Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Emergency Core Cooling of PWR

  1. Dec 6, 2014 #1
    For dealing with different types of loss of coolant accident (LOCA), pressurized water reactors (PWR) have emergency core cooling system (ECCS). ECCS are simply redundant pumps to inject water into core to cool fuel, thus preventing fuel melting in case of an accident (hypothetical) that involve break of primary coolant loop. Now these have further been divided into high head safety injection (HHSI), low head safety injection (LHSI) and accumulator (based on safety analysis study to fill core before LHSI start up). Some plant call HHSI as medium heads safety injection or simply safety injection (SI) because charging pumps flow is sufficient for up to some break size. Now my question after so may years in nuclear field is how all nuclear plant standardized on this arrangement of HHSI and LHSI. I have always wonder why redundant positive displacement pumps (like reciprocating pumps) have not been used to cater to all types of break (small, medium and large break). This way plant capital cost could have been saved. Am I right can some one prove me wrong. Can some one give me an example of other arrangement for ECCS.
     
  2. jcsd
  3. Dec 6, 2014 #2

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    With regard to standardization, please remember that 40+ years ago, there were three PWR suppliers in the US: Westinghouse, Combustion Engineering and Babock & Wilcox. In addition, each had several different designs of different capacities and power densities. The ECCS designs were determined on the basis of postulated accidents. In the intervening years, plants have been uprated and cycle lengths have increased such that the demands on systems have increased.

    Overseas, various foreign corporations licensed technology for W, CE and/or B&W, and there are yet alternate PWR designs but with many similarities.
     
  4. Dec 6, 2014 #3
    "I have always wonder why redundant positive displacement pumps (like reciprocating pumps) have not been used to cater to all types of break (small, medium and large break)." By this sentence I mean why a single pair of redundant positive displacement pumps and not two groups of HHSI and LHSI centrifugal pumps (minimum 04 pumps combined) have been considered. Is there some law for having HHSI and LHSI? I have not studied, but not thoroughly, the gen 4 (I think gen 5 reactors are way different) designs and they too mostly have this arrangement.
     
  5. Dec 6, 2014 #4
    i will further add that i stick to definition of safety class as given by ANS, but peoples define safety related equipment as the one that are used during normal operation (like charging pump and residual heat removal pump), but are also aligned for emergency service. I this case clearly only SI pumps (medium head SI) are in a true sense, meant for only safety service. Based on this assumption I think is it not possible to have a positive displacement pump (with sufficient head and flow rate) to cater to all three types of LOCA (Small, medium and large breaks)? I know this will have advantages as well as disadvantages. Do some one think that this been ever studied.
     
  6. Dec 7, 2014 #5

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    Please provide an example of such a pump.

    Pumps have to be seismically qualified, and have the capacity. One might wish to review the requirements for the HPSI/LPSI systems. In a PWR, the ECCS must inject coolant into the primary system. If there is a LOCA (particularly LBLOCA), at some point, sump pumps may be involved.

    I hope QuantumPion will interject, or anyone else with PWR experience.

    http://nrcoe.inel.gov/resultsdb/SysStudy/HPSI.aspx
    https://nrcoe.inel.gov/resultsdb/publicdocs/CCF/CompBoundaries.pdf
    http://dspace.mit.edu/bitstream/handle/1721.1/31333/MIT-EL-78-031-04842804.pdf

    http://www.nucleartourist.com/systems/eccs.htm
     
  7. Dec 7, 2014 #6
    Astronuc I will check these document first and further refine my question.
     
  8. Dec 8, 2014 #7
    Well, you must have the two RHR pumps anyway for normal shutdown operation, so if you add two ECCS pumps then you will again have four pumps in the design.

    Or, do you mean replace the RHR pumps with large positive displacement pumps? I don't know for sure, but PD pumps with that flow capacity may not be available. Remember that many (most?) of the mechanical components (pumps, valves, heat exchangers, etc) used in the nuclear designs were originally "off the shelf" items already developed for use in other applications. The high-head injection pumps are basically boiler feed pumps; the positive displacement charging pumps were oil-field equipment.

    Plus, it seems positive displacement pumps are mechanical nightmares in comparison to centrifugal pumps - noisy vibrating monsters with pulsating suction and discharge pressures, and lots of maintenance required. The PD charging pumps are bad enough at 44 gpm capacity; I can't imagine the problems in scaling those up to the several thousand gpm capacity needed for RHR service.

    Also, the ECCS analysis and regulations grew / evolved over time. The original thinking was that a system designed for a maximum size break would be able to accomodate a smaller break. When the details of the phenomena associated with smaller breaks were worked out, the need for high-head injection became apparent. But, the flow requirements are much lower, so additional pumps were "tacked onto" the system rather than replacing the low head pumps.

    In the end, it is the overall cost that dictates the design -- cost of the pumps, the piping, the analysis. And there is the cost of delay in licensing. The nuclear field has a large inertia; proposing a system different than the already-licensed systems invites a lengthy regulatory review time and uncertainty in the outcome.
     
    Last edited by a moderator: Dec 22, 2014
  9. Dec 8, 2014 #8

    QuantumPion

    User Avatar
    Science Advisor
    Gold Member

    For SBLOCA you need a pump which can inject water into the 2250 PSI RCS. For LBLOCA you need a pump with very high flow rate at low pressure (< 600 PSI) to refill the RCS. These two types of pumps have mutually exclusive designs, there's no way to make one pump that can handle both regimes.
     
  10. Dec 8, 2014 #9
    To the OP: Why are you recommending positive displacement pumps? Such pumps have pulsating flow, both on the suction and discharge side, and this makes for difficult design, particularly at large flow rates.
     
  11. Dec 8, 2014 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    gmax and Q'pion nailed it.
    power required to pump is basically flowrate X pressure.

    The flowrate that you need to pump into the system is the flowrate that's running out through the leak.
    Small break by definition is a small leak, not much flow meaning pressure will stay high, so you need a small high pressure pump. Ours were a few hundred horsepower.
    Large break is by definition a lot of flow, which will lower pressure, so you need a big low pressure pump that'll move a lot of water. Likewise, ours were a very few hundreds of horsepower. Basically they were irrigation pumps.



    A high flow high pressure pump can certainly be built but it'd take a lot of power. Remember it's the product of flow and pressure.
    You'll soon run into problems powering such a huge machine from a rapid start diesel generator as required for emergency service.
    Our diesels were nominal three thousand horsepower. They had to start from cold and be ready to accept load in ten seconds. Basically they were locomotive engines.
    Our boiler feed pumps were seven thousand horsepower. I don't think you could find a ten second diesel big enough to run them.

    I hope that helps explain the reasoning behind different pumps for the different jobs.

    Now an exercise for you - look into speed/torque characteristics of pumps, both centrifugal and piston, and of electric motors.

    old jim
     
  12. Dec 9, 2014 #11
    Very well gmax. Very well old Jim. Very good explanations. I will look into speed/ torque characteristics. Beware! I think I might study all types of LOCA analysis again.... Hahaha .... My interest would be that after small/ medium break LOCA, generally after how much time operators have reduced primary pressure for LHSI operation (and can this be reduced or some modification to design may be required, and after all this would there be any benefit!).
     
  13. Dec 9, 2014 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    If the leak is small enough that HHSI and Charging Pumps can keep up with it, operators may prefer to cool down and depressurize by normal means... The less you flood containment the easier the cleanup.

    As i'm just an instrument guy, I'll defer further answers to someone who's closer to current off-normal operating procedures.

    old jim

    PS DrD's comment regarding pulsating flow from piston pumps is spot on. The piping is excited at the pulsation rate and any mechanical resonance in the piping system shows itself by tearing pipe hangers off the wall. Pulsation dampers are a must, even on our little 50gpm pumps..
     
    Last edited: Dec 9, 2014
  14. Dec 9, 2014 #13

    QuantumPion

    User Avatar
    Science Advisor
    Gold Member

    If you want to reduce the complexity of safety equipment than you should go with an ESBWR-type design which relies exclusively on passive, gravity fed accumulators and natural circulation.
     
  15. Dec 10, 2014 #14
    There is a small break size where the break flow is not sufficient to remove the decay heat. For breaks smaller than this size, the operators must maintain heat removal via the steam generators and ultimately the RHR/shutdown cooling system. For larger breaks, the system depressurizes to equilibrium with the containment pressure, and the ECCS pumps make up (replace) the inventory lost out the break. Fortunately, there is a lot of overlap in the break size that can be accomodated in either fashion, so the operators dont really have to know exactly how big the break is. The system response tells them which way to go.
     
  16. Dec 10, 2014 #15
    Ok I add, I am a PSA person with some maintenance experience. Nowadays my assignment is related to deterministic analysis. So, it can be said that positive displacement pumps were not used because of their limited flow rate, at high pressure. Astronuc asked about example of positive displacement pump. Well not in a true sense but atleast is example of positive displacement charging pump mostly used for hydrostatic testing of primary system. Some plants around world have modified these for "non safety class' (powered mostly by non emergency bus) emergency need. But i sure their flow rate may be less then charging pump.
     
  17. Dec 10, 2014 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    I'm retired now but nostalgia is irresistible..
    My former plant has positive displacement charging pumps. They've run since 1972 for charging service but take a lot of maintenance. Piston seals wear, and when they ingest air they must be manually vented. There was a hydraulic torque converter between them and their motors which both provides variable speed for flow control and allows their motors to start against reasonable torque.
    We also had multistage centrifugal HHSI pumps that were run only for periodic surveillance testing. With a small recirc line they'll purge themselves of air. Recall also that a centrifugal pump requires remarkably little starting torque making things easier for the diesel generator that has to start them in an emergency.

    There's a LOT of thought behind these systems. You might see if there's an old Westinghouse PWR Technology Manual around, it explains much of the basic reasoning.
    USNRC's "General Design Criteria" are an elucidating introduction to the straight thinking of the industry pioneers (and Rickover).

    In your maintenance experience surely you saw how interconnected all these systems are. Change anything at your own peril.....

    old jim
     
  18. Dec 11, 2014 #17
    Coming from a bwr background, at what point would emergency depressurization be required and how is it accomplished?

    For a BWR plant, we always say "all roads lead to blowdown", at the end of every EOP leg is a blowdown statement, and the blowdown allows you to break cool down rates and use low pressure injection systems. Generally you are blowing down to either maintain adequate core cooling, blowing down to ensure containment integrity is not challenged, or blowing down to reduce or stop an unisolable primary system leak. we blowdown by activating the automatic depressurization system, which opens several Safety relief valves or depressurization valves and venting steam to the suppression pool (basically a containment sump that already has water in it).

    How do Pwrs handle this? How do they handle it with a loss of aux feed and high pressure injection? (No line break).

    I'm just curious. And If anyone is interested in Bwrs let me know.
     
  19. Dec 12, 2014 #18
    I am interested. It's been 3.5 years since Fukushima. Are you guys know for sure you can blow down your BWRs in a prolonged SBO? Did you test it? What sort of batteries, portable generators and/or compressed air tanks will you need to do that?
     
  20. Dec 12, 2014 #19

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    I'm not current on today's ONOP's and EOP's. Will defer to somebody who is.
     
  21. Dec 12, 2014 #20

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    When i went through ONOP course thirty+ years ago
    emergency depressurization was via pressurizer by sprays or as last resort the power operated relief valves (PORV's).
    of course with attention to maintain fifty degreesF of subcooling to keep natural circulation going.
    so to that end i pulled the reactor pressure gauge and added to its face adjacent each major scale division the equivalent saturation temperature, making a 'poor man's subcooled margin monitor'.

    If there's no line break or leak then the system isn't depressurizing itself so you don't need high pressure injection or emergency depressurization.
    If there's a medium break that you cannot keep up with the approach was to make it into a larger break by opening PORVs and letting pressurizer relief tank rupture,
    in other words turn it into a large break that engineered safeguards can handle via accumulators and injection. We could align the high head injection pumps to recirc as well.
    If we lost aux feed (awful to contemplate) then feed with either main feed pumps or if pressure is low enough the condensate pumps.
    Aux feed was so fail-safe and redundant that so long as one could make steam he could use it to run the pumps.
    In the very early days we had a feedwater tie from adjacent fossil units, subsequently removed. The old-timers had put it there for last-ditch backup. It was handy at hot shutdown because the fossil feedwater was preheated which made boiler level easier to control, and especially during turbine rollup and synch.

    Please understand my old memories are now painfully weak in detail and surely procedures have evolved.

    getting old ,

    jim
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Emergency Core Cooling of PWR
  1. APWR and PWR (Replies: 2)

  2. PWR pressure drop (Replies: 4)

  3. TDAFW for PWR (Replies: 4)

Loading...