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Minimal film boiling temperature (Core reflooding)

  1. May 23, 2015 #1
    Hello Forumers and Physics-addicted Fellows!

    I'm an applied nuclear engineering scientist by profession and thermal physicist by education. well nowadays i'm really involved into the describing the processes in Light Water Reactor core during Emergency Core Cooling System in action. In details i'm very interested in the thermal hydraulics of fuel rods and spacer grids during reflooding.

    To simulate the quenching of fuel rods one needs to solve unsteady heat conductivity problem in solids and convection, radiation equation for two-phase flow. Moreover it's required to take into account physico-chemical interaction of zirconium alloy with water (heat effect, new oxide layer properties, variation of surface roughness), cladding ballooning, spacer grids effect.
    In the other words the reflooding process is very complicated.

    One of the main approaches to improve the reflooding process simulation is to predict film and transition boiling regimes of fuel rod more adequate. The criterion of heat regime change is assumed to be minimal film boiling temperature (Tmfb). Its very close to Leidenfrost temperature (TL)

    Could we discuss the influence of any parameters on Tmfb? Perhaps you could have some valuable information, some theoretical ideas about the way of desribing this effect.

    ***Of course any questions about LWR reflooding/quenching are welcome and it would be very nice to discuss and share ideas and knowledge***

    Best wishes,
  2. jcsd
  3. May 26, 2015 #2
    I think you are mixing two things DBA and Severe Accident. If ECCS is initiated mostly for DBA then cooling criteria is clad temperature below 1200 deg C, less than 17% cladding oxidation, amount of Hydrogen generated less than 1% of total Zr-H2O reaction, and core geometry intact for cooling.
    Zirconium reaction and cladding ballooning are studied in severe accident when it is assumed all power sources lost and core must uncover...
  4. May 26, 2015 #3
    Well firstly thanks Muti for reply!
    Secondly i reject your idea of mixing me two accidents (DBA and SA). I know these criteria both for PWR design and VVER design too (The last one is the Russian LWR which is very similar to PWR). By the way in Russian regulatory docs it's pointed that clad oxide layer must be less than 18 % of initial zirconium alloy E110 layer (E110 = Zr + 1 % Nb).

    Actually i'm involved into the applied science research of fuel rod assembly (FA) behavior under DBA and SA conditions. I'm one of the lucky guys who can obtain experimental data on fuel rod thermomechanics, thermal hydraulics and physico-chemical interaction of E110 zirconium alloy with superheated steam. These experiments are carried out at PARAMETER facility in Russia.

    http://itf-mpei.ru/lib/ITF_MPEI_referat_2011_Bazuk.pdf (in Russian)

    While the experiment is being carried out the claddings of fuel rods are ballooned, steam-zirconia reaction is in action but T < 1200 deg C. As we invesigated different SA scenarios T>1200 deg C (up to 1730 deg C) and intensive Zr-H2O interacion took place.

    So i wanna tell you that Zr-H2O chemical reaction is initiated even as T> 700 deg C but it's thermal influence is negligible in comparison with decay heat. T = 1200 deg C is the criteria of uncontrollable chemical reaction starts. But when T > 1200 deg C the Zr-H2O chemical reaction still can be rather weak. The thick oxide layer limit the steam flow rate to pure Zr...

    The conditions you pointed are just simulating one that is they are chosen to investigate specific accident stages. Actually if the accident is intitiated and there's now sufficient heat sink all key events (balloonong, oxidation, temperature rise) take place. And it's still proposed the accident can be neutralized by the water supply into core.

    So i'm interested in quenching of fuel rods under DBA and SA conditions. The main goal now for me is to simulate in details film boiling specific parameters ))

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