In Unit 1, cesium particles of several tens to several hundreds of microns are scattered on the operation floor. Hydratable cesium compounds may have been penetrated the concrete inside from its surface in the form of cesium hydroxide. During the initial stage of core heating, insoluble cesium particles are formed by the solidification reaction between silicon oxide and cesium hydroxide in the gas phase. The amount of insoluble cesium particles produced is considered to be smaller than that of in Unit 2.

In Unit 2, hydrated cesium compounds may have been penetrated the concrete inside from its surface in the form of cesium hydroxide. Insoluble cesium particles of a few micrometers in diameter have been confirmed as originating in the environment of Unit 2. Insoluble cesium particles were formed by the solidification reaction between silicon oxide and cesium hydroxide in the gas phase during the early stage of core heating. The amount produced is limited and estimated to be in the order of 10 kg, and some of the insoluble cesium particles may be remaining inside the PCV and reactor building.

In Unit 3, hydrated cesium compounds may have penetrated inside from the concrete surface in the form of cesium hydroxide. Insoluble cesium particles are produced in the initial stage of core heating by the solidification reaction between silicon oxide and cesium hydroxide in the gas phase, but the amount produced is considered to be smaller than it in Unit 2.

In Unit 1, a hydrogen explosion occurred on the operation floor. It is presumed that FPs released during the accident mainly passed through the PCV, top head flange of the PCV, the reactor well, and the shield plug in this order, to reach the operation floor. It is therefore considered that FPs are unevenly distributed on this pathway. In addition, since the D/W pressure is considered to have been high due to the direct leakage from the RPV to the D/W right after the accident, it is assumed that a large amount of FP was deposited on the D/W as well. It is also estimated that the building wall, S/C wall, or water in the S/C is highly contaminated.

In Unit 2, it is estimated that gas leakage from the RPV to the D/W occurred during the progression of the accident, and that FPs were easily transferred directly to the D/W side. At the operation floor of Unit 2, high dose was observed at the shield plug position. In addition, photographs taken during the accident show that a large amount of steam was released from the blowout panel. Therefore, the FPs are presumed to have passed through the RPV, the PCV, top head flange of the PCV, the reactor well, the shield plug in this order, to reach the operation floor. It is therefore considered that FPs are unevenly distributed on this pathway.

In Unit 3, it is presumed that FPs released during the accident passed through the RPV, the PCV, the top head flange of the PCV, the reactor well, and the shield plug in this order, to reach the operation floor. It is therefore considered that FPs are unevenly distributed on this pathway.

For example, in Unit 2, localized high doses were observed near the CRD rail, in the PCV internal investigation.

In the process of decommissioning (operation floor decontamination) and an accident investigation of Unit 1 to Unit 3, it was confirmed that a large amount of cesium 137 existed in the lower part of the shield plug of each unit (Unit 2 and Unit 3). This amount is comparable to the amount of radioactive material presumed to remain in the PCV. Attention should be paid to this point in the future decommissioning work and accident analysis (mainly on the FP release pathway).