Background

The containment of a VVER 1000 nuclear reactor is the final barrier to radioactive release into the environment. After specific incidents, such as "Loss of coolant accident" (LOCA) hydrogen which is formed by radiolysis of the water of the primary circuit during normal operation is released into the leak-tight containment where it mixes with the containment air. When it reaches a certain concentration (4 volume percent of H2 in air), hydrogen mixed with the containment air is an explosive combination. In order to avoid a dangerous situation the hydrogen concentration needs to be measured and monitored.

The main task of the H2 monitoring system is to provide a representative picture of the location and time dependent hydrogen concentration build-up in the case of incidents in order to take countermeasures before the flammability limit is reached locally or integrally.

The existing H2 monitoring system used in the six units of Zaporozhye NPP measured the H2 concentration at four points in the containment. However four measurement points aren't adequate to monitor sufficiently the hydrogen concentration distribution in the containment and the system was not designed for the requirements and conditions experienced during incidents.
Objectives

The subject of the project was the replacement of the existing system with a new system designed for defined incident conditions (design basis and severe accidents). However the piping and isolation valves of the existing system have been used for installation of the new system. The new system includes

- Measuring of the hydrogen concentration in volume percent at the specified measuring point
(14 measuring points per unit)
- Measuring of the temperature at the measuring point (the pressure is measured independently by existing instruments)

The new H2 measuring device/system shall not be a potential ignition source.

The project includes:

- supply of the basic set of equipment
- the supply of spare parts
- design, manufacture, acceptance tests, certification, delivery, supervision of installation and commissioning and training of End user personnel
Project Results

The project started with the development of the Technical Specifications in 1994. The supply contract with the supplier of the system Siemens (now Areva ANP) for delivery of 8 measurement points per unit was signed in April 1997. An addendum for additional delivery of 6 measurement points per unit was signed in December 1997. The equipment including a total of 14 measurement points per unit and services were provided in accordance with the contract requirements. The Final Acceptance Certificate of the last installation at unit 4 has been signed in August 1999.

The achievement of this project increased significantly the reliability of the hydrogen measurement system by properly measuring the hydrogen concentration during incidents with the required level of reliability under harsh operation conditions (LOCA), which increased the safety and reliability of the plant.