All U. S. nuclear weapons production facilities are presently closed down, and if the various agreements are adhered to, those facilities will never be required except for one critical capability. All modern nuclear weapons use uranium (铀), plutonium (钚), and tritium (氚). Uranium and plutonium have very long half-lives, and there is large surplus of these materials.
Tritium, however, has a relatively short half life of about 12.6 years, so about 5 percent of the amount on hand must be replaced each year to maintain the current inventory. (46) Because of the large retirement of nuclear weapons by the United States in compliance with early agreements and national policy, tritium from retired weapons has been used to make up that lost through natural decay. (47) However, in about 10 to 15 years, depending on future negotiations, the United States will need a guaranteed supply of tritium to maintain its stockpile at whatever level is agreed on.
In antici
All truly great thoughts are conceived
while walking, observed Nietzsche, though I’ve always been a bit suspicious of
the eagerness with which writers and artists celebrate the inspirational power
of taking a stroll. Yet it seems to work. "methinks(我想) the moment my legs begin
to move, my thoughts begin to flow," was how Henry Thoreau described an
experience many of us have had, be it tackling challenging work or worrying over
problems. If we still don’t know why walking inspires clarity and creativity, it’s because there are too many possible explanations, not too few. An evolutionary psychologist might say we’re designed to thrive outside, not at a desk; a scholar of the psychological phenomenon of "priming" might point to studies suggesting that high ceilings-and also, perhaps, the sky—prompt unrestrained thinking. A study in the European Journal A. He wants to challenge things in an extreme way. B. He wants to make violation of writer’s solitude. C. He wants to test his theory of "competing concentration". D. He wants to be forced by something to concentrate. 我来回答: 提交
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