“The accident at the Three Mile Island nuclear power plant on March 28, 1979 was the largest in the history of US nuclear power. Although the radiation consequences were insignificant, this accident greatly changed the US energy policy, completely stopping the development of an entire industry.
Error cost
The accident at the second power unit of the nuclear power plant began at about four in the morning. First, the feed pump of the second circuit stopped, as a result of which the circulation of water stopped and the reactor began to overheat. It was an insignificant incident that would have had no consequences if not for one factor. Due to a gross mistake made during the repair, the emergency pumps of the secondary circuit did not start. As it turned out later, the hard workers who carried out the repair did not open the valves on the pressure, but the operators who monitored the operation of the cooling system could not see this, since the pump status indicators on the control panel were simply covered with pieces of paper!
Presumably, water from one of the condensate cleaning filters through a faulty check valve entered the compressed air system, which was also used to control the pneumatic actuators of the valves. The specific mechanism of the effect of water on the functioning of the system has not been established, it is only known that at 04:00:36 (-0: 00: 01 - time from the conditional reference point) an unexpected one-time operation of pneumatic actuators occurred and all valves installed on inlet and outlet of condensate cleaning filters.
The flow of the working medium of the second circuit was completely shut off, the condensate, feed pumps and the turbine generator were sequentially turned off.
The balance between the thermal power consumed by the second circuit of the station and the power produced in the reactor plant instantly changed, due to which the temperature and pressure began to rise in the latter.
The incipient rise in temperature and pressure in the reactor was a predetermined situation that immediately triggered the automatic emergency protection system. This system immediately drowned out the nuclear boiler. It would seem that the incident could be considered settled, but the human factor intervened, which led to the largest nuclear accident in American history.
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According to the instructions, it was necessary to cool down the reactor. However, since the indicators of the valves of the emergency feed pumps on the control panel were covered with pieces of paper, the NPP personnel could not find their bearings and, not noticing the leak, did everything on the assumption of the perfect operation of the pumps. Operators turned off one of the emergency pumps and limited the water supply, which resulted in a drop in pressure, boiling water and filling the loop with steam (personnel assumed the loop was filling with water).
A little earlier, a safety valve worked, which began to release steam from the reactor, condensed in a special container - a bubble tank, or a bubbler.
However, upon reaching normal pressure, the valve (through which steam was supplied to the bubbler) for some reason did not close, which was noticed only after a few hours.
During this time, the bubbler overflowed, the safety membranes burst in it, and the containment rooms began to fill with superheated steam and hot radioactive water.
Useless engineers
And the staff completely ceased to understand what was happening - the conflicting readings of the sensors completely disoriented them. But the shift ended, and other operators, completely unfamiliar with the situation, began to work.
The "newcomers", who found themselves in a situation developing with the acceleration of an atomic catastrophe, finally managed to determine the malfunction of the pressure compensator valve and eliminate the leak. However, this measure was already late - the rapid oxidation and destruction of the reactor fuel elements had already begun. A new shift of operators tried to start the cooling pumps but failed due to lack of water. The destruction of the reactor core began. The atomic disaster (which would have surpassed Chernobyl and Fukushima combined in scale) was closer than ever. The temperature in the reactor during the accident reached 2200 degrees, as a result about half of all components of the core melted - more than 60 tons of radioactive substance.
Having no instruments at their disposal that made it possible to determine the liquid level directly in the reactor vessel, and not realizing the lack of coolant, the operators tried to resume forced cooling of the core. Attempts were made to start each of the four main circulation pumps. One of the attempts was relatively successful: the launched MCP-2V captured water in the loop of the circulation pipeline and pumped it into the reactor vessel, which made it possible to briefly slow down the fuel temperature rise. However, the injection of about 28 m3 of water into the overheated core caused its instant boiling and a sharp increase in pressure in the installation from 8.2 MPa to 15.2 MPa, and the sudden cooling of the heated fuel led to "thermal shock" and embrittlement of structural materials. As a result, the upper part of the core,consisting of seriously damaged fuel rods, lost stability and sagged downward, forming a cavity (empty space) under the block of protective tubes (BCP).
Compensating for the disturbance in the primary circuit caused by the consequences of turning on the MCP-2V, the operators at 07:13:05 briefly opened the shut-off valve to relieve pressure. Then, apparently in order to maintain it within the operating range, at 07:20:22 the emergency cooling system was manually turned on for about 20 minutes (by this time the coolant covered no more than 0.5 m of the core height. Although the cooling water was supplied into the reactor, the core of the core was practically not cooled due to the surrounding crust of previously melted and solidified material, the temperature of the melt reached 2500 ° C and at 07:47:00 there was a sharp change in the geometry of the core: liquid fuel mass from the center of the core, containing about 50% of its materials,melted the surrounding structures and was distributed in the cavities of the internals and at the bottom of the reactor, and the empty space under the BZT increased in volume to 9.3 m3. Despite the fact that the melt temperature did not reach the melting point, part of the ceramic fuel still passed into the liquid phase when interacting with zirconium and its oxides.
At 07:56:23, the next automatic activation of the emergency cooling system of the reactor took place, now on the signal of an increase in pressure in the containment over 0.03 MPa. This time, a fundamental decision was made: not to interfere with the automatic operation of safety systems until there is a complete understanding of the state of the reactor plant. From that moment on, the process of destruction of the core was stopped.
The situation was saved by the automatic reactor cooling system, which turned on at that moment. The staff, who actually acted at random and did not understand anything, decided not to interfere with her work. This (for the operators who did not understand what was happening) was a desperate risk, but it paid off.
The destruction of the reactor was suspended (in total, about half of the reactor core was damaged), but its cooling was still a problem. The staff already realized that the pumps did not work due to the filling of the areas with steam. An attempt to raise the pressure in the primary circuit for steam condensation failed. Then the operators tried, on the contrary, to reduce the pressure to the lowest possible, but as a result, the re-draining of the core began, so this attempt (fraught with a "restart" of the disaster) was also abandoned.
Nevertheless, by the evening, they managed to start the pumps, after which the critical phase passed. However, abnormal accumulation of hydrogen in the reactor systems was noted. In fact, the threat had been eliminated by that time, but the data leaked to the media about the terrible hydrogen caused a real panic throughout Pennsylvania (people did not understand the intricacies of technical details, but they felt that they were in mortal danger, especially since the radiation has no color. no taste, no smell). They managed to get rid of hydrogen by April 1, and the danger was over.
Effects
If the catastrophe could not be prevented, more than 660 thousand residents of the surrounding cities would be subject to emergency evacuation (in the event of the accident at the Chernobyl nuclear power plant, about 115 thousand were evacuated). However, all work to eliminate the consequences of the accident was completed only by 1993! A large amount of radioactive water escaped from the nuclear reactor, as a result of which the level of radioactivity in the containment rooms exceeded the norm by more than 600 times.
A certain amount of radioactive gases and steam got into the atmosphere, but the most dangerous - the release of highly active nuclides into the atmosphere and water - was avoided, so the area remained “clean”.
On the whole, the Americans got off with a "slight fright" and small (in such an accident) financial losses - the cost of work on the liquidation of the second power unit of the Three Mile Island NPP was estimated at $ 1.26 billion. At present, the Three Mile Island NPP continues to operate - Unit 1 is in operation, which was under repair during the accident and was launched in 1985. But the second power unit is closed, the inside of the reactor has been completely removed and disposed of, and its territory is a “restricted area”. It is assumed that the station will operate until 2034.
However, business Americans even in this case found an opportunity to benefit - in 2010, the turbine generator of the emergency second power unit was sold, removed and transported in parts to the Shearon Harris nuclear power plant, where it took a place in the new power unit! The Americans reasoned that the expensive equipment had worked for only six months, had not suffered during the accident - so the good would not be lost.
An investigation into the causes of the accident resulted in the understanding that the plant operators were not prepared for the incident. As a result, the requirements for NPP workers were tightened, and the training methods were changed.
The results of the accident investigation also led to an increase in NPP safety standards and an increase in supervision of the operation of nuclear power plants.
In the USSR, these results were not given importance - the catastrophe that occurred was attributed to the vices of decaying capitalism. As it turned out later, the Soviet bureaucrats made a big mistake in this case …
Journal: Historical Truth no. Author: Daniil Kabakov