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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 17, 2013 10:59 am 
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NicholasJanssen wrote:
How Many People Have Really Been Killed by Chernobyl?

http://www.slate.com/articles/health_an ... ation.html

not that many... anyone else want to guestimate?


The cancer increase caused by low-level radiation may well be negative. But acute radiation injury is likely to have killed about one Nishapur train blast's worth of people. As said elsewhere,

Any mishap involving a Western power reactor, with designs that comply with the no-Chernobyls rule that Edward Teller and his Reactor Safeguard Committee put forth in the early 1950s, is going to be pretty harmless, and because in the Western world fossil fuels are a large source of government revenue, any such mishap is bound to be tagged with a huge death toll by the best alarmists money can buy.

And perhaps some enthusiastic amateurs, or professionals who don't know they're professional because their money is from government, and governments are pronuclear, right? If there is any doubt, just reassert.

Chernobyl was different: the presiding government did not have a petrodollar conflict of interest, and did have a strong, and well justified, sense of inferiority; and the Chernobyl design had been touted as a triumph of Soviet man.

So its deceptive intent with regard to Chernobyl would tend to be in the direction of downplaying, not, as with governments today and reactor wrecks today, exaggeration.

So we have a heuristic for getting the true Chernobyl toll: it must be much lower than Western fossil fuel tax revenue enthusiasts say, but much higher than the former Soviet Union ever admitted.

My estimate has always been that the antinukes are twice the liars the FSU was, and so when they say 8000 dead and the FSU says about 50, I estimate the true number is 50 to the two-thirds power times 8000 to the one-thirds. About 300.

A few years ago, corroboration for this turned up at http://www.rri.kyoto-u.ac.jp/NSRG/reports/kr79/kr79pdf/Imanaka.pdf:

Quote:
If a dose rate of 2 R/h is used for May 1 instead of 350 mR/h, the average external dose for the inhabitants in Usov village becomes about 2 Sv before the evacuation. In this case, the following description in the secret protocols of the USSR Communist is seriously realistic: ... 2,609 persons are in hospital for treatment, including 471 infants. According to confirmed data, the number of radiation disease is 367 cases ...


"2 Sv" equals 2000 mSv. Recall that Japan evacuated land based on a hundredth of that per *year*.

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 17, 2013 12:49 pm 
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Another way to think about the decay heat load of 13 Watts per fuel rod is to compare that to a standard office fluorescent tube. These operate at a temperature that you can comfortably hold in your hand, unlike incandescent bulbs. The fuel rods are over 4 meters long for BWR fuel, so it generates about 3 Watts per meter of rod. This compares to a 1 meter fluorescent tube of about 30 Watts.

So, the fuel rods produce 10x less heat than fluorescent tube lights.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 17, 2013 6:51 pm 
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Lars wrote:

So we have a tonne of material generating 4kW of power.
Probably we should tie it to the surface area and compare the power density to a incandescent light bulb.

Here goes - sanity check please as this seems incredibly low


I think that the proper surface area is the six sides of the pool. The fuel rods are not in free space. They are next to each other. Perhaps only the the top surface counts because the concrete on the four sides and bottom acts as an insulator. I have no idea whether the geometry of the fuel rods is conducive to air convection. It is possible that most the heat loss is by radiation.

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 17, 2013 7:10 pm 
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pstudier wrote:
Lars wrote:

So we have a tonne of material generating 4kW of power.
Probably we should tie it to the surface area and compare the power density to a incandescent light bulb.

Here goes - sanity check please as this seems incredibly low


I think that the proper surface area is the six sides of the pool. The fuel rods are not in free space. They are next to each other. Perhaps only the the top surface counts because the concrete on the four sides and bottom acts as an insulator. I have no idea whether the geometry of the fuel rods is conducive to air convection. It is possible that most the heat loss is by radiation.


In normal situation the pool is under cooled water right?
If the rods get hot they would expand the air around them and cause the air to rise.
If there are places with less heat (by the sides of the pool) then I would imagine we get a chimney effect.
You are right that the concrete sides would be basically insulators.

It is hard to imagine that these things could get hot enough to melt. But I'll grant you that they will get considerably hotter than one rod free standing in space.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 12:19 am 
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Lars wrote:
It is hard to imagine that these things could get hot enough to melt. But I'll grant you that they will get considerably hotter than one rod free standing in space.


There is a recent paper: "Predictions of Spent Fuel Heatup After a Complete Loss of Spent Fuel Pool Coolant". As I recall, it essentially agrees with you.

The sun goes at 0.000194 kW per tonne, and look how hot it gets.

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 2:12 am 
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pstudier wrote:
Lars wrote:

So we have a tonne of material generating 4kW of power.
Probably we should tie it to the surface area and compare the power density to a incandescent light bulb.

Here goes - sanity check please as this seems incredibly low


I think that the proper surface area is the six sides of the pool. The fuel rods are not in free space. They are next to each other. Perhaps only the the top surface counts because the concrete on the four sides and bottom acts as an insulator. I have no idea whether the geometry of the fuel rods is conducive to air convection. It is possible that most the heat loss is by radiation.


No, convection is the dominant heat transfer. The space between the fuel rods is small, but the fuel rods are arranged vertically with free space below and above. They must be, because they are normally cooled by natural convection of water, so an efficient laminar regime is provided. It's very similar to a home radiator heater: very tight passages are allowed, as the air can be sucked in freely from below and can be rejected freely above. This actually produces a low pressure drop because the regime is always laminar flow. In addition, BWR fuel rods have at least 2 so called water rods, which are empty spaces where normally there is moderator water in the core to provide extra moderation. These rods are efficient convector holes, almost chimney like. They would allow both turbulent and laminar flow.

There is additional surface area in the tie grids, spacers, and fuel rod can sides, that act like additional cooling fins. There is also some space between assemblies, that would further increase convection.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 2:23 am 
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GRLCowan wrote:
Lars wrote:
It is hard to imagine that these things could get hot enough to melt. But I'll grant you that they will get considerably hotter than one rod free standing in space.


The sun goes at 0.000194 kW per tonne, and look how hot it gets.


That's an absurd example. The sun is a bazillion times bigger than a spent fuel pool. Its surface area to volume is terrible, and the distance the energy has to transfer is insane compared to a 4 meter fuel rod.

The appropriate comparison is with something of similar size, like an industrial oven. Without insulation, such an oven would easily lose much more heat than the spent fuel decay heat.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 2:37 am 
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Let's take a look at radiative heat losses.

According to the reference Lars provided, the BWR fuel assembly has a top surface area of 0.0193 m2.

It makes 842 Watts.

With an emissivity of 0.75, a spent fuel building temperature of 50C, and a spent fuel rod hot temperature of 300C, the spent fuel assembly would radiate about 80 Watts. Though it may not get that hot. If it gets to 200C, then radiative heat loss from the top is reduced to 30 Watts/assembly.

So, radiative heat loss from the top is a minor player.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 6:16 am 
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I made a simple model to check the conductive and radiative heat transfer without considering any convection inside the spent fuel assemblies, as a worst case.

In that case, conduction toward the outside surface of the spent fuel is the only heat transfer possible. The model simulates the spent fuel assemblies as a single block, where only convection and thermal radiation outside the block are allowed.

Attachment:
dry spent fuel bay model run.png
dry spent fuel bay model run.png [ 115.51 KiB | Viewed 1017 times ]


The peak temperature of 588 degrees Celsius was reached in the center of the block (of course).

According to the NRC, there will be no fuel cladding failures at this temperature:

http://pbadupws.nrc.gov/docs/ML1302/ML13028A421.pdf

This shows that this fuel will NOT fail, even in a complete dryout scenario. Simply open up a few doors and windows for ventilation and you're fine.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 1:06 pm 
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But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 1:13 pm 
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Burghard wrote:
But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.


That would mean they were producing less heat, wouldn't it? Less buildup of fission products to produce heat when the main fission reaction stops.

edit: Whoops, I guess I misread that.


Last edited by Russ on Sep 18, 2013 1:28 pm, edited 1 time in total.

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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 1:24 pm 
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Burghard wrote:
But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.

I believe the calculation started assuming the fuel rods were as hot as they could be - as if all of them were just removed from the reactor at the end of their useful life just before the tsunami hit. Reality will be considerably lower heat production but requires more detailed knowledge than we have ready access to.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 1:42 pm 
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Burghard wrote:
But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.


It is not correct, but the error is on the side of caution for me in terms of the present analysis: I've assumed all spent fuel assemblies in the unit 4 spent fuel pool to be freshly unloaded core assemblies. That is of course a significant conservatism - the 1600 fuel rods I've assumed can't fit in unit 4's reactor. I think it should be about half that number, so the other half being older. It is true that just after unloading there is more heat load, but then again, we should account for air convective cooling inside that fuel if we really want a realistic analysis. But it is true that really fresh fuel (like a week old, just unloaded into the spent fuel pool) could have some damage, not enough for fuel melting but possibly enough for some cladding rupture. More detailed analysis would have to be done.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 1:57 pm 
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Lars wrote:
Burghard wrote:
But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.

I believe the calculation started assuming the fuel rods were as hot as they could be - as if all of them were just removed from the reactor at the end of their useful life just before the tsunami hit. Reality will be considerably lower heat production but requires more detailed knowledge than we have ready access to.


The 4 kW/ton figure comes from Jaro's image from Kirk's java app. It is for the present day analysis. One week old fuel should make about 4-5x more decay heat. Steam cooling should suffice but dry air cooling might not. The fuel wouldn't melt like the stupid media speculate on, but cladding failures are possible. More detailed analysis would have to be done on just how good the convective cooling of fuel assemblies internals is.


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 Post subject: Re: Fukushima fuel pools
PostPosted: Sep 18, 2013 2:02 pm 
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Cyril R wrote:
Lars wrote:
Burghard wrote:
But is it really correct to take the average heat production of all fuel rods? At least immediately after the tsunami hit the core of unit 4 was freshly unloaded, and these fuel rods must then have been producing a lot more heat than the average.

I believe the calculation started assuming the fuel rods were as hot as they could be - as if all of them were just removed from the reactor at the end of their useful life just before the tsunami hit. Reality will be considerably lower heat production but requires more detailed knowledge than we have ready access to.


The 4 kW/ton figure comes from Jaro's image from Kirk's java app. It is for the present day analysis. One week old fuel should make about 4-5x more decay heat. Steam cooling should suffice but dry air cooling might not. The fuel wouldn't melt like the stupid media speculate on, but cladding failures are possible. More detailed analysis would have to be done on just how good the convective cooling of fuel assemblies internals is.



True that immediately after removal fuel rods should be under water actively cooled. But the context of this thread was specifically the risk that the spent fuel rod holding tanks in Fukushima present to the whole world. In this light every fuel rod is at least 2 1/2 years since removal from the reactor and presumably most are considerably older. That the former ambassador would make such an erroneous statement trying to scare everyone is really irresponsible.


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