BWR fuel assembly question

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Question: So if I am reading correctly what you said then not only does the water leak past the rod drive piston seals but it is intended to do so in order to keep them in a healthy condition and as another way by which to supply water to the reactor , although a rather small amount as compared to the other means still significant as you said enough to keep the decay heat in check.

Response: This is correct. After a few hours the rod drive pumps can supply enough water to manage decay heat losses. For plants with isolation condensers, the control rod drive pumps are the ONLY injection after a main steam isolation until you cool the reactor down (typically below 400 PSIG).

Question: By natural circulation you meant the heat given off by the fuel after scram is not enough to cause boiling under the right circumstances but enough to cause water heating and such heated water travels upwards like in a house boiler and so as it is over the core boundary it can then fall back and cool off via the sides of the reactor between the so called "core shroud" and reactor vessel outer wall where the inlet water pipes are located and the jet pumps through which the inlet pipes enter?

Response: BWR natural circulation is a bit different than a PWR. In a PWR, natural circulation is required to move heat from the core to the steam generators, so that it can be removed from the reactor coolant system. In a BWR, natural circulation is about minimizing stratification and not impacting your cooldown limits. When you have no forced flow, you have a small amount of natural circulation due to the boiling effect. Decay heat WILL boil water, and create sufficient flow to keep the core safe. But it doesn't create enough flow to mix the bottom head of the reactor, and with < 100 degF water being injected there you will have bottom head stratification. You can minimize this stratification by raising reactor water level to the steam dryer (typically 2-3 feet above normal operating level), and whenever the core is subcooled this is a requirement to make sure your reactor coolant temperature measurements are valid.

The concern is if you have bottom head stratification, you could violate the 100degF/hr cooldown limit. Or even worse, you could try to start up a reactor recirculation pump and thermal shock the reactor when that cold water now rapidly moves to a 400+ degF section of the vessel. Tech specs, procedures, and thermal shock interlocks should prevent this, but it's still not a good thing.

The area between the core shroud and the outer vessel wall is called the "Downcomer". The downcomer is where condensation from the steam separators/dryers drains to, where the reactor recirculation pumps and jet pumps take suction from, and is where feedwater is injected to.



Question: I understand this would be highly unlikely but can't there be a case where if the valves or other parts brake in the case of scram reactor water can leak out through the control rod pipes right past the seals if you say the seals are made such that they permit water flow past them? I assume if the pressure from below the CRD mechanism is lost there would be some pressure coming from the high stack of water which is in the vessel and since the control rods are at the bottom all that pressure concentrates there?

Response: The piping for the CRD system is ASME nuclear boiler classified piping. I believe it's class 1 because it is part of the Reactor Coolant Pressure Boundary (RCPB). If you had a failure of one of those pipes, it would potentially be a small break LOCA and would require a unit shutdown and cooldown. It's unlikely though as it is ASME code

Question: Also as you said when the men go to service the CRD mechanism below the reactor vessel say during a refueling, how can one remove a rod with it's piston if the reactor core is under water for biological shielding and heat removal purposes during refueling, wouldn't the water start flowing out through the rod guide tube if the rod and its piston would be removed or say taken out for most of its length, I assume it depends on where the seals are located but I'm not sure about that one, from the drawings in the brochure it seems the piston with seals is located at the down most position of the rod?

Response: The bottom of the control rod has a cone shaped velocity limiter. This cone shape also acts as a sealing surface. When we go to remove a drive mech unit from the bottom of the reactor, we first fully withdraw the control rod and let is sit over the bottom head penetration. The cone shaped velocity limiter creates a seal which prevents water from leaking out of the bottom of the reactor. When we remove the drive units, we unbolt the flanges and pull them out completely, creating a hole to the reactor, but the control rod bottom seals it.


Question: I had a bit of hard time imagining how the collet fingers work in terms of applying opposite pressure to them but I assume that each rod is held in place by more tan just one such finger as they are multiple for each rod spaced equal distance apart?

Response: Take a look at this link: http://patentimages.storage.googleapis.com/pages/US5446774-1.png
29a is the collet finger and 31 is the collet finger springs. The springs push the fingers in place to latch the rod. If you look at the notches (55) each notch in the side is the even number positions (00, 02, 04 ... 48). When you insert a rod, the notches are tapered to allow the collet fingers to be pushed out of the way for an insert signal. When you go to withdraw a rod, the system applies a momentary insert signal to push the fingers out of the way, then pressure on the withdraw header holds the fingers in place. 81 is the withdraw header supply line. If you follow that down, you'll see 69, which is the withdraw pressure to the collet fingers. Follow that up, and you see a small piston below the collet fingers which pushes them up into little slots after they are retracted. Those slots hold the fingers in place so you can continue to withdraw the rod. When the withdraw line is closed, the fingers push back into place and the rod notch settles on the fingers.


Question: Also I understand from what you said that in the case of scram condition when the rods are fully inserted and position is read 00 a valve opens so that no excess pressure is accumulated in the reactor vessel due to decay heat slowly heating up the water and rising pressure? Because if the turbine steam outlets are shut off then another bypass must be opened correct?

Response: This has nothing to do with steam dumping. If you look at that link again, 80 is the insert line, and 81 is the withdraw line. The insert line has two parallel sources of hydraulic fluid, the rod drive system and the control rod accumulator. When a scram occurs the scram inlet valve opens allowing the accumulator to inject to the under piston region and force the rod in.

On the withdraw line you have the normal withdraw valves, then you have a scram outlet valve which opens up. The scram outlet valve directs the over-piston water to the scram discharge volume, which is empty and at atmospheric pressure when the scram occurs. This has nothing to do with decay heat or vessel pressure, it's completely independent from it.

If your turbine steam outlets shut off, there are steam dumps (called turbine bypass valves), which will auto open to control pressure on the main steam header. If the main steamline isolation valves shut, your relief valves will automatically or manually be operated, and operators can also place other systems in service to help control pressure (isolation condenser, RCIC, HPCI, reactor water cleanup, etc).





Question: Lastly speaking about your answer to @rpp answer about rod movement, I read in the "general description" manual that after years of operation about 1980 they made up a new way to use the BWR core called the "CCC" in order to achieve better fuel burnup and lower operator need to adjust control rods for power during operation or fuel lifecycle, now I want to make sure I got it correctly, so the CCC (control cell management) is basically a way to reorganize the BWR core so that after one cycle of the fuel they relocate the most burned assemblies around the control rod blade positions forming arrays, so the highest reactivity assemblies (less burnt fuel) is further away from control blades and lowest reactivity assemblies (most burnt fuel) around the control blades, I understand that by this they achieve the moment where changing the control blade insertion height they get a more smoothed out and less steep response in the core neutron flux and also since the control blades affect the neutron background more to the closest assemblies to the blades the generated heat difference is less if the assemblies closest to the blades are with less reactivity?
and besides this they achieve higher and more even burnup in the fresh assemblies further away from the blades?

is this strategy still used in BWR's and their successors like ABWR etc?

Response: Some background here. The control rods are broken into 4 groups, "A1", "A2", "B1", and "B2". These are the four rod sequences that you can use for operation. In the original core designs, you would start up on one group, and do sequence exchanges every couple of months to equalize fuel burnup. Sequence exchanges take a long time to perform, and require heavily reduced power levels. It was not very efficient, and was a pain on the operators and for capacity factor.

The control cell core is a different fuel loading strategy where you utilize only 1 or 2 rod sequences. The CCC was an improvement. Today we use the "Improved Low Leakage Control Cell Core" design or something similar. We put the oldest fuel on the outer periphery, and it acts like a neutron reflector to the next innermost ring, boosting it's reactivity output. We typically only use 1 rod sequence (my plant is almost always on A2), and we just swap which rods we use in that sequence. You have different "rings" in the core based on fuel loading, and the control rods which are used are based on controlling the reactor's power radially.

We use control rods in the core to help control both reactor power, as well as flux shape. Shallow rods (typically position 32 to 48, that are barely inserted) only impact the axial flux shape of the core. Deep rods (position 16 to 00) affect radial power and full core power. In the middle it depends on the spot in core life and which ring the rods are in. We use rods in combination with burnable poisons in the fuel to shape the flux profile and optimize burnup. Typically we do not use shallow rods anymore. At most, we use them during initial startup/heatup until the fuel is conditioned and xenon is built in sufficiently to allow us to switch to only deep rods (this is why BWRs require a downpower after a day or two of operating, to swap to the normal full power rod pattern).

I do not know what ABWR uses, or the other plants with fine motion control rod drives. I suspect it is similar.


Question: PS. from your last reply about the necessity to move the rods alot etc I assume you are operating one of the older BWR plants with a lower output capacity?
is you facility using the 7x7 fuel assembly dimensions which were then changed to 8x8 referring to rod count on each side of the bundle? Atleast that's what I read int he manual.

Response: Actually I operate a BWR/6, one with a high power output and a very high power density. We are using GNF-2 fuel, which is 10x10 with 92 fuel rods and 2 water rods.
 
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