"Better Water Reactors"

Welcome back to Decouple, the best source for cutting-edge analysis on nuclear energy, with weekly interviews by Chris Keefer. Watch on YouTube, Spotify, or Apple.

This week, we talk Boiling Water Reactors (BWRs) with James Krellenstein, the CEO of Alva Energy. We dive into the engineering, history, and physics of these reactors, how they differ from other designs, and why the United States may have erred in not choosing the Advanced Boiling Water Reactor (ABWR) instead of the Westinghouse AP-1000 for the Vogtle nuclear power plant.

For this episode, we’ve included a glossary below to help with unfamiliar terms.

Watch now on YouTube.

We talk about

• How boiling water reactors (BWRs) function differently from pressurized water reactors (PWRs), with direct boiling in the reactor vessel

• The unique void coefficient of BWRs and how steam bubbles affect reactor moderation and power

• Early experimental work at Argonne National Laboratory and the BORAX tests that established BWR viability

• Why BWRs weren't adopted for naval applications despite their advantages in other settings

• The global deployment patterns of BWRs and why certain countries favored them

• The evolution of recirculation systems, from external loops to jet pumps to internal pumps

• Notable incidents and design challenges, including the Browns Ferry anticipated transient without scram (ATWS) and LaSalle power oscillation

• Containment design considerations and differences between BWR and PWR systems

• The Advanced Boiling Water Reactor (ABWR) and its remarkable construction record in Japan

• Passive safety features and the trade-offs between active and passive cooling approaches

Deeper Dive

The simplest way to understand a boiling water reactor is in the name. Unlike pressurized water reactors where cooling water remains liquid under immense pressure, BWRs deliberately boil water inside the reactor vessel. This creates a direct cycle where steam flows straight to the turbine without an intermediate loop. The approach offers elegant simplicity by eliminating an entire system, but brings unique physics challenges.

Quaint illustrations of a pressurized water reactor (top) and a boiling water reactor (bottom) from the “Nuclear Reactors” booklet by John F. Hogerton, 1963

The relationship between steam voids and reactor power creates a fascinating self-regulating dynamic in BWRs. As James explained, "More bubbles mean less moderation, which means less reactor power. Less bubbles mean more moderation and more reactor power." This negative void coefficient acts as an inherent safety feature, limiting power excursions, but also creates potential stability challenges. The early BORAX experiments in Idaho explored this fundamental question: could a reactor with boiling water achieve stable operation? These tests, conducted at what would become Idaho National Laboratory, not only proved the concept viable but led to the first town powered by nuclear energy when BORAX-III lit up Arco, Idaho in 1955.

BWRs evolved through multiple design iterations, with significant refinement of their recirculation systems. The ability to control reactor power by simply adjusting water circulation is unique to BWRs and enables remarkable load-following capabilities.

James noted that modern BWRs "can change reactor power at about 1.5% per second," making them extraordinarily responsive. Early designs used external recirculation loops, but these were replaced by jet pumps and later by fully internal pumps in advanced designs.

Evolution of the BWR reactor. Source: U.S. NRC

The history of BWRs includes several instructive incidents that shaped their development. The Browns Ferry event in 1980 (not to be confused with the 1975 fire) revealed vulnerabilities in control rod drive mechanisms when operators pressed the scram button but dozens of control rods remained partially withdrawn. The LaSalle Unit 2 power oscillation demonstrated the consequences of unstable operation as reactor power swung rapidly between 50% and 118% in just two seconds. These events led to significant regulatory and design improvements.

"Today's reactor safety wasn't inevitable—it was built from past mistakes, refined through thoughtful engineering." – James Krellenstein

Perhaps the most compelling part of the BWR story is the remarkable success of the Advanced Boiling Water Reactor, which James endearingly calls the “Absolute Best Water Reactor." This reactor design represents the culmination of decades of operating experience from American, Swedish, and Japanese designs. The ABWR achieved what many consider impossible in modern nuclear construction—a 1.35 gigawatt plant completed in just 38 months at Kashiwazaki-Kariwa, and at costs 40% lower than previous designs.

Kashiwazaki-Kariwa Nuclear Power Station, Advanced Boiling Water Reactor. Source: TEPCO, CC BY-SA 2.0

The conversation raises important questions about nuclear technology selection. Despite the ABWR's proven track record, Western utilities have repeatedly chosen unproven designs over it. James makes a compelling case that this represents a costly strategic error, similar to mistakes made during the "nuclear renaissance" of the 2000s.

"We're repeating the exact same error that was made in the first nuclear renaissance, which was passing up a reactor design that actually did the thing everyone said was impossible." – James Krellenstein

Reflecting on the journey of the BWR—from explosive desert experiments to global-scale nuclear innovation—underscores a central truth: nuclear power is shaped profoundly by human choices, politics, and economics, not just technology. The ABWR remains not only a technical marvel but also a stark reminder of how strategic missteps and missed opportunities continue to impact our nuclear future.

Glossary

• ABWR: Advanced Boiling Water Reactor

• ATWS: Anticipated Transient Without Scram

• BORAX experiments: Historical experiments testing reactor limits through deliberate failures

• BWR: Boiling Water Reactor

• COPS: Containment Overpressure Protection System

• CRDM: Control Rod Drive Mechanism

• ESBWR: Economic Simplified Boiling Water Reactor

• FLEX: Diverse and Flexible Coping Strategies (post-Fukushima enhancements)

• FOAK: First of a kind

• IGSCC: Intergranular Stress Corrosion Cracking

• NOAK: N-nth of a kind

• NRC: Nuclear Regulatory Commission

• NSSS: Nuclear Steam Supply System (N Triple-S)

• PRA: Probabilistic Risk Assessment

• PWR: Pressurized Water Reactor

• RCIC: Reactor Core Isolation Cooling System

• RPV: Reactor Pressure Vessel

• SLC: Standby Liquid Control System

Keywords

Boiling water reactor, negative void coefficient, ABWR, reactor stability, recirculation systems, jet pumps, Browns Ferry, power oscillations, natural circulation, passive safety