![]() In this numerical simulation the reactor is turned on at time = 0 and shut down at time = 100. Moles of 135I and 135Xe produced and 235U remaining for each kg of 235U in a reactor. As there are no (or few) neutrons, the only way the 135Xe poison is removed is through its small "natural" decay hole. The parent bucket is full and empties, over time, into the daughter (135Xe) bucket. If you turn a reactor off, or throttle it back, fission and the neutron flux drops. In a reactor running at a steady(-state) rate of k = 1, the amount of 135Xe being made and the amount of 135Xe being lost by neutron capture or natural decay are equal. (135Xe reacts with neutrons about a million times faster than most other nuclei.) 135Xe is called a "reactor poison" because it throttles back k by consuming neutrons, reducing the likelihood of continuing the fission chain. What was understood in the 1940s is that the daughter bucket (135Xe) can also be drained by the neutrons generated in fission, i.e. 135Xe is a reactor poison because it has an extraordinarily large probability to absorb neutrons and convert to 136Xe. One isotope, of one of the pairs, is 135I that decays to 135Xe that in turn decays 135Cs. Control rods, and other neutron absorbing components, allow for this steady-state (k=1) operation. Fission generates pairs of fragments. As more than one neutron is produced, the object of a reactor is to ensure that one (and only one) of the produced neutrons is fed back into the next generation of fission. Nuclear fission is both initiated by and produces neutrons. The mental image is that fission fills a bucket called 135I, and that bucket has a hole in it that fills another bucket labeled 135Xe that also has a hole in it. This nucleus is unstable and decays with a half-life of 6.6 hours to xenon-135, which itself has a "natural" half-life of 9.1 hours. ![]() The energy release comes from the fact that the mass of the products is less than the mass of reactants.) About 6.5% of the fission events produce iodine-135. uranium-235) and a neutron combine, producing two smaller nuclei and a few free neutrons while releasing a lot of energy. In nuclear fission, a heavy nucleus (i.e. In any analogy, k > 1 cannot go on forever. ![]() Consider it this way: k is the amount of fission at one time, compared to what it was an instant before. So, k = 1 means that the fission rate is constant, less than 1 means the rate is spiraling down, and greater than 1 means it is increasing. This is where the term "chain reaction" comes from. Neutrons both initiate fission and are generated by fission. (HBO)Īs explained in episode 5 of Chernobyl, "Vichnaya Pamyat" (Ukrainian for "memory eternal"), there are positive (red cards) and negative (blue cards) drivers of reactor "reactivity." Think of this reactivity, call it "k," as the change in the energy released in the fission process with time. Jared Harris as Valery Legasov describes the positive (red cards) and negative (blue cards) drivers of reactivity. While no short presentation can do justice to the accident, HBO’s depiction of the reactor failure was masterfully done. To nuclear scientists, the idea that you could take a reactor that had been running at normal power, reduce it to an idling power level, and then bring it back up to full power, is foolish and known to be so from experience gained in the early 1940s from the Hanford, WA reactor that produced plutonium for the Manhattan Project. This essay offers a commentary on the HBO series, intended for those who watched all five episodes, focusing on the technical aspects but broaching the subject of how humans and societies construe nuclear technology. The creators of this series were not afraid of technical scientific details, as they realized the story had a scientific core wrapped in how humans, as well as the Soviet bureaucracy, responded to complex truths, both politically troublesome and culturally traumatic. Today, Chernobyl remains a cultural touchstone for the dangers of nuclear technology, serving as a comparison for any nuclear accident.Ī popular HBO series, Chernobyl, was aired this summer detailing, and dramatizing, what happened. ![]() There was a run on iodine pills and a dramatic increase in elective abortions. The reports from the USSR were not informative, but fission products (ash from the nuclear fission process) were being reported in air samples throughout Europe. The newest reactor (#4) at the Chernobyl site exploded, 50 miles north of Kiev in the Ukraine and only a few miles from the border with Belarus. Those of us of a certain age remember that the world shook on April 26, 1986. ![]()
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