Abstract: At 6:45 a.m. on February 18, 2023, an employee was removing trays from a hydrogen reactor. The employee was on supplied breathing air. The employee was attempting to exit the confined space at the end of the shift, when the employee reported to the hole watch that he could not breath. The hole watch told the employee to turn on the emergency bottle. The employee then reported that he was on the bottle. The hole watch told the employee to connect to the emergency escape line. The employee refused and attempted to climb out of the confined space. The employee climbed approximately 30 feet up a fixed vertical ladder. The employee stopped responding to the attendant and other exterior employees. A man down alarm was called, and emergency rescue operations were attempted. The employee was extracted from the confined space. CPR was performed. An AED was applied, and the AED advised not to administer a shock. The employee was transported by ambulance to a nearby hospital emergency department, where he was pronounced dead. The employee was killed by probable asphyxia due to nitrogen gas exposure and/or oxygen deprivation.