TEPCO says the worker's heart stopped, but they do know it's not because of radiation exposure he got by working at Fukushima I Nuclear Power Plant for a year, at 25 millisieverts.
Remember, unless it's an acute radiation sickness, it's not because of radiation.
Also remember how the workers (both of TEPCO and of affiliate companies) massage the radiation exposure levels by various methods, whether they do it voluntarily or are told to do so, while TEPCO on a corporate level maintains plausible deniability by pretending not to notice.
From articles in Mainichi Shinbun (here, here, and here):
The man started working with 5 others to install storage tanks for contaminated water from 9AM on August 22, in full face mask and protective clothes. At about 9:50AM he was resting in the rest area as a precaution against heatstroke, but he complained of feeling ill. At about 10:35AM, another worker found him lying on the floor unconscious.
He started working at Fukushima I Nuclear Power Plant in August last year. August 22 was his first day back from the 1-week vacation. His radiation exposure on August 22 was 0.03 millisievert, and cumulative radiation exposure was 25.24 millisieverts (provisional).
TEPCO announced on August 22 that a man working at Fukushima I Nuclear Power Plant suffered a cardiopulmonary arrest and was taken to a hospital in Iwaki City by ambulance. According to the Fukushima prefectural police, the 57-year-old man died in the afternoon on August 22. TEPCO says, "We do not know how [or why] he collapsed, but it is not from radiation, judging from his radiation exposure level."
TEPCO announced on August 23 that the male worker (age 57) who died after the work at Fukushima I Nuclear Power Plant had an acute myocardial infarction.
(And no one dies inside the plant compound...)