The following is speculation. I have a low level of confidence that if what follows is indeed the case, those responsible for our security would fully disclose the potential risks in order to prepare for worst case mitigation.
The satellite causing all the excitement I believe to be a "Radar Imaging Satellite." This means that it has the capability to use high resolution radar to image instead of using CCDs or powerful cameras.
In the past, some "Radar Imaging Satellites" have used RTGs and Reactors to power the radars because of the size and associated problems related to solar arrays for high power applications.
A RTG is a thermocouple device that uses non-fissile decay of a radioactive nuclide to generate heat that is then converted through the use of a thermocouple to generate electrical power. These devices can be fueled by various types of radio-nuclides to include, but are not limited to, isotopes of 235, 238, 239, or 210.
I am not familiar with the reactor designs used in space applications but conjecture leads me to believe that instead of non-fissile decay, a moderated fissile material is used to generate heat. The significantly higher operating temperature could then be used with thermocouples, or closed loop working fluid generator systems, to produce electrical power. In the case of both the thermocouple and "heat engine" the higher the operating temperature (actually the thermal gradient through which the process operates) defines the base efficiency. In the case of the heat engine this is known as the Carnot Cycle Efficiency, but in both cases encourages the highest possible operating temperatures.
I believe these types of power generation systems could contain anywhere from a few pounds of radio nuclides up to 10s or even hundreds of pounds of material depending on scale.
I consider both RTG and Space Reactor technologies as "mature."
This particular satellite never established initial on orbit radio communication. What this means is, in the case of an accident or normal decay re-entry, the commands to separate the power pack, if it has one on board (which I have no evidence other than the obvious), could not be made to separate the power pack from the satellite as is normally done with RTGs at the life cycle end. The RTG or other type of power pack is then allowed to burn up or is boosted to a burial orbit. This is my understanding (or misunderstanding) of how things were to have supposed to work in the past.
Radio Nuclides may also be used as heaters to maintain constant or minimum temperatures in some applications and one satellite may have anywhere from a couple to a couple dozen small heating devices on board depending on sub-systems needs. These types of devices would contain from less than a gram to few grams of material each. Expected materials used would include isotopes of plutonium or other suitable radio nuclide materials.
The reasons I suspect that this satellite may have nuclear materials on board for use in power production or sub-system heaters are:
1. Past precedence.
2. The power required for "Radar Imaging."
3. The fact that these obvious questions and concerns have not been raised.
Regarding the stated concern for the "Hydrazine"; it is a "propellant" used for attitude control and orbital maintenance and maneuvering. It is not nice stuff but I don't think a half a ton of hydrazine would cause this much excitement.