The Fukushima Daiichi nuclear disaster released significant amounts of radioactive materials, including Cesium-137, into the environment. Cesium-137 is a long-lived radioactive isotope with a half-life of approximately 30 years, making it a major concern for long-term contamination.
There are two likely sources of Cs 137 in these shrimp the most lively is they where feed fish meal pellets made with bottom trawler sourced fish from off the Japanese coast. Cs137 bioaccumulates up the food web taking predator fish and grinding them up to fish meal and feeding that to farmed shrimp absolutely would put this element in them.
The other source is Cs 137 is also sometimes used as a gamma emitter for irradiation of foods.it’s also used in the medical field for nuclear medicine particularly cervical cancers. Its also used in industrial settings for flow meters that cannot contact the fluids flowing, it’s also used in the oil industry as a gamma source for wireline logging using active gamma emissions. It is a fairly common use nuclear isotope.
That said having some of it unconstrained and in contact with water would rapidly contaminate that water stream and anything down stream of it. The water source for these shrimp could have just as easily had a dumped cesium gamma source somewhere in the watershed leaking it into everything and everyone down stream.