Terrestrial Energy CEO converts 511,580 stock units

Terrestrial Energy Inc. CEO and director Simon Irish reported the exercise/conversion of 511,580 restricted stock units into 511,580 common shares on October 21, 2026.

Rhea-AI Filing Summary

Terrestrial Energy Inc. CEO and director Simon Irish reported the exercise/conversion of 511,580 restricted stock units into 511,580 common shares on October 21, 2026. The units represented the vested portion of his award.

Following the transaction, his reported direct common-stock holdings were 2,824,414 shares. The report also lists indirect holdings of 2,818,520 shares through an LLC and 52,255 shares through an IRA.

Insider Trade Summary

511,580 shares exercised/converted

Exercise 4 txns

Insider Irish Simon

Role Chief Executive Officer

Type	Security	Shares	Price	Value
Exercise	Restricted Stock Units F2, F3	511,580	$0.00	$0.00
Exercise	Common Stock F1	511,580	$0.00	$0.00
holding	Common Stock	--	--	--
holding	Common Stock	--	--	--

Holdings After Transaction: Restricted Stock Units — 511,580 contracts (Direct); Common Stock — 2,824,414 shares (Direct); Common Stock — 2,818,520 shares (Indirect, Through LLC); Common Stock — 52,255 shares (Indirect, Through IRA)

Footnotes (3)

1. F1. Represents shares of common stock subject to vested portion of restricted stock unit award.
2. F2. Each restricted stock unit represents a contingent right to receive one share of common stock.
3. F3. 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.

Key Figures

Restricted stock units exercised/converted: 511,580 units Common shares acquired: 511,580 shares Direct common shares after transaction: 2,824,414 shares +2 more

5 metrics

Restricted stock units exercised/converted 511,580 units October 21, 2026

Common shares acquired 511,580 shares October 21, 2026

Direct common shares after transaction 2,824,414 shares Reported following the October 21, 2026 transaction

Indirect common shares through LLC 2,818,520 shares Reported holding

Indirect common shares through IRA 52,255 shares Reported holding

Key Terms

Restricted Stock Units, contingent right, accelerated vesting

3 terms

Restricted Stock Units financial

"vested portion of restricted stock unit award"

Restricted stock units are a type of company reward where employees are promised shares of stock, but they only fully own these shares after meeting certain conditions, like staying with the company for a set time. They matter because they can become valuable assets and are often used to motivate employees to help the company succeed.

contingent right financial

"Each restricted stock unit represents a contingent right"

accelerated vesting financial

"accelerated vesting in certain events"

A contract feature that makes stock awards, options, or restricted shares become owned or exercisable earlier than the original schedule. It shortens or cancels the waiting period so recipients can sell, transfer, or exercise their equity sooner — think of a timed lock that is unlocked ahead of schedule. It matters to investors because it changes when shares enter the market, who controls them, and how much dilution or ownership concentration happens.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many shares did IMSR CEO Simon Irish acquire?

Simon Irish reported acquiring 511,580 common shares on October 21, 2026, through exercise/conversion of restricted stock units. The shares represented the vested portion of the award, and each restricted stock unit represents a contingent right to receive one common share.

What are the vesting terms for Simon Irish's IMSR restricted stock units?

The restricted stock units vest in one-half increments on each of the first and second anniversaries of the grant date, subject to Simon Irish's continued employment through each applicable vesting date, with accelerated vesting in certain events.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 10/26/2026 - 06:30 PM EDT Accepted by SEC EDGAR 10/26/2026 - 06:30 PM EDT Learn about SEC filing dates

SEC Form 4

FORM 4

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
or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL

OMB Number:			3235-0287
Estimated average burden
hours per response:			0.5

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

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

Irish Simon

(Last) (First) (Middle)

C/O TERRESTRIAL ENERGY INC.

2730 W. TYVOLA ROAD, SUITE 100

(Street)

CHARLOTTE NORTH CAROLINA 28217

(City) (State) (Zip)

UNITED STATES

(Country)

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
10/21/2026

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

4. If Amendment, Date of Original Filed (Month/Day/Year)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	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	10/21/2026		M		511,580(1)	A	$0	2,824,414	D
Common Stock								2,818,520	I	Through LLC
Common Stock								52,255	I	Through IRA

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 (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	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 (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (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: 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
Restricted Stock Units	(2)	10/21/2026		M			511,580	(3)	(3)	Common Stock	511,580	$0	511,580	D

Explanation of Responses:

1. Represents shares of common stock subject to vested portion of restricted stock unit award.

2. Each restricted stock unit represents a contingent right to receive one share of common stock.

3. 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.

	/s/ Brian Thrasher, as Attorney-in-Fact	10/26/2026
	** Signature of Reporting Person	Date
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.
* Form 4: SEC 1474 (03-26)