What is the full form of TMI
Three Mile Island
On March 28, 1979, one of the most serious reactor accidents to date occurred in Block 2 of the Three Mile Island nuclear power plant (TMI for short) near the city of Harrisburg in the US state of Pennsylvania. According to the INES scale, which did not exist at the time, the accident would be classified in level 5 as a “serious accident”.
Course of the accident
The accident was initially triggered by the failure of a main condensate pump in the early morning (around 4:30 a.m. local time) and the complete failure of the feed water supply. As a result, deficiencies in the technical design of the plant and incorrect decisions by the operating team resulted in failure of the cooling of the reactor core. The failure of the cooling system led to a partial core melt around two hours after the start of the accident. During the accident, radioactive substances (iodine-131 and radioactive noble gases such as krypton-85) first entered the containment with the leaked coolant and later - via a line that was blocked too late - later escaped into an adjoining building. Around 5% of the noble gases released from the core and a significantly smaller amount of iodine were released into the environment from there. The cooling of the reactor core was finally restored after around 15.5 hours and the accident ended. It then took a few days until some of the hydrogen produced by the core meltdown - the remaining part had burned in the containment during the accident - was removed from the reactor. This was done, among other things, by discharging it into the atmosphere, which in turn released radioactive substances into the environment.
Release of radioactive substances
Due to the release of radioactive materials, the governor of the state of Pennsylvania ordered that pregnant women and children be evacuated from within five miles of the facility at noon on April 30th. Thousands of other people also fled the area. A subsequent long-term study over 18 years on around 30,000 residents of the affected area came to the conclusion that the accident did not cause any damage to health. This has been questioned by various non-governmental organizations and citizens' groups. Other later studies indicate a significant increase in the number of cancers within a few kilometers of the facility.
The work carried out to date to dismantle TMI-2 began in August 1979 and lasted until the end of 1993. So far, only the nuclear fuel has been removed and the reactor dismantled. The rest of the facility, such as the reactor building and other components, are expected to be dismantled together with Unit 1 after it has been decommissioned. The costs incurred so far for the dismantling are estimated at almost one billion US dollars.
Causes of the accident
Investigations into the course of the accident revealed that the accident was caused by an interplay of technical, human and organizational failure. From today's perspective, the design of the system at that time had a number of weaknesses. For example, there was no clear display of the coolant level in the reactor pressure vessel (RPV). During the accident, this led to the operating team at times wrongly assuming a sufficient fill level and therefore stopped the automatic feed of coolant into the RPV. Deficiencies at the organizational level included the lack of suitable emergency procedures and appropriate training. In addition, there was an inadequate safety culture both on the part of the operator and the supervisory authority. Up until the accident, there was a lack of an effective evaluation of previous events and - subsequently - suitable measures to remedy safety-related weaknesses. Specifically, faults in components that had had a decisive influence on the course of the accident had already occurred in a similar form in TMI and another system without any consequences being drawn from them.
The lessons learned from the accident resulted in extensive changes in safety philosophy and technical upgrades in the United States and other countries in the years that followed. In Germany, for example, the above-mentioned Measuring devices for the RPV level, systems for breaking down hydrogen in the containment (so-called recombiners) and devices for filtered pressure relief of the containment have been retrofitted. Corresponding measures, however, were not carried out in all countries. In the reactors of the Fukushima nuclear power plant, for example, there were neither recombiners nor a filtered pressure relief system. Both could possibly have changed or reduced the sequence and consequences of the accident.
On behalf of the Federal Government, GRS has dealt intensively with the TMI-2 accident and its causes in various projects. The subject of the investigations was both the analysis of the causes of the accident and the question of which measures should be derived from the accident. The first “GRS Brief Information” with information on the course of the accident was published on April 3, 1979.
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