Question: Which substance does uranium-238 decay into first?
About the article "Manhattan Project", section "Plutonium".

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Natural concentrations.
Natural uranium consists of three major isotopes: uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%).
All three are radioactive, emitting alpha particles, with the exception that all three of these isotopes have small probabilities of undergoing spontaneous fission, rather than alpha emission.
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth.
Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of 238U atom, and also 49% by 234U (since the latter is formed from the former) and about 2.0% of them by the 235U.
When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of 234U was probably much lower than this.
Uranium-238 is usually an α emitter (occasionally, it undergoes spontaneous fission), decaying through the "Uranium Series" of nuclear decay, which has 18 members, all of which eventually decay into lead-206, by a variety of different decay paths.
The decay series of 235U, which is called the actinium series has 15 members, all of which eventually decay into lead-207.
The constant rates of decay in these decay series makes the comparison of the ratios of parent to daughter elements useful in radiometric dating.
Uranium-234 is a member of the "Uranium Series", and it decays to lead-206 through a series of relatively short-lived isotopes.
Uranium-233 is made from thorium-232 by neutron bombardment, usually in a nuclear reactor, and 233U is also fissile.
Its decay series ends with thallium-205.
Uranium-235 is important for both nuclear reactors and nuclear weapons, because it is the only uranium isotope existing in nature on Earth in any significant amount that is fissile.
This means it can be split into two or three fragments (fission products) by thermal neutrons.
Uranium-238 is not fissile, but is a fertile isotope, because after neutron activation it can produce plutonium-239, another fissile isotope.
Indeed, the 238U nucleus can absorb one neutron to produce the radioactive isotope uranium-239.
239U decays by beta emission to neptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239.
239Pu was used as fissile material in the first atomic bomb detonated in the "Trinity test" on 15 July 1945 in New Mexico. 
238U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are beta particle emitters with half-lives of about 20 days and one minute respectively.
Protactinium-234 (Pa-234) decays to uranium-234 (234U), which has a half-life of hundreds of millennia, and this isotope does not reach an equilibrium concentration for a very long time.
When the two first isotopes in the decay chain reach their relatively small equilibrium concentrations, a sample of initially pure 238U will emit three times the radiation due to 238U itself, and most of this radiation is beta particles.
After all the beta decay is nearly over, the final product of 238U is lead-206 (206Pb).238U radiates alpha-particles and decays (by way of thorium-234 and protactinium-234) into uranium-234.
234U has a half-life of 245,500 years.
The relation between 238U and 234U gives an indication of the age of sediments that are between 100,000 years and 1,200,000 years in age.238U occasionally decays by spontaneous fission or double beta decay with probabilities of 5 and 2 per 100 alpha decays, respectively. 
Uranium (U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234.
The average atomic mass of natural uranium is 238.02891(3) u.
Other isotopes such as uranium-232 have been produced in breeder reactors.
Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.2739 - 99.2752% natural abundance), uranium-235 (0.7198 - 0.7202%), and uranium-234 (0.0050 - 0.0059%).
All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683 years (close to the age of the Earth).
Uranium-238 is an α emitter, decaying through the 18-member uranium series into lead-206.
The decay series of uranium-235 (historically called actino-uranium) has 15 members that ends in lead-207.
The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating.
Uranium-233 is made from thorium-232 by neutron bombardment.
The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons.
The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile. 
Twenty radioactive isotopes of plutonium have been characterized.
The longest-lived are plutonium-244, with a half-life of 80.8 million years, plutonium-242, with a half-life of 373,300 years, and plutonium-239, with a half-life of 24,110 years.
All of the remaining radioactive isotopes have half-lives that are less than 7,000 years.
This element also has eight metastable states, though all have half-lives less than one second.
The isotopes of plutonium range in mass number from 228 to 247.
The primary decay modes of isotopes with mass numbers lower than the most stable isotope, plutonium-244, are spontaneous fission and α emission, mostly forming uranium (92 protons) and neptunium (93 protons) isotopes as decay products (neglecting the wide range of daughter nuclei created by fission processes).
The primary decay mode for isotopes with mass numbers higher than plutonium-244 is β emission, mostly forming americium (95 protons) isotopes as decay products.
Plutonium-241 is the parent isotope of the neptunium decay series, decaying to americium-241 via β or electron emission.
Plutonium-238 and 239 are the most widely synthesized isotopes.
Plutonium-239 is synthesized via the following reaction using uranium (U) and neutrons (n) via beta decay (β−) with neptunium (Np) as an intermediate: formula_1 Neutrons from the fission of uranium-235 are captured by uranium-238 nuclei to form uranium-239; a beta decay converts a neutron into a proton to form Np-239 (half-life 2.36 days) and another beta decay forms plutonium-239.
Workers on the Tube Alloys project had predicted this reaction theoretically in 1940.
Plutonium-238 is synthesized by bombarding uranium-238 with deuterons (D, the nuclei of heavy hydrogen) in the following reaction: formula_2 In this process, a deuteron hitting uranium-238 produces two neutrons and neptunium-238, which spontaneously decays by emitting negative beta particles to form plutonium-238. 
Plutonium-238 was the first isotope of plutonium to be discovered.
It was synthesized by Glenn Seaborg and associates in 1941 by bombarding uranium-238 with deuterons, creating neptunium-238, which then decays to form plutonium-238.
Plutonium-238 decays to uranium-234 and then further along the radium series to lead-206.