Liquid Co2 reactor cooling

tahanson43206 · 2020-09-13 09:43:13

For SpaceNut re #24

Thank you for your continued interest in and support of this (in my view) important subtopic

Please provide a reference for the statement in Post #24 ... I would be astonished if there is any actual data to support such a belief.

However, I recognize your proven search skills, so you may be able to find some.

Failing in that, it seems to me highly likely that the CO2 collecting at the poles of Mars is as pure and as free of contaminants as snow falling in Earth's poles.

I note that there ** is ** material collected from the atmosphere by freezing water as it descends as snow. In fact (my understanding is) that water droplets form around tiny bits of material floating in the atmosphere.

What this discussion would appear to call for is a probe designed specifically for the purpose of examining the dry ice accumulation at both poles.

However, as I read Calliban's explanation of his design for a "natural" Uranium reactor, the presence of material in the dry ice (other than CO2) is NOT a problem for his design, because the process of phase change itself liberates the CO2 and leaves any contaminants behind.

Please provide a link to any reports you may find that show the dry ice at the poles is anything other than "pure as the driven snow".

(th)

Calliban · 2020-09-13 11:49:02

Carbon dioxide is non-polar. It is quite an effective solvent for many organic compounds, but is a poor solvent for ionic salts. Solid dust contaminants can easily be filtered out by drawing the liquid CO2 through a metal mesh.

However, as noted previously, it may be better to cool the reactor using a closed pressurised water loop. CO2 at 30C would have a vapour pressure of 80bar. Water at 100C has a vapour pressure of 1bar. So it may make more sense to boil the CO2 in a secondary boiler, as the aluminium pressure tubes would otherwise need to be much thicker.

The efficacy of this design does depend upon the availability of natural uranium on Mars. If it is not available in concentrated ores, then nuclear power would depend initially on fissile fuels imported from Earth. Assuming it is available, a graphite moderated, pressurised water reactor should be quite easy to build. The primary water cooled side could work using natural circulation, such that no coolant pumps are needed. The secondary boilers would be filled with liquid CO2 injected from the retorts. The secondary boiler tubes would be steel. Corrosion will be controlled by injecting hydrogen to keep oxygen at low levels. On the CO2 side, corrosion would be less of a problem due to the absence of oxygen and water vapour.

Last edited by Calliban (2020-09-13 12:03:37)

tahanson43206 · 2020-09-13 12:21:34

For Calliban ...

Thanks for your work on this (to me very interesting) idea ...

Could you add some tags to your posts?

You can thus make it easier for future searchers to find your work.

In addition, it might be possible for you to register your design with one of the "Free Patent" sites, if you want to increase chances future reactor designers will find your work.

***
As you point out, the efficacy of your design depends upon availability of Uranium.

This forum contains discussion of breeder reactors that would transform Thorium into useful material, and I wouldn't be surprised if you had contributed many of the posts I am remembering.

To make all that work useful (in my opinion) it needs to be indexed in some way, even if the index is no more sophisticated than adding a tag or two.

GW Johnson has demonstrated what that would look like ... if you enter a search for "educationdoneright" with author "GW Johnson", you'll find his set of posts on that specific topic.

(th)

Last edited by tahanson43206 (2020-09-13 12:22:11)

SpaceNut · 2020-09-13 18:36:36

Any dirt or contaminant getting into the boiler pot with the rods will be a problem as the rods must have liquid flow around all surfaces and when the dirt builds up the heat transfer from the covered part of the rods will heat up and become damaged. That is bad even here on earth....This includes water ice in and around the rods as the heat which is there will provide an gap to happen around the rods with no flow of the water to wick the heat away with....

Calliban · 2020-09-15 07:17:34

A first model natively constructed Martian nuclear reactor, may look something like this.
https://en.m.wikipedia.org/wiki/B_Reactor

The original Hanford piles were used to produce plutonium for nuclear weapons production between the mid-1940s to the late 1960s. The waste heat from B Reactor, was not used to produce electricity and was dumped into the Columbia River. The reactor was graphite moderated, with water cooled aluminium fuel channels. The water emerging from the channels had a temperature lower than boiling point, which prevented void reactivity issues. The fuel was aluminium clad uranium metal slugs, which was pushed through fuel channels by hand, using a pole. Spent fuel dropped out the other end into a pond, where it was retrieved for reprocessing. A more elegant design would be a cylindrical pile, with fuel dropping into a central plenum. That way fuel with a higher burn up will be closer to the centre of the core, which will help flatten the flux profile. Not something that was an issue for B Reactor, as they weren't interested in optimising fuel burnup.

B Reactor generated some 250MW of heat in an 1100 tonne graphite moderator block. Not a bad power density for an early graphite moderated reactor. In our design, we would pass the hot water into a boiler, in which liquid CO2 would boil and be superheated to ~100C before passing through a turbine and then into the retorts.

A similar reactor could be built closer to the equator, using a closed low-pressure steam cycle and dumping waste heat into habitats and greenhouses.

Spent fuel would be stored initially in water ponds and would then be dissolved in nitric acid. The actinide nitrates would then directly fuel aqueous homogeneous reactors. These would have substantially greater power density than graphite moderated reactors, but would require the use of stainless steels for reactor vessels and heat exchangers, making them a longer term project.

Strontium fission product, when it is available, could be used to fuel RTGs powering the fusion drive spacecraft that Kbd has described. It would be some time before enough would be available for this purpose. Ceasium-137 would find uses in remote ground based heat sources on the surface of Mars. On Mars, nuclear waste would be a heat generating resource.

This is how nuclear power would develop on Mars if it proves difficult to import enriched fissile fuels from Earth. If enriched fuel is available, things are much simpler and small boiling water reactors may be a better way to go.

Last edited by Calliban (2020-09-15 07:31:35)

tahanson43206 · 2020-09-15 08:37:27

For Calliban re #30

Thanks for the link to that Wikipedia article on the history of the reactors at the Hanford facility! The details of their construction, the safety measures put into place, and the eventual decommissioning were new to me, and i appreciate the opportunity to learn about them.

As a follow up to your suggestion this might be a model for an early design at Mars, I'm curious about how you would plan for use of the reactor. The Hanford reactors were purpose built to make bomb material, so that part of the model seems (to me at least) a poor match for the Mars situation. On the other hand, production of fissionable material for reactors of other designs might be a reasonable application.

This topic is set up (I think) to develop a way of producing power at the poles of Mars, by melting dry ice and capturing energy from the expansion of CO2 as a gas. In that scenario, with the Hanford reactor design as a reference, I'd be interested in knowing what changes (if any) you would make to optimize performance of the reactor for power production.

Other benefits of of operation of the reactor would be interesting.

Finally, would you expect the reactor to be able to stay in operation for an extended period? Would the materials eventually deteriorate to the point the facility would have to be decommissioned in a manner similar to the Hanford ones?

Is there any way to plan ahead to make use of any and all radioactive products? On Mars, as was illustrated by "The Martian", "retired" nuclear fuel can provide useful heat for an extended period. Can that be done safely, to heat homes, for example?

There's a lot of potential for this topic to develop into working plans.

(th)

Calliban · 2020-09-15 14:01:13

In terms of what would change and what would stay the same for a Mars built reactor, I would anticipate the following.

To improve neutron economy, I would make the graphite core cylindrical or spherical, with fuel channels ending in a chute at the centre of the core, through which they would drop down into a pond. I would take advantage of the improved neutron economy by including a number of thorium slugs within the fuel channels. This allows early production of 233U, which has better conversion ratio in aqueous homogenous reactors. The production of 233U therefore allows nuclear capacity to increase more rapidly.

The most significant difference I can see is that we will be building the reactor as a heat source. Rather than discharging the water and heat into a river, we would operate a closed cycle. If we can construct the reactor close to a source of dry ice, then we have the significant advantage that hot water at temperatures <100C can be used to generate power at a respectable efficiency (~20%). We would do this by pumping hot water through a heat exchanger in a boiler, where liquid CO2 would be superheated into supercritical gas. If we are away from the poles, then we would need to generate power using a low pressure steam turbine. The hot water would be pumped into a steam separator tank and would boil at <100C at <1bar. The steam would flow into a low pressure turbine, generating power. The condenser would condense the spent steam back into water at a temperature of about 30C and it would be returned to the steam separator tank. The low temperature heat from the condenser could be used for habitat heating. Total thermal efficiency would be about 10%. Not great, but we get a lot of low grade heat for things like habitat heating, agriculture, melting ice, distilling water, making adobe bricks, etc.

I would probably use ordinary carbon steel as control rod material. The absorption cross section of iron is 50x lower than boron, but the amount of excess reactivity in a natural uranium reactor with thorium inclusions will be low anyway. Another option is to control reactivity by removing reflection into the core and allowing neutrons to stream into steel baffle plates. For a cylindrical core, fuel would be pushed in around the circumference, whereas control rods would pass through vertical tubes through the top. For a spherical core, they would need to be distributed across the surface of the sphere.

Burnup was something that Hanford wanted to minimise, in order to avoid building up 240Pu at higher actinides in the spent fuel. On Mars, we would not have the same priority. One of the reasons I propose a cylindrical or spherical core, is that as fuel slugs burn up and 235U becomes more depleted, they are progressively pushed towards the centre of the core where flux levels are greater. So our reactor would use less uranium per MWh than B Reactor and the the slugs would contain more higher actinides and less 235U when the drop into the fuel pond. We don't really care, because we aren't building bombs.

Building a spherical core may allow a smaller practical size than the Hanford core. This may be valuable for an initial colony, which may not need 250MW of thermal energy, at least initially. The first reactor would likely be a core quite close to minimum critical limits.

Last edited by Calliban (2020-09-15 14:32:49)

tahanson43206 · 2020-09-15 15:21:38

For Calliban re #32 (and topic in general)

SearchTerm:ReactorDesign for Mars
SearchTerm:DesignReactor for Mars

I've set up this post in hopes your thoughts will continue to refine your vision, and (if I have the energy to keep up) it can provide a collection point.

I'd like to open with the first item on the (presumed agenda) ...

Acquiring graphic for this project on Mars.

Do you know of a way to go from CO2 (which is abundant) to graphite?

There may be a simple industrial process, and this topic is a good place to summarize it for future planners.

(th)

Calliban · 2020-09-15 16:29:27

Pyrolysis of methane could be used to produce solid carbon. Methane would be produced using the gas shift reaction with Martian CO2.
https://en.m.wikipedia.org/wiki/Pyrolytic_carbon

To produce green moderator bricks, the pyrolytic carbon dust would be mixed with an organic glue and compressed into a mold. They would then be heated to temperatures in excess of 1000C to pyrolyse the binder, leaving a pure graphite brick. We would need at least a few hundred tonnes of these bricks to build a minimum critical reactor.

Aluminium may be more difficult to acquire. The colonists would need to find a source of relatively pure aluminium oxide and reduce it to aluminium in an electrolysis cell. The quantities needed would be much lower than the amount of graphite. Maybe 10 tonnes.

This article is speculative, but it does suggest that we should be able to find uranium ores on Mars.
https://centaurisky.blogspot.com/2017/0 … nd-it.html

Last edited by Calliban (2020-09-15 16:56:51)

Calliban · 2020-09-15 17:28:28

One interesting application for low grade heat on Mars, could be the production of heavy water. On Mars, deuterium is five times more abundant in water than it is on Earth. On Earth, HDO is typically produced using distillation. HDO boils at a slightly higher temperature than does H2O, due to its slightly greater molecular mass. On Mars, the easy availability of vacuum, makes it possible to boil water at temperatures as low as 30C (4.25KPa). In addition to requiring five times less energy to produce on Mars, deuterium production can take place using lower grade heat.

An aqueous homogeneous reactor using natural uranium dissolved in heavy water, would need about 100 tonnes of D2O to form a minimum critical assembly. Whilst the first reactor built on Mars may be graphite moderated, it is likely to be followed by heavy water reactors as soon as sufficient D2O is available.

Aqueous homogenous reactors have suffered from corrosion problems in the past due the use of sulphate fuels and the high concentration of oxygen ions in the water. One way of avoiding these problems is to use nitrate fuels, to dissolve excess deuterium gas into the heavy water and to keep temperatures relatively low. To keep costs down, it may be possible to use a reactor vessel composed of high silicon cast iron. Something we can more easily make on Mars.

Last edited by Calliban (2020-09-15 17:44:40)

SpaceNut · 2020-09-15 17:38:42

Since we would not want that water source for a crews drinking we are then left with a material that can only be used for that purpose with power generation within the Mars reactor.

So how do we get from nothing and use insitu materials to make this mars reactor?

So if we can make use of a ship which is not being used by a crew and is empty of cargo tat will not return to earth we have some materials and possibly the means to leverage that mars first reactor build by recycling...

We also have a challenge in gathering the raw clean co2 in any form to make use of in the making of graphite moderator rods as well as for the cooling fluid for the internal use of the reactor core and energy creating process.

.

Calliban · 2020-09-15 17:58:16

If the cost of shipping materials to Mars drops to $100/kg, say, then it would be simpler just to import a few hundred tonnes of D2O and perhaps a few tens of tonnes of aluminium from Earth. If uranium can be imported as well, then it is quite easy to build a crude boiling water reactor that will generate a few MW of electric power for a colony. The things that stand in the way of this vision, are the legal bureaucracy that cripples all things nuclear here on Earth. Whilst it is doubtful that an operating licence would be needed for a reactor on another planet, there are all sorts of legal obstacles that would need to be traversed before one could load low enriched uranium fuel pellets onto a rocket.

Last edited by Calliban (2020-09-15 18:05:12)

SpaceNut · 2020-09-15 18:07:58

Part of why Nasa is going with only a 10kw reactor to start out with called KRUSTY of which they have a prototype that did produce power at 1kw with the remaining design making heat....

Calliban · 2020-09-15 19:35:05

One of Canada's first nuclear reactors and the forerunner of the CANDU reactor.
https://en.m.wikipedia.org/wiki/NRX

It produced 42MW of thermal power using some 14 tonnes of heavy water and was of aluminium construction, using aluminium clad 30cm diameter, natural uranium metal bars as fuel. This sort of reactor we could quite easily build on Mars if heavy water and aluminium were supplied from Earth. Given that 1kg of natural uranium releases 500GJ of thermal energy, we could afford to import that as well, at least initially. This reactor is powerful enough to power a large base or small city.

Last edited by Calliban (2020-09-15 19:36:07)

tahanson43206 · 2020-09-16 07:40:46

For Calliban re recent discussion ....

It is encouraging (to me for sure) to see the conversation trending toward "real world" concepts that could be turned into action plans on Mars.

I'll follow the link you provided when I get to a more modern computer.

In the mean time, I'd like to toss out the suggestion that a group wanting to build a reactor along these lines could save a lot of time if engineering specifications are available.

Also ... (before I forget again) .... I noticed that the Hanford reactors were built upon a massive block of concrete (as I recall the Wikipedia article).

One of your recent posts suggested mounting the reactor above a safety pool of water. There is quite a difference in the two foundations.

Is that difference accounted for in a part of your concept I missed, or (perhaps) that you did not have time to include on the first pass?

My understanding is that fissionable material is on the heavy side, which might explain why the World War II engineers decided to build such a massive foundation. But they probably weren't thinking ahead to safety issues, let alone long term sequestration of fission byproducts, as you seem to be doing.

(th)

SpaceNut · 2020-09-16 18:52:14

I should not be surprised that like minds think of such products as here is The application of supercritical CO2 in nuclear engineering: A review

Sort of a typical design for sodium

How to Cool a Nuclear Reactor

Of course getting to the level of co2 is still something to improve on.

https://www.imperial.ac.uk/media/imperi … m-BP-8.pdf

Supercritical CO2Brayton Cycle Control Strategy for Autonomous Liquid Metal-Cooled Reactor

Supercritical CO2-cooled micro modular reactor] Portable size...

SpaceNut · 2021-01-24 17:35:52

Time for a revival of topic in that we did not determine the quantity or process that shows the lowest energy levels to achieve a volume for use in cooling of a nuclear power source.
Of course the volume will be dependent on the size of the reactor to which its important to keep the insitu shielding is a berm ….

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#26 2020-09-13 09:43:13

Re: Liquid Co2 reactor cooling

#27 2020-09-13 11:49:02

Re: Liquid Co2 reactor cooling

#28 2020-09-13 12:21:34

Re: Liquid Co2 reactor cooling

#29 2020-09-13 18:36:36

Re: Liquid Co2 reactor cooling

#30 2020-09-15 07:17:34

Re: Liquid Co2 reactor cooling

#31 2020-09-15 08:37:27

Re: Liquid Co2 reactor cooling

#32 2020-09-15 14:01:13

Re: Liquid Co2 reactor cooling

#33 2020-09-15 15:21:38

Re: Liquid Co2 reactor cooling

#34 2020-09-15 16:29:27

Re: Liquid Co2 reactor cooling

#35 2020-09-15 17:28:28

Re: Liquid Co2 reactor cooling

#36 2020-09-15 17:38:42

Re: Liquid Co2 reactor cooling

#37 2020-09-15 17:58:16

Re: Liquid Co2 reactor cooling

#38 2020-09-15 18:07:58

Re: Liquid Co2 reactor cooling

#39 2020-09-15 19:35:05

Re: Liquid Co2 reactor cooling

#40 2020-09-16 07:40:46

Re: Liquid Co2 reactor cooling

#41 2020-09-16 18:52:14

Re: Liquid Co2 reactor cooling

#42 2021-01-24 17:35:52

Re: Liquid Co2 reactor cooling

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