Form 8-K

8-K — DEEP FISSION, INC.

Accession: 0001104659-26-109201

Filed: 2026-09-21

Period: 2026-09-21

CIK: 0001918102

SIC: 4911 (ELECTRIC SERVICES)

Item: Regulation FD Disclosure

Item: Financial Statements and Exhibits

Documents

8-K — tm2625786d1_8k.htm (Primary)

EX-99.1 — EXHIBIT 99.1 (tm2625786d1_ex99-1.htm)

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0001918102

2026-09-21

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities

Exchange Act of 1934

Date of Report (Date of earliest event reported):

September 21, 2026

Deep Fission, Inc.

(Exact name of registrant as specified in its charter)

Delaware

000-56407

87-4265302

(State or other jurisdiction

of incorporation)

(Commission File Number)

(IRS Employer

Identification No.)

2001 Addison Street, Unit 300

Berkeley, California

(Address of principal executive offices)

94704

(Zip Code)

Registrant’s

telephone number, including area code: (707) 400-0778

N/A

(Former Name or Former Address, if Changed Since

Last Report)

Check the appropriate box below if the Form 8-K filing is intended

to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class

Trading

Symbol(s)

Name of each exchange on which registered

Common Stock, par value $0.0001 per share

FISN

The Nasdaq Global Market

Indicate by check mark whether the registrant is an emerging growth

company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange

Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company x

If an emerging growth company, indicate by check mark if the registrant

has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant

to Section 13(a) of the Exchange Act. ¨

Item 7.01 Regulation FD Disclosure.

On September 21, 2026, Deep Fission, Inc. (the “Company”)

issued a press release announcing its collaboration with Youngquist Brothers, LLC concerning the evaluation and testing of drilling and

borehole-construction methods for the Company’s underground reactor deployment model. A copy of the press release is furnished as

Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

The information furnished pursuant to Item 7.01 (including Exhibit 99.1) shall not be deemed “filed” for purposes of Section

18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that

section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange

Act, except as expressly set forth by specific reference in such filing.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

Exhibit No.

Description

99.1

Press Release dated September 21, 2026

104

Cover Page Interactive Data File (embedded within the inline XBRL document)

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934,

the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

DEEP FISSION, INC.

Date: September 21, 2026

/s/ JON GORDON

Jon Gordon

General Counsel & Secretary

EX-99.1 — EXHIBIT 99.1

EX-99.1

Filename: tm2625786d1_ex99-1.htm · Sequence: 2

Exhibit 99.1

Deep Fission

and Youngquist Brothers Announce Collaboration on Deep, Large-Diameter Borehole Drilling for Underground Nuclear Reactor Deployment

Companies will jointly evaluate and test drilling and borehole-construction methods to support Deep Fission's underground reactor technology

Fort Myers, Fla. – September

21, 2026 – Deep Fission, Inc. (Nasdaq: FISN) (“Deep Fission” or the “Company”), an advanced nuclear

energy company working to place small modular pressurized water reactors (SMRs) in boreholes approximately one mile underground, today

announced a collaboration with Youngquist Brothers, LLC ("Youngquist"), a commercial drilling contractor specializing in large-diameter

boreholes, to evaluate and test drilling technologies for Deep Fission's underground reactor deployment model.

Under the collaboration, the companies

will advance and test methods for constructing large-diameter, deep boreholes for Deep Fission's underground deployment model, drawing

on Youngquist's drilling and borehole-construction expertise. On September 3, the companies successfully completed a related equipment

demonstration in Fort Myers, Florida, where Deep Fission lowered and retrieved its 30-inch full-size prototype reactor canister into

a 34-inch borehole, to a depth of approximately 100 feet using Youngquist’s proprietary drilling technologies & equipment.

The demonstration scale test focused on installation and retrieval mechanics, distinct from the full-depth drilling work intended to

be covered by the collaboration.

“Youngquist brings decades of

hands-on drilling experience that maps directly onto what our deployment model requires,” said Liz Muller, CEO and Co-Founder of

Deep Fission. “Working with an established commercial driller lets us advance the drilling side of our approach using equipment

and expertise that already exist in the field, rather than starting from scratch.”

Youngquist has spent decades drilling

large-diameter boreholes reaching depths of more than 8,000 feet. That experience gives the collaboration a strong foundation for adapting

proven commercial drilling methods to Deep Fission's underground deployment model.

“Large-diameter drilling to these

depths is a method we've refined since 1971,” said Harvey Youngquist, CEO of Youngquist Brothers. “What Deep Fission requires

for its nuclear application is the same discipline we've always applied. We're glad to bring that experience to a project of this scale

and in this unique application.”

Deep Fission's Gravity™ Nuclear

Reactor uses established pressurized water reactor technology and standard low-enriched uranium fuel and secures the reactor roughly

a mile underground in a water-filled borehole. The surrounding rock and water are designed to provide core safety and operating functions,

including containment and cooling. The collaboration with Youngquist is focused on the drilling and borehole-construction methods needed

to support that underground deployment.

The companies intend to continue working

together on borehole drilling and testing as Deep Fission advances toward commercial deployment, which the Company is targeting as early

as 2027.

About Deep Fission

Deep Fission is developing technology

that places a small modular pressurized water reactor in a borehole approximately one mile underground. The Company’s Gravity Nuclear

Reactor™ approach combines established pressurized water reactor technology with a novel underground deployment model designed

to simplify construction, enhance safety, and support scalable commercial deployment. Deep Fission is focused on delivering reliable,

low-carbon baseload power to meet growing electricity demand from utilities, industrial customers, and data centers. The Company is currently

advancing the development of its first reactor project in Parsons, Kansas and was selected for the U.S. Department of Energy’s

Reactor Pilot Program.

Forward Looking Statements

This press release contains forward-looking

statements within the meaning of the federal securities laws. Forward-looking statements include, but are not limited to, statements

regarding the scope, objectives, continuation and anticipated benefits of the collaboration; the parties’ plans to advance and

test technologies and construction methods; the applicability and adaptability of Youngquist’s equipment, expertise and commercial

methods to the Company’s deployment model; the anticipated effect of the collaboration on advancing the drilling aspects of the

Company’s approach; and the Company’s progress toward and anticipated timing of commercial deployment, including its target

as early as 2027. These statements are based on current expectations and assumptions and are subject to risks and uncertainties that

could cause actual results to differ materially from those expressed or implied.

Important factors that may cause actual

results to differ materially include, among others, risks related to the parties’ ability to continue and successfully perform

the collaboration; changes in the scope, timing or objectives of the collaboration; the ability to adapt existing equipment, technologies

and construction methods to the Company’s proposed application and deployment model; technical, engineering, drilling, construction,

regulatory, licensing, financing, supply chain, and deployment risks; and the other risks described under “Risk Factors”

and “Cautionary Note Regarding Forward-Looking Statements” in Deep Fission’s filings with the Securities and Exchange

Commission.

Forward-looking statements speak only

as of the date of this press release. Deep Fission undertakes no obligation to update any forward-looking statements, except as required

by law.

Media Contact:

Chloe

Frader

media@deepfission.com

Investor Relations Contact:

Bob

Prag

IR@deepfission.com

(858) 794-9500

Elevate IR

(720) 330-2829

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