Sunday, 30 April 2017

WW2 german nuclear program

 (Article under construction)

When germany invaded poland on september 1, 1939, the heersewaffenamt (HWA) devised a program to explore the possible development of nuclear weapons. The first meeting was organised by kurt diebner, a nuclear physicist who advised the HWA, and was held on september 16, 1939. The second meeting drew in major players like otto hahn and werner heisenberg, and was held on september 26, 1939. Before the year was out, diebner had assembled a fairly large team of scientists, formed a consenus on what goals they should pursue, and organised everyone into teams to carry out theoretical and applied research. In the early years of the war, the german nuclear program made alot of progress, and was actually ahead of the british and americans until 1942 or so. From that point on, though, the americans began to rapidly outpace them with their manhattan project and the huge resources marshalled toward it. While germany eventually lost the war and any hope of winning the nuclear race, the achievements made by their scientists under the difficult conditions of the war were considerable. Even with limited financial support from the nazi regime, and the frequent bombing of their laboratorys by the allys, the germans were making slow and steady progress towards a nuclear industry. This was especially true for the team run by paul harteck, as we will eventually see. At the time, there were known to be just two approaches to weaponizing the strong nuclear force.

The first method is to enrich natural uranium by separating U-235 from U-238. The U-235 isotope is fissile, but makes up only 0.72% of natural uranium by mass. If uranium is to be used in a bomb, its U-235 concentration must be raised to 90%. The second method is to bombard natural uranium with neutrons and transmute it into plutonium. The U-238 isotope is fertile, and if it captures a neutron, it will turn into U-239, which then decays into Pu-239. If plutonium is to be used in a bomb, its Pu-239 concentration must be raised to 93%. Uranium can be enriched to weapons grade by a variety of techniques, but uranium can only be transmuted into plutonium by a reactor. In hindsight, the german nuclear program made significant steps towards uranium enrichment, but were lagging in their efforts to make a reactor. The details of this subject are complicated and sometimes convoluted, since many historians have offered many appraisals that are mutually exclusive. Authors like paul lawrence rose, for instance, have such prejudice for the nazis that it interferes with their ability to even tell a coherant narrative. Other writers go in completely the opposite direction. As always, though, only some of these appraisals can be corroborated. This article will focus on a number of myths about the german nuclear program and how it measured up to the manhattan project.

A german uranium pile of 1945

The germans never measured the fission cross section of uranium-235: Hence, they were never able to properly estimate the critical mass for an atomic bomb.

This claim is not only illogical, but also contradicted by certain facts. After the conquest of denmark and france in mid 1940 (among other unfortunate victims of the blitzkrieg), germany had access to machines called cyclotrons at vienna, copenhagen, and paris. Cyclotrons are a type of particle accelerator that can be used to separate U-235 from U-238, thus enabling scientists to perform experiments on the isolated sample: In this case, measuring the fission cross section. Moreover, it is known that each of these laboratorys had been visited by german researchers at several points in the war. Apparently, we are expected to believe that the nazis never took advantage of the very machines that would have enabled them to determine the critical mass of U-235, a key parameter for which much of their subsequent work would hinge on! But of course, such a ridiculous claim is unsupported by the evidence. In august of 1941, an individual named fritz houtermanns (who was working in the laboratory of manfred von ardenne) wrote a paper which discussed runaway chain reactions and the possibility of transmuting uranium into plutonium. This paper was circulated among members of the uranverein, eliciting a flurry of discussion. By february 1942, the HWA team run by kurt diebner had published a document outlining the critical mass for a U-235 bomb: The estimate was 10 to 100 kilograms, comparable to the american estimate of 2 to 100 kilograms! However, the HWA report also stated that the difficultys of separating U-235 from U-238 were such that a crash program could not be recommended, because there was no guarantee that such investments would yield a bomb before the wars end. This pessimistic interpretation was reinforced by a meeting that werner heisenberg () had with albert speer in june of 1942, when he flat out stated that a bomb could not be delivered in a reasonable time, and that the nuclear program should only receive modest funding.

The uranverein never developed an effective means of enriching uranium to weapons grade.

This is simply not true. Early in 1943, the research team under paul harteck had created a double centrifuge which was being used to separate isotopes of xenon gas, and then to separate uranium hexafluoride. This machine was able to enrich several grams of uranium to 7%, good enough to warrant funding from the reich research council (RRC). More centrifuges were built, and the design was constantly tinkered with. By may of 1944, a company in freiburg had built and successfully tested the MK III centrifuge, which compelled harteck to move his laboratory there. The team set up a facility in the nearby town of kandern, where a few centrifuges were assembled into a cascade that could enrich several kilograms of uranium to 0.9% each day. After a few months, however, allied bombings forced them to stop work and relocate to a town called celle. Early in 1945, the facility only had 30 or 40 of these machines, but was still enriching 50 grams of uranium to 15% each day, very good for a laboratory effort. While there were plans to put the MK III centrifuge into mass prodution again, the war ended before this could take place. Other research teams in germany had experienced similar ups and downs. By june of 1943, erich bagge had created an 'isotope-sluice' machine that ran uranium hexafluoride through two shutters revolving at high speed, allowing the lighter U-235 to be separated. This was a novel approach which never occurred to the americans, using a combination of electromagnetism, centrifugal force, and thermal diffusion.

While his first two prototypes were destroyed by air raids, bagge was able to relocate to butzbach and set up another machine. By july of 1944, the 'isotope-sluice' had undergone an endurance test lasting 120 hours, yielding several grams of much enriched uranium. The models indicated its efficiency could be greatly increased. At around this same time, manfred von ardenne was testing a magnetic isotope separator, not unlike the calutrons used at the Y-12 plant at oak ridge. Both machines used magnetic fields to deflect charged particles and separate them based on differences in mass, but the germans used an ion source to sublimate the uranium, a design which was superior to the calutrons. Ardennes laboratory was located underground in his manor, which protected it from air raids. And since he was supported by the post office, work on it was able to continue unimpeded. However, only one of these small machines were made during the war. Putting the technical details aside for now, it should be clear that the uranverein had made major progress in their knowledge and ability to produce U-235. The problem was that these efforts were all confined to laboratorys, and were never expanded to the industrial scale that was needed for an atom bomb. There were not enough scientists and engineers working on uranium enrichment, and there was not enough funding from the RRC to produce these machines in anywhere near the numbers required.

The germans never generated a self-sustaining nuclear chain reaction, much less a working reactor.

This point requires some background. One of the things needed for a nuclear reactor is a substance which can act as a neutron moderator, and allow a chain reaction to continue unabated. During WW2 there were only two known substances that could fulfill this role: Graphite and heavy water. Allied and axis scientists investigated each of them. In january 1941, walther bothe had performed experiments on the purest graphite available, to see whether it could slow down the neutrons without absorbing them. Eventually, he determined that the capture cross-section of graphite was too large to make it an effective moderator. The americans actually came to the same conclusion as him, but would quickly learn that this was due to trace amounts of boron, which could be removed by making the graphite out of petroleum instead of coke. The germans never did this extra step, and were now totally dependent on a supply of heavy water, which was synthesized at only one location in all of europe: The norsk hydro plant. By may of 1942, enough heavy water had been assembled to make a uranium pile at leipzig. Heisenbergs teams detected a neutron increase of 13 percent, meaning that the pile emitted more neutrons than what had been injected into it. 

This was a step in the right direction. Unfortunately, the containment vessel exploded soon after the experiment, leaving the scientists with no heavy water left. While the basic research problems had been overcome, no new reactors could be built without an adequate supply of heavy water. Progress on this area stalled as a result, and results came at an agonisingly slow pace. Conditions were only worsened when the allys conducted raids against the norsk hydro plant, interrupting the supply of heavy water. The months and years dragged on, and optimism soon gave way to pessimism. By early 1945, the germans had two crude 'reactors' that were on the verge of sustaining a chain reaction. One of these uranium piles was at stadtilm under diebner, while the other uranium pile was at haigerloch under heisenberg. Both of these reactors were assembled under very difficult circumstances while the scientists were on the run from allied armys, and contained only what materials could be loaded onto truck. The cavitys they were lowered into had been pre-constructed, but everything else was basically assembled on the fly. Heisenbergs reactor could have went critical if he had been given 50% more uranium and heavy water, which was locked away in a warehouse for safety. And diebners reactor may have briefly went critical, before being forced to shut down.

Tuesday, 28 February 2017

Animorphs FAQ

This is another entry into what is quickly becoming a large catalogue of thoughts on the animorphs series. The first post was an overview of what the storys were about, how they set a high bar in childrens sci fi, and have never really been surpassed in terms of thematic content. I also offered some thoughts I had about certain dilemmas that had plagued readers of the series. The second post was an attempt to determine whether the animorphs hometown had a real world analogue. I was able to firmly prove that the descriptions of their city matched those of ventura, californa. This post will take more of a FAQ format, and answer some questions that were never addressed in the books. Mostly about those big, sciency questions that people tend to shy away from. Alot of this is about the yeerk species, and how they evolved in a biological and cultural manner. When K.A. applegate sat down to wrote the animorphs series, her aim was to provide the young heros with a unique race of aliens to fight against. At first glance, they are your standard group of evil alien parasites trying to enslave humanity, thus yielding the moral high ground to the humans who were fighting in self defense. But as the series progresses, applegate presents the yeerks in an increasingly sympathetic light. Their natural bodies are so inadequate that they cannot see, hear, or do any of the things that humans take for granted. They are a race of cripples that have no choice but to enslave others.

Alien races from the series

How is it possible that a self aware race like the yeerks evolved to become parasites, and control the functions of another creatures brain? 

Were they parasites before they became sentient, or were they sentient before they became parasites? This is something that has always perplexed me. Yeerk nature is at the core of the animorphs series, the reason they came to earth and put humanity in their sights. Their parasitic nature is what compelled them to infest other self aware beings, denying them freedom so that they could be free themselves. I have a speculative theory on how this might have happened. While details from the book are scarce, what we do know is that they are hermaphrodites that reproduce through epigamy. Putting it simply, they only reach sexual maturity in very specific conditions, because the metamorphosis is irreversible and reproduction is fatal. During the spawning process, yeerks merge together and somehow 'fuse', and from them emerge juvenile 'grubs.' There appear to be both male and female yeerks, even though they carry both pairs of sex organs. Most importantly, they are an aquatic species that can move through an animals ear canal and into their actual skull, sinking into its every crevice. By attaching their neurons to those of the hosts brain, the yeerks are able to take complete control over their bodys and use them as a puppet. Can natural selection provide a explanation for such a bizarre race? My answer is an emphatic yes. 

Before we go any further, though, I would like to posit two separate claims. 1) That the ancestors of yeerks had a 'neural patch' as part of their anatomy. 2) That these proto-yeerks could only reproduce by migrating to some ancestral spawning ground. If both these claims are true, then we can venture the following theory. Like salmon, the yeerks must travel from an ocean up into a river, and hence, swim against strong tides and up into waterfalls. I imagine this would require incredible exertion on their part, and that some of the yeerks wouldn't complete the journey back. If so, then the bottleneck for reproduction is limited to those who can succesfully migrate back to the spawning grounds. Maybe the yeerk ancestors faced a crisis at some point, something which caused the journey to become more dangerous and more difficult? If so, it would hardly be surprising if some of the yeerks managed to cheat and find another way to get upriver... Maybe they swam into the ear canal of some creatures (like the gedd) who were taking a drink, using their 'neural patch' to direct them towards the spawning ground? * This approach would have given the yeerks in question a major advantage over their peers who were trying to swim upriver the hard way. Perhaps it created a selection pressure great enough that the yeerk anatomy changed, so successive generations had a larger 'neural patch' that could take more and more control over the gedd brain? We have seen similar instances of parasitism here on earth.

Did the yeerk empire have conflicting needs when it came to the species they tried to conquer?

Absolutely. After they set up their colony on the hork-bajir world, and their population grew to number in the billions, the yeerks faced a catch-22 situation with regards to their hosts. They have five different classifications for potential host species. Class 1 are those which are physically unfit for infestation. Class 2 are those which are fit for infestation, but suffer from severe drawbacks. Class 3 species are those that would make excellent hosts, but are few in number and can't be bred quickly. Class 4 are those that would make excellent hosts, but are too formidable to conquer. Class 5 are those fit for infestation, are large in numbers and able to breed quickly, and cannot fight back. The existential crisis plaguing the yeerks is that despite decades of searching, they had seen no examples of the desperately needed class 5 species. You see, it all comes down to agriculture. If a species doesn't have agriculture, then its population will number roughly the same as neolithic humans, I.E about 5 or 10 million. This is why the hork-bajir were rated as class 3. Their small numbers and lack of technology made them easy to conquer, but it also meant that they couldn't supply the full needs of the empire. But if a species did have agriculture, then its population would quickly explode in numbers, developing better technology and a state apparatus to manage them. (This usually implys a military) This is why humans were rated as class 5. They were harder to conquer than class 3 species, but also yielded a much greater payoff for the empire.

The huge problem here is that within just a few centurys, a species can go from type 5 to type 4 status, just as the andalites did. Technological evolution would rapidly allow a type 5 species to leave its planet, build an armada of spaceships, and colonize other worlds, which means they are no longer type 5! The central paradox for the yeerks is that the type 5 species they coveted so badly were unstable and transitionary: They were nothing more than a brief period of adolescence that was eventually grown out of. Both the yeerks and andalites believed that humanity was nearing the end of its type 5 status, and would quickly mature into a type 4 species like them. This must have been incredibly frustrating to the yeerks. Even after they had picked a needle from a haystack and discovered earth, they would not get an effortless victory over mankind. Conquering them would require scarce resources that were needed elsewhere. And the situation wouldn't have been any better if they had somehow went back in time and discovered humanity back in the 1800s (when its population only numbered 1 billion). The yeerks would have an easier time conquering earth, yes, but the payoff in hosts would be smaller. It takes about 18-19 year for humans to grow to maturity, and they literally didn't have the time to wait that long. The empire had an immediate need for huge numbers of hosts to stem back the advancing tide of the andalite fleet. It was an unenviable situation for them.

*In the movie avatar, the alien na'vi use a neural cord that can attach to those of other species and control their behaviour. Its an interesting analogue, if nothing else.