Assessing and anticipating natural risks
Storms, floods, heat waves or earthquakes ... To ensure the safety of its radioactive waste disposal facilities and the protection of human health in the face of natural hazards, Andra relies on cutting-edge expertise and applies the principle of "defense in depth".
On a daily basis, at its existing or planned disposal facilities, Andra has one essential objective: to limit the impact of radioactive waste on people and the environment. And to do this, it must take into account a multitude of parameters and in particular protect against what are called "external aggressions", such as a plane crash or natural hazards, as provided for in the regulations. "We have to analyze a list of risks," explains Mathieu Laplanche, head of the safety studies department. And for each of them, we must comply with standards as well as rules and guides produced by the Nuclear Safety Authority (ASN). "
The list is the same for all basic nuclear installations (BNIs) and must be considered for the phases of operation, closure and monitoring of radioactive waste disposal facilities.
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WHAT THE REGULATIONS SAY
The decree of February 7, 2012 setting the general rules relating to basic nuclear installations requires in particular to protect the facilities against internal or external attacks, namely "any event or situation which originates respectively inside or outside the building. outside the basic nuclear installation and which may directly or indirectly cause damage to elements important for protection or jeopardize compliance with the requirements defined ”for the safety demonstration.
Hazard assessment
This requires first of all to determine a level of hazard which the installation must withstand. "Rigorous work for which we obviously surround ourselves with experts for each natural event listed," emphasizes Mathieu Laplanche. For the risk of flooding, for example, we look at what would happen in the event of extreme rain. To do this, we must first define an intensity of rain that could fall on each of our sites. "
The starting point for these studies is to look for events that have already occurred in the past, using data recorded over long periods of time. “Typically, one of the rules for designing stormwater networks is to take into account what we call 100-year rains, that is to say the most intense rains that are likely to occur on average one day. times every 100 years. Using statistical tools, we are increasing this intensity so that our facilities can withstand even heavier rains ", specifies the head of Andra's safety studies department. This margin applied for each natural hazard thus makes it possible to take into account the unknowns specific to the climate or geology, and the uncertainties identified in the current state of knowledge.
Obviously each hazard is defined according to a specific geographical area. For earthquakes, strong winds, snow episodes or any other natural phenomenon, the level of hazard determined will therefore depend on the location of each Andra installation. Estimated locally and based on an extensive history, each hazard will then allowto verify that Andra's facilities are properly sized to deal with natural risks.
Anticipate all possibilities
Andra then relies on a principle of defense in depth, which is based on several levels of protection. "First, we will do everything possible to limit the risk of an accident being caused by the occurrence of a natural event. As part of the periodic safety reviews of our Aube and Manche repositories, if the authirized level of hazard has changed with the regulations, we check the robustness of our existing structures. For projects under design, such as the Cigeo geological disposal center project, each element of the installation is sized according to the risks, "continues Mathieu Laplanche.
For flooding, for example, Andra teams ensure that the equipment is properly sized in the event of heavy rain in order to guarantee the protection of areas containing radioactive substances. "Regarding the temperature in Cigeo, the underground installation must be ventilated, this will include protecting the electrical rooms that supply the fans from a high level of outside temperature. Redundant cooling systems for these premises are thus planned, ”adds Mathieu Laplanche. Another example: for strong winds, storms or even tornadoes, Agency experts are studying the impact of high wind speeds and projectiles on the installations. “Obviously knowledge improves and standards evolve accordingly. Initially, some nuclear facilities may therefore have been sized on the basis of levels of rain, wind or temperature lower than the current reference. But - this is their goal - the ten-year periodic safety reviews is the occasion to check whether, on the basis of updated standards, the installation is still suitable and to make modifications if necessary ", points out Mathieu Laplanche.
« Using statistical tools, we are increasing this intensity so that our facilities can withstand even heavier rains. »
Despite all these precautions, the principle of defense in depth requires considering that an incident could still occur and Andra ensures that it is able to detect and react quickly if the installation is weakened. or threatened. Alarm systems are thus put in place. Moreover, they are not specific to natural hazards alone and are part of the standard measures that Andra takes to guarantee the safety of its centers.
And because there is no such thing as zero risk, Andra, like any nuclear operator, sets up emergency procedures and interventions so that in the event of a breakdown or accident, it can be maintained under control and its consequences limited.