We do it all the time. It is called hydrolasing. (a reference to the water jet being focused and intense like a laser beam). Mostly we use it inside refuel cavities, which you don't have. I did use one once at Pickering in the irradiated fuel bay, but it was all underwater.
Some things to consider:
The water spray will create a mist which can often carry contamination aloft and deposit it outside the area.
The jet can be dangerously powerful. It can cause injury if not used properly.
It can take off more material than you might have intended. If the surface is painted or coated, a lot of that will be removed if you aren't careful. Some of it will be removed even if you are careful.
If you get a little Karcher at Canadian Tire, it may not have enough power to do the job on a large area, but it could do quite well for smaller things like tools and parts.
If your machine has a steam option, do not use it unless you need to remove oily or greasy contamination. Otherwise, it won't help you clean and it will increase the airborne contamination a lot.
If you are using it to clean parts or tools inside a glove box, it is a really effective method for getting off loose contamination in tight areas, but CO2 pellets or sandblasting are better for removing fixed.
They use incredibly high volumes of water, thereby increasing your liquid rad waste volume and eating up your pure water supply.
It would be helpful to know what you intend to use it for.
I hope this helps.