Asvad is a safety valve –unique in the world– specifically designed to avoid the nitrogen injection complication.

It is fully PASSIVE and AUTOMATIC. It does not need any external power or operator action to fulfill its safety function.

Asvad is the SAFEST solution to a risky -and not properly evaluated– complication during a nuclear accident in the PWR nuclear plants.

This complication is the nitrogen injection to the core from the safety accumulators. This gas once inside the cooling pipes, will disturb –or even impede– the core cooling.

1. It can permanently stop the “natural circulation” flow avoiding the core cooling.
   
2. It will strongly diminish the steam condensation in the Steam Generator tubes.
3. It will increase (and maintain) the pressure in the system. Increasing the leaks and avoiding the low-pressure safety pumps recovery work.
4. It will accelerate the fuel cladding degradation diminishing the time available to avoid the core damage.
5. It will disturb the recovering pumps. Air binding, cavitation, and water-hammer issues.

It is very likely to suffer this complication because:

• This gas IS ALREADY INSIDE the system.
• There are BIG QUANTITIES  of it inside the accumulators.
• It has an OPEN and DIRECT path to the core.
• As is pressurized, during ALL the TIME it is trying to enter in the cooling pipes! … So it’s a question of time that it can reach the core.

Probably you know what happens when there is air inside the cast iron radiators at home… there is no heat transfer until the air is vented.  So, in a nuclear reactor it happens the same, but in this case during the accident, no venting are possible!

The only way to avoid it is to prevent from happening.

ASVAD is always sensing the accumulator pressure. During the accident, as the accumulator empties its water, its internal pressure decreases. Only when the accumulator is depleted, and the nitrogen pressure is residual, the ASVAD valve acts venting all the gas to the atmosphere… away from the reactor pipes.

The next 1 minute video is very descriptive. The ASVAD acts when the pressure drops below 18 bar. Look at the manometer. The depressurization is complete.