Centre News

On Tuesday, 13^th May, the ESCP Energy Society organised a visit to the Saint-Laurent Nuclear Power Plant, located in the Loir-et-Cher region of France. This plant comprises four nuclear reactors. The first two built were UNGG (Uranium Naturel Graphite Gaz) reactors – an early French reactor technology that uses graphite as a moderator and carbon dioxide gas as a coolant.

However, due to growing concerns about the long-term safety and efficiency of this reactor type, EDF and the French Atomic Energy Commission (CEA) decided in the early 1980s to adopt a more proven design. As a result, two Pressurised Water Reactors (PWRs), each with a capacity of 900 MWe, were commissioned in 1981.

What is a PWR?

The first part of the visit, led by Lucile Amary and Delphine Vidal from EXIRYS, focused on understanding how a typical French PWR operates. A PWR is a reactor type that uses pressurised water both as a coolant and as a neutron moderator.

Key Characteristics:

• The primary circuit water is heated to 328°C under a pressure of 155 bar, preventing it from boiling even at such high temperatures.
• The reactor operates through three separate circuits:
1. Primary circuit
2. Secondary circuit
3. Cooling circuit

More information about PWR is available here.

1. The Primary Circuit

The primary circuit is a closed-loop system in which water becomes radioactive as it flows through the reactor core, absorbing heat generated by nuclear fission, as explained to us by our guides.

• The nuclear fuel consists of fuel pellets made of uranium dioxide:
• 97% UO₂-238 (non-fissile)
• 3% UO₂-235 (fissile)
• These pellets are stacked inside zirconium alloy tubes, known for their high resistance to heat and corrosion, making them ideal for nuclear applications.
• The hot pressurised water from the primary circuit flows through the steam generators, where it transfers its heat to the secondary circuit without mixing with it.
  This process produces clean, high-pressure steam that drives the turbines to generate electricity.

Reactor Control Methods

The reactor’s power output is controlled by three main methods:

1. Boron Solution (Chemical Control):
• The water in the primary circuit contains boric acid, which absorbs neutrons and slows down the fission chain reaction. By adjusting the boron concentration (measured in ppm), operators can control reactor power.
2. Control Rods (Mechanical Control):
• Control rods, made of neutron-absorbing materials, are inserted or withdrawn from the reactor core to increase or decrease power output.
3. Moderator Properties (Physical Control):
• The pressurised water itself acts as a neutron moderator, slowing down neutrons to sustain the chain reaction.

Additionally, the reactor vessel is fully submerged in a water pool, providing passive cooling and an extra safety barrier.

2. The Secondary Circuit

The secondary circuit, as Lucile Amary and Delphine Vidal detailed, is a non-radioactive steam-water cycle that converts heat from the primary circuit into mechanical and then electrical energy.

• The steam generated in the three steam generators (each reactor has three) is sent to a turbine group consisting of:
• One high-pressure turbine stage
• Two low-pressure turbine stages
• This drives the alternator, producing electricity.
• After passing through the turbines, the steam is condensed back into water in the condenser, ready to be reheated in the steam generators.

3. The Cooling Circuit

The cooling circuit removes heat from the condenser in the secondary circuit, typically using river water or cooling towers.

• At Saint-Laurent, river water is used as the cooling source with a pumping capacity of 1,500 litres per second, out of which 1,000 litres per second are returned to the river after passing through the system.

Site Infrastructure and Safety

During the visit, Lucile Amary and Delphine Vidal guided us as we observed the external structures of:

• BR (Reactor Building)
• BK (Fuel Building)

The Reactor Building (BR) stands about 50 to 55 metres high and is constructed with prestressed concrete and internal steel liners, designed to withstand:

• Seismic events
• Terrorist attacks

This building is part of the three physical safety barriers designed to contain radioactive material:

1. The Reactor Vessel (Core Containment)
2. The Primary Circuit
3. The Reactor Building (BR) 

The Secondary Circuit in a PWR

After learning about the reactor and the primary circuit, the visit continued with a deeper exploration of the secondary circuit, where electricity generation takes place. This circuit is non-radioactive, meaning the water and steam in this loop do not come into direct contact with radioactive materials.

How the Secondary Circuit Works:

1. Steam Generation:
• In the steam generators (there are three per reactor), the hot, pressurised water from the primary circuit transfers its heat to clean water in the secondary circuit.
• This heat exchange produces steam without mixing the two water circuits, keeping the secondary steam non-radioactive.
2. Steam Expansion in the Turbine:
• The high-pressure steam is directed to the turbine group, which at Saint-Laurent consists of:
• One high-pressure turbine stage
• Two low-pressure turbine stages
• As the steam expands and loses pressure through these stages, the turbine blades rotate rapidly. This mechanical energy is converted into electricity via an alternator connected to the turbine shaft.
3. Steam Condensation:
• The steam enters the condenser – a large heat exchanger – where the steam is cooled and condensed back into liquid water.
• This condensation process is critical to maintaining the vacuum required in the condenser, maximising the efficiency of the turbine.
4. Condensate Return and Reheating:
• The condensed water (called condensate) is then pumped back towards the steam generators to restart the cycle.
• Before re-entering the steam generators, the condensate may pass through preheaters, which use extracted steam from the turbine to increase the water temperature, improving the overall thermal efficiency of the plant.

“This visit provided an excellent overview of the complex systems and robust safety measures involved in operating a French PWR nuclear power plant, as well as insight into the transition from older technologies like UNGG reactors to other tech like PWR systems” – Juan Pablo Revuelta, MSc in Energy Management Student

Useful Links:

ESCP Business School
MSc in Energy Management
ESCP Energy Society
EDF – Saint-Laurent-Des-Eaux Nuclear Power Plant