Three Mile Island is a island of 3,3 km ² on the river Susquehanna, close to Harrisburg, Pennsylvania with the the United States. Its name is associated with a Nuclear accident which occurred the March 28th 1979 in the Nuclear plant of the island, when the engine n°2 (called TMI-2) partly melted. This accident was classified on level 5 of the international scales of the nuclear events (INNATE).

The nuclear accident of Three Mile Island

First minutes of the accident

The principal pumps of water supply of the secondary cooling system (or secondary circuit) broke down around 4 a.m. the morning (T = 0) the March 28th 1979. This breakdown modified instantaneously the conditions Thermodynamique S in the Steam generator , decreasing its capacity to cool the primary education cooling system (primary education circuit). The pressure in the primary education circuit (which crosses the heart) increased then immediately because of the rise of temperature. In order to prevent that the pressure does not increase too much, the spill valve of the pressurisor of the primary education circuit automatically opened (T = 3s) then the turbine and the engine was cut automatically (T = 8s). This valve should then have been closed as soon as the pressure would be gone down again, but in spite of the automatic order of closing, it was not the case. Worsening factor, the indicators of the operators showed the valve in closed position (the indicator indicated in fact that the order of closing had been given, but not that the operation had been carried out). Consequently, the pressure continued to decrease in the primary education circuit, which was emptied by this valve remained open (loss of the second containment).

The decrease in pressure in the primary education circuit involved the automatic starting of the circuit of injection of safety (T = 2min 01s), charged to bring water in the primary education circuit. However, at the same time as the pressure dropped, of the “vacuums” (of the steam in fact) were formed in the primary education circuit. These vacuums generated complex water movements which, paradoxically, filled the water pressurisor (however in top of the circuit). The operator, having the information which the pressurisor was full, concludes from it by error that all the primary education circuit was it also and manually stopped the circuit of injection of safety (T = 4min 38s). Little time after, water started to boil at the exit of the heart (T = 5min 30s).

In parallel, another problem had appeared elsewhere:

• the back-up system of water cooling of the steam generators had been tested 42 hours before the accident. During this test, a valve had been closed, and was to be reopened at the end of the test. But, following a human or administrative negligence, the valve was this time not reopened, preventing the cooling system of help from functioning. The closed valve was finally discovered and manually open (T = 8min 18s), making it possible the back-up system to correctly function, to cool the steam generators, and consequently the primary education circuit.

• the mixture of vapor and water which escaped from the valve of the pressurisor was directed towards a reserve of discharge. However, at the end of a certain time (T = 14min 48s), this tank was completely full, bringing to the rupture of the discs of discharge planned for this situation. As from this moment, the primary education circuit was emptied directly in the containment (third and last containment of the radioactivity).

During the hours which follow

After more than one hour of slow increase in the temperature and draining of the primary education circuit, the pumps of the primary education circuit started to tremble because they pumped more vapor than of water. They were then cut (T = 1:13 for the first, T = 1:40 for the second), because the theory provided that the natural convection would make it possible water to continue to circulate. Actually circulation was almost stopped by the hydrogen already trapped in the steam generators, and the evaporation of the water of the primary education circuit still accelerated. At the same time, the top of the heart started to emerge from water. The temperature supported the reaction between the vapor and the coating in Zirconium of fuel, formant of the Hydrogène, strongly degrading the sheath of fuel and bringing to the relaxation of radioactive elements in the primary education circuit (loss of the first containment).

But in control room the operators started to react. To include/understand the sequence of this accident, it should well be understood that the operators practically blind (were drowned under alarms) and were not able to include/understand what was placed (very complex situation, stress, pressure, too many people in room of the orders, etc).

An isolating valve located downstream from the valve of the pressurisor was closed, which stopped finally the draining of the primary education circuit (T = 2:22). Then, the operators also decided to start a pump of the primary education circuit (T = 2:54) whereas there was to remain approximately only one meter of water in the heart: the movement of mixing strongly degraded combustible matter, mainly emerged and extremely heats (even already partially molten).

The pump was finally stopped (T = 3:12), and the operators decided to reopen 5 minutes the isolating valve which closed the valve of the pressurisor. The primary education circuit started again to be emptied in the enclosure, but this time Ci with water very strongly contaminated, which set off the alarms of irradiation. Including/understanding whereas the heart had been strongly degraded and that the circuit thus missed surely water (and starting to include/understand the situation), the operators gave in service the injection of safety (T = 3:20), giving the heart, partly melted, underwater. By doing that, they took the risk to generate a vapor explosion or to cause the rupture of the tank because of the thermal shock, but nothing of all that arrived: the tank held good and the heart was again underwater (T = 3:45), stabilizing the situation.

The circuit of injection of safety sending of water to very high pressure in the primary education circuit, it was necessary, in the hours which followed (between T = 5:00 and T = 9:00), to open and close successively the isolating valve in order to maintain a pressure acceptable (what was the role of the faulty valve normally). This still brought to slacken hundreds of cubic meters of water contaminated in the containment.

Last major event (T = 9:50): the hydrogen, generated by the reaction between and the zirconium steam of fuel then slackened in the containment, exploded, but without making particular damage (the only evidence of this event was the detection of a peak of pressure in the containment).

During the hours which followed, the operators tried to fill the primary education water circuit, which was difficult since great quantities of hydrogen were trapped in the high points of the steam generators. Lastly, the situation was stabilized, and the pumps of the primary education circuit were given in service (T = 15:49). The state of the engine was very degraded, but allowed nevertheless to cool the fuel which, even partially melted, was going to continue to produce heat during years…

Assessment of the posterior studies to the accident

Years of studies on this accident made it possible to discover that with the final one:

• 50% of the heart had melted

• 20% had run at the bottom of the tank, which however resisted!

Concurrently to this sequence of mechanical failures, human errors and design defects, it as should be retained as in spite of the extreme gravity of the accident, the containment having remained integrates, the relaxation of radioactive products in the environment remained weak. It is however difficult to find figures reliable to quantify it (for the simple reason which they could not, essentially, being measured at the time).

In addition, an evil for a good, this accident brought the owners of similar power stations of design (and in particular EDF in France, even if its power stations present good number of differences) to deep reflections. Indeed, contrary to Tchernobyl, TMI was very instructive and allowed to advance safety. Moreover, if a projection had to be retained, it would be that of “control by state”.

The operators of TMI had procedures to apply according to such or such incident (one speaks about “event-driven procedures”). It was seen that in real situation, it could not make a diagnosis and that has in fact worsened the situation (stop of the injection of safety, restarting of the primary pumps with an emerged heart etc). All the procedures of accidental control were thus re-examined with a completely new approach: not to more ask the operators to include/understand what occurs (because there are very great probabilities so that they are mistaken, also qualified are they), but to give them actions to be made according to the parameters they have: pressure, temperature, water level, rate of radioactivity or others. It is what is called “the approach by state”, which is used today in very many nuclear plants all over the world.

See too

• List of the nuclear accidents

External bonds

• Technical informations and description of the accident
• the lesson of Three Mile Island
• Seen satellite of the power station, Google Maps.
• Report/ratio of the '' Nuclear Regulatory Commission '' (NRC) on the accident of Three Mile Island and with format pdf
• Seen satellite on Wikimapia: The site of Three Miles Island

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