It doesn’t need to be a liquid per say. Perhaps the reactor becomes ‘contaminated’ much like how a boiler gets calcified and requires a cleaning tool, that tool would then need to be probably treated, and the cleaning process would be cheaper than disposing of it and building a new one?
Or a container that fills via percentage over time, again cleaning the container designed to be cheaper than disposing of the container and building a new one?
If you can’t stop someone voiding the waste, you need to provide an incentive to discourage the behavior.
EDIT
Perhaps, and I’m just saying this without considering the sizable coding it might take, but you could work in a process similar to how swamp water takes over normal water. That the reactor needs to be connected to a river or ocean and would contaminate the river/ocean over time. I say river rather than a water source as you seem to have methods in place to check if it is a river/ocean or not.