什么是核反应堆衰变热
Sophia发表于2011-03-17 12:20:23
为什么核电站的冷却系统出了问题，会给核泄漏带来如此巨大的影响？而反应堆的热能到底来源于哪里？
核反应堆发电的原理与传统的火力发电原理相似，都是以蒸汽驱动涡轮旋转发电。它们的区别在于热力的产生。传统火电站用锅炉燃烧煤产热，而核反应堆通过核裂变产热。日本福岛核电站应用的是沸水式核能发电模式，原子能使水加热，水沸腾形成水蒸气，然后水蒸气带动涡轮运转从而产生电力
核反应堆的主要热量由同位素铀235和钚239裂变产生。当中子激发同位素发生裂变的时候会释放出大量的热能，以及更多的中子。中子激发更多的核裂变，在冷冻剂的作用下不断产生热能发电。平均而言，大约80%的能量由核裂变产生，这些裂变在很短的时间内再促进更多的裂变。余下的约20%的热量由超热中子或是其他形式的辐射产生。
当所有的控制棒完全插入抑制核反应的时候，核反应被关闭，裂变反应基本上被终止，热量水平以最快的速度下降，几乎降低到原有水平的7%。能量水平不是降低到零，因为裂变产生的放射延迟会产生一些热量。裂变产物除了热量还有一些电磁辐射，如γ射线、β粒子和α粒子。衰变辐射也能释放部分热量，这种热量被称为衰变热。只有当放射性同位素持续衰变，越来越多的放射性同位素处于稳定状态并停止产生电磁辐射能，衰变热才能彻底停止。
要彻底消除衰变热必须在衰变热产生之初就迅速冷却，否则核反应芯又会重新加热。正常情况下，在核反应被控制棒关闭后，冷却系统还要工作一段较长的时间以确保中和衰变热。然而，福岛地震导致的海啸破坏了核反应器的冷却系统，导致衰变产生的热量很难被控制。
衰变热会在核反应关闭后一段时间慢慢减少。以下这张图显示的是不同时间福岛核电站衰变热的变化趋势。这并不是福岛核电站关闭反应后的实际衰变热变化趋势，而是根据以往核反应关闭后的衰变热变化建立的模型推测所得的结果。
What is Decay Heat?
Posted on
March 16, 2011 7:01 am UTC by
mitnseExplanation of Nuclear Reactor Decay Heat
Nuclear reactors produce electricity in a similar way to conventional coal plants in that they heat steam to drive a turbine that spins an electric generator. However, they differ on how that heat is produced. Coal plants burn coal to heat a boiler that produces the steam while nuclear reactors use nuclear fission to create the heat. The Fukushima reactors are boiling water reactors (BWRs) that produce the steam directly in the reactor core, which then drives the turbines.
The heat in an operating reactor is produced mainly by the fission of fissile isotopes such as uranium-235 and plutonium-239. When a neutron causes one of these isotopes to split, a large amount of energy is released, which is then deposited in the fuel, cladding, coolant, and structures. On average, approximately 80% of the energy released in a fission reaction is imparted to the two or more fission products and these deposit their energy in the fuel since they have a very short range. The rest of the energy is released in the form of neutrons, and other forms of radiation.
When there is a SCRAM, where all the control rods are inserted and the reactor is shutdown, the fission reactions essentially stop and the power drops drastically to about 7% of full power in 1 second. The power does not drop to zero because of the radioactive isotopes that remain from the prior fissioning of the fuel. These radioactive isotopes, also called fission products, continue to produce various types of radiation as they decay, such as gamma rays, beta particles, and alpha particles. The decay radiation then deposits most of its energy in the fuel, and this is what is referred to as decay heat. As these radioactive isotopes continue to decay, more and more of them reach a stable state and stop emitting radiation, and thus no longer contribute to the decay heat.
The decay heat must be removed at the same rate it is produced or the reactor core will begin to heat up. The removal of this heat is the function of the various reactor core cooling systems that provide water flow through the reactor core and then reject the heat elsewhere. However, at the Fukushima site the integrity of these systems were compromised by the large tsunami that resulted from the earthquake, and made it difficult for the operators to keep up with removing the decay heat.
The amount of the decay heat expected at various times after shutdown is well known. Below is a figure and a table that show an estimate of the decay heat of Fukushima Units 1-3 in MW as time has progressed since the earthquake. This data is not produced from measured data on the actual reactors at Fukushima, but from using a well established model that is routinely used to estimate decay heat from shutdown reactors.
Approximate reactor decay heat vs. time. The curves begin after the SCRAM of the reactors (complete and rapid control rod insertion) that occurred immediately after the earthquake.
Tabulation of approximate decay heat for the Fukushima reactors from 1 second after the scram caused by the earthquake until 1 year after the event.
Date/Time (Fukushima Time) Fukushima Daiichi-1 Decay Heat (MW) Fukushima Daiichi-2 & 3 Decay Heat (MW) Percent of Full Reactor Power
3/11/11 2:46 PM 92.0 156.8 6.60%
3/11/11 2:47 PM 44.7 76.2 3.21%
3/11/11 2:48 PM 36.9 62.8 2.64%
3/11/11 2:50 PM 31.4 53.5 2.25%
3/11/11 3:00 PM 24.1 41.0 1.73%
3/11/11 3:30 PM 19.1 32.5 1.37%
3/11/11 8:00 PM 12.8 21.9 0.92%
3/12/11 8:00 AM 10.1 17.3 0.73%
3/12/11 8:00 PM 9.1 15.5 0.65%
3/13/11 8.5 14.5 0.61%
3/14/11 7.8 13.2 0.56%
3/16/11 6.9 11.8 0.50%
3/20/11 6.1 10.5 0.44%
4/1/11 5.2 8.8 0.37%
7/1/11 3.7 6.3 0.26%
10/1/11 3.3 5.6 0.23%
3/11/12 2.9 5.0 0.21%
福岛第一核电站的发电功率为460兆瓦，第二和第三核电站的功率为784兆瓦。由于热力学和实际限制，发电的效率只有33%。在这种情况下产生的热功率大约是电功率的3倍，这些热功率就是必须被清除的，热功率显示在上面的图和表中。在核反应产生的热量降至平时的2%后的第一天，衰变热就降低得很缓慢乐。1年后，衰变热大约是核反应产生的0.2%。
如果衰变热没有停止就会导致核然料重新加热，那么最可怕的结果就可能出现，如锆合金外壳快速氧化 (1200℃)，外壳熔化（1850℃）和燃料熔化（2400-2860℃）。
Fukushima unit 1 has an electrical rating of 460 MWe and units 2 and 3 have an electrical rating of 784 MWe. However, due to various thermodynamic and practical constraints, the efficiency of the plants is only about 33%. Therefore, they have thermal ratings (MWth) about 3 times that of the electrical ratings and this thermal energy is the energy that must be removed, and is what is shown in the figure and table above. The decay heat drops off very slowly after about 1 day where the decay power is already below 2% of the operating power of the reactor. After a year the decay power is about 0.2% of the operating power of the reactor.
If the decay heat is not removed then the reactor fuel begins to heat up and undesirable consequences begin as the temperature rises such as rapid oxidation of the zircaloy cladding (~1200C), melting of the cladding (~1850C), and then the fuel (~2400-2860C).
