I have just discovered that the Chernobyl related controls which were imposed on sheep farms in England and Wales have just been lifted. The radioactive cesium from Chernobyl will have become weaker because of the radioactive (physical) decay, while the cesium-137 will only have decayed slightly the cesium-134 will have decayed a lot by now.
I imagine that by now that the cesium will have migrated deeper into the soil, this makes the cesium less able to enter the grass. Also I am hoping that the cesium will be more tightly bonded now to the soil minerals, one of the problems with the hill sheep farms is that the soil is poor in potassium and the minerals which will bind the cesium are not present in large amounts. This will lower the Kd value for cesium in the soil as a result the plants will be more able to absorb the cesium via their roots.
I saw with interest that Naofumi Kozai, Toshihiko Ohnuki, Makoto Arisaka, Masayuki Watanabe, Fuminori Sakamoto, Shinya Yamasaki & Mingyu Jiang published an article (Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi Nuclear Power Plant accident) in the Journal of Nuclear Science and Technology, 2012, volume 49, issue 5, pages 473 to 478.
In this paper the authors report some leaching experiments which have been done on cesium contaminated soil from Japan, they found that the majority of the cesium stayed in the soil even when the soil was treated with ammonium chloride and acetic acid solution. This suggests to me that much of the cesium is locked up inside the soil which will prevent both the easy washing of the cesium out of the soil and also prevent the plants absorbing the cesium with ease.