the poor irradiated worker...
...let's analyze his case a bit more. :)
i'll drop some hints and facts:
the sievert is the energy dose (gray, joule/kg == energy/absorbing mass) multiplied with the radiation weighting factor. the radiation weighting factor (which determines the "effectiveness" for damaging human tissue) for beta and gamma radiation is 1, so sievert == gray.
it's different for alpha or neutron radiation, but as alpha radiation is shielded by even a piece of paper (or skin) and neutron radiation is not as much of a concern, let's ignore that.
so, let's have a look at all the isotopes in nuclear reactors. I-131, Cs-137, and the like. they emit gamma radiation. yes. but only because the daughter nuclide is in an excited state after radioactive decay. the above famous "nuclear reactor products" both undergo BETA decay, while the I-131 betas have a mean range of <50cm in air, and the Cs-137 range ~150cm in air. in AIR, mind you, not in water. Cs-137's average betas can only travel 20cm in water... and there are isotopes like Sr-90 in NPPs, too, which are (almost) pure beta emitters.
...so, the big difference is the RANGE in a medium. gamma radiation can travel very far, while particle radiation - such as beta radiation - has a rather limited range.
okay, next fact: tactical dosimeters are to be worn in a "representative" spot for the body. and that is NOT the leg. within my country, the law says civil defense crews / firefighters HAVE to wear their personal dosimeters on the UPPER TORSO POCKET of their jacket / protective gear. that's quite far away from the leg.
so, let's remind ourselves... beta particles... low range in air, and even more so in water... and a distance of approx. 90cm between the lower leg and the upper chest level, where the dosimeter must be located.
also, let's see... the bad protective gear was to be blamed - the water got into the shoes of the workers and contaminated their legs. only because of that, they received these radiation burns. they should've been wearing rubber boots, like some of the other workers.
...hey, wait. if RUBBER BOOTS made the difference between one worker being fine and the other one having a radiation burn on their leg, guess who's to blame - yes, the beta particle! a gamma ray would not care about boots, it just moves right through that rubber. to shield HALF of Cs-137's gamma radiation, you'd need to make your boots from 3mm+ lead, not from rubber!
...one extreme example is Litvinenko. he "ingested" Polonium-210, a PURE alpha emitter. had he been wearing a dosimeter such as the workers in fukushima have, it would've read ZERO mSv as he died. now, do you think he died of the bad british food...? :-)
by the way, beta emitters are widely used in radiation therapy, too. we make people ingest I-131 to treat e.g. thyroid cancer - the thyroid will receive such a high dose, it can - in the best case / optimal and homogenous uptake - DESTROY the organ (and tumor) completely due to the high dose of radiation - however, e.g. the BRAIN (which is not all that far from your neck) stays safe. you'll just have a bit of a throat inflammation, but that's all it can get at.
also, if somebody ever looked into how teletherapy (radiation therapy from a distance - Cs-137 can and was used for that, too, to keep in line with mentioning our "fukushima fallout isotopes"), you'll know a normal (!) daily (!) dose (LOCAL dose, of course) of 2000-3000 mSv (!) will not do anything at all at first. it will take a week or two until even the slightest "sunburn" (radiation dermatitis) occurs. thus, to cause immediate (!!) redness and even blistering of the skin (like a severe sunburn), you'd have to expose your skin to much, MUCH more than that. so yeah, you'll work out of radiation therapy, with the tumor on your leg having received its target dose of 80,000 mSv (80 Gy; common dose over the course of a few weeks of therapy), your leg will likely be suffering from radiation dermatitis, but the dosimeter on your chest will still just show... 170 mSv. :-P
well, okay, that's enough with the hints - go figure. :-)