SCHEDULE 13D

The principal business office of the Reporting Person is c/o Deep Fission, Inc., 2831 Garber Street, Berkeley, California 94705.

The Reporting Person is a member of the Board of Directors (the "Board") and the President and Chief Executive Officer of the Issuer.

During the last five years, the Reporting Person has not been (i) convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) and (ii) a party to a civil proceeding of a judicial of administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

During the last five years, the Reporting Person has not been (i) convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) and (ii) a party to a civil proceeding of a judicial of administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

The Reporting Person is a citizen of the United States.

All of the shares of Common Stock to which this Schedule 13D relates were acquired by the Reporting Person in connection with the consummation of the closing on September 5, 2025 (the "Closing Date") of the transactions contemplated by the Agreement and Plan of Merger and Reorganization dated as of September 5, 2025, by and among Surfside Acquisition Inc. ("Surfside"), Deep Fission Acquisition Co., a wholly-owned subsidiary of Surfside ("Acquisition Sub") and Deep Fission Nuclear, Inc. (fka, Deep Fission, Inc.) ("Legacy Deep Fission") (the "Merger Agreement"). Pursuant to the terms of the Merger Agreement, a business combination of Legacy Deep Fission and Surfside was effected by the merger of Acquisition Sub with and into Legacy Deep Fission, with Legacy Deep Fission surviving the Merger as a wholly owned subsidiary of Surfside (the "Merger"). In connection with the consummation of the Merger on the Closing Date, Surfside changed its name from Surfside Acquisition Inc. to Deep Fission Inc. Pursuant to the terms of the Merger Agreement, each outstanding share of Legacy Deep Fission common stock issued and outstanding immediately prior to the effective time of the Merger was converted into the right to receive 17.32141 shares of Common Stock, rounded to the nearest whole share.

The Reporting Person acquired the Common Stock in connection with the Merger. The information contained in Item 3 of this Schedule 13D is incorporated herein by reference. The Reporting Person serves as a Director and President and Chief Executive Officer of the Issuer. Accordingly, the Reporting Person may have influence over the corporate activities of the Issuer, including activities that may relate to items described in clauses (a) through (j) of Item 4 of this Schedule 13D. Subject to the Registration Rights Agreement described in Item 6 of this Schedule 13D, the Reporting Person may, from time to time, purchase or sell securities of the Issuer as appropriate for her personal circumstances. Except as described in this Schedule 13D, the Reporting Person does not have any present plans or proposals that relate to or would result in any of the actions described in clauses (a) through (j) of Item 4 of this Schedule 13D. The Reporting Person reserves the right to formulate plans and/or proposals and to take such actions with respect to her investment in the Issuer, including any or all of the actions set forth in clauses (a) through (j) of Item 4 of this Schedule 13D.

The aggregate percentage of Common Stock that the Reporting Person may be deemed to beneficially own is determined in accordance with the rules of the SEC and is based on 50,874,809 shares of Common Stock issued and outstanding as of September 5, 2025, as reported in the Issuer's Current Report on Form 8-K filed with the SEC on September 11, 2025 (the "Super 8-K"). Based on the foregoing, the Reporting Person may be deemed to beneficially own 20.6% of the Issuer's Common Stock in the aggregate, rounded to the nearest tenth. As of the Closing Date, the Reporting Person beneficially owned, in the aggregate, 10,541,757 shares of Common Stock, comprising (i) 5,196,423 shares of Common Stock, held directly by the Reporting Person, (ii) 253,661 shares of Common Stock issuable for vested and exercisable stock options issued pursuant to the Issuer's 2025 Plan, (iii) 33,821 shares of Common Stock issuable pursuant to stock options exercisable within 60 days of September 19, 2025, and (iv) 5,057,852 shares of Common Stock held by the Muller Family Trust, for which Ms. Muller is a trustee.

As a result of the Transactions, the Reporting Person is the beneficial owner of, and has the sole power to vote or direct the vote and to dispose or to direct the disposition of 10,541,757 shares of Common Stock, which includes 33,821 shares of Common Stock underlying options which are exercisable within 60 days of September 19, 2025.

Except as set forth in this Schedule 13D, there have been no other reportable transactions with respect to the Common Stock within the last 60 days by the Reporting Person.

Except as set forth herein, no other person has the right to receive or the power to direct the receipt of distributions from, or the proceeds from the sale of, securities covered by this Schedule 13D.

Not applicable.

Registration Rights Agreement In connection with the Merger, on September 5, 2025, the Issuer entered into a registration rights agreement (the "Registration Rights Agreement"), pursuant to which, the Issuer has agreed that promptly, but no later than 45 calendar days after the date on which the Super 8-K is first filed by the Issuer with the SEC, the Issuer will file, subject to customary exceptions, a registration statement with the SEC, covering, among other securities, the shares of Common Stock issued to Legacy Deep Fission's stockholders as a result of the Merger and the related share conversion. Lock-Up Agreement The Reporting Person has entered into a lock-up agreement pursuant to which it has agreed not to, except in limited circumstances, offer, pledge, sell, contract to sell, transfer or dispose of, directly or indirectly, or engage in swap or similar transactions with respect to, any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock, including, as applicable, shares received in the Merger and issuable upon exercise of certain options, for a period of 180 days after the listing of the Common Stock on a national securities exchange.

Exhibit 1 Agreement and Plan of Merger and Reorganization, dated September 5, 2025, by and among the Issuer, Acquisition Sub, and Legacy Deep Fission (incorporated by reference to Exhibit 2.1 to the Issuer's Current Report on Form 8-K (File No. 000-56407), as filed with the SEC on September 11, 2025). Exhibit 2 Form of Registration Rights Agreement by and between the Issuer and the parties thereto (incorporated by reference to Exhibit 10.4 to the Issuer's Current Report on Form 8-K (File No. 000-56407), as filed with the SEC on September 11, 2025). Exhibit 3 Form of Lock-up Agreement (incorporated by reference to Exhibit 10.11 to the Issuer's Registration Statement on Form S-1 (File No. 333-290438), as filed with the SEC on September 23, 2025)