Walk Away Safe Nuclear Design

I am a bit skeptical. Most of the dimensions are due to protection walls and heat exchangers The "machines" inside do not occupy a lot of volume.

To eliminate 45 MW to air and maintain a temperature low enough an area of 600,000 sqft is required considering free convection since no more electrical power is available.

If anything more is available I would like to have a look in order to understand how they imagine the heat transfer.

The principle of "fail safe" is applied in many domains and is nothing new. Of course it would be great if for the nuclear power plants same solution could be applied.

'...To eliminate 45 MW to air and maintain a temperature low enough an area of 600,000 sqft is required considering free convection....'

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Do you mind explaining how you arrived at that specific area? What assumptions were made (for example, what temperature is 'low enough' )?

It doesn't seem like an area of 600,000 sq ft would be that difficult, so long as there is no requirement for the area to all be only one layer thick and horizontal. Common automotive radiators probably have around ten times or more surface area for heat transfer than the exterior dimensions alone would suggest.

The heat transfer equation would be P(kW)= α (W/m²K)*Δθ (°C)*A(m²)

Due to power non availability (power off) the convection has to be considered as natural thus limited at around 10 W/m²K.

The temperature can be a function of pressure in the circuit. At atmospheric pressure under 100°C to avoid bubbles which have a bad heat transfer coefficient in the circuit and could generate hot-spots. For higher pressures the temperature can be higher.

The value I mentioned is a rough estimation and has ONLY an orientation value.

As for the the comparison with automotive heat exchangers I have to remind that they work at least with a forced convection due to the movement of the vehicle. When the mobile is not moving in general an air flow is sustained by a fan. We are in a case where the heat exchanger stays and a fan is not usable thus the very low convection coefficient. It would be not clever to count on wind for a fail -safe feature since wind is not a reliable "value".

As for the position since free convection has to be considered the surfaces must be VERTICAL : free convection is based on density difference between hot and cold air. In case of horizontal exchange surfaces the efficiency is much lower. The problem is much more complex due to the boundary layer thickness evolution over the height and the risk of dust and other insulating "stuff". In fact I should have counted α≈3..5 W/m²K

'...The temperature can be a function of pressure in the circuit. At atmospheric pressure under 100°C to avoid bubbles which have a bad heat transfer coefficient in the circuit and could generate hot-spots. ....'

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Okay. There are the two assumptions that would lead to thinking such a large area is required.

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In a natural circulation coolant loop, the heat source is always below the heat sink. Pressure under water increases by roughly one atmosphere every 33 ft. As difference in height is the driving factor in natural circulation, you can assume that the highest point on any fuel element will be far below the maximum height of the coolant in the loop.....meaning that the coolant around the fuel element will be be far above atmospheric pressure.

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Phase change heat transfer can be far more efficient than single phase heat transfer. Specifically in water, heat transfer rates for nucleate boiling dwarf those for heat transfer under similar conditions where no phse change occurs (a little higher pressure, for example).

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Nucleate boiling also can significantly increase flow in natural circulation systems as well.

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Surface area needed to keep primary coolant at 100 C isn't necessary. 100 C is far below what can be safe.

I am not the nuclear professional you seem to be so that I can make errors but I know that one of the problems which were encountered was the bubble generation since at the place where a bulb appears the local heat transfer is reduced and the wall material suffers a higher thermal strain. This was the reason to consider temperature low.

If you look at the sketch of the proposed generator it is a LOT smaller so that also the available water column is smaller it will thus generate less static pressure. I visited a lab for nuclear elements research and the "sticks" were not so tall may be 20 ft (it was perhaps a research sample only) so that the pressure increase is not as much as you assume-at least according to what I saw.

As I wrote it was an estimation under certain assumptions you are free to think that my assumptions are too on the safe side, Fukushima occurred for same reason nobody imagined that a wave could go over the usual height. Any way I do not yet intend to build a nuclear plant so that all is a speculation.

I am surprised that you do not consider my wind assumption also as too exaggerated.

The accident at Fukushima is tragic and devastating on many levels. The poor decisions made harm the environment not only from the release of fission products, but more significantly and possibly longer term by the default increase in coal burning plants each of which will continue to put tons of mercury, lead, thorium and uranium as extremely fine particulate into the atmosphere.

There certainly needs to be better, more cautious planning for for nuclear plants. And there definitely need to be more nuclear power plants as opposed to coal.

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About the heat transfer related to boiling, you aren't wrong, if things are carried far enough. There is an interesting and important characteristic you might be missing though. Check out this graph of heat transfer vs delta T with the various condition types labeled:

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Notice how rapidly heat transfer increases when nucleate boiling begins to occur.

In natural circulation systems the effect is even more pronounced because nucleate boiling creates a marked decrease in density which both increases mass flow rate (removing heat more efficiently) , and decreases neutron moderation (less heat is created).

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Beyond critical heat flux, things make a turn for the worse. Just as you suggest, departure from nucleate boiling, or boiling a channel dry can quickly damage fuel elements.

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Just another quick observation..... The total height of the fuel assembly is not as important as how far below the highest fluid point in the system is above most elevated part of any fuel element. Having the heat exchangers far above the highest part of any fuel not only contributes to increased pressure, it also drives stronger mass flow rates in natural circulation systems.

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I'm not sure which wind assumptions you are discussing.

I'm not trying to be mean in my comments, I was at first asking for justification or calculations supporting your assertion. After that I was suggesting that as far as safety, that the area your stated is needed was in my view very excessive. Not being mean, just speaking up when I see something that doesn't look right.

OK I misunderstood your reaction. What you say is partly right. It strongly depends on the transfer surface position. If it is vertical then of course the ascending of vapor bubbles generate as well an ascending fluid flow thus the transfer increase but it has a limit when the bubbles become too big the wall region only in contact with vapor grows and during the time it has no contact with the fluid the temperature can grow since vapor alone has a low convection coefficient (about 10 to 30 times less). This leads to the decrease all is function of the flux magnitude, in such systems for different reasons flux is quite high thus the risk.

As for the safe side design I think that it is no way it has to consider the unimaginable in order to avoid consequences. For instance years ago the concrete envelope was not designed for big plane impacts to day they are.

but more significantly and possibly longer term by the default increase in coal burning plants each of which will continue to put tons of mercury, lead, thorium and uranium as extremely fine particulate into the atmosphere.

Not many people know that coal plants release Hg, Pb, Th, and U, along with other stuff, into the atmosphere. How can we inform them so they won't be so resistant to anything with the word "nuclear" in it?

I wish I knew a good way to more efficiently disseminate the information.

I believe, regardless of what powers the electrical generation, that rules regarding emissions should be the same. This is clearly not the case right now.

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If all power plants were held to the same standards as nuclear power plants, coal fired plants would be shut down immediately. The cost to bring emissions in line would be cost prohibitive....Coal based power would no longer be price competitive.

Most other sources would have no problem....hydro, solar, wind, natural gas...would all be unchanged.

Some crude oil sourced electricity (very low percentage in the US) would have to make some changes, but it probably wouldn't price it out of competitiveness for its niche.

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The average 1000MW coal fired plant releases over 5 tons of uranium and 12 thorium as ultra fine particulate into the environment...EVERY YEAR! Thorium and uranium are alpha emitters, and definitely not something you want to inhale or ingest.

Even if the additional tonnage of mercury, lead and other heavy metals released were not a problem, the alpha emitters constitute a large long term hazard building cumulatively.

As "truth . . ." explains, 100 degrees C is not required because of the pressure increase due to depth. However, the fast reactor is even better since it is cooled by a liquid metal such as sodium. I don't know its boiling temperature off hand, but is sure is much higher than water, meaning that the system can run at a higher temperature. I'm weak on thermo, but during normal operation, I think higher temperature means higher efficiency.

It's basically a large coil floating in a pool of water, the natural convection of the water thermal facilitates cooling....The process I'm told, is described in the Robert Hargraves book, "Thorium - Energy cheaper than coal"...

"Liquid Fluoride Thorium Reactors" an article by Hargraves and Ralph Moir in American Scientist....

http://home.comcast.net/~robert.hargraves/public_html/2010-07Hargraves2-1.pdf

What we had up till now.....

http://en.wikipedia.org/wiki/Passive_nuclear_safety#Examples_of_reactors_using_passive_safety_features

I first heard about liquid thorium reactors in a long video, which has edited here to about 5 minutes in length. Sounds like a great idea. (This guy is obviously a promoter; the skeptic in me wonders if there is another side to this.)

http://www.youtube.com/watch?v=uK367T7h6ZY

Another side ? I think this is a good idea, and a common sense solution to future energy needs of this planet....

http://en.wikipedia.org/wiki/Flibe_Energy

PS: Thanks for another interesting link.....

Here a promo for the documentary "The Good Reactor"...

http://www.youtube.com/watch?v=9smV3lcmBdY

This is definitely a step in the right direction.

Considering we have yet to actually build anything that utilizes mechanical motion, electrical energy operated devices, and/or basicaly anything that requires energy in one form or another to operate without it experiencing failure.

My major concern is with the fact that concrete cement will crack sooner or later and if exposed to severe temperature change the cracks tend to be extreme.

How do we contain the contaminated water and keep it out of the groundwater if a leak occurs?

There are reasons besides the possibility of an accident to keep fluids relatively clean. Regularly removing impurities from coolant is needed:

-to minimize corrosion/erosion,

-to minimize activation of impurities,

-to minimize deposition of activated corrosion products or other sources of radioactivity throughout system making maintenance difficult

-to facilitate indication/notice of rapid degradation in the system (through sudden increase in contaminants)

-to facilitate accurate control of plant chemistry.

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A heavily shielded system to purify the coolant with filters and ion exchange resin can be highly effective in capturing and confining the vast majority of the dangerous stuff that becomes mobile in the coolant.

This sounds like a conventional technology thermal neutron plant with changes to the cooling system. I favor a new technology fast neutron plant, which is many times better, and I understand will also shut itself down passively because it develops negative reactivity in the core. It also cools by thermo-siphon (convection.) One huge feature is that it will use as fuel the "waste" from the conventional thermal plants we now have--that is, the fuel is "free."

I do like the conversion of waste to fuel of fast reactors.

It is a good idea to remember that the 'experts' describing the advantages of their particular system are not likely to be impartial.

A good example of this is the 'negative reactivity coefficient' claimed as an attribute of fast reactors to demonstrate their inherent safety and ease of control.

The truth is (compared to thermal reactors) control of a fast reactor is not so much a strength as a hurdle.

Thermal reactors rely on delayed neutrons for control. Delayed neutrons make up less than one percent of fission neutrons, but occur corresponding to a half life around 15 seconds as opposed to occurring within 10^-14 (prompt neutrons). This Operating prompt sub-critical / critical only with the addition of delayed neutrons, allows the system to be designed such that changes in reactor power can be limited to a few percent a second...a rate that an active system or even people can reasonable be expected to respond to and control effectively. Additionally in thermal reactors, the coolant can be a moderator, such that as the coolant gets hotter, it is less dense and becomes less effective as a moderator, which decreases reactivity (negative coefficient of reactivity).

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In Fast reactors, because delayed neutrons have reduced effectiveness, compared to prompt neutrons, at causing fast fission (because delayed neutrons have much lower energy, along with some other factors), delayed neutrons play a much smaller role in reactivity. This combined with the fact that no moderation of the neutrons occurs, yields very short neutron generation time. This has the effect that power in the reactor, instead of changing by a few percent in a second, can be capable of increasing by a factor of 20,000 or more in a second. Controlling that rate of change actively would be no trivial task. Additionally, even though the coolant cannot be an effective moderator (as it would remove too much energy for fast fission to remain at criticality), most coolants still moderate to some degree. This means that if the coolant boils off or otherwise becomes substantially less dense or moves away, the void created adds positive reactivity, working to increase power (positive coefficient of reactivity).

Fast reactors typically end up being designed to rely on expansion of the fuel elements as temperature increases to provide a negative coefficient of reactivity sufficient to allow control.

The other big hurdle with fast reactors is the coolant. The choices are a mixed bag. Sodium metal....if this developed a leak, the chemical reactivity of sodium metal would be hell to deal with. Lead, which gets activated creating long lived very hot toxic lead waste. Helium, scarce and difficult to keep around.....

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The supercritical water reactor offers a lot of the benefits of both thermal and fast reactors and seems to skip a few of the downsides of each.

It is a good idea to remember that the 'experts' describing the advantages of their particular system are not likely to be impartial.

Truth, This was a fascinating post to me (month and a half ago!). But the sentence above tells me that my learning on the subject has been one-sided since my reading so far has been almost all pro-IFR. Can you suggest where I can find some more general information on the subject.

Thanks.

The many of the various technologies have been around a long time and textbooks are common in larger libraries.

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Look for something like Reactor Principles or Nuclear Reactor Engineering in the title.

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I was looking for old textbooks I could recommend that might be available online, but unfortunately it appears Google Books has recently curtailed the access previously available.

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I'll let you know if I do run across anything decent that is accessible online.

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Another option might be to read what all the competing technologies say about each other.... the truth probably lies somewhere in between.

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EDIT: found something along the lines of what we were discussing. Not exhaustive, but a decent intro; Starting on page 24 of the document (page 34 of the PDF) .