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Burlington Atomic Energy Week, 1946-1950
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This Fateful Atom... Looking back twelve months to the birth of Year 1, Atomic Age, we begin to sense the majestic import of the atomic bomb that blasted the naked desert at Alamogordo, N. M., on July 16, 1945. There man first shattered atoms in an explosive fast-chain reaction. Then came Hiroshima and Nagasaki. In every case the fateful atom was either uranium 235 (U235), or plutonium derived from the action of U235 on U238. Every pound of U235 atoms split in these unprecedented blasts yielded the energy of 11.4 million kilowatt-hours, or 1400 tons of coal - slightly more for plutonium. No matter where one mines uranium ore, the purified natural uranium (Fig. 1) always contains 99.3% of the "garden" variety U238, and a mere 0.7% of the precious U235. An atom is like our solar system. The central sun is the nucleus -- a bunched mass of protons and neutrons, each weighing one unit. The planets are electrons. Each proton has one plus electrical charge - each electron an equal negative charge. There must be as many negative protons electron planets as positive protons in the nucleus. This is also the "number" of the atom. Neutrons have no charge but add weight. The atomic number of uranium is 92 because the uranium atom always has 92 nuclear protons and 92 electron planets. The isotopes U238 and U 235 differ only in the number of neutrons; U238 has 146 neutrons, and weighs 92 + 146 = 238. U235 has 143 neutrons, and weighs 92 + 143 = 235 units. Ordinary chemical reactions, such as TNT explosions, release only a fraction of the modest energy of the whirling electrons in the outer atom. Nuclear reactions unlock the immensely greater energies which bind together the nucleus. Even the gentle tap of a slow-moving neutron bullet will split the atom of U235 or of man-made plutonium into two medium-weight atoms, yielding also one to three spare neutrons plus energy. Thus these fissionable materials supply both their own bullets and a highly sensitive lot of high-explosive targets - a perfect setup for a chain reaction (Fig. 2). Chain reactions work like chain letters. Neutrons from one nuclear explosion hit and explode other nuclei. But, since atoms are mostly open spaces a chain started in a small block of U235 or plutonium quickly dies out because most of the released neutrons escape from the block. The bigger the block, the smaller will be the percentage of escaping neutrons, and the more left to split other nuclei. When the block is rapidly built up beyond a certain secret size the fragments of 1000 nuclear fissions split many more than 1000 additional nuclei. Then fissions multiply geometrically, and the block disintegrates with explosive speed and violence - as in a bomb (Fig. 3). This bomb explosion is a fast-neutron chain. For economy and ease of control, uranium piles for the gradual release of nuclear energy for commercial purposes will normally use a lean fuel - that is U235 or plutonium diluted with U238, thorium or other less costly materials. To maintain a chain reaction such piles must be large and artificially stimulated by using carbon blocks or some other moderator (Fig. 4) to slow many of the neutrons. Slow neutrons make more hits than fast neutrons because there is more time for them to be swerved from a straight path by the attraction of nearby nuclei, as shown below. [[Figure 1 text]] ORE TO U235 Only 0.7% of natural uranium is U235 URANIUM ORE 140 LB NATURAL URANIUM 139 LB U 238 = 99.3% ILB U235 = 0.7% 92 ELECTRON PLANETS NUCLEUS 92 PROTONS 143 NEUTRONS U 235 ATOM [[Figure 2 text]] CHAIN REACTION Fragments from earlier nuclear explosions smash other nucleii [[Figure 3 text]] WHY BOMB EXPLODES When block of rapidly assembled U235 passes secret critical size it explodes spontaneously HARMLESS WEIGHS X MINUS WEIGHS X MINUS EXPLODING WEIGHS X PLUS [[Figure 4 text]] SLOW NEUTRONS MAKE MORE HITS A slow neutron is more easily swerved from a straight line SLOW NEUTRONS FAST NEUTRONS U235 CARBON
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This Fateful Atom... Looking back twelve months to the birth of Year 1, Atomic Age, we begin to sense the majestic import of the atomic bomb that blasted the naked desert at Alamogordo, N. M., on July 16, 1945. There man first shattered atoms in an explosive fast-chain reaction. Then came Hiroshima and Nagasaki. In every case the fateful atom was either uranium 235 (U235), or plutonium derived from the action of U235 on U238. Every pound of U235 atoms split in these unprecedented blasts yielded the energy of 11.4 million kilowatt-hours, or 1400 tons of coal - slightly more for plutonium. No matter where one mines uranium ore, the purified natural uranium (Fig. 1) always contains 99.3% of the "garden" variety U238, and a mere 0.7% of the precious U235. An atom is like our solar system. The central sun is the nucleus -- a bunched mass of protons and neutrons, each weighing one unit. The planets are electrons. Each proton has one plus electrical charge - each electron an equal negative charge. There must be as many negative protons electron planets as positive protons in the nucleus. This is also the "number" of the atom. Neutrons have no charge but add weight. The atomic number of uranium is 92 because the uranium atom always has 92 nuclear protons and 92 electron planets. The isotopes U238 and U 235 differ only in the number of neutrons; U238 has 146 neutrons, and weighs 92 + 146 = 238. U235 has 143 neutrons, and weighs 92 + 143 = 235 units. Ordinary chemical reactions, such as TNT explosions, release only a fraction of the modest energy of the whirling electrons in the outer atom. Nuclear reactions unlock the immensely greater energies which bind together the nucleus. Even the gentle tap of a slow-moving neutron bullet will split the atom of U235 or of man-made plutonium into two medium-weight atoms, yielding also one to three spare neutrons plus energy. Thus these fissionable materials supply both their own bullets and a highly sensitive lot of high-explosive targets - a perfect setup for a chain reaction (Fig. 2). Chain reactions work like chain letters. Neutrons from one nuclear explosion hit and explode other nuclei. But, since atoms are mostly open spaces a chain started in a small block of U235 or plutonium quickly dies out because most of the released neutrons escape from the block. The bigger the block, the smaller will be the percentage of escaping neutrons, and the more left to split other nuclei. When the block is rapidly built up beyond a certain secret size the fragments of 1000 nuclear fissions split many more than 1000 additional nuclei. Then fissions multiply geometrically, and the block disintegrates with explosive speed and violence - as in a bomb (Fig. 3). This bomb explosion is a fast-neutron chain. For economy and ease of control, uranium piles for the gradual release of nuclear energy for commercial purposes will normally use a lean fuel - that is U235 or plutonium diluted with U238, thorium or other less costly materials. To maintain a chain reaction such piles must be large and artificially stimulated by using carbon blocks or some other moderator (Fig. 4) to slow many of the neutrons. Slow neutrons make more hits than fast neutrons because there is more time for them to be swerved from a straight path by the attraction of nearby nuclei, as shown below. [[Figure 1 text]] ORE TO U235 Only 0.7% of natural uranium is U235 URANIUM ORE 140 LB NATURAL URANIUM 139 LB U 238 = 99.3% ILB U235 = 0.7% 92 ELECTRON PLANETS NUCLEUS 92 PROTONS 143 NEUTRONS U 235 ATOM [[Figure 2 text]] CHAIN REACTION Fragments from earlier nuclear explosions smash other nucleii [[Figure 3 text]] WHY BOMB EXPLODES When block of rapidly assembled U235 passes secret critical size it explodes spontaneously HARMLESS WEIGHS X MINUS WEIGHS X MINUS EXPLODING WEIGHS X PLUS [[Figure 4 text]] SLOW NEUTRONS MAKE MORE HITS A slow neutron is more easily swerved from a straight line SLOW NEUTRONS FAST NEUTRONS U235 CARBON
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