Risks faced by Zaporizhzhya

Russia and Ukraine recently accused each other of planning to attack the occupied Zaporizhzhia Nuclear Power Plant in the southern Ukraine.

About Zaporizhzhya Nuclear Power Plant

• Location: On the banks of the Dnieper river,200 kilometers from the conflicted Donbas region.
• Significance: One of the four operating NPPsin the country since 1984, accounts for one-fifth of Ukraine’s annual electricity production.
• Composition: Consists of six Pressurised Water Reactor (PWR) units with a gross electrical capacity of 1,000MW each.
  
  

  What is a Pressurized Water Reactor? Most common type of nuclear power reactor in the world that uses ordinary water as the moderator and coolant. Composition: Pressurised Heavy Water Reactors (PHWRs) are fuelled by Natural Uranium.  Light Water Reactors (LWRs) are fuelled by Low Enriched Uranium. Operating System: A PWR has two water systems: Primary Reactor system: Retrieves heat generated in the reactor.  Secondary Turbine system: Produces electric power with steam generated by that heat.

Reactor design at Zaporizhzhya

• Reactors: Zaporizhzhya NPP has six VVER-1000 reactors with a capacity of 6 GW total power-generation.
• Primary cooling circuit: Consists of the reactor vessel with fuel rods submerged in water.
  • Control rods are inserted at the top.
  • A pressuriser holds the water at a constant pressure (around 150 atm), preventing it from boiling.
  • Water acts as both coolant and moderator.
• Secondary cooling circuit: Accumulates the heat from boiling, where it converts a separate resource of water into steam which feds the turbines to generate electricity.

Comparison between Zaporizhzhya and Chernobyl

• Composition: At Chernobyl, the coolant and the moderator were different (light water and nuclear graphite, respectively) unlike the PWR at Zaporizhzhya where water acts as coolant as well as moderator.
• Radioactive Coolant: The coolant having been exposed to the nuclear fuel became radioactive, flowed out of the reactor vessel while water at Zaporizhzhya never leaves the reactor.
• Chernobyl Disaster: One of the reasons of the Chernobyl disaster was that when the reactor was breached, the superhot graphite caught fire after coming into contact with air.
  • Unlike Chernobyl, the VVER-1000 reactor and its power-generation units at Zaporizhzhya are placed inside a large airtight chamber called a containment.

Risk at Zaporizhzhya

As per the Bellona report:

• Worst-case scenario: If the containment is completely damaged with a projectile striking a reactor while it is generating power could depressurise the primary water circuit as steam and escape into the air, along with radioactive material and other volatile substances.
  • Iodine-131: This mixture will contain the isotope iodine-131 that damages the thyroid gland in humans.
  • Caesium-137: A breach and depressurisation would also release caesium-137, which was responsible for Chernobyl disaster as well.
  • Threat would be less as compared to Chernobyl as the design differences between the two NPPs could keep the fallout to within around a few hundred kilometres in Zaporizhzhya.
• Fukushima scenario: When the NPP is disconnected from the external power grid with nuclear reactions not occurring in reactor, the nuclear fuel has to be cooled otherwise fuel could become hot enough to melt through the reactor’s bottom, contaminating soil, air, and water.