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Homework Help: Natural Circulation and Decay Heat

  1. Apr 10, 2014 #1
    1. The problem statement, all variables and given/known data

    I am having issues with some problems relating to a plant theoretically shut down on natural circulation, and calculating the core ΔT and natural circulation flow rate.. Can anyone provide some equations or theory I could use to assist me? I'm not familiar with this material and have spent hours searching Google and cannot find much.

    Problem 1:
    Initial Decay heat = 2.5% rated thermal power.
    Initial Core ΔT = 14 deg F
    Final Decay Heat = 1% rated thermal power
    Find: Final Core ΔT.

    Problem 2:
    Initial natural circulation flow rate = 3.5% full power flow rate.
    Initial Core ΔT = 15 deg F
    Final Core ΔT = 8 deg F
    Find: Final natural circulation flow rate

    2. Relevant equations

    I dont know any equations for this, I have searched for hours and cannot seem to find anything. If someone could reference me to any material they know of I can attempt a solution on my own first.

    3. The attempt at a solution

    (see 2. above)
  2. jcsd
  3. Apr 11, 2014 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    thought about this overnight

    make it a simple fluid flow problem.

    piping configuration does not change.

    driving head is in proportion to ΔT

    flow is in proportion to √head

    heat removed (power) is product of ΔT and flow

    should be able to ratio things out

    old jim
  4. Apr 12, 2014 #3
    Jim Hardy has outlined the correct approach. You will find that delta T and flow are proportional to fractional powers of decay heat. I will leave the specifics to you as an exercise! (Hint: the exponents involve 2 and 3). At SONGS, I had to do this evaluation to show the NRC that operating at 70% power led to a better natural circulation flow to power ratio. Alas, we never restarted.
  5. Apr 12, 2014 #4
    You might also check any licensing submittals that NuScale Power has made to the NRC for their SMR as their design does not used forced circulation and relies on natural circulation.
  6. Apr 14, 2014 #5
    Okay I got the equations that we are supposed to use...

    For power and core temp:
    where, Q=power (%)

    For natural circulation flow rate and core temp:

    From here, the problems are incredibly easy. So yes, I guess it is just a ratio. Not sure how the equations come about, but I guess proofs are not my main concern at the moment. However... It's me, and I'm curious.

    Thanks for the responses!
  7. Apr 15, 2014 #6
    Her's a derivation:

    W = Sqrt(Delta P)

    W=Sqrt(Delta (rho*g*h))

    W=Sqrt(Delta T*g*h) = Sqrt(Delta T)

    Q=W*Delta T

    Delta T = Q/W


    W^2 = Q/W


    So, W=Q^(1/3)

    Q=Q^(1/3)*Delta T

    So, Delta T = Q^(2/3)
    Last edited: Apr 15, 2014
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