Chernobyl
On April 26th 1986 the largest nuclear disaster, in the history of man, took place in Chernobyl, Ukraine 100Km north of Kiev. The disaster has been attributed to faulty reactor design, a lack of safety culture, and human error. The environmental ramifications from this disaster will be felt in this area for centuries to come.
It is important to note that the RBMK-1000 reactors that were in use at the Chernobyl NPP were similar to the reactors used by the United States to make the fissile material for the nuclear bombs used on Hiroshima and Nagasaki, Japan. This type of reactor was common in the early 1940’s and 1950’s in the US, but represented extremely antiquated technology for the 1980’s. This was partly due to the USSR isolating themselves both physically and politically from the rest of the world. On the night of April 26th 1986 the number 4 reactor was to be shut down for routine maintenance. It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the main core cooling water circulating pumps, until the diesel emergency power supply became operative. The aim of the test was to determine whether cooling of the core could continue to be ensured in the event of a loss of power. This test had been carried out the previous year, but the power to the turbine ran down too rapidly, so new voltage regulator designs were to be tested. By noon on the day before, in preparations for conducting the test, effective output of the reactor was reduced to 50% and one of the two turbine generators stopped. Further reduction in reactor power was forbidden by the grid dispatcher due to a delay in the start-up of another power plant on the grid. Resumption of the shutdown was finally permitted at approximately 2300, April 25th. The test program called for power to the reactor to be stabilized at 700-1000 MWt, but on April 26th at approximately 0028 power fell precipitously to 30MWt. Per INSAG-7, the precipitous fall in reactor power was caused by operator error. Efforts to compensate for the rapid loss of power were exasperated by a combination of xenon reactor poisoning, reduced cooling, and graphite cool down. Many of the control rods were extracted from the core to compensate for these conditions, resulting in a violation of the Operational Reactivity Margin (ORM) at 0100, though the reactor operators may not have known this. At 0103 the reactor stabilized at 200 MWt, and the decision was made to go forward with the test at this power level. Calculations performed after the accident showed that the ORM at 0122 was equal to eight manual control rods. The minimum permissible ORM stipulated in the operating procedures was 15 rods. The test commenced at 0123; the turbine stop valves were closed and the four pumps powered by the slowing turbine started to run down. The slower flow rate, together with the entry to the core of slightly warmer feed water, may have caused boiling at the bottom of the core. Combined with xenon burnout this may have resulted in a runaway increase in power. An alternative view is that the power excursion was triggered by the insertion of the control rods after the scram button was pressed at 0123. At 0123, the power excursion rate emergency protection system signals came on and power exceeded 530 MWt and continued to rise. Fuel elements ruptured, leading to increased steam generation, which in turn further increased power because of the large positive void coefficient. Damage to even three or four fuel assemblies would have been enough to lead to the destruction of the reactor. The rupture of several fuel channels increased the pressure in the reactor to the extent that the 1000-ton reactor support plate became detached, consequently jamming the control rods, which were only halfway down by that time. As the channel pipes began to rupture, mass steam generation occurred as a result of depressurization of the reactor cooling circuit. A note in the operating log of the Chief Reactor Control Engineer reads: "01:24: Severe shocks; the RCPS rods stopped moving before they reached the lower limit stop switches; power switch of clutch mechanisms is off." Two explosions were reported, the first being the initial steam explosion, followed two or three seconds later by a second explosion, possibly from the build-up of hydrogen due to zirconium-steam reactions. Fuel, moderator, and structural materials were ejected, starting a number of fires, and the destroyed core was exposed to the atmosphere. There were 30 deaths directly credited to the Chernobyl NPP accident. Two of the deaths came were directly attributed to the explosions of the reactors. The casualties included firefighters who attended the initial fires on the roof of the turbine building. All these were put out in a few hours, but radiation doses on the first day were estimated to range up to 20,000 millisieverts (mSv), causing 28 deaths – six of which were firemen – by the end of July 1986. The accident caused the largest uncontrolled radioactive release in the environment ever recorded for any civilian operation, and large quantities of radioactive substances were released into the air for almost 10 days. The entire spectrum of noble gases was ejected during the explosion and subsequent discharge of materials from the reactor core. To this day, the area surrounding Chernobyl is considered an uninhabitable wasteland. But, over the last 25 years, there has been a boom of biodiversity despite popular knowledge of Chernobyl existing in a state of ecological disrepair. Populations have reemerged and flourished in Chernobyl for several reasons, the least of which has been the absence of human activity. With the evacuation of nearly 135,000 people agricultural plots and disturbed habitats were able to naturally return to grasslands. It is noted that the amount of radiation present at the Chernobyl site has had a negative impact on almost all types of mammals, but it appears that the resurgence of wildlife in the exclusion zone is mainly attributable to the absence of humans. The abundance of animals in the exclusion zone is comparable to a protected natural reserve. During scientific excursions into the exclusion zone there were noted many moose, roe deer, Russian wild boar, foxes, river otter, and rabbits. The scientists spent a comparable amount of time outside the exclusion zone, searching for signs of animals and noted only one rabbit outside the area. The top predators, eagles and wolves, were also noted to be more abundant within the exclusion zone than outside the area.