Should the NRC Revise the Aircraft Rule for SMRs?

In a cordial email exchange this week with Rod Adams, long time blogger at Atomic Insights, and more recently the Managing Partner at Nucleation Capital, we discussed whether the Nuclear Regulatory Commission should revise the rule (ML09022052) that contains requirements for nuclear reactor containment structures. (Consideration of Aircraft Impacts for New Nuclear Power Reactors)

Specifically, Adams is focused on the part of the rule which requires the massive concrete containment structures to be able to withstand the impacts of a wide body jet airliner. The risk addressed by the NRC is a very serious safety and security concern since terrorists destroyed the twin towers of the World Trade Center with just such an attack.

My interest in the topic goes back to October 2009. I wrote a piece for the Energy Collective titled “Westinghouse gets a scare from NRC.” The piece predicted some of the costs and challenges that would result from the imposition of the Aircraft Impact Assessment rule.

Adams writes, after reviewing it, says it in his email to this blog that with the NRC’s new found perspectives of dealing with adaptation of current rules to emerging reactor designs, e.g., small modular reactors (SMRs), “We know a lot more about the costs of complying with the rule today than we did when it was being developed. We have a lot more reasons to want to reduce the effort it takes to perform necessary safety reviews. We also should recognize that expending resources on design features that do not contribute to adequate protection isn’t a good investment.”

For instance, recently the Breakthrough Institute (BTI) and four other pro-nuclear NGOs made issued a joint statement about the NRC’s delay in issuing a revised rule on SMR emergency planning zones (EPZs). Shortly thereafter the NRC announced it would address the issue in favor of a modified EPZ rule for SMRs and not apply the rule as intended for full size reactors.

While coincidence is not necessarily a driver of causation, the close proximity in terms of dates of publication of BTI’s policy brief and the NRC’s action announcing progress on the EPZ rule represent refreshing progress on the regulatory front.

Is the NRC Aircraft Rule (Parts 50 & 52) as is Still Appropriate for SMRs?

The question is whether the aircraft rule for containment structures, as it exists, is still appropriate for SMRs or whether changes to it would reduce construction costs for them?

At the time the NRC rule was under consideration, the heart of the issue for Westinghouse was not solely the containment rule for aircraft itself. A key point of differences between Westinghouse and the NRC was the method proposed by Westinghouse for building the containment structure.

The Wall Street Journal in 2009 reported the NRC decision was a setback for new modular construction techniques. Westinghouse’s modified design, which the NRC rejected, proposed that the containment structure would be assembled from steel-and-concrete sections fabricated elsewhere and shipped to the reactor site. The containment structure would be built in modular block fashion piece-by-piece.

Previously, the approach for building a nuclear reactor containment wall was to have it poured continuously on site over a dense steel mesh of rebar with entire effort subject to strict AMSE quality and NRC constructions standards.

The NRC staff told the WSJ the construction method proposed by Westinghouse has not been used before and there was inadequate testing to prove it is strong enough to satisfy safety requirements. The agency told Westinghouse it would have to change the design and conduct tests to prove the modifications would meet agency safety requirements.”

Critics of the Westinghouse “lego” method of using pre-fabricated concrete panels said the NRC has a legitimate concern about the modular approach because it potentially created multiple points of failure along the seams of the panels compared to the strength of a web of steel embedded in a continuous concrete pour. That point of contention could come up again if the issue is reopened for SMRs.

What’s New About SMRs and Containment Structures?

What’s new for SMRs is whether taking a page from the logic used relative to changing the EPZ rule for SMRs, the approach could be to ask the NRC to consider adopting a revised rule for SMRs related to containment structures. This could be relevant for designs like NuScale’s and others where the reactor site is partially underground (see image below). Many of the mini reactors, e.g., < 25 MW, will also be essentially “plug and play” in much smaller underground silos.

Of course, not all SMR and mini-reactors will be built in underground silos which come with their own challenges. According to Rod Adams, a short list of issues includes the risk of flooding, the structural requirements of digging and maintaining the equivalent of a five-to-ten story underground space that will stand up to soil pressures at these depths, and the overall challenges of managing a reactor in a confined space underground,

Adams offers an interesting metaphor. He says that the underground space planned by some SMR developers will be the rough order equivalent of standing up a nuclear submarine vertically and sinking it into a huge shaft in the ground.

He called it a “ship in a bottle” which even if you just consider just the power plant and propulsion system is an visually compelling image.

Adams noted that his intent is to illustrate the difficulty of conducting maintenance and repair work in confined spaces. Submarines are confined spaces in their horizontal layout, with thick hulls that sometimes require cutting to provide access – with later weld repairs to fix the intentional hole. Fully underground SMRs would be in confined spaces without the fallback plan of cutting into the side if access is needed.

What Will Be the Impact of the Aircraft Rule on SMRs?

Leaving aside the issue of underground bunkers for SMRs, it gets problematic relative to the aircraft rule and SMRs for the mid-size reactors, e.g, 300 MW and larger. Examples include the GEH BWRX300 and the newly announced Westinghouse AP300 both of which are expected to be delivered to customers in multiple units.

Except for NuScale, none of the other SMR developers have reactor designs formally entered into the NRC’s process to grant a license although prelicensing work with the agency is underway for many of them.

These vendors, and others, are the ones most likely to be interested in a change to the aircraft rule. In the UK the Rolls-Royce 470 MW PWR will also face this issue with the Office of Nuclear Regulation as part of its generic design assessment.

There’s actually a pile up of pending licensing applications for SMRs coming the NRCs’ way and the agency has promised to complete each and every one of them in 42 months more or less.

The NRC’s aircraft rule is potentially on a collision course with an estimate of as many as two-to-three dozen SMRs that will need to be licensed over the next five-to-ten years. Here’s a short list.

• TerraPower’s 345 MW advanced “Natrium” reactor is being developed to replace a coal-fired power plant in Wyoming. With Pacific Corp, its customer, Terrapower has announced plans for five additional projects within the utility’s service area.
• X-Energy has plans to offer four of its Xe-100 advanced reactor for its customer Dow Chemical at that firm’s manufacturing site in Texas. It also has plans to build 12 units in the Pacific northwest with the first unit in the series to be located at the Columbia Generating Station near Richland, WA.
• TVA has plans for 800 MW of SMRs at the Clinch River site using the GEH BWRX-300 design.
• Duke Energy announced it will consider deploying two SMRs to replace coal fired power plants. The first one will be in North Carolina.
• NuScale says it will build a six-pack of its 77 MW light water reactor design for its customer UAMPS at planned the Idaho site.

Duke and TVA have long experience with containment structures for full size reactors. It would be in their interests to assess taking action to ask the NRC to reconsider the aircraft rule for SMRs. A change in the rule could have significant benefits in saving money during construction of SMRs without compromising safety or site security for the final built installation.

On the other hand, all of the full size reactors that could be built in the future in the US – AP1000, ESBWR1500, and APR1440 – have completed their safety design reviews using the current containment rule. These vendors and their potential customers are unlikely to want to reopen the rule making for reactors of their size due to the associated costs and uncertainties of also opening the door to other changes that might not be in their interests.

What About Risks for SMRs Slated for Export?

Moving overseas from the US regulatory space, all of the SMR developers are planning to pursue export deals and several of them have secured marketing beachheads in Europe including in Poland, Romania, and UK.

This raises a new question about risk and airplanes because there is a war raging in Ukraine. Currently, shelling in Ukraine around the six full size reactors at the Zaporizhzhia nuclear plant raises the prospect of the vulnerability of plants in war zones. The six reactors there make it one of the largest nuclear power plants in Europe. All of them are loaded with irradiated fuel and there are spent fuel ponds at the site as well. An attack on the reactor site with high velocity munitions could hit one of the spent fuel ponds with the resulting explosion producing a large fast moving cloud of radioactive derbies.

In December 2017 in the UAE Houtihi rebels from Yemen claimed they fired Iranian built long range missiles at the Barakah nuclear power plant which at the time was under construction. The UAE denied about a month later that any of the rockets hit the site. The rockets also hit other UAE locations around Abu Dhabi.

The UAE authorities also said at the time of the attacks that nuclear fuel had not yet been loaded at any of the four units under construction. Note the UAE’s four 1400 MW nuclear reactors have containment structures that meet the NRC’s final rule for the risk of aircraft crashing into them.

What may have deterred the Houthis from launching additional missile attacks once the UAE reactors were commissioned with loaded fuel is the prospect of being blamed for a disastrous radioactive release with regional and international blowback on Iran for supplying the missiles.

It is unclear whether Iran anticipated that the Houthis would attack the nuclear site. We can only assume that it was alarmed by the consequences of being blamed for causing a major nuclear disaster and that this stopped them from supporting additional missile attacks from the rebel groups in Yemen on the UAE reactors.

These kinds of incidents in Ukraine and the UAE will influence regulatory agencies having jurisdiction over licensing of SMRs in Europe and elsewhere. They will likely take the potential of hostilities between nations or insurgent groups into account in setting requirements for containment structures.

SMR developers in the US, mindful of the fact that the NRC’s safety rules are considered to be the gold standard for acceptance of US nuclear technologies, have an incentive to engage with the agency on the aircraft containment rule sooner rather than later due to the global reach of their industry.

Summing it up, it seems that the NRC, in its efforts to adapt current regulations for full size reactors in the US to SMRs, and having established a precedent with emergency planning zones for them, might benefit its pursuit of safety and security for SMRs to take the next step. This would be to have a serious look at whether the aircraft rule for full size reactors also needs a review for the next generation of small LWR type SMRs and advanced designs of SMRs. There’s a lot to consider and a lot interest waiting in the wings for an answer.

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Addendums

• Exceptions to the Aircraft Rule – Anonymous

Those vendors who think they wouldn’t need to consider it, can do an analysis to show that if an aircraft impact would occur, it would not create an accident or severe hazards that would result in exceeding release limits.

Also the threat basis considerations (DBT) are of so low-low probability, that the design or site specific application need not consider it/ For example no large aircraft/flights are in the vicinity of a remote mining site.

• Safety of TRISO fuel – Jonathen Facemire

TRISO used in advanced reactors is the Safety Related piece of “functional containment”. Qualified up to 1800C, in theory, testing of TRISO as functional containment could meet the intent of the aircraft impact rule without the need for a large concrete structure. That might be the only way for microreactors to be licensable at a reasonable cost.

• Airplanes can’t destroy nuclear containment domes – Anonymous

Considering that a jet airplane’s structure is basically a thin hollow aluminum tube, impact, even at 500 mph into a concrete containment structure, would result in the equivalent of a crushed soda can and some scorch marks on the side.

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