Location of fuel debris estimated from plan parameter trend

The heat source of RPV (fuel debris) was estimated based on the trend of the temperature around the RPV in the post-accident condition, water temperature of S/C, water injection amounts of feedwater (FDW) system and reactor core spray (CS) system. Figure 1 shows difference between flow paths of the FDW system and CS system. FDW system is a system that introduces the cooling water during the normal operation of BWR into the RPV. After flowing into the RPV, cooling water is accumulated in the space between reactor core shroud and the RPV (annulus), it will be flowing into the jet pump at the time when the water level reaches to the upper part of the jet pump mixer. If the integrity of the bottom of the RPV is maintained, the cooling water flowed in will accumulated inside the RPV and water level of the accumulated water will be raised. However, since increase in the water level was not observed, the bottom of the RPV has been damaged and the cooling water is estimated to be flown down from the damaged portion to the inside of the pedestal. That is, FDW system water injection cannot cools down the BWR reactor core portion but the bottom part of the RPV, even after the accident. While on the other hand, CS system is a core spray system during the coolant loss accident and is installed along the walls immediately above the core reactor core shroud. In the CS system water injection, the cooling water is flowing down the space from the reactor core to the bottom of the RPV and the scape can be cooled down. Based on the above, the fuel debris locations were estimated for each Unit.


Figure 1 Flow path of FDW and CS systems

Shown below is the fuel debris distribution for each unit estimated from the trend of the plant parameters.

a. Evaluation results for Unit 1

Figure 2 shows temperatures at several positions inside the PCV of Unit 1 along with changes in the quantity of injected water and measurement locations. In response to the changes in the quantity of injected water, the following characteristic changes in temperature were observed.

  1. (1) Compared with Units 2 and 3, the ambient temperature of the RPV decreased at a fast rate, which decreased to below 100°C five months after the accident.
  2. (2) The ambient temperature of the RPV did not rise at a rate that corresponds to the decrease in the amount of the injected water for the FDW system.
  3. (3) With increases in the quantity of injected water for the FDW system, the ambient temperature of the RPV dropped to below 50°C and the S/C water temperature rose.
  4. (4) With decreases in the amount of injected water for the FDW system, the ambient temperature of the RPV rose.

With the characteristics shown in (1), (2), and (3) above, it is estimated that the heat source is probably is small inside the RPV. From (3) and (4), it is assumed that a heat source may exist in the water injection channel for the FDW system and the heat removed in response to water injection has transferred to the S/C.


Figure 2 Changes in Plant Parameter and Measurement Locations for Unit 1

b. Evaluation results for Unit 2

Figure 3 shows the temperatures at several positions inside the PCV of Unit 2 along with changes in the amount of injected water and measurement locations. In response to the changes in the amount of injected water, the following characteristic temperature changes were observed.

  1. (1) Compared with Unit 1, the ambient temperature of the PRV is high, which was higher than 100°C even six months after the accident.
  2. (2) The temperature of the lower RPV head sensitively responded to the decreases in the amount of injected water for the FDW.
  3. (3) With the start of water injection for the CS system, the ambient temperature of the RPV decreased and the S/C water temperature rose.
  4. (4) With decreases in the amount of water injection for the CS system, the ambient temperature of the RPV rose, around the lower RPV head, in particular.
  5. (5) With increases in the quantity of injected water for the CS system, the ambient temperature of the RPV dropped.

With the characteristics shown in (1), (3), (4), and (5), it is estimated that a certain quantity of heat source may exist inside the RPV; from (2), it is assumed that the lower RPV head is closer to the heat source than the water supply nozzle (N4B). From (3), it is deemed that the removed heat had transferred to the S/C.


Figure 3 Changes in Plant Parameter and Measurement Locations for Unit 2

c. Evaluation results for Unit 3

Compared with Unit 1, the ambient temperature of the PRV is high, which was higher than 100°C even six months after the accident; as with Unit 2, it is assumed that a certain percentage of fuel debris exists in both of the RPV and PCV. The procedure for the above estimation is shown below:

  1. (1) Compared with Unit 1, the ambient temperature of the PRV is high, which stayed at a level higher than 100°C even six months after the accident.
  2. (2) Although the amount of injected water is highest for the FDW system, the ambient temperature of the RPV decreased at a low rate.
  3. (3) With the start of water injection to the CS system, the ambient temperature of the RPV sharply dropped.
  4. (4) With a decrease in the amount of injected water for the CS system, the temperatures of the water supply nozzle (N4B) and the lower RPV head rose.

With the characteristics shown in (1), (3), and (4) above, it is estimated that a heat source may exist inside the RPV.


Figure 4 Changes in Plant Parameter and Measurement Locations for Unit