VI. Ответьте на вопросы письменно.

1. Why did Magellan lose the favour of the king of Portugal?

2. What did Ferdinand Magellan do after he was dismissed from service to the king of Portugal?

3. How did the Pope divide New World lands between Spain and Portugal?

4. How was the passage found?

5. When did the ships cross the International Date Line?

6. How long did the voyage across the Pacific Ocean last?

7. Why was Magellan killed?

8. What did Magellan's expedition prove?

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ИФиРЭ

Nuclear Reactors

In order to establish a nuclear chain reaction, it is necessary to set up conditions radically different from those prevailing in the earth's crust. In the crust, although uranium is present and stray neutrons are to be found in the atmosphere, no chain reaction exists, or, as-far as we can tell, ever has existed.

The reason for this is that when an atom of uranium undergoes fission (either spontaneously or through absorption of a neutron), the neutrons liberated are absorbed by surrounding atoms. Most of these surrounding atoms are not uranium and are not themselves nudged into fission. The neutrons from fissioning uranium are thus absorbed and no neutrons are re-emitted, so that the potential chain reaction is effectively quenched. There is enough non-uranium material in even the richest natural concentration of uranium to quench any potential chain reaction at once.

What was necessary, then, if a nuclear chain reaction was to have any chance at all, was to make use of pure uranium, in the form of an oxide or even as the metal itself. In the metal, where almost all atoms would be uranium atoms, any neutron liberated by one uranium atom undergoing fission would stand an excellent chance of being absorbed by another uranium atom and therefore of bringing another fission - the next link in the chain.

Shortly after the idea of uranium fission had been accepted Niles Bohr pointed out that on theoretical grounds uranium-235 was much more likely to undergo fission than uranium-238 was. Experiment soon showed Bohr to be right.

To give a nuclear chain reaction a decent chance, uranium would have to be prepared in which uranium-235 was presented in greater, than usual amounts. Such a preparation would involve isotope separation, a difficult task - particularly, if it is to be carried through on a large scale.

Atomic Pile

It was soon realized that a nuclear chain reaction could not be set up in a limited number of uranium. Even uranium-235 atoms will not necessarily always absorb a neutron that comes toward the uranium atom. The neutron may merely bounce off, unabsorbed.

If in the process of bouncing from atom to atom the neutron manages to make its way out of the uranium and, into the open air, it is lost. If enough neutrons do so, the nuclear chain reaction will be quenched. To prevent thus, one must see to it that the chances of loss of neutrons to the surrounding environment, before absorption and consequent fission have a chance to take place, are minimized. The simplest way to do this is to increase the size of the uranium core. The larger its size the more bounces a neutron must undergo before reaching the edge of the core and the greater the chance of its absorption.

If the core is just large enough to lose so few neutrons that the nuclear chain reaction may just keep going, it is said to be at critical size. A smaller core, one of "subcritical size" cannot maintain a "self-sustaining nuclear reaction".

Toward the end of 1942, the first attempt was made to set up a self-sustaining nuclear reaction. This took place under the guidance of Enrico Fermi.

At the time, some pure uranium was available in both metallic form and in the form of the oxide. It was not enriched and so the critical size was extraordinarily high. A very large "atomic pile" had to be built, (it was called a "pile" because it was a pile of bricks of uranium, uranium oxide, and graphite.)

When this first nuclear reactor was completed, it was 30 feet wide, 32 feet long and 21 1/2 feet high. It weighed 1400 tons, of which 52 tons were uranium. The uranium, uranium oxide, and graphite were arranged in alternate layers with, here and there, holes into which long rods of cadmium could be fitted.