Admiral Grace Hopper to ZMC-2

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So far I've written 149 entries in this blog. Some of those were basically content-free, but others covered multiple topics, so that's a reasonable estimate for the number of things we've talked about. Some of the topics I focused on were predictable (for me anyway), like space history and structural failure, while some were a bit of a stretch, like nuclear physics and the Iranian hostage crisis. Some were fun and crossed over between multiple topics, like space, history, and nuclear bombs, or geology, natural history, and nuclear engineering. Some stuff was just weird. I think that I gave a passable description of orthodox Christian things, which is cool because that's not at all my area of expertise.

The top-five most popular posts of 2013 are as follows:
1) Naked mole rat
2) Space elevator
3) Aloha Airlines flight 243
4) The Hyatt Regency walkway collapse
5) Project Azorian

Given that greatest hits list, it sounds like animals, advanced space concepts, plane crashes, failure, and spy stuff are popular topics. Should I write more about these things? I mean, I can write about whatever I want, but presumably I do this for a reason. Feedback is appreciated, especially since the comboxes on this blog are typically pretty quiet.

The idea when I first started this blog was that I'd post daily, or as close to daily as feasible. That's probably not very realistic, and my actual on-base percentage was closer to 40%. That seems reasonable, and my objective for next year is to achieve something close to that but more evenly-spaced throughout the year. Thanks to everyone who's read this space so far, and I hope to see y'all back in 2014.

The Megatons to Megawatts Program

Over the last two decades about 10% of the electricity generated on the American power grid came from decommissioned bomb material produced by the Soviet Union during the Cold War. The program to convert Russian bomb-stuff into American power-stuff was officially known as the "Agreement between the Government of the Russian Federation and the Government of the United States of America Concerning the Disposition of Highly-Enriched Uranium Extracted from Nuclear Weapons," but its unofficial title had a catchier ring to it. You can read more about the program here:
The Megatons to Megawatts Program

Less than 1% of natural uranium on Earth is useful for unleashing nuclear power. Some reactor designs can get by with the natural scarcity of fissile uranium, but most require the ratio of (fissile) uranium 235 to (fertile, but non-fissile) uranium 238 to be artificially enriched to about 5% to run. Bombs require enrichment to 90% U-235 for the explosive chain reaction to work, and during the Cold War the Soviet Union stockpiled an enormous quantity of highly enriched uranium (HEU) as a precursor to the manufacture of nuclear and thermonuclear bombs.

What the Soviets actually planned to do with all this uranium isn't exactly clear. By the breakup of the union, Russia had a stockpile of 500 metric tons of HEU, enough to build at least 20,000 nuclear warheads. Short of fending off a fleet of invading alien starships, it's difficult to conceive of any possible justification for that much firepower, and weapons-grade uranium sitting inert in a warehouse wouldn't have done much good strategically or tactically had the Cold War ever gone hot. Since evil invading aliens don't seem to be on the way, American diplomats proposed downblending the weapons-grade stock to reactor-grade uranium, and consuming the converted explosive material in reactors across the United States. Isaiah wasn't far from the mark when he talked about beating swords into plowshares. For the last 20 years Americans have been lighting our houses with the weapons that were meant to destroy them.

Earlier this month the last shipment of downblended reactor fuel arrived from Russia. Since about half of the nuclear industry's supply of low-enriched uranium since 1993 has come from the Megatons to Megawatts program, another source of uranium will need to be found soon to keep the price of nuclear fuel from climbing. A small amount of the American stockpile of HEU has been converted into reactor uranium since the end of the Cold War, but much of it remains the stuff that makes mushroom clouds and fallout rather than the stuff that runs lightbulbs and air conditioners. Both sides of the former Iron Curtain still have hundreds of tons of plutonium, used in the primary stages of thermonuclear weapons, and this has potential for power generation as well. For now, there are many more swords in the arsenal that would better serve us as plowshares.

Advanced Gas-cooled Reactor

Once nuclear chain reactions were understood and harnessed for bombs and ships, nuclear engineers on both sides of the Iron Curtain turned their attention to the mundane but practical task of making electricity. Most nations adapted their submarine engines to the task, but the British found a different solution. 15 of the UK's 16 commercial nuclear power plants use high-temperature carbon dioxide rather than water to cool the reactor's core and heat its steam generator. You can read more about that here:
Advanced Gas-cooled Reactor

Mark's Standard Handbook for Mechanical Engineers has this to say about the thermodynamics of nuclear reactors:

...A nuclear reactor must be treated as a heat source which differs from a chemical heat source in that no oxygen is required and the heat does not have to be removed from gaseous combustion products which possess poor heat-transfer properties.

Most nuclear reactors in the United States, France, Japan, and the former Soviet Union (the largest producers of nuclear power) use water to channel the immense heat released when uranium atoms split to engines that convert some useful fraction of this heat into electricity. Liquid water is conductive, stable, nontoxic, and abundant where most (but not all) nuclear reactors operate, and its ability to absorb tremendous amounts of energy per unit volume lends compactness to any design using water as a coolant. Since most reactors used for commercial power share a heritage with ship and submarine propulsion systems, where space and power are precious things, much of the nuclear world gets its power from pressurized- and boiling-water reactors. The British decided to go a different route.

All else equal, the hotter a heat engine runs, the more efficient it is. Though water-cooled reactors are compact, they suffer a major problem here since water boils at a temperature well below the allowable temperatures of the structural metals that reactors are made of. Steam is far less dense than liquid water, and is impractical for use inside the reactor's core, so the boiling temperature of water imposes an upper limit on how hot water-cooled reactors can run. Increasing coolant pressure (standard practice in the pressurized-water reactors that make most American nuclear power) delays water's boiling to higher temperatures, but there are practical limits to this approach as well. The British technique of using carbon dioxide as coolant allows the reactor to run at much higher temperature and efficiency than can be achieved by the American, Russian, and French water-cooled designs. The AGRs are bulkier than their water-based cousins, but that poses little issue for powering the grid.

An interesting design feature of the AGR is that it outputs steam at exactly the same temperature, pressure, and power as previous coal-fired power plants built in the UK. This allows the generator-driving turbine to be agnostic to its power source. Heat from uranium is indistinguishable from heat from coal, but you'll breathe a little easier without all that soot in the air. If only someone would remind the the power that be in London of this...

Luna 24

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On August 22, 1976, a Soviet robotic sample-return spacecraft landed on the surface of the Moon. As of this writing this is the most recent time that a human-built craft landed on the Moon, and you can read more about it here:
Luna 24

Luna 24 was the last in a long, distinguished, and problematic series of missions for the Soviet Union. The missions saw the far side of the Moon, touched its surface, and landed gently there for the first time. The early Luna program was a source of tremendous pride and achievement for Soviet science despite the frequent failures, but later it became clear that the Soviet space program would be unable to compete with NASA's effort to send astronauts to the Moon by the end of the decade. Since no cosmonauts would be going there anytime soon, a program of robotic rovers and sample return missions was launched in the late-1960s and early '70s.

Like the early Luna probes, most of these larger and more sophisticated landers failed. The Soviets knew their trade better in 1976 than they did in 1959, but there's a lot more that can go wrong when the mission entails collecting a sample and sending it back to Earth rather than simply crashing into the Moon. To be sure, shipping 170 grams of lunar regolith to Siberia is an impressive achievement, but the scientific return of the late Luna missions was dwarfed by the hundreds of kilograms of carefully-picked rocks imported by astronauts during the Apollo program. Fortunately the Apollo haul has been quite a lot to work with, since there have been no trips to the lunar surface, even robotic ones, since the last of the Lunas.

With luck, that will soon change. Earlier today the China National Space Administration launched Chang'e 3, a solar-powered robotic rover toward Sinus Iridum. If successful, it will be the first time China's landed a spacecraft on another world, and the first time any nation has done so on the Moon in 37 years. It's about time.