![]() ![]() We have just seen how extremely improbable an accident of that magnitude should be. ![]() In the wake of the Chernobyl accident, the primary question on American minds was can it happen here? Let us try to answer that question. It is difficult to imagine how anything worse could happen to a reactor from the standpoint of harming the public outside. In that accident, a substantial fraction of all of the radioactivity in the reactor was dispersed into the environment as airborne dust its most dangerous form. If that should come to pass, a history of energy production written at that remote date may well record that the worst reactor accident of all time occurred at Chernobyl, USSR, in April of 1986. However, based on everything we know now, one can make a strong case for the thesis that nuclear fission reactors will be providing a large fraction of our energy needs for the next million years. It is very difficult to predict the future of scientific developments, and few would even dare to make predictions extending beyond the next 50 years. Timeline for the disaster.Next=> THE CHERNOBYL ACCIDENT CAN IT HAPPEN HERE? So the phenomenon had been dealt with from the earliest days of our experience with nuclear fission, and should have been known by anyone who was in control of a nuclear reactor. They found that they had to increase the fuel concentration to overcome the xenon poisoning. In fact, it was dealt with in the original Manhattan Project where it presented itself as a dilemma - the researchers expected a given configuration to maintain a chain reaction and it failed to do so. ![]() The "xenon poisoning" of the reaction rate had been known for many years, having been dealt with in the original plutonium production reactors at Hanford, Washington. ![]() The increased power then burned away the xenon and also caused voids in the cooling water, both of which rapidly increased the reaction rate, driving it out of control. They apparently did not have the understanding that the failure to increase was due to the absorption of neutrons by the xenon, so they completely removed the control rods to force the increase. When the persons conducting the tests on the Chernobyl reactor tried to increase the power at some point in their tests, it would not respond. It would eventually peak and decrease, but with a 9.2 hour half-life, that decrease would come too late! But when the power level was drastically lowered at the Chernobyl reactor, the xenon-135 concentration began to increase because the parent iodine-135 was near full-power equilibrium concentration to produce it and the neutron flux necessary to "burn it away" was not present. There is an equilibrium concentration of both iodine-135 and xenon-135. Iodine-135 is produced, decays into xenon-135 which absorbs neutrons and is therby "burned away" in the established balance of the operating conditions. In the normal operation of a nuclear reactor, the presence of the xenon-135 is dealt with in the balancing of the reaction rate. The xenon-135 has a very large cross-section for neutron absorption, about 3 million barns under reactor conditions! This compares to 400-600 barns for the uranium fission event. But it has a half-life of about 6.7 hours and decays into xenon-135 (half-life 9.2 hours). It has a rather small probability for absorbing a neutron, so it is not in itself a significant factor in the reaction rate control. Iodine-135 is a rather common fission product, reportedly amounting to up to 6% of the fission products. One of the extraordinary sequences in the operation of a fission reaction is that of the production of iodine-135 as a fission product and its subsequent decay into xenon-135. It is a delicate balancing act requiring detailed knowledge and careful control. To control the chain reaction, neutron absorbers in the control rods limit the rate of reaction, and the moderator (graphite in the case of Chernobyl) slows down the fast neutrons to enable the reaction to be sustained. Neutron absorption is the main activity which controls the rate of nuclear fission in a reactor - the 235U absorbs thermal neutrons in order to fission, and produces other neutrons in the process to trigger other fissions in the chain reaction. "Xenon Poisoning" or Neutron Absorption in Reactors Xenon PoisoningĪ major contribution to the sequence of events leading to the Chernobyl nuclear disaster was the failure to anticipate the effect of "xenon poisoning" on the rate of the nuclear fission reaction in the Chernobyl nuclear reactor. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |