The safety of dumping radioactive waste and the concerns of the above mentioned nations intersects if the dumping is done in a region which will have a direct effect on exploitation of resources by the aforementioned nations. That's what their concern is essentially stemming from.
If dumped in a remote corner in the middle of nowhere, it will be safe enough to do so and the claims that it might affect the entire Pacific ocean seem far fetched and contrary to known data on the subject.
Also, do you disagree that to quote international laws and to speak to uphold them one must not selectively choose aspects of it as per convenience. it's either all of it or none of it.
I would also recommend reading up on water, radiation and how the two interact. It's always better to learn, make mistakes and then come to a conclusion rather than agree on a topic simply because of opinions of those who you consider superior say so.