自20世紀(jì)50年代起,人們就一直在利用核反應(yīng)堆發(fā)電。2003年初時(shí),世界上的30 個(gè)國家中共有441臺(tái)核電機(jī)組在運(yùn)行,總裝機(jī)容量為359GW。
Nuclear reactors have been producing electricity since the 1950s and, in early 2003, there were 441 nuclear reactors operating in 30 countries with a total installed capacity of 359 GW.
核反應(yīng)堆
Nuclear reactors
核反應(yīng)堆的運(yùn)行依賴于中子與燃料的原子核之間的核反應(yīng)。鈾作為幾乎所有反應(yīng)堆和鈾-238。早期的反應(yīng)堆使用天然鈾作為燃料,它所包含的這兩種同位素的質(zhì)量百分組成分別為0.7%和99.3%。目前大多數(shù)正在運(yùn)行的反應(yīng)堆使用的是富集鈾,它所包含的鈾-235約占2.5%。
Nuclear reactors depend on a reaction between neutrons and the atomic nuclei of the fuel for their operation. Uranium the fuel for almost all reactors consists principally of two isotopes uranium-235 and uranium-238. In natural uranium, the fuel for early reactors, those isotopes are in the proportion of 0.7 per cent and 99.3 per cent, respectively, by weight. The enriched uranium used in most currently operating reactors contains about 2.5 per cent of uranium-235.
鈾-235的原子核吸收中子并發(fā)生裂變時(shí)就釋放能量。所謂裂變,就是鈾-235的原子核分裂為兩個(gè)攜帶著巨大能量的碎片(或稱裂變產(chǎn)物),并伴隨著釋放幾個(gè)高能中子(或稱快中子)及一些Y射線。這些中子在反應(yīng)堆中被減速,以便誘發(fā)鈾-235進(jìn)一步裂變。此類中子常被稱作熱中子,依靠熱中子工作的反應(yīng)堆被稱為熱中子反應(yīng)堆。與鈾-235不同的是,鈾-238的原子核吸收快中子后，并不發(fā)生裂變而是變成鈾-239,最終衰變?yōu)轭?239。钚-239吸收熱中子后可以發(fā)生裂變,也可以俘獲中子后形成其他的錒系元素,如镅或鋦。目前正在考慮的一個(gè)問題是給某些反應(yīng)堆供應(yīng)混合氧化物燃料(記作MOX),它是富集鈾和從乏燃料后處理中回收的钚的混合物。這被認(rèn)為是使燃料回用和控制可用于制造核武器的钚的儲(chǔ)量的一種好方法。
Energy is released when a uranium-235 nucleus absorbs a neutron and undergoes fission, that is, it splits into two large energetic fragments or fission products, accompanied by the release of several high energy or fast neutrons and some gamma radiation. The neutrons are slowed in the reactor so that they induce further fissions in the uranium-235. Such neutrons are often called thermal neutrons and the reactors that rely upon them thermal reactors. By contrast, when a nucleus of uranium-238 absorbs a fast neutron, it becomes uranium-239, which ultimately decays to form plutonium-239. This will also fission or capture neutrons to form isotopes of additional actinides, such as americium or curium. Consideration is currently being given to fuelling some reactors with mixed oxide fuel (known as MOX which contains enriched uranium mixed with plutonium recovered from spent fuel by reprocessing. This is seen as a way of recycling fuel and controlling stockpiles of plutonium that can be used to make nuclear weapons.
燃料在核反應(yīng)堆中被組裝成陣列狀的堆芯。堆芯內(nèi)還包含稱作慢化劑的材料,通常為水或石墨,它能減緩中子的速度,或稱作 使中子熱化。燃料中由裂變產(chǎn)生的熱量靠冷卻劑(通常為水或氣體)導(dǎo)出,然后讓冷卻劑通過熱交換器以產(chǎn)生蒸汽,再由這些蒸汽 驅(qū)動(dòng)汽輪機(jī)發(fā)電。
The fuel in a nuclear reactor is assembled in an array called the core which also contains the moderator, a material, generally water or graphite that slows or thermalizes the neutrons A coolant, usually water or gas, conducts heat away from the fuel and then passes through heat exchangers to make steam The steam then drives turbine generators to make electricity.
燃料密封于金屬包殼中,堆芯被置于壓力容器中(在某些設(shè)計(jì)中,燃料元件被裝在單獨(dú)的壓力管中)。厚重的混凝土屏蔽有助于 吸收運(yùn)行期間和運(yùn)行之后堆芯發(fā)射的強(qiáng)烈輻射。多數(shù)反應(yīng)堆還附加了一個(gè)密封的安全殼, 把反應(yīng)堆以及熱交換器圍了起來。
The fuel is sealed in metal containers and the core is contained in a pressure vessel (or, in some designs, fuel elements are contained in separate pressure tubes). Massive concrete shielding helps to absorb the intense radiation emitted by the core during and after operation. Most reactors also have an additional containment building surrounding the reactors and usually the heat exchangers.
新燃料僅有輕微的放射性,不需屏蔽即可裝卸。但是,一旦裝入反應(yīng)堆中,反應(yīng)堆運(yùn)行后其活度即大大增強(qiáng),這主要是由于在燃料中產(chǎn)生了裂變產(chǎn)物。這意味著,反應(yīng)堆一旦發(fā)生事故就有可能釋放出大量的放射性物質(zhì)。用過的燃料稱作乏燃料,從反應(yīng)堆中取出之后仍然還有余熱,因此除了要加以屏 蔽以減少輻射照射外,還必須加以冷卻以防止乏燃料熔化。
Fresh fuel is only mildly radioactive and can be handled without shielding Once in the reactor, however, there is an enormous increase of activity due mainly to the fission products that have been generated in the fuel; this means that an accident at the reactor could release significant amounts of radioactive material. After removal from the reactor the spent fuel remains hot and must be cooled to prevent melting, as well as shielded to reduce radiation exposure.
雖然對(duì)于所有的核電站而言安全都是至關(guān)緊要的,但自從發(fā)生了切爾諾貝利事故和蘇聯(lián)(USSR)解體以來,人們特別關(guān)注WWER 和RBMK這兩種反應(yīng)堆的安全性。在許多國際合作項(xiàng)目的支持下,由于東歐和前蘇聯(lián)專家們的努力,在提升這些反應(yīng)堆的安全性方面已取得了很大的進(jìn)展。
Although safety is a crucial issue for all nuclear power plants, there has been a particular focus since the Chernobyl accident and the breakup of the USSR on the safety of WWER and RBMK reactors Thanks to the efforts of specialists in eastern Europe and the former Soviet Union supported by many international cooperation projects, great progress has been made in upgrading the safety of these reactors.
