Passive protection for the EPR

INTERNAL HAZARDS HAVE PLAYED AN important role in the safety of nuclear plants, one that has been managed for years and is always undergoing improvement. Among these hazards, fire is one of the major issues, since in the case of fire both damages and consequences may be huge.

In France, the design of nuclear power plants has been guided by codes for decades. The RCC-I code applies to 900MW to 1450MW nuclear plants, the ETC-F code to existing EPRs and the RCC-F to future plants.

Initially written by EDF fire experts, ETC-F and RCC-F have been produced by a dedicated working group within the AFCEN organisation (France’s code management body for design, construction and in-service inspection rules
for nuclear island components). More than 650 experts from the nuclear market work with AFCEN to work out the guideline codes on electrical, mechanical, civil engineering and fire issues. The more recent release of the RCC-F fire code was published in 2017 and a new release is expected for 2020.

The fire protection is based on the defence in depth approach following the three main principles:

• Prevent fire (use of materials with low reaction to fire, limitation of fire loads, use of cable with low fire propagation. etc)
• Limit damage and propagation of fire, including suppressing fire loads, fire compartmentation and preventing common mode failures (routing, functional fire protection, containment of unfriendly environmental substances);
• Mitigate fires (using fire detection, fire extinguishing systems and manual fire fighting).

The code gives only general principles and requirements (see Table 1) but does not prescribe the technical solutions, products, materials or systems that allow these principles and requirements to be fulfilled. It makes the code very easy to adapt to local regulations (such as the UK’s EPR at Hinkley Point C).

Fire load reduction is part of the fire prevention approach. It involves preventing cables (or other equipment) from burning so a local fire will not grow to a generalised fire. Fire load reductions on cables are achieved by using a reactive material wrapped around cable trays.

The passive fire protection approach is the second fire barrier, after prevention. Passive fire protection will include fire compartmentation as well as functional protection against common mode failure in fire conditions.

Compartmentation is done with fire resistant doors, penetration seals, structural stability and fire dampers that must be tested and classified according to European standards.

Concerning functional fire protection, when the Flamanville EPR was designed, there was no standard, so EDF wrote its own test specifications based on more than 30 years of experience in the field. Functional fire protection can be achieved on cable trays, electric cabinets or any other equipment. Fire-resistant penetrations are made with non-fibrous plain materials such as mortar, foam or elastomer. They must be tested according to EN 1366-3 standard for fire.

Fire compartmentation is achieved on ventilation ducts using fire dampers or fire resistant ducts. The EN 1366 tests are mandatory for fire resistance.

Functional fire protection is different from fire load reduction or compartmentation as continued operation of cables or equipment must be guaranteed in case of fire. Electric cabinets or cables may be protected.

In the event of heat release, the fire protection system must be able to dissipate the internal heat.

For all these fire protections, there are many other test specifications for additional requirements, such as seismic, irradiation, thermo-hydric, pressure and air/water tightness and most of the time these refer to the same prototype.

The fire load may be reduced by installing fire load reduction products that are resistant to localised fire only. This kind of protection is not intended to keep cables in operation.

Fire resistant compartments are guaranteed by passive fire protective materials or systems tested according to European fire test standards and with the required fire classification. However, some additional requirements must be considered that are specific to a nuclear power plant.

In the case of a common mode risk of failure, additional passive fire-resistant materials can allow one or more train to remain operational in the event of a fire.