Deep Fission Names Day & Zimmermann to Build Its Mile-Deep Borehole Nuclear Reactor in Parsons, Kansas

Overview

Advanced-nuclear startup Deep Fission has lined up an established engineering firm to build the surface portion of its unconventional reactor, which is designed to operate roughly a mile underground. As World Nuclear News reported, Day & Zimmermann and Deep Fission have announced a partnership in the construction of Deep Fission’s Gravity reactor, with the engineering firm set to oversee pre-construction planning and perform above-ground construction for it.

The reactor is bound for the Great Plains Industrial Park in Parsons, Kansas, where, as Interesting Engineering reported, Deep Fission has begun drilling the first of three planned data acquisition wells for its pilot facility.

What We Know

The underground siting is central to the engineering pitch. Rather than relying on the heavy steel pressure vessels that surface reactors use to contain their primary coolant, Deep Fission lets the earth do the work. As Interesting Engineering explained, the weight of the water column directly above the reactor naturally produces the 160 atmospheres of hydrostatic pressure needed for standard operation. The same source reported that the surrounding bedrock provides natural geological containment and radiation shielding.

Each unit is small. The Department of Energy’s pilot-program documentation describes the design as the Deep Fission Borehole Reactor-1, a 15 MWe pressurised water reactor, per World Nuclear News. The concept is meant to scale by repetition: World Nuclear News reported that 100 reactors would produce 1.5 GWe, and Interesting Engineering noted that up to 100 units can be co-located on a single site.

In statements released with the partnership, both companies framed the work as a first-of-its-kind effort. Ross McConnell, Day & Zimmermann’s Chief Nuclear Officer, said the combination of “Deep Fission’s advanced reactor technology, combined with Day & Zimmermann’s quality programme and nuclear expertise, will bring first-of-a-kind nuclear construction to reality,” according to World Nuclear News. Mike Brasel, Deep Fission’s Chief Operating Officer, said that “Partnering with Day & Zimmermann will bring proven nuclear construction expertise and execution discipline to Deep Fission’s deployment,” the same outlet reported.

The Federal Backdrop

The Parsons project is one slice of a federal push to get privately developed reactors running quickly. Deep Fission is part of the US Department of Energy’s Reactor Pilot Program, which selected it as one of ten program participants, Interesting Engineering reported. When the DOE unveiled the program, it announced an initial selection of 11 advanced reactor projects, naming Deep Fission Inc among the developers and setting a goal to construct, operate, and achieve criticality of at least three test reactors using the DOE authorisation process by July 4, 2026, according to World Nuclear News.

Deep Fission has publicly embraced that deadline. When the company signed its agreement with the DOE, Liz Muller, the company’s CEO and Co-Founder, said Deep Fission was “working hard toward our goal of achieving criticality by July 4, 2026, and with the DOE’s support, we’re proud to show that our Gravity Nuclear Reactors can strengthen America’s energy leadership,” according to Deep Fission. Muller called the signing “a major milestone for Deep Fission and a meaningful step forward for advanced nuclear in the United States,” the company said.

The pilot program has already produced its first reactor to reach the criticality bar, though not Deep Fission’s. Antares Nuclear’s Mark-0 achieved zero-power criticality at Idaho National Laboratory’s Reactor and Critical Experiment facility, becoming the first advanced reactor to reach that milestone under the program, POWER Magazine reported. The same race was the focus of earlier Machine Herald reporting on the US military’s first airlift of a microreactor, and on Aalo Atomics completing the first reactor built on DOE land in 50 years.

What We Don’t Know

The partnership announcement does not lay out a construction schedule for the Parsons facility or say when above-ground work will begin. Deep Fission has only just started drilling the first of three planned data acquisition wells, per Interesting Engineering, and neither company has published a date for when the underground reactor itself would be emplaced or licensed to operate. The borehole approach also remains unproven at commercial scale: no reactor has yet operated a mile down using a water column for its primary pressure, and the announcement does not address how refueling, maintenance, or decommissioning would work at that depth.

Analysis

The significance of the Day & Zimmermann deal is less about the reactor’s physics than about its credibility. Deep Fission’s core claim is that it can skip the large pressure vessels and sprawling containment structures of conventional plants by burying a reactor where the planet supplies the pressure for it. That is an attractive cost story on paper, but it is also a construction story that demands a partner who has actually built nuclear infrastructure. Bringing in an established engineering firm signals that Deep Fission is trying to move its concept out of the realm of renderings and into a buildable project, even as the hardest milestone, an operating reactor underground, remains ahead of it.

The timing matters too. With the federal pilot program already notching its first criticality and a July 4, 2026 target looming, the construction partnership reads as an attempt to keep a borehole-based design in the conversation alongside the more conventional microreactors that have moved faster toward power production.