Small Modular Reactor Decision Made With Inadequate Information


Los Alamos continues to receive carefully spun information on the Small Modular Reactor (SMR) project.  For example, the “approval” of the NuScale SMR “design” ballyhooed in recent news stories is actually only the Final Safety Evaluation Report of the Design Certification Application by Nuclear Regulatory Commission (NRC) staff and a recommendation to the NRC Commission, which will meet later to consider approving it.

This action tentatively approves only the safety aspects of the design, which seems at odds with a report in  Reuters Events on September 1,  which quoted NuScale:  “A certification means the reactor has gone from ‘this is a really cool something that we’ll see in the future’, to ‘here it is, in a packaged box, I can sell it to you.’ That’s a big difference in the market.” There are at least two problems with this.  The NRC seems to have a limited definition of “the design,” because it appears that the design of the nuclear core is not complete, which I will discuss shortly.  But first, there may be a serious safety concern that was left out of the press release.

A scientist of the NRC ONRR has filed a “design non-concurrence” based on his evaluation of the safety of the reactor in a shutdown scenario.  “It is the author’s view that the [NuScale] reactor could reach fuel failure and prompt criticality condition for a wide range of initial conditions. … Based on event analysis and the identified modeling deficiencies, the author believes that the NuScale reactor will most likely experience core damage … even in the case without operator actions ….additional design changes are needed from NuScale to avoid … catastrophic core damage … and improve the safety margin.”

This non-concurrence was apparently overridden in the evaluation, and of course there was no mention of it in the press releases.  Many critics of the project have focused on the rush to judgement by the NRC of this brand new, unprecedented, first of a kind nuclear reactor, including an early decision not to require a number of traditional safety precautions. Is this rejection of an unanticipated safety concern another example of the abolishment of venerable safeguards that has come to characterize the current regulatory environment in Washington?

Now, what’s the status of the rest of the design?  NuScale’s marketing pitch has repeatedly stated that this design is based on “well-known and established pressurized light water reactor technology”, while in the next breath emphasizing that it is a First-Of-A-Kind (FOAK) design.  Nowhere is this contradiction as obvious as in the nuclear core. 

All PWR’s constructed to date have used pumps to circulate the coolant that transports heat from the reactor core to the load.  A unique feature of the NuScale and some other new designs is the use of natural convection to circulate the coolant, hence there is no need for pumps and the concomitant extraordinary ability of this design to shut itself down without external power in the event of an accident (think anti-Fukushima). 

But without the pumps the flow rates are outside the parameter space of the computer models used to model the heat transport in PWR’s.  NuScale is doing a series of tests to determine new values for  parameters in the codes and the NRC has certified this process, but the fact that this had to be done illustrates the fallacy of saying that this is the same as other PWR’s. 

In fact it is a First-of-a Kind and on close examination almost nothing will be the same as existing PWR’s.  The fuel assemblies may be the traditional 17 x 17 array of fuel rods favored by NuScale’s chosen fuel designer,  but the number,  spacing, and other physical dimensions and materials must be re-calculated and can not be determined until the new parameters are put into the models.

Even the nuclear fuel, which NuScale incongruously, occasionally still  claims is the same ole ceramic UO2 used in PWR reactors, is under redesign as NuScale has contracted a fuel design company and announced over the last few years that it will use a more highly enriched fuel, and is considering a metallic fuel, and had tested (in a computer model) MOX fuel.  The physical characteristics of the nuclear fuel and other materials contained in the fuel rods, and cladding material diameter and thickness, and the coolant itself, all may be in play as designers explore the new parameter space driven by the lower flow rates available from natural cooling.  NuScale indicates that these factors will be addressed by modifications to the standard design awaiting approval by the NRC.

In response to a question from a committee of the Parliament of Victoria, Australia,  NuScale testified that “Since NuScale’s SMR technology is based on well-known and established pressurized light water reactor technology NuScale neither intends, nor is it required under U.S. regulations, to first construct a prototype or demonstration plant.”  OK  they are not a crook. 

I don’t think this is a safety or legal problem, it’s just common sense and good science: NuScale is justifying skipping testing a prototype by its similarity to established PWR technology.    But I have just shown that this reactor is a long way from traditional PWR technology.  Reactor cores, the associated circulation hardware and vessels, and coolant composition and circulation comprise many interacting parts that are carefully tuned and balanced.  It seems highly likely that a lot of redesign and rework is going to be necessary when they put together all the new pieces of a FOAK for the first time.  They have allowed a year for this. The schedule is likely to slip – again.

In its testimony to the Parliament committee, NuScale blamed the lengthy schedule and changes in the schedule on the conservatism and power needs of UAMPS.  NuScale said it could deliver a module sooner to another customer.   If that’s the case they should have time to create by 2026 a prototype and test it and not take a chance on missing the 2030 date of the existing schedule – which has slipped 6 years in the past six years.

Los Alamos has determined to continue in this project, but I believe that the decision was made with inadequate information.  The Board of Public Utilities and Council should insist on transparency from NuScale and UAMPS especially with technical information and scheduling decisions.  The documentation that NuScale provides is unhelpful, responses to questions at public fora are unsubstantial, and these have the effect of making the factual basis for the project indecipherable.  Los Alamos ought to demand and receive accurate information.  We should not have to go to Australia to find by happenstance a clear statement of an important aspect of the project.  Important decisions should be relayed to us in documents that convey accurate information, not fluffy press releases.

George Chandler
Los Alamos