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Re: Does load-following require much temperature change?

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  • gumsh0e
    ... For the most part, a change in the feedwater injection rate is a consequence, not a cause, of a power change (see below). ... In a BWR, the principle means
    Message 1 of 8 , Feb 12, 2008
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      --- In Know_Nukes@yahoogroups.com, "csceadraham" <csceadraham@...>
      wrote:
      >
      > I would have thought adjusting the power output
      > of a nuclear power plant whose reactors are
      > water-cooled would normally be done by
      > adjusting the rate of cool water injection
      > to the reactor.

      For the most part, a change in the feedwater injection rate is a
      consequence, not a cause, of a power change (see below).


      >
      > Starting with a uniform water injection rate and an
      > exactly critical reactor and then slowing
      > the water injection rate would slightly
      > increase the temperature of the fuel and the
      > water around it, causing the reactor to go
      > minutely subcritical, so that the power would
      > ease down to restore the original balance.
      > No large temperature change, I would have thought,
      > would occur.

      In a BWR, the principle means of changing power (once the desired
      control rod pattern is established) is to vary the amount of
      recirculation flow in the core. Increasing the flow briefly "sweeps"
      the water voids higher up the fuel channel, which increases neutron
      moderation, which increases power, which increases the back pressure
      of the voids within the channel, which causes the voids to build back
      to nearly the same level you started with until a new equilibrium is
      established. Increasing the power level increases the steaming rate,
      which the level control system compensates for by increasing the
      feedwater rate (so the "goes-ins" equal the "goes-outs").

      I'm not as up to speed on PWRs, but I believe increasing the load on
      the main turbine causes more heat to leave the steam generator, which
      cools the cold leg, which increases moderation, which increases
      power...




      >
      > But in the Michael Keller essay just now linked by
      > David Walters, Keller says,
      >
      > "A conventional nuclear unit is not really designed
      > to follow load because the flow of coolant
      > is essentially constant. To reduce load,
      > the coolant (and fuel) temperature must be reduced."

      I confess I haven't read the link so I don't know the context of this
      comment but the above seems somewhere between misleading, incomplete,
      and wrong. Nuclear plants are typically the first to be dispatched,
      and prefer to run flat out. However, when the weather is particularly
      mild, even they may be asked to drop power because of low demand. I
      suspect jockeying the power around too much could result in Xenon
      building in or burning out in a way that is out of synch with where
      you are trying to get the power to go or the feedwater system may
      start to oscillate as it tries to find its "happy place".
      Consequently plants are typically limited, physically and/or
      administratively, in how quickly power is allowed to change
      (generally only a couple percent per minute).

      Naval reactors, with their small cores, are designed to be a lot more
      nimble.



      >
      > If that's true, going from 100 percent thermal power to,
      > say, 70 percent must imply a reduction in turbine mechanical
      > power output by significantly more than 30 percent
      > because of reduced T_hi in the heat engine. Really?
      >
      >
      > --- G.R.L. Cowan
      > http://www.eagle.ca/~gcowan/boron_blast.html --
      > let the baby play with matches in the fuel storage room!
      >
    • schedule80
      A question about BWRs. In the start up procedure for a PWR, the primary plant is brought up to temperature first, before the reactor is taken critical. The
      Message 2 of 8 , Feb 12, 2008
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        A question about BWRs. In the start up procedure for a PWR, the
        primary plant is brought up to temperature first, before the reactor
        is taken critical. The reactor coolant pumps provide enough heat to
        get the reactor coolant system and steam generators at, say, 545
        degrees F with 1000 psia in the steam generators before any nuclear
        heat is added by the reactor. How does the start up procedure work for
        BWRs? Are the recirculation pumps run for several hours to heat up the
        reactor, and establishing the steam bubble in the top, before bringing
        the reactor critical? If so, how is it going to work for the ESBWR,
        which doesn't have recirculation pumps?

        - Pete


        --- In Know_Nukes@yahoogroups.com, "gumsh0e" <gumsh0e@...> wrote:

        > In a BWR, the principle means of changing power (once the desired
        > control rod pattern is established) is to vary the amount of
        > recirculation flow in the core. Increasing the flow briefly "sweeps"
        > the water voids higher up the fuel channel, which increases neutron
        > moderation, which increases power, which increases the back pressure
        > of the voids within the channel, which causes the voids to build back
        > to nearly the same level you started with until a new equilibrium is
        > established. Increasing the power level increases the steaming rate,
        > which the level control system compensates for by increasing the
        > feedwater rate (so the "goes-ins" equal the "goes-outs").
        >
        > I'm not as up to speed on PWRs, but I believe increasing the load on
        > the main turbine causes more heat to leave the steam generator, which
        > cools the cold leg, which increases moderation, which increases
        > power...
        >
        >
      • gumsh0e
        BWRs are started up cold . BWRs don t have (or need) anything comparable to a pressurizer. Think in terms of the level control system on the secondary side of
        Message 3 of 8 , Feb 12, 2008
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          BWRs are started up "cold". BWRs don't have (or need) anything
          comparable to a pressurizer. Think in terms of the level control
          system on the secondary side of the steam generators in a PWR - heat
          is transfered directly from the fuel rods to boil the water just as
          heat is transfered from the steam generators tubes to the secondary.
          The BWR level control system is analogous to the steam generator level
          control system (and the level control system in a coal plant boiler).
          Steam generators and coal boilers aren't allowed to "go solid" either.

          After rods are pulled to make the reactor critical and a positive
          period of a minute or or is established, the operators simply wait for
          power to increase to "the point of adding heat" (where the moderator
          coefficient begins to have an effect), at which the operators start
          pulling rods again to counteract the effects of the moderator (and
          eventually the void and doppler) coefficient(s).

          Lenny

          --- In Know_Nukes@yahoogroups.com, "schedule80" <schedule80@...> wrote:
          >
          > A question about BWRs. In the start up procedure for a PWR, the
          > primary plant is brought up to temperature first, before the reactor
          > is taken critical. The reactor coolant pumps provide enough heat to
          > get the reactor coolant system and steam generators at, say, 545
          > degrees F with 1000 psia in the steam generators before any nuclear
          > heat is added by the reactor. How does the start up procedure work for
          > BWRs? Are the recirculation pumps run for several hours to heat up the
          > reactor, and establishing the steam bubble in the top, before bringing
          > the reactor critical? If so, how is it going to work for the ESBWR,
          > which doesn't have recirculation pumps?
          >
          > - Pete
          >
          >
          > --- In Know_Nukes@yahoogroups.com, "gumsh0e" <gumsh0e@> wrote:
          >
          > > In a BWR, the principle means of changing power (once the desired
          > > control rod pattern is established) is to vary the amount of
          > > recirculation flow in the core. Increasing the flow briefly "sweeps"
          > > the water voids higher up the fuel channel, which increases neutron
          > > moderation, which increases power, which increases the back pressure
          > > of the voids within the channel, which causes the voids to build back
          > > to nearly the same level you started with until a new equilibrium is
          > > established. Increasing the power level increases the steaming rate,
          > > which the level control system compensates for by increasing the
          > > feedwater rate (so the "goes-ins" equal the "goes-outs").
          > >
          > > I'm not as up to speed on PWRs, but I believe increasing the load on
          > > the main turbine causes more heat to leave the steam generator, which
          > > cools the cold leg, which increases moderation, which increases
          > > power...
          > >
          > >
          >
        • bj_yakman
          A bit rusty but I believe for PWRs Tavg is a constant and Tcold/Thot vary as power level changes. The cores have negative temperture moderation so that an
          Message 4 of 8 , Feb 13, 2008
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            A bit rusty but I believe for PWRs Tavg is a constant and Tcold/Thot
            vary as power level changes.

            The cores have negative temperture moderation so that an increase in
            steam demand to the steam generator causes Tcold to drop slightly
            increasing core power. Similarly a decrease in demand results in an
            increased Tcold and a drop in core power. Larger power shifts are
            accomodated by rod movement (short tem) and chemical (boron) shim (long
            term)

            Nuke NSSS systems can load follow fairly well. I think the French, and
            some US as well, have made changes to the BOP, the turbine in
            particular, to accomodate load following.

            Nukes are usually base loaded, I think for economical reasons, i.e.
            they are a cheap source once up and running. High end fossil plants
            (oil and gas) are used to cover peaks. I also think that larger coal
            plants are not the easiest to load follow either. Combustion
            conditions, thermal stresses, and air pollution systems all tend to
            make load changes a challenge.
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