U1.02/01A OSA South Ukraine NPP - PIP Lot1/3

Boron-10 is a neutron absorber. In water cooled - water moderated power reactors, slow changes in reactivity are controlled using liquid boric acid dissolved in the coolant.

Continuous monitoring of boron concentration enables a rapid response to reactivity disturbances and plays an important role in nuclear reactor control and safety. The on-line measurement principle is based on the absorption of neutrons by the isotope boron 10, which depends on the boron content of the coolant.

The subject of the project was focus on the upgrading of the boron concentration monitoring capability during all modes of operation (normal operations, power transients, shutdown and accidental conditions).
The objectives of the project were:

• to perform on-line neutron absorption boron concentration measurement,
• to prevent spurious boron dilution during all modes of operation,
• to improve the boron measurement performances (measurement stability, accuracy, reliability, response time, quantity of effluents to waste),
• to reduce NPP operators’ exposure to radiation,
• to solve the spare-parts procurement difficulties and to increase the life-time of the boron concentration monitoring system.

The proposed modernisation included the installation, in each Unit 1 and 2, of three fully redundantneutron absorption boron-meters with the following main features:

• a measurement range up to 20 g/dm³ boric acid,
• a better accuracy: less than 3 % of the measured value,
• an automatic temperature and neutron source ageing compensations,
• a response time less than 2 minutes,
• an insensitivity to the gamma activity of the environment or of the sampled stream,
• a minimised maintenance requirement,
• a remote calibration and operation,
• a minimised operator exposure to neutron and gamma rays as the monitor does not require any intervention from the operator for maintenance during power operation,
• the recycling of the sampled stream to the primary system (via, for example, controlled leaks collection tank), to avoid the production of liquid wastes,
• the independent supply of power from safeguard busses.

The sampling arrangements to the boron-meters had to be such that each boron-meter would be capable to monitor all sampling points, providing full redundant monitoring capabilities, by using remotely controlled selectors.

The project included:

• 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

The project started with the development of the Technical Specifications in 2004 under the service contract On-site Assistance to South Ukraine NPP – Bridging contract - U1.02/01. The contract with the supplier of the system, Mostro As, Czech Republic, was signed in December 2004. The modernization included the installation of six fully redundant neutron absorption boron-meters in the primary sides of Units 1 and 2 provided in accordance with the contract requirements. Last Final Acceptance Certificate has been signed in August 2007.

The achievement of this project increased significantly the reliability of boron concentration measurement which enables a rapid response to reactivity disturbances, and thereby increases the safety and reliability of the plant.