VVER - Design

Design

The Russian abbreviation VVER stands for water-cooled, water-moderated energy reactor. This describes the pressurised water reactor (PWR) design. The main distinguishing features of the VVER compared to other PWRs are:

• Horizontal steam generators
• Hexahedral fuel assemblies
• No bottom penetrations in the pressure vessel
• High-capacity pressurisers providing a large reactor coolant inventory

Reactor fuel rods are fully immersed in water kept at 15 MPa of pressure so that it does not boil at normal (220 to over 300 °C) operating temperatures. Water in the reactor serves both as a coolant and a moderator which is an important safety feature. Should coolant circulation fail the neutron moderation effect of the water diminishes, reducing reaction intensity and compensating for loss of cooling, a condition known as negative void coefficient. Later versions of the reactors are encased in massive steel pressure shells. Fuel is low enriched (ca. 2.4–4.4% 235U) uranium dioxide (UO₂) or equivalent pressed into pellets and assembled into fuel rods.

Reactivity is controlled by control rods that can be inserted into the reactor from above. These rods are made from a neutron absorbing material and depending on depth of insertion hinder the chain reaction. If there is an emergency, a reactor shutdown can be performed by full insertion of the control rods into the core.