Wednesday, March 16, 2011

Post # 32: Understanding and Comparing Boiling Water Reactors (BWRs)

The situation at Fukushima Dai-Ichi continues to evolve and world attention is focused on the unfolding events.  There web is rampant with misinformation and often confusing information.

In the midst of this unfolding tragedy, I think it's important to pause and clarify the most basic facts about the plants themselves and their potential relevance to our plants in the United States.

First, it's important to understand how rare it is for any two nuclear plants at different sites to be identical – particularly for the vintage of plants we're concerned with here.  So even when reactors have the same "make and model number" (to use automobile language for a moment), they are usually not identical units.

Boiling Water Reactor (BWR) Nuclear Plant Nomenclature

First, some nomenclature that is very important.  BWR nuclear plants have a "make and model".  In terms of organizational roles or "makes", every nuclear plant has a designer, a manufacturer, a constructor, and a licensee, and an operator.  Sometimes the same organization plays multiple roles.  Due to the complexity of modern power plants, there are actually hundreds, if not thousands, of equipment suppliers involved in making these plants a reality.

With regard to the physical configuration of the plants, BWRs are characterized by the reactor "type", and the primary containment "type".  Based on historical General Electric nomenclature, there have been six "vintages" of commercial BWRs (not including the ABWR and ESBWR).  These are referred to as BWR-1, BWR-2, BWR-3, BWR-4, BWR-5, and BWR-6 reactors.  Then, there's the primary containment.  These are referred to as Mk-I ("Mark-One"), Mk-II ("Mark-Two"), and Mk-III ("Mark-Three") designs.

Now let's look at how all of this is relevant to today's events.

Fukushima Dai-Ichi Plant

As indicated on Wikipedia ( ) Fukushima Dai-Ichi consists of six boiling water reactor or "BWRs".  Unit 1 is a BWR-3 / Mk-I system.   Units 2, 3, 4, and 5 are BWR-4 / Mk-I systems, and Unit 6 is a BWR-5 / MK-II system.  Thus there are actually three different operational plant configurations at the Fukushima Dai-ichi site.  In addition, two Advanced Boiling Water Reactors are under construction at the site.


There are 35 operating BWRs in the U.S.  As seen below (based on a sort of plant information taken from the U.S. NRC web site Appendix A @ ).  These thirty five plants can be divided into 6 generic categories based upon their reactor / primary containment combinations.

As can be seen from the table, there are six US plants (yellow) with the same generic configuration as Fukushima Dai-Ichi 1, fifteen plants with the same generic configuration as Fukushima Dai-Ichi Units 2-5 (light blue), and 4 US plants with the same generic configuration as Fukushima Dai-Ichi Unit 6 (green).

Generic Plant Type Is Important But Plant Specific Differences Matter

I've noted there are a plethora of significant design details that differ even between plants of the same generic type.  To illustrate, this, I've noted in the table below some significant design differences between four US BWRs of the same generic type (BWR-4 / MK-I) as Fukushima Units 2-5 (excerpted from Table 1.7-4 of the Browns Ferry FSAR):

A quick look at the last three rows of the table will aptly illustrate my point about plant specific differences.

The Bottom Line

As we discuss the events on-going in Japan, we should not jump to conclusions about applicability of events there to plants here in the US.  Similarities with regard to generic plant types are important and very relevant, but these similarities can be overwhelmed by differences in accident initiators, siting, plant design details, plant operating procedures, and a host of other factors.  The DETAILS MATTER.

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