Nuclear Friday: Dry Fuel

Last week I wrote about the Ivy Mike test and how it didn’t really produce anything that could be turned into a viable weapon. Liquid deuterium, a naturally occurring heavy isotope of hydrogen, is a cumbersome material that requires refrigeration plants to maintain cryogenic temperatures and high pressures. The fusion secondary is made out of a vacuum insulated thermos, and it always leaks. Sounds impractical, but this was the fifties, a time when the US tried to build almost every type of bomb that could be thought of. Between the Ivy test series in 1952 and the Operation Castle series in the spring of 1954 the US did make a few of the air-deliverable TX-61 “Jughead” devices pictured in the header image. These were just barely functional bombs needed to be topped off and pressurized in-flight by a cryogenic refrigerator built into the B-36 bombers modified to carry them. One Jughead was scheduled for testing during the Castle series. The test was cancelled. The Jughead would never be deployed. The Bravo shot, the first of the Castle tests, was successful beyond expectations. The United States had entered the age of dry fuel.

Convenient fuels and breeder reactions:

One way to get a lot of hydrogen in a solid form is to use a chemical compound that contains it. Water would seem an obvious choice. It would have to be water that contained only the deuterium isotope of hydrogen, but refining such water dates back to the late thirties. Water won’t work though, oxygen is a large and heavy atom compared to hydrogen. By mass, there’s not much hydrogen in water.

Lithium hydride seems like a much better idea. Lithium is the lightest metal by weight, so Lithium hydride has a high proportion of hydrogen. Obviously, almost all that hydrogen would have to be deuterium, that’s the isotope that fuses easily. This chemical compound is called Lithium deuteride, or LiD. Now you might think that even though we’ve packed a lot of deuterium into a small space, the lithium might get in the way. And it would, except that lithium has a secret. When conditions are right, it can undergo a breeder reaction.

Edward Teller was the first to realize this. In 1945 Teller was speculating about a fusion based device he called “Super”. He was at Los Alamos then, but didn’t do much work on the fission bombs developed there. Instead, he analyzed all the data and theoretical work on neutron absorption and discovered that Lithium 6, the second most common isotope of lithium, would split apart fairly easily under neutron bombardment, yielding tritium and helium. The helium doesn’t do anything, but tritium, a rare heavy isotope of hydrogen with two neutrons, can react with deuterium to release large amounts of energy. A nuclear reaction that makes a product that can enter into new nuclear reactions is called a breeder reaction. This reaction is very unusual because it’s one of the few cases where fission of an element lighter than iron releases rather than consumes energy. It’s not a lot of energy compared to the fusion reactions, but lithium 6 does its part.

That lithium 6 is a breeder fuel is the first secret of lithium. There’s a second secret too, but that would discovered at Bikini Atoll in 1954.

The Soviet Layer Cake:

The USSR was the first nation to harness the power of lithium deuteride in a usable thermonuclear weapon. The Soviets had a lead in working with LiD because Teller’s former assistant at Los Alamos, Klaus Fuchs, had passed his notes from a secret 1946 conference hosted by Teller to his Soviet contacts. During that conference Teller had detailed the lithium 6 breeder reaction and proposed a speculative design for a partly fusion based bomb  he called Alarm Clock.

The Soviets were very concerned about this. Not only did they not yet have a fission based bomb, it seemed the US was ready to start building new types of weapons. The US wasn’t really interested in any variation of Teller’s “Super” projects at that time, but how would the Soviets really know this? Fuchs was their best intelligence contact, but he was no longer close to center of nuclear research as he had been at Los Alamos. After a second communication from Fuchs in early 1948, the USSR would would form a design team to investigate thermonuclear weapons. And keep in mind, this happened a year before the Soviets even had a fission bomb of any kind. 

Shortly after the team formed, Andrei Sakharov proposed his first design, the Sloika, the layer cake, made from alternating layers of enriched uranium and fusion fuel. By 1949 he proposed a refined version of the Sloika design based on better data he acquired from the Soviet fission bomb research team as they neared their first successful detonation. Sakharov was put in charge of Soviet thermonuclear research. It wasn’t that well funded, the USSR just wanted to be able to produce some kind of H-bomb quickly in case it was

Sakharov put a lot of effort into studying lithium 6 and used his funds to make very pure samples of that isotope. Separating lithium 6, which is makes up about 7.5% of natural lithium, from the more common lithium 7, is an expensive and time consuming process. Sakharov wanted pure and reliable materials if he were given the order to produce a bomb.

That order came in i952 after the US Ivy Mike thermonuclear test. The USSR tested a fully weaponized dry fuel bomb called the RDS-6s based on the Sloika design in August 1953, less than a year after Ivy Mike.

The Sloika design was only 400 kilotons, compared to Mike’s ten megatons. But as the Russians pointed out, Ivy Mike was an 80 ton thermonuclear installation, not a weapon. No one in the USSR knew about the Jughead, but the TX-61 had not been tested and the bombers modified to carry it were not ready.

One downside to the Sloika design is that it cannot be scaled up much over 400 kilotons, but it was still quite a shining moment for the Soviets during the arms race. Sakharov’s attention to detail and lithium 6 at a high level of purity had paid off. But that purity had kept Sakharov from discovering the second secret of lithium.

Back to Bikini:

Bikini Atoll had been the site of the first postwar nuclear tests by the US in 1946. Nuclear testing would return to Bikini in early 1954. The first test in Operation Castle was the Bravo shot, the first dry fuel hydrogen bomb detonated by the US. It was March 1 at the site, February 28 back home. Expected yield was around 6 megatons. Actual yield was an astounding 15 megatons. This was the second secret of lithium. No one had any idea that lithium 7 could be a breeder fuel as they knew lithium 6 could.

US designers were not so scrupulous about lithium isotope separation as Sakharov had been. Bravo’s “shrimp” bomb had lithium deuteride refined to only 40% lithium 6, the remainder was lithium 7. Physicists soon understood that most of the unexpected yield came from an unknown breeder reaction. Lithium 7 can be split by a neutron to make tritium and helium just like lithium 6. This reaction “eats” energy from the system, but that loss is made back many times over by having more tritium available as fusion fuel. The reaction also makes a new neutron, and neutrons always help make more kaboom.

Here’s a pretty good video that’s a “fan edit” of public footage released by the US government:

It covers all the basics, including how the Marshall Islanders were put at risk by this test. I think if it had just been the Islanders and US service members endangered by this test, we would know nothing about it. There would have been a complete cover up. The harm that came to the crew of the Japanese fishing boat Lucky Dragon 5 turned Castle Bravo into an international incident. The Japanese government was outraged, for obvious reasons, and publicized the plight of the crew.

One would think that the Castle series would have been suspended out of respect for the injured. It was only slightly delayed. There were five more tests at what was left of Bikini. Some of the tests had to be conducted on barges since there wasn’t much stable ground left anywhere in the atoll. Romeo was the second shot of the series. It was similar in design to the Bravo shot, and was expected to be even lower in yield than the original projections for Bravo. After the the shocking results of Bravo, projections for Romeo were revised upward and appropriate safety measures taken.

Two of the Castle tests were cancelled. One of these was a test of a Jughead device. There was no need for a cryogenic flying fridge of doom.

The Day Sakharov’s Life Changed:

After the detonations at Bikini became public, Sakharov was put in charge of building a similar device. He would later go on to win the Nobel Peace Prize. He was set on the path to that award in November 1955 when he observed the first test of his new design. Here’s what it looked like:

I’m pretty sure the older male voice at the beginning of the video is from a Sakharov radio interview in the late 80’s, shortly before he died. Be sure to watch the three people standing  in the plaza at the 2 min mark. It’s quite eerie. They survived.

Soviet planners had insisted that every safety measure had been taken. Sakharov had designed a lower yield version of the RDS-37 estimated to be 1.6 megatons instead of the 3 mT planned for the standard military version. And his estimates were correct. Why two people killed, and hundreds more injured during this test? Political pressure to get this test finished before winter fully settled on the Kazakh-Siberian border was intense. They already sent the bomb up once only to cancel the test due to bad weather. The day of the second attempt was judged to be the last chance before Spring, and that would be politically unacceptable. There was a temperature inversion, a layer of warm air on top of cold air on the ground. This was known to be a risk factor. The cold air would focus the blast wave downward more strongly than the scientists had calculated. But the test simply had to be done. The nearby city of Kurchatov was damaged by the blast. But only two died.

Sakharov left the weapons design team and began work on non-military nuclear applications. By the early sixties he became a peace activist and public critic of the government. That’s a dangerous lifestyle choice in the Soviet Union. He was able to avoid being silenced by using his status as a public celebrity. But he was eventually sentenced to internal exile in the city of Gorky. He would not be allowed out until 1986.

The New Age:

The test of the RDS-37 began a new age of escalation. After this point, a full nuclear exchange would be a serious threat to global civilization. And this age has not ended. The US and Russia still have about 7,000 weapons each. Not that much has changed since the late 50’s. If Sakharov came back from the grave he’d yell at us. And we’d deserve it.

Useful research links:

Operation Castle footage made semi-public in the 50’s

Heavily censored Castle Bravo footage released in 1999

Complete list of Operation Castle devices and results.

Article about Sakharov’s reaction to the RDS-37 test.

Post with links to my Simulated R7 missile attack on central Texas. Warhead is the RDS-37.

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