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PostPosted: Jul 16, 2010 3:11 pm 
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Brainstorming The Lithium Homogeneous Reactor


Reasons, Objectives and Priorities


As a general rule, the TRISO fuel format is well tested, understood and accepted by the nuclear industry. For the practical reactor designer, this political priority overshadows the downsides in its use in breeder reactor design. However, many new and exciting ideas come from revolutionary concepts such as the nuclear airplane engine from which the MSR/Lftr is inspired.


IMHO, the one thing that is glossed over in discussions about the PB-AHTR is recycling/reprocessing of its TRISO format based seed and blanket pebbles. This reprocessing job will be especially difficult because the TRISO fuel format is very rugged and specifically engineered to be sequestered in underground storage for a minimum of 1,000,000 years in a once through uranium fuel cycle, with the pebble design made so robust that it serves as a containment structure for long term nuclear waste storage.



TRISO fuel is generally considered highly proliferation resistant because of the technical difficultly in getting at its fissile component. Furthermore, in this time when nuclear waste is to be minimized, the TRISO fuel format does just the opposite.


Because of all this above, the use of the TRISO fuel format just does not fit well into a closed fuel cycle systems engineering model; I contend that the economics of a solid fueled liquid cooled reactor can be improved upon by looking at other pebble fuel formats that are more compatible with nuclear fuel breeding and waste processing.


The technical challenge of recycling TRISO fuel requires shipment of PB-AHTR fuel off the local reactor site to and from a central depot where a large amount of expensive equipment is required to reprocess and prefabricate TRISO fuel.


A top priority in a solid fuel reactor design is to make the associated fuel format simple enough to allow fuel fabrication as well as waste handling possible on site local at the reactor, in the spirit of the Lftr.


Another downside in the use of TRISO fuel is the large design overhead in the recycling and inspection of TRISO pebbles during on-line operations. If this large amount of specialized equipment could be simplified or eliminated, the cost of the solid fueled salt cooled reactor could be greatly reduced.


On the other hand, the PB-AHTR does have many exciting and innovative design characteristics that solve some intractable and long standing problems in two fluid breeder designs, specifically the elimination of the core blanket barrier.


It is also very small with a high power density and it uses inexpensive nuclear industry standard material in its construction. This alone makes it cost effective as compared to other current reactor designs.


I think that it is possible to discard the costly downsides of this design but retain the most cost effective aspects of this concept.


I also think I see a way to meet or exceed all these high temperature solid fueled thermal breeder reactor priorities and objectives in the context of the Lithium Homogenous Reactor concept as improved by using the aerogel pellet slurry nuclear fuel format.


This thread will describe the aerogel pellet slurry nuclear fuel format and list the advantages it has in solid fueled base breeding and nuclear waste isolation and processing.

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PostPosted: Jul 16, 2010 4:11 pm 
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Step 1: What is aerogel

Aerogel is the lowest bulk density of any known porous solid that has so far been manufactured. It is nicknamed frozen smoke solid smoke, solid air or blue smoke due to its translucent nature and the way light scatters in the material; however, it feels like expanded polystyrene (styrofoam) to the touch. Its geodesic like internal structure makes it strong and resilient in the extreme.

it is very strong structurally. Its impressive load bearing abilities are due to the dendritic microstructure, in which spherical particles of average size 2–5 nm are fused together into clusters. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores just under 100 nm. The average size and density of the pores can be controlled during the manufacturing process.


Image
A 2.5 kg brick is supported by a piece of aerogel weighing only 2 grams


Like foam, aerogel can be open cell or closed cell in nature. I am interested in open cell aerogel. The most common aerogel constituent material is silicon dioxide. This stuff is use by NASA for many aerospace applications.

Image

But more recently, many other types of aerogels have been developed. Of most interest to the aerogel fuel format is rare earths and metals like thorium. Using aerogel, a metal structure can be made so light, feathery and vacuous that is can be made to float on water.

Also multi-stage manufacturing procedures exist that can infuse pockets of gas in the aerogel if required.

Of particular interest is something called FOGBANK. This is a code name given to an aerogel based material made from beryllium and boron used in nuclear weapons such as the W76, W78 and W80.

Unclasified Department of Energy Nuclear Explosive Safety documents simply describe it as a material "used in nuclear weapons and nuclear explosives" along with lithium hydride (LiH) and lithium deuteride (LiD), beryllium (Be), uranium hydride (UH3), and plutonium hydride”


It was originally manufactured in Facility 9404-11 of the Y-12 National Security Complex in Oak Ridge, Tennessee from 1975 until 1989, when the final batch of W76 warheads were completed. After that the facility was mothballed, and finally slated for decommissioning by 1993. Only a small pilot plant was left, which had been used to produce small batches of FOGBANK for testing purposes.


So it seems to me that current Aerogel manufacturing technology makes fabrication of aerogels made of alloys of thorium, beryllium and U233 to serve as 100 micron pellets that can be made feathery enough to float within lithium deuteride reactor coolant and yet strong enough to withstand years of operations and neutron damage inside a reactor.

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PostPosted: Jul 16, 2010 6:56 pm 
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Step 1: What are the design advantages of the aerogel pebble


I intent for the aerogel pebble concept to be an improvement over the Nano-grain pellet design proposed by Liviu Popa-Simil.


I am greatly taken by the simplicity and the elegance of the Popa-Simil pellet concept and think that it solves many liquid reactor based waste problems that are regular points of discussion on this forum.


The fission recoil separation effect (FRSE) makes nuclear waste handling during on-line reactor operations easy so its preservation and optimal application is a top design priority. What make FRSE function is to have a nuclear fuel and thorium form factor configured and confined in a structure that is about one nanometer or slightly more in thickness. This allows the release of both nuclear waste and protactinium into the surrounding liquid lithium medium highly probable after an atomic event.


The dendritic microstructure in the aerogel pebble, in which spherical particles of average size 2–5 nm are fused together into clusters allows for this very small nano dimensionality to be uniformly achieved.

Attachment:
nanofoam.jpg
nanofoam.jpg [ 671.84 KiB | Viewed 3974 times ]



The manufacture of aerogel pebbles is far more straightforward being amenable to volume production and therefore vastly more inexpensive compared to a complex structure like the Nano-grain pellet.


Aerogel manufacture is based on chemistry whereas the Nano-grain pellet would be manufactured using more expensive micro production technology.


Aerogel takes on the material character of its continuant material. Admixture of fissile and fertile nano-particularized metals can be alloyed together to provide structure that is more impact resistant structure that standard aerogel. Because standard aerogel is made of silicon dioxide, pressing firmly enough will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass—a property known as friability.

For example, metal foam made of metals like steel or aluminum can be used to cushion the impact of an auto collision.


In addition, the aero pellet is open in all directions to the flow of lithium coolant throughout its entire volume. All its dendritic walls both on its outside as well as deep within will be constantly bathed by liquid lithium. This constant coolant flow will carry waste both solid and gaseous, protactinium in pure elemental form immediately after its production, deuterium nuclear moderator to and from, and heat from the interior of the aerogel pellet.


Its huge relative surface area will allow the aerogel pellet to be buoyed on the currents of lithium coolant to support both nuclear waste removal and straightforward pellet segregation from the lithium flow in the support of damage inspection, and characterization through the use of a size graduated screening regime that is typically found in common use today at road gravel production plants.


The ease of this inspection mechanism relative to moving and inspecting aerogel pellets will eliminate the cost that would be incurred doing a like function in the PB-AHTR reactor.


Much PB-AHTR equipment and cost can be eliminated together with all the difficult issues associated with the movement and inspection of the TRISO pellet.


Along another line, I have always considered the LHR to be and upscale version of the originally designed Hyperion reactor. The design intent is to maintain all the negative void characteristics and self regulation characterize by the Hyperion hydride design. But in this Hyperion thorium design embodiment, thorium hydride slurry just sits in place unmoving where its fissile content is gradually consumed to burnout and waste accumulates inside the slurry.


But in the Lithium Homogeneous Reactor (LHR), waste removal is an ongoing on-line function and new fuel can be added as needed to maintain criticality. An added bonus is optimized protactinium breeding and off-line transmutation to fissile U233 all done in the context of a deeply thermal and optimized neutron thorium breeding spectrum and the highest achievable U233 breeding ratio equal to that of the Aqueous Homogeneous Reactor (AHR).

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PostPosted: Jul 17, 2010 11:45 am 
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Aerogel pebbles are proliferation resistant



One of the real strengths of the PB-AHTR is its very high degree of resistance to proliferation. This proliferation resistance is maintained by aerogel pebbles.

This resistance is centered on multiple proliferation barriers as follows:

    1. A very high bulk factor relative to the fissile content makes accumulation of fissile constituent material content difficult and detectable. The mass of the fissile content in the pebble is very small compared to the total weight of the pebble. One would need to accumulate many tons of pebbles to get access to a dangerous/critical amount of fissile material.


    2. Pebbles support accounting procedures done by regulators. They can count pebbles one at a time and write the total down in an accounting record. If some pebbles are missing the regulators will immediately know it.

    3. Since U233 is alloyed with thorium and beryllium in the aerogel pebble, complex and dangerous chemical processes are required to extract the fissile content from the pebble. This process will expose the perpetrator to self protecting U232/U233 for a fatal period. Nations might try this sort of thing but this is outside the capability of sub-national groups.


    4. It takes a great deal of technical knowhow and easily controlled and detectable specialized equipment to manufacture aerogel pebbles. The aerogel based reactor will function only using aerogel pebbles. So the reactor cannot be diverted by substituting other types of material in a clandestine weapons program.

In conclusion, the aerogel pebble as used in the Lithium Homogeneous Reactor (LHR), retains all these TRISO pebble type proliferation barriers.

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PostPosted: Jul 19, 2010 4:03 pm 
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Regarding aerogel pellet structure melt down.



IMHO, the melting point of solid nuclear fuel can’t ever be high enough. One way to increase the melting point of the thorium/uranium /beryllium alloy that forms the dendritic structure of the aerogel pebble is to use the metal oxide compounds of these metals. For example, this should push the melting point within the interior of the aerogel pellet to beyond 2000C.

The inner workings inside the aerogel pellet are a complicated subject, on the order of a nano sized nuclear reactor in its own right.

Unlike a TRISO pebble, the lithium inside the aerogel pellet flows with little viscosity and near its boiling point it is extremely low. Its thermal conductivity also increases greatly with temperature and is at its maximum near boiling. It fact, it can even vaporize. All this is very good for the thermodynamic behavior of the aerogel pebble.

When liquid lithium vaporizes inside the aerogel pellet, this vapor produces less moderation than the liquid form. The nuclear reaction will stop for a short period until the vaporized volume becomes a liquid again. The heat of vaporization of liquid lithium is very high.

For example, the heat of lithium vaporization: is 147.1 kJ /mol compared to its specific heat capacity of 3.58 J/ g- K..


Little heat transfer will follow the metal oxide dendritic structure of the aerogel pebble. Almost all the heat will be carried away by the high conductivity of the liquid lithium.

So at this time it looks to me that the lithium boil off mechanism will stop the aerogel pellet structure from melting down when fission occurs in a self controlling negative void behavior.

Because of this self controlling negative void behavior in the interior of the aerogel pebble, it also seems to me that the temperature inside the aerogel pellet will never exceed 1450 C, the vaporization temperature of lithium.

In addition, if a direct energy conversion of fission energy to electric power is also used, this will reduce the heal load on the aerogel pebble substantially.

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PostPosted: Aug 02, 2010 2:18 pm 
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I have found a reactor design that uses lithium hydride and pebble micro slurry that has been considered in depth by Department of Energy's Office of Naval Reactors (DOE-NR) and NASA and/or the DOD and are still considered viable.

The MITEE family of compact ultra light weight nuclear thermal propulsion engines is designed for planetary transport and for exploration and commercialization of space.

But the MITEE and its predecessor the NOTV-PBR Concept still needs to be evaluated in real mission scenarios since this system's future home base of operation would be designed primarily for spacecraft power and propulsion. MITEE was derived from work done earlier on Particle Bed Reactor (PBR) which uses 7LiH (lithium 7 hydride) moderator with high power density (30 MW/Liter) particle bed fuel elements.

I bet Kirk has worked on this design during his time of employment at NASA or at least hear about it.

You don’t hear much about reactor designs like this as follows:
Quote:
DOE-NR: While the U.S. is committed to developing this technology for civilian uses, it could be used by others for military purposes. It is in our national security interest to protect those specific details that would enable a terrorist or an unfriendly nation to use this technology. Protection of this technology under the International Traffic in Arms Regulations and other applicable laws and regulations is appropriate.

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PostPosted: Aug 05, 2010 6:04 am 
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A homogenous reactor is, by my understanding, a reactor with fluid fuel which will let gaseous neutron poisons from fission product escape. This could be molten salts, aquous solution or metal eutectic solution. Can LiH form the base for metallic fuel???


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PostPosted: Aug 05, 2010 8:37 am 
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jagdish wrote:
A homogenous reactor is, by my understanding, a reactor with fluid fuel which will let gaseous neutron poisons from fission product escape. This could be molten salts, aquous solution or metal eutectic solution. Can LiH form the base for metallic fuel???



The duck test: "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."

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