Heat transferred from water to air in Containment Building.

[MPHinman]

Hello,

I am a new Mech. Engineer at a nuclear plant and I am having a hard time getting started with a specific calc that I am not allowed to use a computer program for.

Basically, I have a pool (fuel transfer pool in a nucler plant) with a source of heat in the bottom (nuclear fuel assemblies). Above the pool is air in a closed containment building. If we lose the ability to remove the heat from the water that is being generated by the fuel assemblies, the water starts to heat up. As the water heats up, the air begins to heat up, which changes the pressure in the air.

The goal is to find the amount of heat escaping the water at different conditions so I can determine what the air pressure will be at given conditions at different times.

I think Q=h*A*delta(T) is the way to approach this problem, however, I am having a hard time identifying "h" and checking myself.

Any tips will be greatly appreciated.

Thanks
MPH

 

[racookpe1978]

Yes, the air heats up, which will change the relative humidity in the building above the fuel transfer pool.

But I really question any assumption that the "air pressure" will increase due to the transfer pool water heating up. Even if the fuel transfer pool were continuously boiling, the containment building atmosphere (overall!) would not heat up enough to change the containment building air pressure enough to change the heat transfer rates from the pool to the air above it.

Your containment building has a known volume, sized to restrain the "blow out" of the primary water from NOP/NOT plus a reserve for the replacement cooling water, right.

Try this: calculate the change in air temperature (from thermo PV/t = P2V2/T2 needed to change the pressure of air in the containment by 5%. from 14.7 psia (ambient) to 15.5 psia.

Compare the heat transfer properties of air (and steam) at 14.7 psia and 15.5 psia.

Now, find out the amount of heat energy (btu's) needed to heat the containment building air to that temperature - even if you assume no heat loss across the concrete to outside, and if you assume no fans, no cooling water, no power. (And, if the containment gets vented, then there is no change in pressure possible.)

Compare the btu's required to change the pressure in the containment bldg to the btu's available in the transfer pool, assuming worst case fuel rod coming from the hottest (most burned out) part of the core.

--

Second thought path: The water "boiling" out of the fuel transfer pool will condense on the cooler steel-lined concrete walls of the containment bldg. Once condensed, it will drain back to the floor - then where? If it falls back in the fuel pool (and it will eventually unless pumped out of the containment bldg drains - which are implied to be shut.) If the pool is being "replenished" by condensed water, then your problem will be mitigated.

 

[Bribyk]

Wow... I know nothing of nuclear design but that's a lot of air pressure change for a building. I take it people don't inhabit this space? A 0.8 psi pressure change puts about 2200 lbs of force against a standard door.

 

[racookpe1978]

Right.

That's why I'm questioning whether there is any meaningful change in air pressure from heating the pool.

(Yes, if the steam pipes blow, there are very large, very sudden pressure and temperature rises that must be resisted by the air locks and their doors, the buildings drains, inlet and cooling pipes, etc. (I don't recall the actual design pressures - and won't try to guess them.))

My intention was to use that 0.8 psig as an INDICATION of what new pressure MIGHT be, then see how much air and steam properties change IF pressure could go up that much, then see whether there is enough energy present in the fuel pool to increase temperature that much.

 

[MintJulep]

If you are assuming that the containment building is actually sealed, then don't forget that water expands as it gets hotter.

 

[racookpe1978]

Don't know if you're familair with a containment building:

Consider a "typical" 1150 Mwatt plant:

The containment "vessel" (inside the outer 3 ft thick concrete shield wall) is a 1.0 - 1.5" thick steel dome 120 ft in dia. This 120 ft domed pressure vessel is on top of a vertical steel wall about 115 feet tall, with an inside volume of 1.19 x 10^6 cubic ft volume. Design pressure is 15 psig.

The refueling pool is an open "pool" averaging around 30 ft deep and 80 x 30 or so across. His fuel transfer pit is an extension of that pool into the fuel storage/fuel handling building - but is ized to ahndle one bundle at a time. It's very deep, but relatively narrow, relatively short - maybe 4-6 ft wide x 20 long x 40 ft deep.

As I understand his question - and this understanding may be incorrect! - His boss's fundamental concern would be losing all cooling ability for this "open transfer pool" for a very long period of time while the reactor is shutdown, and potentially boiling the water in the small pool.

(When the reactor is operating - nothing (no fuel) is "stored" in the transfer pool. Nothing is in the fuel pool either for that matter. Its just sits there full of very pure, chemically treated water. So reactor operating heat loads are meaningless - in this particular case.)

 

[Bribyk]

I don't really understand what the OP's trying to determine but, I agree, this is starting to sound like one of those problems that's easier to back-calculate.

If it's a boiling issue, it's easiest to take the conservative approach and assume the room pressure is constant (i.e. leaking out) and at the lowest forseeable absolute pressure. Then figure out if the water boils or not.

I would hope a nuclear facility would have safety factors large enough to cover the effect of changing atmospheric pressure on the boiling point of water. If not, please let me know where this is so I can make sure I'm outside the contamination/blast radius.

 

[MPHinman]

Thanks for all of your input!
Yes we do have plenty of actions that can prevent anything like this from happeneing. However, we need to be ready for worst case scenario.

Containment pressurization is an issue in this event because it increases the chance that we may allow containment inventory to escape into the atmosphere... a no no in nuclear of course.

This problem is being used in my Calc Qual. I need to follow a procedure to become qualified and the problem is really irrelevant. Just an interesting one my supervisor proposed.

I appreciate your inputs.

 

[racookpe1978]

I'm smiling because this problem is almost identical to one on my PE exam twenty plus years ago.

After several pages of analysis predicting water temperatures and heat rates based on power history, fuel reload history and storage patterns, when the water level above the spent fuel rods would be reduced bu boiling to the point where radiation levels would start increasing ... (I had already made some assumptions about reduced shielding above the rods from the lower density of the water at 212 degrees, etc ....)

I couldn't figure out to "stop" and solve the problem: fuel rods would eventually be exposed if I "did" nothing to solve the problem.

So I wrote down, in the last paragraph on my PE exam, words to the effect of: "In the final case, to prevent fuel rod exposure, I would back up a fire truck up to the spent fuel pool and refill the pool with whatever (clean) fresh water supply was available. The original borates and chemicals would still be in the pool after boiling reduced the earlier water level, so the new water chemistry is close to being correct; and the time gained by refilling the pool allows the plant to bring in temporary power supplies for the cooling system and new water filters."

Was that answer correct?

(I dunno, but I passed the Principles and Practices test.)

 

[HamishMcTavish]

The critical safety function is to maintain the decay heat removal capability why are you even worrying about air pressure? Your systems for the core pond should have the ability to make up water level which will vary from normal evaporation anyway.

Also, surely the fuel pond area has some sort of ventilation system through banks of HEPA filters so how will pressure vary significantly?

confused as to what you're trying to achieve, HM

No more things should be presumed to exist than are absolutely necessary - William of Occam

 

[gmax137]

maybe too late to help you, but... I would do a heat balance on the pool: decay heat in - heat lost by evaporation => rate of heatup in pool. You need some kind of correlation to figure heat lost by evaporation. Or, neglect evap and calculate a time to boil. Once it is boiling you can say heat lost to atmosphere = decay heat. One little trick - the pool contains water and alot of stainless steel (racks etc). Check the volumetric heat capacity of SS vs that of water - save yourself from figuring out the volume of all that steel.

write back & tell us how your qualification quiz went.

 

[sailoday28]

Also needed is a cooreleation of "rate" of evaporation (ebulation??)