Seite - 93 - in Water, Energy, and Environment - A Primer

amounts of hydrogen into massive amounts of helium and releasingmassive amounts of energy. It has been doing this formore than four billion years, and is expected to continue doing this for about another five billion when its hydrogen supplywill finallydwindle.At this latter point the fusion reactions in the core of the Sunwill no longer be able to offset the gravitational forces acting on the Sun’s very large mass and the Sunwill explode as the CrabNebula did in 1054. Itwill thenexpandandswallowuptheEarthand itsotherplanets. 7.2.2 Numbers Tounderstand thequantityof energy released: everycubicmetre of seawater, on average, contains 30 grams of deuterium. As mentioned in Chapter 1, there are 300 million cubic miles of water on Earth, 97% in the oceans. Each deuterium nucleus (one proton+ one neutron) weighs so little (3.3millionths of a trillionth of a trillionth of a kilogram) that these 30 grams amount to close to a trillion trillion nuclei. Each time one of these nuclei is fused with a tritium nucleus (one proton+ two neutrons) 17.6MeV (millions of electron volts) of energy is releasedwhich can be captured as heat. NowMeV sounds like a lot of energy but it isn’t– aBtu, amore commonenergy unit, is 6.6 thousand trillionMeV). Now this is a lot of numbers, somevery small and somevery large, but taking them all together that cubicmetre of seawater can lead to the production of about 7 million kWh of thermal energy, which if converted into electricity at 50% efficiency corresponds to 3.5 million kWh. If one were to convert the potential fusion energy in just over 1 million cubic metres of seawater (a small fraction of a cubicmile) one could supply the total annual US electricity production of 4 trillion kWh – and remember that our oceans contain several hundredmillion cubic miles of water. This is why some people get excited about fusion energy. Nuclearpower 93