Terrestrial Energy Inc美股招股说明书(2026-01-05版)

SUPPLEMENT TOPROSPECTUS FOR UP TO 151,970,541 SHARES OF COMMON STOCK, AND18,350,000 WARRANTS TO PURCHASE SHARES OF COMMON STOCKOF TERRESTRIAL ENERGY INC.(FORMERLY KNOWN AS HCM II ACQUISITION CORP.) ThisSupplement No.1(this“Supplement No.1”)updates,amends and supplements the definitive proxystatement/prospectus dated September 26, 2025 (as supplemented or amended from time to time, the “Prospectus”), whichforms a part the Registration Statement on Form S-4 (Registration No. 333-288735, the “Form S-4”) filed by TerrestrialEnergy Inc. (formerly known as HCM II Acquisition Corp., or “HCM II”). Capitalized terms used in this prospectussupplement and not otherwise defined herein have the meanings specified in the Prospectus. The attached information updates, amends and supplements certain information contained in the Prospectus in order toupdate the Prospectus for use in connection with the issuance following the closing of the Merger of securities registered onthe Form S-4 in connection with the Business Combination, including the issuance of shares upon the exchange ofExchangeCo shares, the exercise of certain warrants, and if applicable, the settlement of the contingent value rights issued inthe Merger, as contemplated by the Prospectus. To the extent information in this Supplement No. 1 differs from, updates orconflicts with information contained in the Prospectus, the information in this Supplement No. 1 is the more currentinformation. This Supplement No. 1 is not complete without the Prospectus. This Supplement No. 1 should be read inconjunction with the Prospectus, which is to be delivered with this Supplement No. 1, and is qualified by reference thereto,except to the extent that the information in this Supplement No. 1 updates or supersedes the information contained in theProspectus. Please keep this prospectus supplement with the Prospectus for future reference. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 12 of theProspectus. NEITHERTHE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIESREGULATORY AGENCY HAS APPROVED OR DISAPPROVED OF THE SECURITIES DESCRIBED IN THEPROSPECTUS OR THIS PROSPECTUS SUPPLEMENT OR PASSED UPON THE ADEQUACY OR ACCURACYOFTHEDISCLOSUREINTHEPROSPECTUSORTHISPROSPECTUSSUPPLEMENT.ANYREPRESENTATION TO THE CONTRARY CONSTITUTES A CRIMINAL OFFENSE. The date of this prospectus supplement is January 5, 2026. This Supplement No. 1 should be read in conjunction with the Prospectus, which should be read carefully and in itsentirety. SUPPLEMENT NO. 1 TO THE DEFINITIVE PROXY STATEMENT/PROSPECTUS RISK FACTORS See Annex A included as part of this prospectus supplement under the heading “Risk Factors,” which is incorporatedherein by reference. UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION See Annex A included as part of this prospectus supplement under the heading “Unaudited Pro Forma CondensedCombined Financial Information,” including the unaudited pro forma condensed combined balance sheet as of September 30,2025, and the unaudited pro forma condensed combined statements of operations for the nine months ended September 30,2025 and for the year ended December31, 2024, and the related note thereto, which are incorporated herein by reference. INFORMATION ABOUT TERRESTRIAL ENERGY See Annex A included as part of this prospectus supplement under the heading “Information About Terrestrial Energy,”which is incorporated herein by reference. MANAGEMENT’S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OFOPERATIONS OF TERRESTRIAL ENERGY See Annex A included as part of this prospectus supplement under the heading “Management’s Discussion and Analysisof Financial Condition and Results of Operations of Terrestrial Energy,” which is incorporated herein by reference. DESCRIPTION OF NEW TERRESTRIAL ENERGY’S SECURITIES See Annex A included as part of this prospectus supplement under the heading “Description of Capital Stock,” which isincorporated herein by reference. BENEFICIAL OWNERSHIP OF SECURITIES See Annex A included as part of this prospectus supplement under the heading “Beneficial Ownership of Securities,”which is incorporated herein by reference. CERTAIN RELATIONSHIPS AND RELATED PERSON TRANSACTIONS See Annex A included as part of this prospectus supplement under the heading “Executive and Director Compensation ofTerrestrial Energy,” which is incorporated herein by reference. MANAGEMENT OF NEW TERRESTRIAL ENERGY FOLLOWING THE BUSINESS COMBINATION See Annex A included as part of this prospectus supplement under the heading “Management,” which is incorporatedherein by reference. INDEX TO FINANCIAL STATEMENTS See Annex A included as part of this prospectus supplement under the heading “Index to Financial Statements,”including the audited and unaudited financial statements contained therein, and the related note thereto, which areincorporated herein by reference. EXCHANGE OF EXCHANGECO SECURITIES