How To Build A Nuke Plant
 

Several years ago I read an interesting report from the IAEA about operational experience from unconventional nuclear power plants that have been built around the world, which basically means molten salt or liquid metal cooled reactors. One thing that stood out was the number of problems with pumps and heat exchangers. In fact with one exception, every sodium cooled plant ever built has had a leak with resulting molten sodium fire. And with one exception, every piece of equipment in the loop has had a leak and fire. The exception being the reactor vessel itself, none of which have ever leaked. While little to no radiation was released, this is beyond bad optics.

With that in mind, and with the added goals of greatly reduced fissionable inventory and an order of magnitude reduction in size and complexity, there is an interesting option.

Back around the early 1900's, internal combustion engines took over the market from steam engines primarily due to about a 50 times reduction in size and weight. While the engines themselves were not much lighter, internal combustion engines don't need a boiler. Instead they get the heat transfered by direct contact between the fire and the working gas. Applying the same notion to nuclear power requires a very large surface area between the hot nuclear fuel and the working gas.

Translated to hardware it looks like this: A molten salt reactor with an output of around 750 MW at 1300 F. Molten salt passes upward through the reactor and into the float bowl of a large updraft carburetor with a 3 ft diameter venturi. Approximately 700 cubic feet per second of argon from a centrifugal compressor picks up a 20 cubic foot per second spray of salt passing through the venturi. The salt is then removed by a panel of vortex tubes similar to the ones used on helicopter and tank turbines. The still liquid salt then flows into a downcomer back to the bottom of the reactor with no circulation pump required, while the heated argon passes through a centrifugal turbine for about 250 MW of power production. The cooled and expanded argon then passes through a cycle cooler and back to the compressor inlet. While a 1% slipstream of salt spray does carry over through the cyclone tubes, the primary loop is built to handle it.

A radial flow compressor and turbine are used due to the very simple and rugged shape, and because of the bearings. Multi-nozzle aerostatic bearings supplied by high pressure argon can easily handle the load and speed of a turbine this size. While this type of bearing does require several thousand horsepower of pumping power, that is acceptable at less than 1% of the power output. Shaft seals are non contact labrynth seals using argon flush gas. Power takeoff is via a second argon compressor wheel mounted between the bearings. So it looks like an extra wide 12 foot high turbocharger. The only materials involved are fuel salt, austenitic stainless steel and hastelloy-n, and argon. And nothing touches the turbine and shaft except gas, which means the service life can be measured in centuries.

The power takeoff compressor passes about as much flow as the primary loop, and delivers power to a remote turbine in the form of pressure times flow rate. The power takeoff flow also picks up heat from the cycle cooler on the way, cooling the primary loop while acting as the bottoming cycle for a significant increase in thermal efficiency. This allows use of a simple compact single stage primary loop while still providing good fuel efficiency.

A power plant of this type could operate with a very small amount of fuel, potentially less than 100 lb of fissionables. The neutron counter types can design any arrangement of reactor they like as long as it puts out 750 MW of hot salt. Molten salt reactors tend to be constant temperature load following devices, so nothing special is required there. Every component is a well understood time tested design that has been extensively used in similar applications. Every part of the plant can be accessed for maintenance or replacement with nothing buried in concrete. And the overall plant size is dramatically smaller and cheaper than existing designs. Of course everyone's head would explode at NRC and it would take about 100 years to get so much as a test rig approved. But it would work, and do it efficiently and safely.

I'm missing something in your pivot from molten sodium to salt. What type of salt? Nitrate?

According to Wikipedia, fluoride is the anion of choice because fluoride salts are poorly soluble, stable in their molten state, don't become radioactive under bombardment, and moderate neutrons well. Apparently chlorides are less desirable because they have to be pure Cl³⁷ to prevent the buildup of dangerous and unstable SCl₄.

I know there have been some molten salt reactors and there was one at Oak Ridge National Labs in the 60s. The problem sited for not using them is corrosion, but materials science is way better than it was in the 1960s.

Everything old is new again. This one's using fluoride salt:
https://tennesseelookout.com/2023/12...clear-reactor/

This is mostly a heat exchange design. I make no pretensions of being a neutron counter. The salt would be a compound of fluorine, lithium, and beryllium because:

• It doesn't burn on exposure to air
• It has the right viscosity, density, and specific heat properties to work in a carburetor
• It has the right atomic properties to work in a reactor

Materials would be less of a challenge because:

• 40 years of R&D
• No hot heat exchanger
• The operating temperature is right in sweet spot for stainless steel
• The venturi and vortex tubes which handle high speed salt flow can be made of ceramic
• And mainly, every single component is accessible for replacement and there aren't that many of them

You may note that there still is a big whopping heat exchanger, which is there because I can't weasel out of it. However it is less of a problem because:

• It is on the cool side of the loop (800 F)
• It operates at about half the pressure as the hot side of the loop (400 psi)
• It mostly handles argon which is an inert noble gas
• Cool gas is denser than hot gas, plus about a third of the heat gets converted to work in the turbine, so it would be less than half the size of a hot heat exchanger
• Nearly all of the salt slipstream can be removed with a hot electrostatic precipitator located between the turbine exhaust and the heat exchanger inlet
• Nothing catastrophic happens if it does spring a leak

The Kairos design is a solid fueled salt cooled reactor with a moderate output temperature of 1200 F. They made an interesting design choice by using a steam turbine operating at around 2800 psi for energy conversion. That gets them higher thermal efficiency compared to a conventional light water reactor which operates closer to 650 F and 1100 psi. But thats a lot of pressure, damn near supercritical. They are building on a lot of prior research there, but lets just say they know the tradeoffs better than I do. The other way of solving the problem is a gas turbine which allows efficient temperature with more reasonable pressure.

I was speaking more to the return of salt cooled reactors re: Wolf's mention of them falling out of favor because of corrosion, not making a direct comparison to your design concept, Jag. Though on 2nd read I'm not sure if Wolf was referencing cooling with the salts.

Research on molten salt reactors in the 60's culminated in a reference design for a 1000 MW breeder reactor using circulating fuel salt moderated by graphite. A secondary salt heat transfer loop circulated to a rack of high pressure steam generators feeding a steam turbine. Onsite fuel processing was planned. Primary outstanding questions at the conclusion of the study centered around reprocessing, tritium handling, remote maintenance, and long term neutron irradiation of structural materials (at the astounding rate of around 2 1/2 quadrillion zoomies per second per square inch for 30 years). Corrosion was pretty much a so!ved problem by then, although erosion remained a concern in high velocity locations.

Feel free to raise questions and concerns, BTW. This is a mental chew toy at this stage, so go ahead and chomp.

Fun Fact: the Three Mile Island thing happened on my third birthday :science:

Over my head, not that it's not interesting.

Expanding a bit more on the logic that leads to this design, power plants take a monumental amount of heat to operate. A typical coal fired boiler looks like a hollow six story building with a carefully managed white hot ball of fire inside. The goal is to convert as much of that as possible into take-home twist using the first law of thermodynamics.

A heat engine runs off a flow of heat just like a water wheel runs off a flow of water. What is often called waste heat is totally necessary or the thing won't run. Just like if you took a water wheel and built a lock downstream to prevent any "waste" water from leaving. Soon downstream is at the same level as upstream and the wheel is stalled. The goal for fuel efficiency is to make the hot side as hot as possible and the cold side as cool as possible, just like a larger drop will get more work per gallon of water.

Every heat engine operates off a compressible fluid of some kind whether it is gas, steam, or (if they ever get the bugs worked out) supercritical fluid. They all work because it takes less work to compress cool gas than you get from expanding hot gas. And unless you are using outside air or water as your working fluid, that means heat exchangers (boooo, hisss). The standard example is the steam engine, where the working fluid circulates through the boiler, the engine, and the condenser. As compared with the gasoline engine where the working fluid gets drawn in from an ocean of air, compressed-heated-expanded, and then exhausted with no need for heat exchangers. The radiator is to keep the engine from melting and theoretically is not strictly necessary.

Unfortunately, air is around 200 times less effective than water for heating and cooling, and more like 4000 times less effective than steam. Which means that if you are going to use gas as your working fluid in a closed loop cycle, your heat exchangers get to be the largest and spendiest items in the plant. Plus they require high tech alloys that are a bastard to weld, but yet have have miles of welds that can't be allowed to leak, and they tend to be built into the center of the plant. When you need one, you need one. But we hates them, yes we do.

Steam plants work just fine but are limited to lower temperature due to steam pressure which is currently limited to around 1200 psi. Also the cooling towers on a typical steam plant draw around 50,000 gallons per minute of fresh potable water to spray on theoutside of the cooling pipes. Thats the steam you see floating away, and is enough to supply a fair size town. In desert areas thats a problem.

The Chinese are going to solve the problem by using a molten salt reactor to provide heat to a standard off the shelf gas turbine straight out of a regular old peaking plant. Only one heat exchanger but its a doozy. Plus that is an open cycle which is only possible because they don't have Sierra Club dipshits diving in front of dozers. But what if we could do better and use a closed cycle too? And thats how we get to this approach.

BTW, supercritical fluids really are a lot different. Above 3200 psi, it is actually possible to have an underwater fire.

Instead we use a noble gas. Helium is the perfect stuff from every angle except long term availability. Helium is co-produced from natural gas wells and is generally released during use. And then floats to the top of the atmosphere and gets blown away by solar wind. However 1% of the atmosphere is argon. It catches too many neutrons to be used directly in a core but otherwise works just fine and we'll never run out. A noble gas means no corrosion or fouling or chemical problems with the salt.

Another thing that should be spelled out is proliferation, aka keeping the fissionables away from the bad guys. There are 114 different elements in the table so far, but more than 2600 known isotopes. An isotope is an atom with extra neutrons. For example hydrogen, deuterium, and tritium are chemically nearly identical but the weight is different. Chemical separation is difficult but doable with commercially available technology. Isotope separation is so hard that it takes entire governments years of effort to get it done in useful amounts.

Why we care is because making bomb grade material from natural ore requires isotope separation. But spent nuclear fuel has a bunch of nifty fissionables in it which can be extracted using chemical separation (aka partitioning). Unforch, that knowledge is available around the world. Only the difficulty of isotope separation prevents well financed garage tinkerers from building a DIY bomb out of uranium ore off ebay. Spent nuclear fuel tends to be self-guarding since it radiates a fatal dose in less than 5 minutes. But its also well guarded by folks who totally are authorized to shoot you with a 30mm autocannon.

Current reactor designs only use about 5% of the fissionables due to fission and decay products that catch too many neutrons and stop the show. To put that in perspective, 99% of the material out of a typical uranium mine is overburden. Then of that 1% uranium metal, only about 1/2% is the right isotope. So for every ton of fissionable metal delivered, we have a 20,000 ton hole in the ground. Throwing away over 95% of that is just stupid expensive. But even using light water reactors, there is more energy contained in known uranium ore deposits than all the coal, oil, and gas that has ever been burned. And its nearly carbon-free, not just carbon neutral.

The alternative, and the tiger we must ride, is chemical partitoning of spent fuel. Or in the case of a salt reactor continuous processing of a small sidestream of molten salt. The concern is if some clever bastard steals a gram here and a gram there, it only takes a few years to build a bomb. And if a rogue government takes over a power plant, there's all the equipment lined up to separate fissionables. A primary goal of all modern nuke plant designs is to make it verifiably impossible for any of that to happen in secret. We watch like a hawk and if there's any funny business then we send in the rude boys.