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Is a power supply needed to cool the reactor after its shutdown?

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Is a power supply needed to cool the reactor after its shutdown?


After the reactor shuts down, the core can be cooled by simple natural convention, i.e. without any source of electrical energy.

Once the reactor shuts down, in other words once the fission chain reaction in the uranium fuel stops, the core nevertheless continues to release energy because of the radioactivity of the fission products it contains. This energy per unit of time is known as the residual core power. As this residual power is due to the radioactivity of the core, it therefore decreases with time as this radioactivity decreases.

After a normal operating cycle, i.e. after 46 days at full power of 57 MW, the reactor core gives off the following residual power:

  • 2 MW immediately after reactor shutdown;
  • 0.55 MW after one hour;
  • 0.18 MW after 24 hours;
  • 0.08 MW after one week.

This residual power is not only low but also decreases very rapidly. This is due, on the one hand, to the reactor’s low operating power (57 MW) and, on the other, to its short operating cycle (46 days). The fuel element, a single fuel rod which constitutes the whole of the reactor core, must then be changed. As the reactor cycle is short, the level of radioactivity of the long-lived fission products (those with long half-lives) does not have time to accumulate.
Nevertheless, the core still has to be constantly cooled to prevent its temperature from rising and hence avoid any risk of fuel meltdown.

During a normal shutdown, the main coolant pumps are left to run for an hour, providing a water flow rate through the core of 2400 m3/h. Once these are shut off, the pumps of the shutdown cooling system continue to operate alone, providing a water flow rate through the core of 150 m3/h. The cooling water is water taken from the river Drac. The heat from the core is transferred to this water via heat exchangers, an operating mode which avoids any thermal stress on the fuel element or the reactor vessel.

In the event of the loss of external power supplies, the main pumps of the primary cooling system will stop. This triggers the automatic shutdown of the reactor but also the automatic startup of the emergency backup diesel generators. These generators make it possible to restore power to the main systems, in particular the shutdown cooling system.

Finally, even in the event of the total loss of external and emergency backup power supplies, the pumps of the shutdown cooling system continue to function for one hour, as they have batteries which guarantee autonomous operation for this length of time. If these batteries fail, cooling is no longer provided by means of forced convection but by simple natural convection, which is perfectly adequate to ensure that the reactor core is properly cooled.