Sodium safety

The entry into operation of the BN-800 fast reactor, which is unit four at Beloyarsk, Russia, is a major step for Rosatom. Commercial operation of the reactor began late last year following trial operation for the first time in August 2016.

The fast reactor, which has sodium coolant and runs on mixed oxide (MOX) hybrid fuel, is the newer, more powerful incarnation of the BN-600 unit at Beloyarsk 3. Its sodium coolant is highly reactive with air and water, so it poses unique challenges for fire safety.

Rosatom told NEI that fire safety at unit 4 is based on the concepts applied to next- generation nuclear units. Beloyarsk 4 was designed and is operated using the defence-in-depth concept based on the barrier sequence system. This standard concept has been adapted to unit 4 to ensure fire safety, as shown in the table.

The fire safety systems at BN-800 have been developed using Russian regulatory requirements and the International Atomic Energy Agency guide ‘Protection Against Internal Fires and Explosions in the Design of Nuclear Power Plants’. The plant complies with all mandatory fire safety requirements laid down in Russian federal law, as well as national standards and codes.

A spokesperson for Rosatom said: “The fire safety systems at unit four of Beloyarsk nuclear power plant were designed and constructed considering the best practices, including foreign practices of recent years, as well as fire safety research and development and solutions.”

The systems have been developed based on sodium fire analysis findings, in research such as ‘Fire safety specifications for spaces with sodium coolant at unit 4 of Beloyarsk NPP’ (2007), carried out at the A.I. Leypunsky Institute for Physics and Power Engineering in Obninsk. Compliance with fire safety requirements is assessed and controlled by the Federal Environmental, Industrial and Nuclear Supervision Service of Russia, the supervisory department of Russia’s defence and emergency body EMERCOM and the Rosenergoatom Fire Safety Service.

At BN-800 the individual fire hazard within the premises must not exceed 10-6 per year; the individual fire hazard in the residential area near the unit must not exceed 10-8 per year; and the societal fire hazard in the residential area near the unit must not exceed 10-7 per year.

“As part of the project, the deterministic and probabilistic safety analyses were made with computer models used for risk assessment, including fire hazards,” the Rosatom spokesperson said.

Beloyarsk 4 is compartmentalised into fire zones, which could be groups of rooms, an area or part of a room. These zones are classified according to whether flammable material is stored continuously or periodically, including during disruption to normal operations. The zones are separated by distance or by fire barriers.

For equipment spaces that contain sodium, Rosatom said that fire safety principles and criteria require the development of fire safety systems that take account of the ultimate accident for each closed unit within a space. The design ensures that in the event of a sodium leak, heat is removed to the environment until the sodium temperature falls below its flash point.

The probability of overheated sodium interacting with air, because of leakage, is a reduced by using stronger equipment and pipeline components, for example by using contingency casings. Spaces are designed to minimise leaks, with metal cladding on walls, floors and ceilings. Doors, and wall and flooring penetrations are all sealed. Concrete structures within sodium spaces are protected with thermal insulation and steel cladding, preventing direct impact from sodium and from thermal effects.

Sodium interaction with water and water vapour is excluded in the design and there are wireless systems that provide emergency release of sodium-water interaction products, as well as fast water-shutoff devices.

There is also a special fire ventilation system for heat and combustion product removal via special filters in the event of a fire.

Electrical cables at Beloyarsk 4 have non- combustible or low-combustible insulation. Fire barriers with specified fire resistance grading are used to divide cable structures into segments less than 50m in length and 20m in height.

Cabling through walls and floors is either made using special modular systems or it is installed in pipes, while the penetrations are sealed with certified fire-resistant materials with the same fire resistance grade as that of the enclosures, for example by Hilti, Pyro-safe or Promat.

The instrumentation and control system includes a totally automated active fire safety system, including automatic fire alarm, fire suppression systems, fire dampers at ventilation systems, smoke removal system and a fire annunciation and evacuation management system.

Beloyarsk 4 has a special fire panel for fire system equipment operation that is attended 24 hours a day. In the event of a fire at BN- 800, information is communicated via the fire panel between the facility fire brigade and the Rosenergoatom crisis centre.

Spaces with sodium coolant from the primary and secondary circuits, the turbine hall, and some ducts are equipped with a fire detection system, based on hypoxic air, which is a supersensitive aspiration automatic fire alarm.

Rosatom also said that the Beloyarsk 4 site has design-based systems for flushing sodium from sodium-contaminated components removed from the reactor.

Experience and technology use from BN-600 have influenced the BN-800 design. Rosatom said that fire safety systems that are suitable for BN-600 are used in BN-800, including special fire ventilation. Automatic fire alarms of the VESDA type, which were trialled earlier in spaces with high radiation fields at BN-600, are also used at Beloyarsk 4.

The BN-800 fire safety design, construction and operation are based on the experience of BN-600 fire services. One fire brigade covers BN-600 and BN-800, and it now has more firefighters to meet the needs of the new fast reactor.

By using the defence-in-depth approach and experience from BN-600, BN-800 operators have established a fire safety system for the new reactor that is tailored to the unique challenges of sodium as a coolant in this type of fast reactor.