He and a fireman , both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants , approached the reactor building stairs .
At 9 : 17 p.m. , a health physicist arrived .
health physicist and a fireman , both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants , approached the reactor building stairs .
At 9 : 17 p.m. , a health physicist arrived .