The MIGHTR project

a tale of cycles

Inception and general aspects

The MIGHTR, Modular Integrated Gas High Temperature Rector, is a High Temperature Gas Reactor (HTGR) created to be the natural technical continuation from the Next Generation Nuclear Plant (NGNP) program. While some countries have advanced further in HTGR development in the last decades, those lagging behind could adopt a smarter design strategy rather than replicating or slightly modifying existing models with more resources.

I believed MIGHTR had strong potential because, thanks to the focus on constructability, it represented a logical continuation from the NGNP and MHTGR projects, as well as other HTGR initiatives worldwide. This approach allowed us to build on the extensive work already done in these multi-decade projects, rather than starting from scratch, while modifying some aspects of the design to improve constructability dramatically.

The term HC-HTGR is a general label for the concept, similar to how LPSR refers to the AP1000 or LASR the APR-1400 and similar. MIGHTR was the name we chose for the potential product at the project’s inception.

The inception of MIGHTR began with a heated debate between Robbie and me in the spring of 2019. Our research group at MIT had selected the NGNP design (SC-HTGR) for a technology scoping project aimed at supplying process heat in Japan. While the group was generally enthusiastic about this technology, there was no consensus on its feasibility as a deliverable project. Robbie was initially frustrated with my criticisms of the NGNP design, but eventually, he listened to my arguments and asked, “Alright, help me make this design better. What would you change to address the issues you’re pointing out?”

From that moment, we dedicated hours outside of our responsibilities within the Japan Next Project and any MIT-related activities to sketch a reactor design that could overcome the challenges faced by the NGNP project. When we informed our professors about our extracurricular efforts, they not only encouraged us to continue but also offered enthusiastic support.

A few weeks later, our professors encouraged us to present our idea at a student competition at ICAPP Abu Dhabi in Spring 2020 and at the MIT Energy Night in October 2019. Then we decided to file a provisional for protection in 2019 and the patent application in 2020. We presented the concept at many stages between 2019 and 2021. The culmination was a publication on the Applied Energy journal.

At the start of 2020, we began to develop the MIGHTR concept more seriously. By summer, we had convinced Professor Koroush Shirvan to submit a proposal for a three-year conceptualization project to the US Department of Energy (DOE) under the Advanced Reactor Demonstration Program (ARDP). In December 2020, the DOE announced a $4.9 million funding ward for the development of the MIGHTR/HC-HTGR concept under the ARDP program, ARC-20 track.

We worked independently from the DOE-funded project members until August 2021. From September 2021 onwards, the DOE-funded team joined the project, each member advancing a specific aspect of the design. Over these three years, the development progressed much faster than before.

We are immensely grateful to the DOE for funding this project and to all the participating institutions for their contributions. Special recognition goes to MIT’s leadership. We have summarized all the major aspects of MIGHTR in ANS publications.

ARC-20 overview

The ARC-20 team comprised MIT, UB, UM, MPR Associates, and ANL, with MIT leading the project under the guidance of Professor Koroush Shirvan. Within the consortium, BA subcontracted to MIT, and SGH subcontracted to UB. The consortium began its work in September 2021.

This project encompassed various aspects, including cost modeling, printed circuit heat exchanger development and testing, neutronic design and modeling, shielding and activation analysis, mechanical structural analysis, fabricability, licensability, reactor cavity cooling system thermal hydraulics, core thermal hydraulics, primary system thermal hydraulics, seismic analysis, civil design, and construction assessment.

Tasks outside the ARC-20 project

The work undertaken by BA outside of the ARC-20 project includes:

• Refueling and Tooling: We subcontracted Sam Quemby, an engineer with over five years of experience in nuclear robotics. Deft Dynamics and Southern Research (now Kratos SRE) collaborated on projects related to MIGHTR robotics under ARPA-E contract DE-AR0001157.

• Integration of Disciplines: We integrated the work of various systems, structures, and components to create a consistent Nuclear Power Plant. Our unique strategy for this integration was published in ANS.

• Safety and Licensing: We prepared for NRC engagement to ensure safety and compliance.

Where the MIGHTR is now

Over the past four years, we’ve made significant strides in addressing the major feasibility questions we initially faced. At an early stage of this type of project, it is important to focus on evaluating the feasibility of the different changes proposed with respect to the reference design from where the project starts. This is particularly challenging, for obvious reasons, if it is a white paper project, i.e., there is no reference technology to compare to. In our case, we have the MHTGR design as a reference. Every task faced an array of constraints and objectives, but here is a simplified version of them:

Key Achievements:

• Reflector structural mechanics: Objective: show the graphite reflector blocks could last 10+ years to avoid replacement while meeting their dose limits, fabrication tolerances, shielding constraints, structural requirements, interfacing requirements and layout limitations. Avoiding replacement greatly simplifies the operations and design for operations. After three years of iterations, the structural mechanics of the reflector look feasible. While some redesign or block splitting may be needed, our conservative stress assumptions suggest we’re close to our limits.

• Core control: Objective: control the core reactivity using only the outer reflector to limit the temperature exposure during design basis events and maintain access for the refueling equipment. Thanks to the work from UM and MIT, we are quite sure we can control the core from the outer reflector, showcasing their exceptional neutronic analysis capabilities.

• Seismic advantage: The seismic robustness of the low-rise MIGHTR building, and its reactor primary system and internals, was evaluated using numerical analysis by the team at UB. Two-dimensional and three-dimensional seismic isolation solutions were explored to mitigate the impact of the seismic load case.

• Seismic resilience of graphite core: The objective is to deploy a shear-key based design for the graphite assemblies that eliminate the need for dowel pins common to other HTGRs. UB’s seismic testing of the model assemblies demonstrated that the proposed design resists seismic forces through the controlled rotation of the blocks, thus minimizing the bearing stresses on the shear keys. No chipping/damage of shear keys was observed even after many cycles of repeated and rigorous testing with peak base accelerations exceeding as high as 0.8 g. Test videos can be accessed at this link.

• Helium leak management: The integrated design of MIGHTR introduces a mid-vessel flange that poses different design loads than the flanges in NGNP. Objective: ensure that this flange can meet helium leakage limits, fabrication requirements, and design loads. Our mid-vessel flange can likely be fabricated to maintain helium leak rates to a limit that does not invalidate the concept, ensuring the design remains practical. If it lost too much helium, the design would be impracticable. We defined feasibility to mean <10X the leakage rate of the Fort St. Vrain HTGR, and we found that a much lower rate is possible. In the future, this aspect should be included in the technoeconomic analysis to identify the exact helium leak rate and where the threshold for acceptable cost is.

• Efficiency hit due to by-pass flow: The novel fuel block design and control drums created new bypass flow paths through the core that is not directly or efficiently heated by the fuel. Objective: limit the flow lost through by-pass to minimize wasted helium pumping energy. By-pass flow appears to not result in a show-stopping efficiency drop.

• Accident thermal management: Objective: show that temperatures variations during major design accidental conditions are manageable. The back of the envelope analyses we have done show that in a horizontal SMR HTGR, Pressurized Conduction Cooldown (PCC) and Depressurized Conduction Cooldown (DCC) yield much lower temperature differences along internal components (10s of °C) compared to vertical designs (100s of °C), making them easier to manage. In vertical HTGRs, during PCC accidents, the top gets hot because of hot gas accumulation. This poses challenges and requires engineering solutions.

• Fabrication and installation: We can likely fabricate, transport, and install all core and primary system components without major issues. Thank you MPR Associates for your assistance at figuring this out.

• RCCS performance: The Reactor Cavity Cooling System (RCCS) comprises a set of ducts, panels and tanks that can remove decay heat and protect RPV, internals and civil works around from excessively high temperatures in different accidental conditions. Vertical HTGRs have the benefit of long heating distances to generate enough driving force for natural circulation. The reduced heating height from a horizontal HTGR brought the question at the beginning of the project on whether this would be problematic to guarantee RCCS performance. The work conducted by ANL in the project suggest that the RCCS works well with our reduced height constraint, potentially outperforming vertical HTGRs in some aspects, like dynamic behavior of raiser tubes and panels.

• Power output: Our core operates at 150 MWth, with potential to uprate to 200 MWth+ through value engineering and design optimization, enhancing economic viability. This uprate would potentially make MIGHTR the most economical HTGR on the planet, capable of supplying process heat for ammonia, cement, chemicals and many other products. The objective from the project was hitting the 150 MWth, so we were on target.

• Refueling machine: Objective: design a machine capable of placing and removing every fuel and inner reflector block every 18-24 months. The refueling machine at this stage is a multi-degree of freedom robotic arm railed to a cantilever beam that retrieves and replaces fuel blocks during refueling. A machine vision scheme shows promise for the refueling machine.

Major Perceived single Remaining Challenges:

• Mid-Vessel Flange: Assessing its impact on the economic potential for a MIGHTR in the 150-200 MWth range.

• PCHE/HX/Secondary system: Deciding whether to use a Printed Circuit Heat Exchanger (PCHE) or simply a Heat Exchanger (HX) and with what scope plus the integration of this with the rest of the secondary system, as well as planning operation and maintenance of the different valves in the primary system and integration with the circulators. There is a lot of decisions still to be made in ducting, valves, penetrations, thermal expansions management, maintenance, fabrication limitations management, etc.

• Refueling/Tooling: Developing specific operation, maintenance, assembly and decommissioning tools for the MIGHTR architecture, which may necessitate redesigning some reactor parts.

Integration:

The most challenging aspect is integrating all these elements into an operable, constructible, licensable, and cost-competitive design.

Where the MIGHTR is going

What would be best for the project now?

1.        Specify every system based on the work done

2.        Engage with the regulator

3.        Start building prototypes

Development is expected to progress slowly. There’s a possibility that MIGHTR may continue its development in Europe. Robbie will be dedicating most of his time to a separate entity focused on LWRs, reducing his involvement in MIGHTR to minimal management duties at Boston Atomics (expect a future post on this).

I will maintain my role at MIGHTR but with a new approach. I will now sign consulting and R&D contracts with companies and institutions, working on projects that benefit both the institution and MIGHTR. This could involve developing micro-reactor HTGRs or assisting with HTGR cost-modeling, all of which will advance MIGHTR and HTGR technology.

For instance, the MIGHTR patent remains open up to 20 MWth, allowing for the possibility of a lower power demo as part of the concept development conducted by an external entity for its own benefit. Any external work on this low-power version would be welcomed and beneficial to the project.

Additionally, I will continue to push the ongoing technical work for MIGHTR, dedicating a fraction of my time to ensure its progress.

Final notes

The MIGHTR project has had ups and downs. In 2019 and 2020 the MIGHTR team was tiny. For 3 years, the project has been relatively big and advanced steadily. Now it looks like the resources into the MIGHTR project are going to be small again. As stated before, I don’t expect to be able to demonstrate MIGHTR at full scale before the late 2030s. However…I am waiting for our opportunity. When the right people realize this is the best way to build an HTGR we will gather the needed resources and demonstrate the technology.