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Filtered Vents and Boiling Water Reactors: It’s Not About the Costs

Recent media reports accusing the nuclear energy industry of opposing the installation of filtered vents on Mark I and II boiling water reactors (BWRs) are incorrect. What is really at stake is taking the most meaningful steps at nuclear plants to prevent and manage an accident like the one at Fukushima.

The nuclear energy industry continues to learn a great deal about what happened at Japan’s Fukushima Daiichi nuclear power plant on March 11, 2011, and is applying the lessons to make America’s nuclear plants even safer.

Commercial nuclear power plants in the U.S. must be able to prevent and manage an accident like the one at Fukushima.

Clearly, the best way is to not have an accident in the first place.  As documented in a report by an independent commission of Japan’s national legislature, U.S. plants are much better regulated, run and prepared than their counterparts in Japan.  In addition to the myriad systems installed in U.S. plants to prevent accidents and cope with a severe accident, U.S. plants have become even better prepared since the Fukushima accident.  Even if all installed cooling systems were to fail, new portable power and water systems will cool the reactor core and protect spent uranium fuel in storage pools at the facility.  These new systems—part of a new diverse and flexible safety strategy (FLEX)—also include additional pre-staged pumps, generators, and other equipment at other nuclear plants and two new regional response centers.

But, what happens if these measures don’t prevent a meltdown?  The image of Fukushima that sticks with many people is the explosions at three of the plant’s buildings. These explosions were later found to be caused by a buildup of hydrogen, which was produced as uranium fuel overheated but was not released by plant operators from the reactor containment by the installed venting system.  Instead, hydrogen collected in the buildings surrounding the containment structures, where it mixed with oxygen in the air and was ignited.

Excess pressure in the reactor containments also resulted in radioactive releases (mostly not via the vent) that contaminated the surrounding area.  However, evacuations due to the tsunami warning and Tokyo Electric Power Co.’s emergency response plans effectively relocated nearby residents prior to the release of radiation.

Based on lessons learned from Japan, the U.S. Nuclear Regulatory Commission (NRC) ordered 31 reactors with the same basic design as those at Fukushima to install hardened vents that could be used to release the buildup of hydrogen and other gases.

In the extremely unlikely event that an accident progresses to the point of the fuel melting through the reactor vessel onto the containment floor, it is imperative that water be injected into containment to cool the fuel debris on the floor.  If not, radiation releases will occur from numerous locations in the containment building and bypass the vent.  If the vent is bypassed, it doesn’t help if there is a filter on the vent, because it, too, will be bypassed.

Last November, the NRC staff issued a policy paper recommending that the commission also approve the installation of external filters on boiling water reactors.

The external filters favored by the NRC staff are large tanks partially filled with water that are located outside the containment building.  Gases that are moved from the containment building using the vent system are released through the water, which filters the radionuclides.

The decision on filters is pending before the five NRC commissioners.

Is there a better solution? Yes.

One approach marries the need to cool the uranium fuel debris by injecting water into the containment building with filtering radioactive material from the vent gases with that water.

A September 2012 report by the Electric Power Research Institute (EPRI) evaluated several potential radiation filtering strategies and assessed their abilities to avoid radiation releases.  The report emphasized the need to cool the fuel debris during a severe accident.  Otherwise, the melted fuel will rupture containment and releasing radioactive materials into the environment bypassing the vent and any filter that might be installed.

Ensuring core cooling this way has the added benefit of “filtering” radioactive material inside the containment building.  The water in containment would work the same way that it would in an external filter.  Thus, keeping damaged fuel cool through the injection of water, with its inherent filtering capability, challenges the basis for compelling external filters at 31 reactors.

This is not a new concept.  In the late 1980s, when the NRC wanted the BWR Mark I plants to add containment vents to prevent over-pressurization during an accident, the vents were piped through the internal suppression pools (water already in containment as part of the existing design to cool and reduce the pressure of the containment atmosphere during accidents).  NRC staff reviews dating back to 1993 stated that an “external filter would not significantly increase the removal of radioactive material….”

In light of the significant body of research on this issue, the industry has recommended that the NRC consider less-prescriptive filtering strategies. Consistent with the EPRI research, filtering strategies focus primarily on providing additional ways to get water to the fuel during a severe accident and to control pressure in containment.  Rather than a prescriptive solution like external filters that has limited benefit and may not provide for additional water to cool the core, the industry envisions that each company would assess the issue on a site-by-site basis and develop an approach that best suits each site.

As pointed out numerous times industry testimony and letters to the NRC, filtering strategies based on the individual plant evaluations could result in the installation of a vent filter if that’s what makes sense for a given plant.

Even the NRC’s Advisory Committee on Reactor Safeguards — an independent body that reviews NRC staff activities and provides independent recommendations to the Commission – supports the proposal to look at performance-based solutions at each reactor.

Filtering strategies will require plant upgrades.  The nature and ultimate cost of those upgrades will depend on the result of the commissioners’ vote on the NRC staff recommendation.  These upgrades will involve making sure that cooling systems can put water in containment to cool the fuel debris and filter radioactive material when and where needed.

Adding this additional layer of safety is the right thing to do, but the industry must have the necessary flexibility to take the right steps based on the unique characteristics of their facilities.

It’s not about the money.  The Fukushima Daiichi accident reinforced the fact that no one can predict Mother Nature. We expect the unexpected and prepare beyond it, to ensure the swiftest and safest response to protect the public and the environment. That’s why we are constantly reviewing and improving the technology and systems we have in place to ensure we have backups to every backup, and practiced emergency response plans for the most extreme situations.

This issue is about providing yet reliable methods to cool the fuel debris by injecting water into containment and filtering the radioactive material from the vent gases with water inside the containment building.

The following article was written by Jason Zorn and Steven Kraft of the Nuclear Energy Institute. Zorn is NEI’s assistant general counsel. Kraft is senior technical advisor.

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