A risky accident in a Nuclear Reactor is the Extended Loss of AC Power accident (ELAP). This accident is also known as Station Black-Out (SBO).  At this accident, the control of ALL the equipment in the plant is completely lost for a long time. This happened at Fukushima with their catastrophic consequences.

But this accident has a worse side; it directly leads to another serious accident: The Loss of Coolant Accident (LOCA).

After a few minutes, the high temperature reached will degrade the Reactor Coolant Pump seals. Then, it will begin a moderate -but continuous- loss of coolant across these damaged seals. This leak will be PERMANENT during all the time. But not just during the accident event… also during the whole recovery time. Only when operators can access again to the containment building, this leak can be fixed… ant this can take years!

The safety accumulators inject soon their borated water, recovering during some time the leaked coolant… This is good while there is water inside the accumulators.

But sooner or later, the water runs out. Now the pressurized nitrogen which has driven the water injection, begins entering into the cooling system.  This Nitrogen injection will complicate the accident recovery disturbing the cooling systems.

This is a bad situation, because this nitrogen in the cooling circuit will produce further problems such as disturbing the Natural Cooling, increasing the pressure, and greatly increasing the chances of core melting.

Once this nitrogen reaches the pipes, it will remain inside during the rest of the accident, disturbing all the cooling efforts even during the long recovery time.

It’s a similar situation when an air bubble gets into a vein. It will disturb the normal blood circulation. If the bubble is big enough, it can permanently stop the blood flow. It’s the embolism that can lead to the death.

It’s easy to understand that avoiding this undesired nitrogen injection is a very important task to recover the reactor’s safety.

WHY NITROGEN IS A DANGEROUS THREAT?

-Having nitrogen in the system cooling pipes is a very dangerous situation, as it can greatly difficult the core cooling.

1. It can permanently stop the “natural circulation” flow. This is the main way to cool the core in these circumstances. Natural circulation is a physical process that produces a constant flow from the core to the heat exchangers without the need for pumps. It’s a passive way to extract the core heat.
2. It also can make difficult another cooling mode named “reflux cooling”. Nitrogen will diminish the steam condensation in the Steam Generator tubes, increasing the temperature in the core.
3. It will increase (and maintain) the pressure in the system. This gas will be heated by the steam from the core. This will increase the pressure in the system increasing the leak and avoiding that the low-pressure safety pumps (LPSI) can recover the core level.
4. It will accelerate the fuel degradation. If the core temperature rises enough, the nitrogen presence will accelerate the fuel cladding oxidation, diminishing the time available to avoid the core damage.
5. It can disturb the recovering pumps. Sooner or later the power will be restored again. But with the nitrogen inside, it can reach the cooling pumps and cause them to become air bound with little or no flow, rendering the pump inoperable. Air binding can affect more than one pump when pumps share common discharge or suction headers or when the gas accumulation process affects more than one train. This situation will greatly increase its risk significance.
6. Gas accumulation can result in water hammer or pressure transients, particularly in pump discharge piping following a pump start, which can cause piping and component damage or failure.

HOW BIG IS THE PROBLEM?

Then, it’s easy to understand that the nitrogen injection is a dangerous complication, but… How much?

We can do some simple calculations:

Each accumulator has around 14 m3 of nitrogen (@ 45 Bar @ 35ºC).  This means around 640 kilograms. This nitrogen obviously follows the ideal gases laws (P * V = n * R * T), and during the accident its temperature T will rise in the system, and at the same time, the leaks will make the pressure P falling continuously.

This means that this nitrogen will expand a lot. From the initial volume of 14 m3, It can reach until 585 m3 (@ 1 Bar @ 35ºC).

On the other side, the total volume of the RCS is around 340 m3 at 4 loop plants. At 3 loop plants or 2 loop plants, it is even smaller.

This means that just one accumulator has enough nitrogen to fill the whole RCS volume. But this can be worse, because there is one accumulator in each loop.

Then, the problem can be THREE OR FOUR TIMES BIGGER!!!. Nitrogen can easily fill the entire RCS volume displacing the water. There is too much nitrogen inside the accumulators!!

And all this nitrogen has a DIRECT PATH to the core!  It will constantly disturb the core cooling from its inside.