Terrestrial Energy Inc. /DE/

Rule 10b5-1 Trading Plans

Except as stated below, none of the Company’s directors or executive officers adopted, modified or terminated any “Rule 10b5-1 trading arrangement” or any “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408 of Regulation S-K, during thethree months ended September 30, 2026:

On September 10, 2026, Simon Irish, the Company’s Chief Executive Officer, adopted a new Rule 10b5-1 trading arrangement (the “New Trading Arrangement”) intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act that replaces the prior 10b5-1 Trading Arrangement entered into on June 10, 2026. The New Trading Arrangement is intended to provide for “eligible sell-to-cover transactions” (as described in Rule10b5-1(c)(1)(ii)(D)(3) under the Exchange Act) to satisfy tax withholding obligations arising exclusively from the vesting of equity awards and related issuance of shares of the Company’s common stock. The number of shares that will be sold to satisfy applicable tax withholding obligations upon vesting is not currently determinable as the number will vary based on the market price of the Company’s common stock and the extent to which vesting conditions are satisfied. The New Trading Arrangement also contemplates the sale of up to 56,800 shares of the Company’s common stock. The expiration of the New Trading Arrangement is March 23, 2027, subject to early termination for certain specified events set forth in the arrangement.

On September 10, 2026, Brian Thrasher, the Company’s Chief Financial Officer, adopted a Rule 10b5-1 trading arrangement (the “Thrasher Trading Arrangement”) intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act. The Thrasher Trading Arrangement is intended to provide for “eligible sell-to-cover transactions” (as described in Rule10b5-1(c)(1)(ii)(D)(3) under the Exchange Act) to satisfy tax withholding obligations arising exclusively from the vesting of equity awards and related issuance of shares of the Company’s common stock. The

number of shares that will be sold to satisfy applicable tax withholding obligations upon vesting is not currently determinable as the number will vary based on the market price of the Company’s common stock and the extent to which vesting conditions are satisfied.