Chernobyl Accident Chernobyl is located in Ukraine, part of the Former Soviet Union. It was the sight of the worst nuclear accident in the history of

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1 Chernobyl Accident Chernobyl is located in Ukraine, part of the Former Soviet Union. It was the sight of the worst nuclear accident in the history of the world. But before we get into the details of the accident, the question can it happen here should be answered, and the answer is NO. This particular type of reactor was very different in its design from reactors licensed in the rest of the world. It was not actually designed to make electricity. It was designed to make Plutonium for the Soviet Unions nuclear weapons programs. The plutonium needed for weapons is the isotope with a mass of 239, or Pu-239. This material is produced by the capture of a neutron in Uranium-238 which is actually much more abundant in the core than the fissile U-235 that operates the reactor. This production path is very desirable as it takes less Pu-239 to make a weapon compared to the U-235, and because there are few isotopes that require the difficult process of isotope separation to be developed. In order to make Pu-239, the reactor also could not be shutdown for refueling, this lead to a design that did not have a formal containment dome, rather it was essentially located inside of an industrial building that could confine a small scale accident. This was not the case when the accident occurred. An accident in the US at the Three Mile Island nuclear power plant that had a containment dome had a full core meltdown, the worst case accident for a US PWR and it was able to contain the vast majority of the radioactive materials within the containment system. This accident resulted in at most one additional statistical deaths in our country.

2 On 25 April, prior to a routine shut-down, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines would spin and supply power following a loss of main electrical power supply. Similar tests had already been carried out at Chernobyl and other plants, despite the fact that these reactors were known to be very unstable at low power settings. A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. As flow of coolant water diminished, power output increased. When the operator moved to shut down the reactor from its unstable condition arising from previous errors, a peculiarity of the design caused a dramatic power surge.

3 The fuel elements ruptured and the resultant explosive force of steam lifted off the cover plate of the reactor, releasing fission products to the atmosphere. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames.

4 There is some dispute among experts about the character of this second explosion. The graphite burned for nine days, causing the main release of radioactivity into the environment. A total of about 12 x 1018 Bq of radioactivity was released. Some 5000 tonnes of boron, dolomite, sand, clay and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles. The timeline of the incident at Chernobyl goes as follows: April 25: Prelude 01:06 The scheduled shutdown of the reactor started. Gradual lowering of the power level began 03:47 Lowering of reactor power halted at 1600 MW(thermal).

5 14:00 The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later. The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly. 14:00 The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MW(t) and the experiment was delayed. Without this delay, the test would have been conducted during `day shift'. 23:10 Power reduction recommenced. 24:00 Shift change. April 26: Preparation for the test 00:05 Power level had been decreased to 720 MW(t) and continued to be reduced. It is now recognised that the safe operating level for a pre-accident configuration RBMK was about 700 Mwt because of the positive void coefficient. 00:28 Power level was now 500 MW(t). Control was transferred from the local to the automatic regulating system. Either the operator failed to give the `hold power at required level' signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MW(t). 00:32 (approximate time). In response, the operator retracted a number of control rods in an attempt to restore the power level. Station safety procedures required that approval of the chief engineer be obtained to operate the reactor with fewer than the effective equivalent of 26 control rods. It is estimated that there were less than this number remaining in the reactor at this time. 01:00 The reactor power had risen to 200 MW(t). 01:03 An additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure). 01:07 An additional pump was switched into the right hand cooling circuit (part of the test procedure). Operation of additional pumps removed heat from the core more quickly. This reduced the water level in the steam separator.

6 01:15 Automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor. 01:18 Operator increased feed water flow in an attempt to address the problems in the cooling system. 01:19 Some manual control rods withdrawn to increase power and raise the temperature and pressure in the steam separator. Operating policy required that a minimum effective equivalent of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of manual rods was reduced to less than this (probably eight). However, automatic control rods were in place, thereby increasing the total number. 01:22:10 Spontaneous generation of steam in the core began. 01:22:45 Indications received by the operator, although abnormal, gave the appearance that the reactor was stable. 01:23:04 Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test. 01:23:10 Automatic control rods withdrawn from the core. An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves. Usually this decrease is caused by an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. The expected decrease in steam quantity did not occur due to reduced feedwater to the core. 01:23:21 Steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power. 01:23:35 Steam in the core begins to increase uncontrollably. 01:23:40 The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core. The insertion of the rods from the top concentrated all of the reactivity in the bottom of the core. 01:23:44 Reactor power rose to a peak of about 100 times the design value. 01:23:45 Fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels. 01:23:49 Fuel channels ruptured. 01:24 Two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapor. The explosions lifted the pile cap, allowing the entry of air. The air reacted with the

7 graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted. Thereafter, over nine days: Some 8 of the 140 tones of fuel, which contained plutonium and other highly radioactive materials (fission products), were ejected from the reactor along with a portion of the graphite moderator, which was also radioactive. These materials were scattered around the site. In addition, cesium and iodine vapors were released both by the explosion and during the subsequent fire. Radioactive materials in the environment can enter you body through as wide range of mechanisms. The primary paths are listed below.

8 The picture below depicts the shell of the reactors confinement structure. The lack of a containment lead to the wide scale distribution of radioactive materials throughout the countryside.

9 This image is of the reactors fuel sitting in the basement of the building. It melted its way through several concrete floors to finally end up in this structure known as the elephants foot. The accident site is currently covered in a concrete structure called the Sarcophagus. This structure is slated for replacement in the near future with one that will be the tomb of the reactor for the next hundred years. This accident was a horrific accident that could have been avoided if it were not for the cold war mentality that drove its construction and operation. Reactors of this kind cannot be licensed in our country and the FSU is currently in the process of taking these reactors out of service.

10 Which Nuclear Energy Champion said the following? There is now a great deal of scientific evidence showing nuclear power to be an environmentally sound and safe choice. A doubling of nuclear energy production would make it possible to significantly reduce total [greenhouse gas] emissions nationwide. In order to create a better environmental and energy-secure future, the [United States] must once again renew its leadership in this area." Guess who? The pro-nukes manifesto came from GREENPEACE founder Patrick Moore. The head of Greenspirit Strategies testified before Congress this past April, and he's not the only big-foot environmentalist who's rethinking nuclear power. When Britain's Hugh Montefiore, a longtime trustee of Friends of the Earth was ready to make a pro-nukes pronouncement ("I have now come to the conclusion that the solution [to global warming] is to make more use of nuclear energy"), his colleagues made him resign. Spent Reactor Fuel (high level Nuclear Waste) After several years in the core of a nuclear reactor, the uranium dioxide fuel needs to be replaced to ensure that the safe and effective operation of the reactor. This fuel is highly radioactive when it is removed from the core as it contains all of the fission products that accumulated over the life cycle. When removed from the core, the fuel still retains a higher level of enrichment with the fissile Uranium-235 than naturally occurring uranium 1.4% verse 0.7% for natural uranium. In addition to the U-235 in the fuel, exposure of U-238 to thermal neutrons results in the production of Plutonium-239, a material with fission characteristics similar to those of U-235. In fact, the amount of fuel that is produced in the core of a power reactor from the fission of Pu-239 at the end of a reactors core useful lifetime is close to half.

11 Just as the Russian RMBK reactors were designed to make Pu-239 for nuclear weapons production, reactors can be optimized to actually produce more nuclear fuel than they use. This is possible when you once again allow neutrons to interact with U-238 to produce Pu-239. Pu-239 can also be used to operate a nuclear power plant, so reactors that have their cores lined with fertile U-238 can be used to breed new fuel in the form of Pu-239 which is fissile. This type of reactor is often call a breeder reactor. The North Koreans have a reactor that is dedicate to this operations cycle where Pu-239 is being produced. Pu-239 is chemically separable from uranium. This characteristic allows for rogue nations to get their hands on a material that can readily be used to make a nuclear weapon. With this capability in mind, President Jimmy Carter declared it illegal to reprocess fuel. The then president wanted to limit the proliferation of nuclear weapons. If the US did not engage in this technology, it was hoped that we could lead by example, and show the rest of the world that paths leading to the development of nuclear weapons was not required to have an economically viable nuclear power program. The spent nuclear fuel would merely be buried in the ground and left to decay. Note, President Jimmy Carter was a Nuclear Engineer by training. He served as an officer on a nuclear powered submarine prior to becoming president Close to this time, the US government also started taxing nuclear power plant operators for the disposal of their spent fuel. The DOE charged all nuclear power plants several thousandth of a cent per kilowatt hours of power generated. The US government has taken in Millions of dollars for the disposal of these materials, yet they remain on site at the plants where they were produced. It also does not appear that the plants will be shipping the spent fuel for disposal in the near future. The current Senate Leader, Harry Reid of Nevada, has categorically stated that the disposal facility will not be allowed to operate. This is also after Nevada has accepted more than $2 Billion in funds to develop the facility. The government is being sued by the utility companies that have the spent nuclear fuel on site for the storage costs. The materials were initially supposed to be being

12 shipped in the 1990 s. Due to the contractual clauses in the taking of the money associated with this program, the government has also been loosing rather harshly, being required to expend funds to pay for the storage of the fuel at the reactor sites. The spent nuclear fuel is first stored in pools where the energy from the decay of the fission products can safely be removed. It the fuel is not constantly cooled for several years it could potentially melt and release the radioactive materials. This is a picture of a spent fuel element. Each grid position contains a spent fuel assembly or element. The element is composed of a series of rods containing the Uranium oxide fuel pellets.

13 The dry cask storage systems are considered to be an excellent means to store these materials that will be effective in containing the radioactive materials for the next years. Where/What is Yucca Mountain? Aside from the fact the spent nuclear fuel from domestic nuclear power reactors in the United States cannot be reprocessed other solutions to the long term disposal of these highly radioactive materials have led to the application of an underground repository where the materials will be geologically isolated for thousands of years.

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