boiling heat transfer in the coolant. Although existing transient boiling databases form a highly valuable source of information, sometimes conclusions from different authors are quantitatively and qualitatively in disagreement with each other. This is likely due to differences in experimental setups and also limitations in the accuracy of diagnostics available in those experiments. In order to clarify these discrepancies and shed light on the mechanisms of transient boiling heat transfer, a new experimental study of both pool and flow boiling under exponential power input were carried out respectively. Exponential power escalation periods from 5 ms to 100 ms, bulk temperatures from saturation to 75 K of subcooling and Reynolds number from 25000 to 60000 have been explored at ambient pressure in both pool and flow boiling conditions. What makes this study unique is the use of synchronized state-of-the-art diagnostics such as IR thermometry and high speed video (HSV) which has improved the accuracy of measured quantities, and has provided a new and unique insights into the transient boiling heat transfer, including detection and modeling of the onset of nucleate boiling, better understanding of the transient boiling process and revelation of a new heat transfer mechanism that leads to higher CHF. (authors)

• OSTI ID: 23042947
• Journal Information: Transactions of the American Nuclear Society, Journal Name: Transactions of the American Nuclear Society Vol. 115; ISSN 0003-018X
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
22 GENERAL STUDIES OF NUCLEAR REACTORS
COMPUTERIZED SIMULATION
COOLANTS
DOPPLER EFFECT
EXPERIMENTAL REACTORS
HEAT TRANSFER
MITIGATION
NUCLEAR FUELS
NUCLEATE BOILING
POWER INPUT
REACTIVITY COEFFICIENTS
REACTIVITY INSERTIONS
REYNOLDS NUMBER
SUBCOOLING
TIME DELAY
TRANSIENTS