Boiling water reactors

Accident at the Fukushima power plant on March 11, 2011

This article was completed before any lessons could be learned from the accident. Readers wishing to learn more about the reactors that crashed should consult the article [B 3 130] from 1979 by Alain Guyader (archives), as well as paragraphs 4.8 (Core meltdown, long-term containment) and 4.9 (Earthquake resistance) of this article.

The design of the Boiling Water Reactor (BWR) is based on that of the Pressurized Water Reactor (PWR) developed for naval propulsion; in its civil version, for power generation, the reactor is no longer subject to the same constraints of compactness, resistance to shaking and changes in trim that could disrupt the stability of a water-steam interface, if any had existed in the vessel.

Freed from these constraints, it was thought possible to create a power reactor that would be less costly and more efficient than the PWR, by allowing water to boil in the reactor core. This paved the way for the direct water-steam cycle – "Dual-cycle" of GE Co's BWR 1 (Boiling Water Reactor), then for the complete elimination of steam generators (starting with GE Co's BWR 2). Research began in 1945 at the Oak Ridge (ORNL) and Argonne (ANL) laboratories in the USA. The EBWR (Experimental Boiling Water Reactor) prototype commissioned at Argonne in 1956 demonstrated the feasibility of the concept.

The Boiling Water Reactor (BWR in the USA) was launched on the world market in the 1960s by GE Co, while at the same time Westinghouse was promoting its Pressurised Water Reactor (PWR).

The BWR was not as commercially successful as had been hoped, as the material of the recirculation loops soon began to crack (intergranular stress corrosion of austenitic stainless steel), resulting in significant loss of availability in operating reactors. In addition, some electricity producers feared that the direct cycle would lead to high radioactivity in the condenser –, but operating experience proved otherwise.

While GE Co faded from the commercial scene (the worldwide installed base of BWRs is only a third of that of PWRs), the German company AEG, which had acquired the GE Co license, and the Swedish company ASEA-Atom took over the reactor design in the 1970s. In addition to safety advances, such as the adoption of three sets of safeguard systems, the most significant European contributions were the introduction of fine motion control rods and the elimination of external recirculation loops, which had proved to be defective. The core cooling water recirculation pumps were then installed directly in the bottom of the reactor vessel – major...