No credible source claims that Sellafield released more radioactive material than Fukushima. Fukushima released 340,000 to 780,000 TBq. At it's peak, Sellafield was releasing 5,200 TBq in 1975 [1]. The Magnox reprocessing lasted from 1964 to 2022. Even if we assume this peak release was constant through the entirety of operation (it wasn't), that's still 1 to 0.5 times the release of Fukushima over Sellafield's entire 58 years of operation. This is nowhere near the 4x you claimed, and it's actually even lower since Sellafield was not releasing 5.2 PBq per year throughout its entire operation.

All the fluid flow models show that the radiation released rapidly disperses to negligible concentrations. But even the best fluid flow model isn't as good as real world experimentation. Fukushima gave us that real world experiment. At least as much radiation (probably many times more) as Sellafield released over it's 6 decades of Magnox reprocessing was released all at once when Fukushima melted down. Scientists and environmental organizations all over the world tested their waters for the levels of radioisotopes. And even just a few kilometers off the shore of Fukushima, the concentrations were less than one percent of safe levels. Maps like these [2] can be so misleading because they use a Log scale, and neglect to mention that the safe levels of Cs137 is 7,400 Bq per cubic meter. Most of that map is under 10 Bq per cubic meter.

This is the same deal with bioaccumulation. We test fish. We measure the concentration of toxins. Because unlike radioisotopes, there are toxins like mercury and algae byproducts that do pose real health risks. We have all the infrastructure in place to discover bioaccumulation of radiation. But no such increases are being observed. Because it isn't happening. Or are all the governments across the world colluding to keep fish contamination under wraps?

Again, I come away with the conclusion that you have fundamentally skewed perceptions about the dangers of radiation. We released 2,566,087 PBq of radiation as part of nuclear weapons tests. This is equivalent to 5,000 Sellafields releasing radiation at its peak rate of release for 98 years. And these nuclear tests weren't spread out over a century, they were done between 1945 and 1963. And guess what? It resulted in no ocean bioaccumulation, no contaminated oceans. And in reality Sellafield did not average 5 PBq per year throughout it's operation, and not all reprocessing plants are as badly run as Sellafield. Even if we take the most pessimistic scenario possible it'd still result in radiation releases less than released from nuclear testing every century. If this level of radioactive release impacted the global environment, we'd have had ample opportunity to detect it. So unless you're claiming that there's a global conspiracy to keep a lid on ocean contamination from nuclear weapons tests, there's no reason to think that fuel reprocessing will result in contamination even if every reprocessing facility is as bad as Sellafield.

Radiation is dangerous when it's concentrated. Parts of Nevada are still irradiated from nuclear tests because the radioactive material landed on the ground and then it stays there. Radioactive material released in the ocean dilutes to negligible levels. Radioactive material in the human body typically amounts to 8,000 Bq [3]. Yet you're freaking out over 5 Bq per cubic meter in the ocean. I'm sorry but all this conversation has done is conveyed just how pervasive misinformation about the risks of radiation is these days.

1. https://en.wikipedia.org/wiki/Sellafield#Radiological_releas...

2. https://earthsky.org/earth/tracking-fukushima-radiation-acro...

3. https://www.radioactivity.eu.com/site/pages/Activity_Doses.h...