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Thermal Hydraulics: Heat, Water, Nuclear Power and Safety

Scott Krepel
Reactor System Engineer
 

One of the most important safety questions in a nuclear power plant is: Can you cool the very hot nuclear fuel in an accident when normal cooling is disrupted? The scientific field best equipped to answer this question is called “thermal hydraulics.”

bwrThe first part of the term, “thermal,” relates to heat transfer, such as the movement of heat from the burner on a stove to the water in a pot via the metal of the pot. The second part, “hydraulic,” relates to the flow of a fluid such as water. The combination, “thermal hydraulics,” can be applied to systems where both the flow of fluid and the transfer of heat are important – such as a nuclear power plant.

I work in the NRC’s Office of Nuclear Regulatory Research as part of a team dedicated to expanding our understanding of thermal hydraulics and applying that understanding in nuclear power plant safety. Over time, we’ve put much effort into incorporating existing knowledge into the NRC’s thermal hydraulics computer simulation program, TRACE. This program allows NRC staff to construct computer models of the cooling systems of a nuclear power plant and then simulate accidents such as pipe breaks (but not wildly improbable events such as the considerable destruction caused near the end of a typical superhero action movie).

TRACE is constantly being pushed to become more accurate, reliable and versatile. Universities and test facilities around the world are conducting experiments and accident simulations to collect real-world data that can be used to determine TRACE’s ability to accurately predict specific phenomena. We use the outcomes to update the program as needed to make it more accurate and to better capture certain phenomena.

Sometimes, new safety issues may result in further investigation of certain scenarios and further evolution of TRACE. Ultimately, the goal of this work within the research arm of the NRC is to continuously expand our understanding of situations which may impact the cooling of the nuclear fuel. This knowledge can then be used to ensure that the public and the environment are protected in the unlikely event of an accident at an U.S. nuclear power plant.

43 responses to “Thermal Hydraulics: Heat, Water, Nuclear Power and Safety

  1. Sergey August 1, 2014 at 3:24 pm

    Good article. Thanks for the information.

  2. jonycad@gmail.com February 8, 2014 at 1:16 pm

    Great article, it is most helpful. I`v been asked to do a model simulation on the very same question: “Can you cool the very hot nuclear fuel in an accident?”

  3. Lisa researching hydraulic safety January 14, 2014 at 10:07 pm

    I’m having trouble believing the “unlikely” part of a disaster happening at a U.S. power plant.

  4. Tai game avatar June 22, 2013 at 3:25 am

    Until the NRC talks about core meltdown, steam explosion and who knows what other make-believe processes and use incorrect models, defining the cladding temperature incorrectly for the steam bubble covered state and until the NRC pushes for Hydrogen recombiners for the supressed Hydrogen generation rates calculated by incorrect codes, instead of looking into the real ignition and burning of Zircaloy cladding in the steam there will be no safe nuclear power plant.

    Even it would be relatively easy to achieve: the hot leg side injection ports could be used for the depressurization vent lines in the PWRs and the safety relief line in the BWRs. Whenever the state of the reactor is unknown, the forced circulation through the core is lost or the heat transfer to the ultimate heat sink is severed the operators should open the depressurization vent and allow the reactor to depressurize rapidly and the staged injections of borated water to operate. Only question remains: do we have sufficient gravity emergency core cooling water reserves?

  5. richard123456columbia March 7, 2013 at 12:44 pm

    Statement: The AP1000 and ESBWR plant designs are generation 3+ designs which DO utilize gravity fed passive cooling systems, and are walkaway safe for 72+ hours.
    Only if not jamed by earthquake or other even. Are many or all rods configured to one support or is each rod independed, knowing the extra cost for independent rods are a problem for profit I would guese not.

    • Aladar Stolmar March 8, 2013 at 2:10 am

      What I am calling for is a direct rapid depressurization vent allowing the operators to vent the steam directly out of the upper part of the reactor before a stagnant steam bubble would extend down into the core.

      In the PWR the accumulator injection (ECCS) ports connected to the hot leg side could be utilized for such vent, in the BWR the existing safety relief lines could be rerouted to the vent stack.

      The use of this rapid depressurization vent is proposed in three cases:(1) the state of the reactor is unknown, (2) the forced circulation through the core is lost or (3) the heat transfer to the ultimate heat sink is severed

      And please, add these to the existing plants too with sufficient gravity water reserves.

    • David Andersen. March 8, 2013 at 10:42 am

      Each control rod has its own operating mechanism, they are operated in groups under normal conditions but in the event of a reactor trip each will insert into the core independently assisted by a spring and gravity.

      • richard123456columbia March 8, 2013 at 2:35 pm

        If that is the case it sounds good. Arnie Gunderson has had concerns about AP1000’s earthquake stability, have they improved them from those that are being built now?

  6. Aladar Stolmar March 5, 2013 at 10:36 am

    Nuclear guy and The Firestorm Fighter – the issue is that the real process when a stagnant steam bubble forms in the reactor core is an ignition and firestorm of the zirconium-steam reaction as it was modeled in the cited experiment as well as in the PBF SFD tests, most correctly in the scoping test of this later series. Which was indeed the real process in the TMI-2 accident, Chernobyl-4 accident, Paks 2 refueling vessel incident and in Fukushima 1, 2 and 3 reactors.

    What I want to achieve is the addition of depressurization vent and operator response to depressurize the reactor and avoid the steam bubble formation in the core of BWR and PWR types, therefore exclude the possibility of the firestorm.

    Until the NRC talks about core meltdown, steam explosion and who knows what other make-believe processes and use incorrect models, defining the cladding temperature incorrectly for the steam bubble covered state and until the NRC pushes for Hydrogen recombiners for the supressed Hydrogen generation rates calculated by incorrect codes, instead of looking into the real ignition and burning of Zircaloy cladding in the steam there will be no safe nuclear power plant.

    Even it would be relatively easy to achieve: the hot leg side injection ports could be used for the depressurization vent lines in the PWRs and the safety relief line in the BWRs. Whenever the state of the reactor is unknown, the forced circulation through the core is lost or the heat transfer to the ultimate heat sink is severed the operators should open the depressurization vent and allow the reactor to depressurize rapidly and the staged injections of borated water to operate. Only question remains: do we have sufficient gravity emergency core cooling water reserves?

    And yes, NRC is responsible for the Fukushima.
    “Even “Zirconium being one of the strongest reducing agents in the periodic table” page 148 of NRC-2012-0022-0002 “At the same time, Zr is also reacting with steam from concrete decomposition, producing hydrogen gas,”
    Zr + 2H2O = ZrO2 +2H2
    But the reaction heat of 5 MJ/kg Zr reacted is missing as well as the reaction of steam not from concrete, but the coolant itself! WHY? NRC does not allow the steam from coolant react with the zirconium, just with the steam from the concrete?! And the nature just follows the orders of NRC?! As we saw it in Fukushima Daiichi, indeed!” – cited one of my comments…

    • nuclear guy March 5, 2013 at 1:50 pm

      Just some information:

      In BWRs, the automatic depressurization system will AUTOMATICALLY depressurize the reactor should water drop below level 1 (low-low-low alarm level) and a low pressure coolant pump is activated and ready for injection. This system is highly reliable and will automatically blowdown the core to the suppression pool for low pressure injection.

      Now there are some issues with your comment that simply depressurizing the core will fix all problems. First off, depressurizing with a hot containment/suppression pool will catastrophicaly damage your containment. This is undesirable and could greatly complicate the consequences (dose) to the public from an accident. Second, is depressurization will provide some steam cooling during the blowdown, but will also lower your water level much lower than it originally was. If a low pressure cooling system is not available, as was the case in Fukushima, then depressurizing has no benefit, and will just serve to hasten fuel fragmentation and progression of the accident. Depressurization also is only analyzed for 1 use in the life of the vessel, after which a full vessel reanalysis needs to be performed to see if the vessel is still capable of functioning. This isn’t something that you can just do willy nilly, it is only to be there to ensure design basis accidents can be automatically controlled using ECCS.

      Loss of heat sink does not immediately cause a steam bubble. This “steam bubble” you talk of, in BWRs, happens when water drops BELOW 2/3rds top of active fuel for some extended period of time. If the normal heat sink is lost, the suppression pool becomes the interim heat sink, either through relief mode valve lifts or safety mode valve lifts. It’s not until most of the core is uncovered that you have an instance where the zirconium is even possible. In all cases, for design accidents, ECCS prevents this from ever happening.

      • Aladar Stolmar March 6, 2013 at 9:59 am

        nuclear guy, I thought I spelled out clearly that only in 3 situations I want to give an opportunity to the operators to intervene, prevent the core damage with that dedicated depressurization vent: “Whenever the state of the reactor is unknown, the forced circulation through the core is lost or the heat transfer to the ultimate heat sink is severed”.

        In Mark I BWR unfortunately the torus is used for the suppression pool as well as for water reserve for injection and when the cooling of it was lost in Fukushima it resulted in the brake down of RHIC and failure of the core cooling (in a day or so). Here the venting of the steam starting immediately after the loss of the cooling was realized (which was not that radioactive before the core damage) to the dry well and to the vent stack even would help the situation. That is my proposal.

        For the PWR the vent is equal to a large LOCA, which not suppose to cause such problems as You describe and vent to the environment should be a possibility, when there is no core damage. What I’m calling for is to provide staged passive injection reserves (or just check, that it is there) all the way to gravity injection, sufficient to cool the core for a relatively long time to prevent damage to the fuel rods.

        I expected that someone objects that I did not mention the core exit temperature measurements, which are there to detect the steam bubble in the core. I do not want the operators to wait until they measure above the saturation temperatures there, when there is a real danger of the core damage from the above problems. Vent the steam bubble before it enters into the core, please!

        I know that the ECCS is there to handle the design basis accidents. And yes, I think so that one size should fit all, the rapid depressurization and vent with well staged passive injection reserves all the way to the gravity injections should do that. Make the nuclear power plants, reactors unquestionably safe.

      • Hiddencamper March 6, 2013 at 6:00 pm

        At Fukushima, they had to get permission for venting (different from US, where operators are required to vent by procedures). They also had problems getting their rupture disks to open, which is something the US plants to not use. They were TRYING to vent, and it failed due to their specific design of the containment vent. I think We both fully agree that venting containment is a critical strategy to protect the plant.

        With BWRs, you have a steam void in the core at all times (it is a boiling reactor). There’s no incore temperature monitors. There are only water pressure and water level monitors.

        ECCS is designed to automatically perform a rapid depressurization, followed by a transition to low pressure cooling systems. Existing plants do not have the required designs to support passive cooling systems. The RCIC/HPCI/IC steam powered cooling systems are designed to cool the core until power is restored or portable pumps are connected.

        The AP1000 and ESBWR plant designs are generation 3+ designs which DO utilize gravity fed passive cooling systems, and are walkaway safe for 72+ hours.

      • Aladar Stolmar March 7, 2013 at 9:39 am

        Hiddencamper, I am talking about venting the steam from the top of the reactor, above the core and not just venting the containment.

        You also cited that the new design incorporates the passive injection. They should.

        Why the NRC does not want to talk about upgrading the existing plants, also?

        Zircaloy Mass in Fuel Cladding [kg / lb] 16,465/ 36,300 in the PWR and 40,580 /89,500 in BWR from NRC-2012-0022-0002 and NRC-2012-0022-0003.

        Zr (91) + 2 H2O (36) = ZrO2 (123) + 2 H2 (4) + 5 MJ/kgZr
        Water required for complete reaction for the PWR 16,465 * 36/91 = 6513,6 kg or about 6.5 m3 (available), it produces 16,465 * 123/91 ZrO2 = 22,255 kg zirconium dioxide and 16,465 * 4/91 = 723.7 kg Hydrogen and 82,325 MJ heat. For a 10 second firestorm duration it gives 8GW power… or twice the full power of the reactor…
        Water required for complete reaction for the BWR 40,580 * 36/91 = 16053,6 kg or about 16 m3 (available), it produces 40,580 * 123/91 ZrO2 = 54,850 kg zirconium dioxide and 40,580 * 4/91 = 1784 kg Hydrogen and 204,250 MJ heat. For a 10 second firestorm duration it gives 20GW power… or five-six times the full power of the reactor…
        Considering that NRC does not require a top of the reactor depressurization vent to prevent the zirconium firestorm in the reactor, the above back of the envelope calculated worst case scenario should be considered.

      • hiddencamper March 7, 2013 at 2:35 pm

        BWR plants have safety valves located in the steam lines, above the reactor. These valves will manually or automatically open to depressurize the reactor to release steam, and will fully depressurize the reactor under certain conditions (Low water level 1 alarm + low pressure injection systems available). I am talking about the steam IN the reactor steam dome, NOT the containment.

      • Aladar Stolmar March 8, 2013 at 1:57 am

        Hiddencamper, if You blow the steam into the torus and heat-up the water what You want to inject into the reactor, You will cause that a stagnant steam bubble extends down into the core, because You will not be able to pump the boiling water. Does not matter that You vent the steam or not, if You will deplete the water reserves, makes it unusable.

        So this is why the proposed by me rapid depressurization vent is different from the existing solution: Blow the steam to the stack and have gravity water reserves for injection. (beside the now usable suppression pool water in the torus, not heated-up.)

        Yes, please add some gravity water reserves to Mark I, too!

  7. richard123456columbia February 27, 2013 at 10:06 am

    So they will keep building plants not knowing the risks till they gather data on failures to be plugged into TRACE. This to me is scary that they do not know the potential outcome of these plant designs but go ahead with the risk. They seem to ignore what they do not understand and hope for the best results. Is this proper engineering and design practices? Not in the Engineering school I went to.

  8. american2018@gmail.com February 26, 2013 at 10:57 pm

    Lots of talk. The reality is the atmosphere, the Pacific Ocean, and the dairy pastures on the west coast are all now contaminated. Here’s a question: are the pumps at Fort Calhoun submersible, since they are on a flood plain? Here’s another: What’s the NRC done about the faked quake test results that whisteblowers reported? Here’s another: How many babies died in the womb, or shortly after birth from Fukushima fallout?

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