Much of the the already radiation-hardened electronic components of the Juno Spacecraft needed to be enclosed within a thick and heavy (~200 kg) titanium vault (see also Juno Radiation Vault) in order to protect them from one or two dozen passes through the radiation from particles trapped in Jupiter's magnetic field, lasting (roughly) a few days each, and radiation damage of some components less effectively shielded is still what is likely to degrade and perhaps destroy several of Juno's instruments by the end of the mission.
This total dose is nothing compared to the radiation effects being a few meters away from a nuclear fission reactor for months or years!
To address @Hobbes's comment with an envelope-back-class answer, according to Wikipedia 235U fission releases about 200 MeV per fission event. A one-kilowatt source would then have something like 3E+13 decays per second.
The article says that for each event, about 7 MeV of that is released as gamma rays. Assuming 0.5 MeV per photon, that works out to 33,000,000,000 gamma rays per second per square centimeter at 1 meter, or almost 3 Watts of gamma rays per square meter, 0.2% as bright as the Sun's 1360 W/m^2 of radiation, but 100% ionizing!
That's also about 2,000,000,000 neutrons per square centimeter per second, which are can also be damaging and problematic.
I'll update this and compare with proximity to Jupiter as well as deep space background if I can find/locate all of the sources and conversion factors. While this answer makes it look easy by quoting a post-nuclear survivalist's blog about bomb shelters, this it's actually not a simple calculation to do correctly unless you do it for a living.
However, I can estimate that a person standing 1 meter from this reactor absorbing only 10% of the 3 Watts/cm^2 of gamma rays would receive about 500 Sv per day, or about 100x higher rate than the rate for "colonists" on Jupiter's moon Europa.
For more about Juno's orbit through Jupiter's radiation field, see for example:
below: Image from Installing Juno's Radiation Vault Cropped and complete (click for full size view). Credit: NASA