Regular Article

ASVAD

A new safety element to avoid the complications of the undesired nitrogen injection to PWR reactors

ASVAD INTL SL Tarragona Spain

^* e-mail: alaborda@asvad-nuclear.com

Received: 20 February 2025
Received in final form: 1 May 2025
Accepted: 27 June 2025
Published online: 22 July 2025

Abstract

During a Long-Term Station Blackout (LTSBO) accident like the Fukushima accident, the main way to remove decay heat from Pressurized Water Reactors (PWRs) is the Steam Generators (SGs). During these accidents, a Loss of Coolant Accident (LOCA) can also occur, as the main pump seals degrade quickly without proper cooling. To cope with LOCA, all current PWRs are equipped with 3 or more accumulators. Each accumulator consists of a tank filled with subcooled borated water. These tanks are pressurized with nitrogen to a pressure of around 4.5 MPa. When the primary pressure falls below the initial accumulator pressure, its water is pushed by the pressurized nitrogen into the primary system, increasing the mass inventory and cooling the reactor. This system has the advantage of being fully passive. However, nitrogen may flow into the primary system once the water has been depleted. To avoid gas intrusion, the isolation valve must be closed in time. However, this is very difficult to do when there is no power. If the valve isn’t closed in time and successfully, nitrogen will enter the primary system as soon as the accumulator is depleted. This nitrogen soon reaches and accumulates in the SG tubes. Here, the gas will significantly decrease steam condensation, which is the main method of cooling the core. This leads to a sudden increase in primary pressure and a strong decrease in natural circulation, threatening core cooling. To avoid all these complications, a special valve has been designed. The ASVAD valve is fully passive and automatic. It automatically vents the nitrogen at the correct moment, when the accumulator empties, preventing nitrogen from reaching the SG tubes. Being fully passive and automatic guarantees its proper operation without any operator action even during LTSBO scenarios. With ASVAD, operators are relieved from managing nitrogen injection and can remain focused on recovery tasks. By allowing further primary depressurization, the valve facilitates accident recovery, gives a longer coping time. The overall safety of the PWR reactors can be improved by installing ASVAD on each accumulator.