Sec Form 3 Filing - BUCKMAN FREDERICK W @ Terrestrial Energy Inc. /DE/ - 2025-10-28

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

1. Name and Address of Reporting Person BUCKMAN FREDERICK W 2. Issuer Name and Ticker or Trading Symbol Terrestrial Energy Inc. /DE/ [ IMSR] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 2730 W. TYVOLA ROAD, SUITE 100 3. Date of Earliest Transaction (MM/DD/YY) 10/28/2025 (Street) CHARLOTTE, NC28217 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock, par value $0.0001 per share ("Common Stock") 6,669 ( 1 ) D Common Stock 14,930 ( 1 ) I Through Trust

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 1.4764 ( 6 ) ( 2 ) 10/01/2036 Common Stock 22,352 D Stock Option (Right to Buy) $ 2.0133 ( 2 ) 11/19/2037 Common Stock 111,758 D Stock Option (Right to Buy) $ 2.237 04/01/2024( 3 ) 04/01/2043 Common Stock 67,055 D Stock Option (Right to Buy) $ 2.237 02/03/2026( 3 ) 02/03/2045 Common Stock 67,055 D Warrants (Right to Buy) $ 2.237 ( 4 ) 07/31/2028 Common Stock 22,352 D Warrants (Right to Buy) $ 2.237 ( 4 ) 07/31/2028 Common Stock 44,703 I Through Trust Contingent Value Right ( 5 ) ( 5 ) ( 5 ) Common Stock ( 5 ) D Contingent Value Right ( 5 ) ( 5 ) ( 5 ) Common Stock ( 5 ) I Through Trust

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other BUCKMAN FREDERICK W 2730 W. TYVOLA ROAD, SUITE 100 CHARLOTTE, NC28217 X Signatures /s/ Steven M. Millsap, pursuant to Power of Attorney 11/07/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Upon closing of the business combination (the "Business Combination") between Terrestrial Energy Inc., a Delaware corporation formerly known as HCM II Acquisition Corp. ("New Terrestrial"), and Terrestrial Energy Development Inc., a Delaware corporation formerly known as Terrestrial Energy Inc. ("Legacy Terrestrial"), the reporting person acquired these securities in exchange for the reporting person's securities in Legacy Terrestrial pursuant to the terms and conditions of the Business Combination Agreement, dated as of March 26, 2025, by and among HCM II Acquisition Corp., HCM II Merger Sub Inc. and Legacy Terrestrial. ( 2 )Consists of options granted pursuant to the Legacy Terrestrial Second Amended and Restated 2024 Stock Option Plan. The options are currently exercisable. ( 3 )Consists of options granted pursuant to the Legacy Terrestrial Second Amended and Restated 2024 Stock Option Plan. The options vest in one-third increments on each of the first, second and third anniversaries of the grant date, subject to the reporting person's continued service through each applicable vesting date. ( 4 )Consists of Legacy Terrestrial warrants to purchase common stock that are currently exercisable. ( 5 )Consists of contingent value right issued upon conversion of Legacy Terrestrial convertible notes in connection with the Business Combination. Each contingent value right entitles the holder to receive additional shares of Common Stock in the event that the volume weighed average price of the Common Stock for the twenty (20) trading days beginning on the trading day immediately following expiration of the lock-up period contemplated by the applicable Legacy Terrestrial convertible note is less than seventy-five percent (75%) of the redemption price ($7.905). ( 6 )Price is expressed in Canadian dollars.

Remarks: Exhibit List - Exhibit 24.1 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.