If you want high power density nuclear reactors in zero gravity, you need efficient heat transfer. Boiling liquid metal is about as good as you can get. Here are some boiler tube geometries studied by Pratt & Whitney in 1964 (a 🧵). First up: the serpentine tube. The twisted ribbon insert tube

Do you prefer advanced nuclear reactor designs because they’re newer and better than the old-fashioned ones that make up today’s fleet? Then buckle up, you’re in for a surprise! A 🧵 on the weird way that advanced nuclear today is actually a “Back to the Past” story... High temperature gas-cooled reactors were first conceived in 1944, and then developed by the Europeans in a multinational collaboration that resulted in the Dragon reactor in the UK. It went critical in 1964. This program developed the beloved coated microsphere fuel (TRISO) and everything. Many other HTGRs followed, such ML-1, Peach Bottom, AVR, Ft. St. Vrain, THTR-300. Today, the HTTR, HTR-10, HTR-PM are all operational outside the US (Japan and China). China has 2 6-packs of them under construction.

A nuclear reactor on the moon!? Yes, this is a great idea, and totally doable!

You need lots of power on the moon for people to live there full-time. They need heat, closed-cycle life support, and oxygen from oxides in the soil or ice. Here's a nuclear-powered lunar base 🧵 The habitat might be a 16 m inflatable ball with 1 meter of radiation shielding. You need shielding from cosmic rays and solar flares anyway, and yes it helps with the reactor radiation too.

Everyone says nuclear power is over-regulated. With word of the big nuclear EOs looming, I spent a few weeks talking to people in the nuclear industry to find out which reforms they thought would be most helpful, and which they were nervous about. Here are the top 12 (a🧵...) 1:🌳Keep fixing NEPA. We should default to Environmental Assessments instead of Environmental Impact Statements on sites with previous or generic EIS from within the last ~10 years, and for low-risk reactors. We should accelerate the ongoing implementation of the Fiscal Responsibility Act, and remove/reduce the need for power and alternates analyses sections for any reactor.

Specifically, someone could ask the NRC staff to proceed with rulemaking to update/modernize 10 CFR Part 51. Overlaps nicely with ongoing work to implement FRA. See SECY-24-0046. Remove the requirement in 10 CFR 51.20 to require an environmental impact statement for nuclear power plant applications and power uprates. Provide allowance for categorical exclusions for advanced reactors and power uprates in 10 CFR 51.22. In lieu of categorical exclusions, allow for environmental assessment for first-of-a-kind facilities in 10 CFR 51.21 and a categorical exclusion for nth-of-a-kind facilities and power uprates in 10 CFR 51.22.

Crazy story: in the early 1990s, the USA purchased 6 TOPAZ-II space nuclear reactors from the USSR/Russia and flew them to New Mexico for testing. These reactors had thermionic cells around each of their 37 fuel pins: "Thermionic Fuel Element"! (a 🧵...)
The 115 kWt reactors used 93% enriched annular UO₂ fuel elements, which transferred heat through a cesium gap, converting about 5% of the heat to electricity. Outside each pin, they had electromagnetically-pumped liquid metal sodium-potassium eutectic coolant.

Let's talk shielding of microreactors. Here's an operable 3.3 MWt nuclear microreactor on a flatbed (the ML-1). a 🧵 Looking inside that tank, we see numerous shield structures surrounding the core. 2 inches of lead, 'shield solution', more lead, and 2 feet of 2% borated water. Optimization suggested putting 3" of tungsten in there with the lead.

Calder Hall, the world's first full-scale atomic power station, was opened by Her Majesty the Queen on October 17th, 1956 in the UK. 67 years, 7 months, and 12 days later, you now can read this neat 1961 informational booklet about it that I just scanned (short thread 1/n). Eventually, 4 units were built. They were natural uranium metal fueled, CO₂-cooled, graphite-moderated, and clad with a magnesium alloy called 'Magnox'. (2/n)

The Sodium Reactor Experiment (SRE) was a prototype reactor by LA for sodium-cooled graphite-moderated reactors (SGRs). It was thought that high-temperature, low-pressure sodium coolant with fuel-conservative neutron moderation would make low-cost nuclear plants. (a thread, 1/n) Then, as today, the best way to verify that your economic suppositions are correct was to build and operate a prototype. Originally the SRE was going to dump all its heat to air, but they ended up hooking it up to generator and putting electricity on the local grid. (2/n)

Let me tell you about Aqueous Homogeneous Research Reactors. These reactors with fluid fuel: an aqueous solution of uranium. Aka "water-boiler" type reactors. The first few were made at LANL during the Manhattan Project. Later, Atomics International sold them for research. (1/n) These reactors were nice for research because they were small, inherently safe due to strong negative reactivity feedbacks, "easy" to maintain, and could be operated with a small crew. Here's one being built at the Armour Research Foundation in Chicago (now @IITRI_Chicago) (2/n)

Check out one of the more truly advanced reactor concepts ever seriously considered: the Mobile Lithium Cooled Reactor Experiment (LCRE) Powerplant. (1/n) (2/n) You may recall the nitrogen-cooled/water-moderated ML-1 truck-mounted microreactor that was operated out in Idaho by the US Army. It was built and operated and could kick out 3.3 MW of heat and ~400 kW of gas turbine shaft horsepower. But that wasn't enough power...

How can you leverage nuclear energy to propel vehicles? In 1963, the US Army knew direct nuclear plants would be too heavy for normal vehicles, and very large vehicles would have "serious tactical disadvantages"...

And so the Army focused on 'the energy depot' concept, where a nuclear reactor and associated equipment would be used to manufacture chemical fuels from elements universally available in air and water. (kinda like what @isaiah_p_taylorv just announced with Valar Atomics).

I got it! The PM-1 Final Summary Report was delivered to me yesterday after I filed a FOIA request to the military library that had it cataloged (at their recommendation). This is the pure-gold lessons-learned report from operating a 1 MWe military microreactor in WY for 4 years. It contains cost and performance information, summarizing which systems worked well and which ones did not. This is a follow-up to some searching I did after getting this summary film digitized.

https://twitter.com/whatisnuclear/status/1675950732175118336

The Advanced Boiling Water Reactor (ABWR) is the Dark Horse of today's nuclear reactors. It's modern, efficient, resilient, broadly licensed, and proven buildable in record time. (a 🧵...) Having been built before, it's (obviously) a fully-complete design, already licensed and ready to rock in the USA, Japan, and the UK. Here's its design certification from the NRC: nrc.gov/reactors/new-r…

Almost unbelievably, there are 3500 exajoules of nuclear energy in these 48.3K tonnes of uranium, accessible using breeder reactor tech we demo'd in 1952. The world uses about 600 EJ of primary energy per year (including everything!!), so that's about 6 years of world energy.

https://twitter.com/JesseJenkins/status/1584325272530485248

Rather than breeder reactors, we use non-breeder reactors today, which get about 1% of the energy from the mined uranium. Our earlier attempts at deploying more breeder reactors were thrwarted by anti-nuclear activism and minor technology complexities whatisnuclear.com/recycling.html

Another cool thing about nuclear energy is just how ridiculously flexible it is. It can be used basically everywhere, in any environmental condition imaginable. A 🧵... (1/9) 2/ The first obvious example is deep under the sea. After the Navy made nuclear-powered submarines, they immediately went on grand adventures previously unheard of. The Nautilus went to the North Pole under the ice, and the Triton circumnavigated the world without surfacing.

Paul Dorfman, one of @greenpeace's main nuclear commentators, just blocked me (a Ph.D. scientist) after pointing out a glaring and egregious error in his analysis. He said nuclear wasn't low carbon. I pointed out, calmly and patiently, that it was.

https://twitter.com/whatisnuclear/status/1449025281948217344

Basically he was taking the lowest number and the highest number out of a Yale meta-analysis and saying that the 'mean' was the midpoint (around 57). But the real mean is 17.8 and the median is 12.

This is so shockingly anti-scientific given the numbers at hand. Sunlight creates 2,000,000,000,000 Becquerel of C-14 in our atmosphere every single day and @Greenpeace says that this water, with 2-200 Bq/L, is going to alter human DNA. @ShaunBurnie wut!? doi.org/10.1002/2015GL…

https://twitter.com/CNN/status/1320032343122464771

Natural sunlight produces the entire net quantity of C-14 in all the tanks (63 GBq) every 43 minutes in our atmosphere.