Since I've been paged.
I'm a current senior reactor operator at a US boiling water reactor.
I'm going to be talking from a boiling water reactor perspective, but most of this is applicable to any primary coolant system. Also, heavy water reactors will have some different answers as they have a lot of tritium content.
Your water itself does get some increased levels of tritium due to neutron capture, however for the most part, you are primarily concerned with things that are soluble in the water. Stuff like fission products which diffuse out of the fuel cladding or leak from a failed fuel element, or corrosion products from piping which pass through the core and are irradiated (cobalt 60 is a big one). You also build up a large Nitrogen-16 inventory, which as a very short half life (completely gone in a couple minutes), but produces a very large amount of dose.
Primary coolant water needs to be extremely pure to prevent accelerated corrosion of the reactor internals or fuel cladding. So you have a continuous cleanup system running. For a BWR plant, the reactor water cleanup system is designed to clean up about 1% of the reactor's inventory per hour. The cleanup system utilizes filter/demineralizers which are resin based. The resin is designed to accept various electropositive or negative ions and deionize the water, pulling radioactive ions with it.
The water we boil to steam in our reactor goes back to the condenser, then gets pumped through condensate filters/polishers and sent back to the reactor. So it's really not "fresh" water, it's the same water getting reused over and over again.
Your resin based demineralizers will absorb radioisotopes and other impurities until they begin to get expended. You know they are going to expend because they tend to leach out silica or other compounds first, as those are weakly bound to the resin, and the more potent ions will kick them out. So when you see those levels come up, you take the demineralizer off service and backwash the resin off of it. You then put a new resin bed on the filter.
The spent resin then gets sent for radioactive waste processing. It's in a water/resin slurry at this point. You send to your radwaste system to some storage tanks. Let it settle, decant clean water off the top and send it back to the condensate storage tank for later use, then transfer the settled sludge off the bottom to your waste-sludge system. Next you recirculate the tank and use a dewatering system to separate the solid resin beads/powder from the liquid, getting clean/pure water back, and leaving a nearly solid compact waste product that you ship to a storage facility.
That's all for the reactor though. The spent fuel pools are similar, you have a continuous cleanup and filtering system that operates using resin as well. You get contamination in the spent fuel pool for a few reasons: Fuel leaks/leeching, some activated "tramp" uranium which was on the outside of the fuel rod that got activated, corrosion products from the reactor core that ended up coming out with the fuel rods, "CRUD" which stuck to the fuel rods during operation breaking loose in the spent fuel pool. It's definitely not pure/clean water, but it's pretty close to it. A healthy plant with few or no fuel leakers and continuous filtering is likely to have water that's low enough in activity that you would be able to wash it off pretty easily if you did fall in, without much absorbed dose. A plant like LaSalle which has had one of their two units with a leaker the majority of the last 15 years, their pools are much less desirable to fall into.
During refueling activities the spent fuel pools can be very ugly in terms of dose. There's potential for a CRUD burst while moving fuel, especially if you scrammed going into the outage. Your reactor water cleanup system may be out of service for maintenance during the outage too, heavily reducing the cleanup capability of the combined reactor/cavity/spent fuel pool.
As for fresh water, typically you just add condensate storage tank water to the pool to make up for evaporative losses. Condensate water is contaminated water which is allowed to interact with the reactor, primary coolant system, spent fuel pool, high pressure coolant injection/core spray/isolation cooling systems. If you did put fresh water in, you would immediately consider it contaminated based on the fact that there is a known inventory of radioisotopes in the pool to begin with.
Hope this helps
I believe the cooling liquid is recycled and maintained inside the facility. This liquid is most likely passed through a heat exchanger where its heat is passed to an outside source of cool water (why nuclear reactors are usually placed near a large body of water) that remains uncontaminated - like a car radiator transfers the heat of the engine coolant to the air passing over it. Of course, in this case it's liquid to liquid.
Most if not all of the primary coolant is in a closed loop, that loop has a stage where the contaminated water exchanges heat with the uncontaminated water. If I had to guess i would say modern reactors contain more than 99% of their radiation. Unless something catastrophic happens chances of any sort of contamination are slim.
Edit: It seems I've misread the question. Spent fuel used by the US Gov. Is sent to a staging area in a mountain to be burried. Spent fuel used by private industry is to be sealed up and kept on sight, since the fuel lasts so long the private reactors are only just starting to run out of space, the department of energy is figuring out how to dispose of the waste. As for the water... idk. :(
Filtering to remove contaminants. That's most of your work, physical and chemical filtering. And honestly, if fuel elements are in good shape, the sfp water is pretty safe to drink or bathe in. So long as you don't get close to the elements on the wall or floor. A damaged element would probably be isolated. Paging u/hiddencamper for backup.
To remove heavy water? That's a different process since the water is chemically identical to regular water.
1. Dilute with truly clean water until below minimum detectable. Now you can release it in accordance with local regulations. (Usually never)
2. Reprocessing. Which I believe requires a centrifuge. Expensive, slow, and difficult since you have to transfer large amounts of rad waste to the processing facility. This may include going to new countries.
Japan is trying to do #1 with Fukushima since there's so much that #2 is impossible to keep up.
Primary coolant water is purified regularly and radioactive elements are removed during this process. At least it is in Navy nuclear power plants and I'm sure it's the same for commercial plants. Admiral Rickover, the spearhead of the navy nuclear program, once drank a glass of primary coolant discharge to prove it's safe. Like most are saying, the primary system and steam producing system are completely isolated. Steam system or secondary system water is completely free of radioactivity.
Former nuclear reactor operator of the US Navy.
u/Hiddencamper replied in depth to somewhat similar topic [What if I took a swim in a typical spent nuclear fuel pool](https://what-if.xkcd.com/29/
) in this reddit [discussion about it](https://np.reddit.com/r/askscience/comments/175j8j/this_weeks_xkcds_what_if_claims_that_it_is_safe/
>All reactor grade water in nuclear power plants contains some levels of radioactive material (aka contamination). The spent fuel pool is no exception. Radioactive material in the spent fuel pool can come from a variety of places: [...]
>The spent fuel pool has a cleanup system which consists of resin bed demineralizers and filters. The water from the spent fuel pool can also be cleaned (depending on specific plant design), by the plant's radioactive waste system, the reactor water cleanup system, or the condensate filter demineralizer system. [...]
And in the reply another link was provided to similar discussion: [Is water used in the decontamination of nuclear fuel, or even just used in the general nuclear energy process, more radioactive than standard water?](https://np.reddit.com/r/nuclear/comments/174mio/is_water_used_in_the_decontamination_of_nuclear/
>The water itself, no. However, water absorbs things which are soluable. Many radioisotopes are soluable, and as such the water will contain radioactive material.
>So if I use water to spray down something which was contaminated, for all intents and purposes, the water is now contaminated. The water can be filtered and purified to remove radioactive material from though (we do this in nuclear plants to reuse water).
>The water itself, no, the stuff mixed in the water, yes.
Plenty of good information.
I learnt about this a little in school. There are different types of nuclear storage facilities, they different have levels of contamination and different ways of dealing with the different levels of radioactivity.
They recycle the water that is contaminated, but if necessary store it in a tank that isn't connected to a water supply until it reaches safe levels. They have leaked in the past and safe levels are disputed, but the water isn't that dangerous.