Following up to this article, from the further investigation,there is a possibility that europium-152 is under the ground.
Europium-152 is a material of control rod in a reactor. If that is true, a piece of the control rod flew from reactor 3 and got into sewage pipe in Setagaya.
After they measured 110 micro Sv/h, they measured higher radiation level of 170 micro Sv/h at 60cm away from the boarder to the next area.
They also suggested the possibility that highly radioactive sewage water is stocked under the supermarket.
If it it europium-152, it proves the reactor 3 had a nuclear explosion as it has been suggested.
(Source)