Sec Form 3 Filing - Irish Simon @ 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

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1. Name and Address of Reporting Person Irish Simon 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 X __ Officer (give title below) _____ Other (specify below) Chief Executive Officer (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") 2,295,934 ( 1 ) D Common Stock 2,818,520 ( 1 ) I Through LLC Common Stock 52,255 ( 1 ) I Through Defined Benefit Plan

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) $ 2.0133 ( 2 ) 06/16/2038 Common Stock 24,140 D Stock Option (Right to Buy) $ 2.237 ( 2 ) 01/24/2031 Common Stock 107,288 D Stock Option (Right to Buy) $ 0.2013 ( 7 ) ( 2 ) 12/31/2029 Common Stock 178,812 D Stock Option (Right to Buy) $ 0.2013 ( 7 ) ( 2 ) 12/31/2034 Common Stock 2,279,850 D Stock Option (Right to Buy) $ 0.2013 ( 7 ) ( 2 ) 12/31/2028 Common Stock 178,812 D Stock Option (Right to Buy) $ 0.2013 ( 7 ) ( 2 ) 12/31/2034 Common Stock 2,172,563 D Stock Option (Right to Buy) $ 2.237 05/19/2023( 3 ) 05/19/2032 Common Stock 98,347 D Stock Option (Right to Buy) $ 2.237 ( 2 ) 01/01/2043 Common Stock 21,905 D Stock Option (Right to Buy) $ 2.237 ( 2 ) 01/01/2044 Common Stock 26,286 D Stock Option (Right to Buy) $ 2.237 ( 2 ) 01/01/2045 Common Stock 4,381 D Stock Option (Right to Buy) $ 2.237 04/01/2024( 3 ) 04/01/2043 Common Stock 89,406 D Stock Option (Right to Buy) $ 2.237 07/14/2024( 3 ) 07/14/2033 Common Stock 178,812 D Restricted Stock Units ( 4 ) 10/21/2026( 4 ) 10/21/2035 Common Stock 1,023,160 D Warrants (Right to Buy) $ 2.237 ( 5 ) 07/31/2028 Common Stock 44,703 D Warrants (Right to Buy) $ 2.237 ( 5 ) 07/31/2028 Common Stock 89,406 I Through Defined Benefit Plan Contingent Value Right ( 6 ) ( 6 ) ( 6 ) Common Stock ( 6 ) D Contingent Value Right ( 6 ) ( 6 ) ( 6 ) Common Stock ( 6 ) I Through Defined Benefit Plan

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Irish Simon 2730 W. TYVOLA ROAD, SUITE 100 CHARLOTTE, NC28217 X Chief Executive Officer 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 and accelerated vesting in certain events. ( 4 )Each restricted stock unit represents a contingent right to acquire one share of Common Stock. The restricted stock units vest in one-half increments on each of the first and second anniversaries of the grant date, subject to Mr. Irish's continued employment through each applicable vesting date and accelerated vesting in certain events. ( 5 )Consists of Legacy Terrestrial warrants to purchase common stock that are currently exercisable. ( 6 )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). ( 7 )Price is expressed in Canadian dollars.

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