Terrestrial Energy Inc美股招股说明书(2026-05-14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 This Supplement No. 3 (this “Supplement No. 3”) updates, amends and supplements the definitive proxystatement/prospectus dated September 26, 2025 (as supplemented or amended from time to time, the“Prospectus”), which forms a part the Registration Statement on Form S-4 (Registration No. 333-288735,the “Form S-4”) filed by Terrestrial Energy Inc. (formerly known as HCM II Acquisition Corp., or “HCMII”). Capitalized terms used in this prospectus supplement and not otherwise defined herein have themeanings specified in the Prospectus. This Supplement No. 3 is being filed to update, amend and supplement the information included in theProspectus with information contained in our Quarterly Report on Form10-Qfiled with the SEC on May14, 2026, which is set forth below. To the extent information in this Supplement No. 3 differs from,updates or conflicts with information contained in the Prospectus, the information in this Supplement No.3 is the more current information. This Supplement No. 3 is not complete without the Prospectus. ThisSupplement No. 3 should be read in conjunction with the Prospectus, which is to be delivered with thisSupplement No. 3, and is qualified by reference thereto, except to the extent that the information in thisSupplement No. 3 updates or supersedes the information contained in the Prospectus. Please keep thisprospectus supplement with the Prospectus for future reference. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page12ofthe Prospectus. NEITHERTHE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATESECURITIESREGULATORY AGENCY HAS APPROVED OR DISAPPROVED OF THESECURITIES DESCRIBED IN THE PROSPECTUS OR THIS PROSPECTUS SUPPLEMENT ORPASSEDUPON THE ADEQUACY OR ACCURACY OF THE DISCLOSURE IN THEPROSPECTUS OR THIS PROSPECTUS SUPPLEMENT. ANY REPRESENTATION TO THECONTRARY CONSTITUTES A CRIMINAL OFFENSE. The date of this prospectus supplement is May 14, 2026. This Supplement No. 3 should be read in conjunction with the Prospectus, which should be readcarefully and in its entirety. (MarkOne)☒QUARTERLYREPORTPURSUANTTOSECTION13OR15(d)OFTHESECURITIESEXCHANGEACT OF 1934 For the quarterly period ended March 31, 2026 or☐TRANSITION REPORT PURSUANT TO SECTION13 OR 15(d)OF THE SECURITIESEXCHANGEACT OF 1934For the transition period fromtoCommission File Number: 001-42252 TERRESTRIAL ENERGYINC. (Exact name of registrant as specified in its charter) 98-1785406(I.R.S. EmployerIdentification Number) 2730 W. Tyvola RoadSuite100Charlotte, NC 28217(Address of Principal Executive Offices)(646) 687-8212(Registrant’s telephone number) Securities registered pursuant to Section12(b)of the Act: Indicate by check mark whether the registrant (1)has filed all reports required to be filed by Section13 or 15(d)of the SecuritiesExchange Act of 1934 during the preceding 12months (or for such shorter period that the registrant was required to file such reports),and (2)has been subject to such filing requirements for the past 90days.Yes☒No☐ Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submittedpursuant to Rule405 of RegulationS-T (§232.405 of this chapter) during the preceding 12months (or for such shorter period that theregistrant was required to submit such files).Yes☒No☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smallerreporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smallerreporting company,” and “emerging growth company” in Rule12b-2 of the Exchange Act. Emerging growth company☒ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period forcomplying with any new or revised financial accounting standards provided pursuant to Section13(a)of the Exchange Act.☐ Indicate by check mark whether the registrant is a shell company (as defined in Rule12b-2 of the Exchange Act).Yes☐No☒As of May 6, 2026, there were 105,935,254 of the registrant’s ordinary shares outstanding. TABLE OF CONTENTS Cautionary Note Regarding Forward-Looking StatementsPart IFinancial Information4Item 1.Financial Statements (Unaudited)4Condensed Consolidated Balance Sheets as of March 31, 2026 and December 31, 2025(Unaudited)4Condensed Consolidated Statements of Operations and Comprehensive Loss for thethree months ended March 31, 2026 and 2025 (Unaudited)5Condensed Consolidated Statements of Stockholders’ Equity (Deficit) for the threemonths ended March 31, 2026 and 2025 (Unaudited)6Condensed Consolidated Statements of Cash Flows for the three months ended March31, 2026 and 2025 (Unaudited)7Notes to Condensed Consolidated Financial Statements (Unaudited)8It