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Home arrow Environment arrow Reflections on the Fukushima Daiichi Nuclear Accident

Present Situation of Cores and PCVs of Units 1–3

Unit 1

Water is being injected to Unit 1 from the Core Spray (CS) and feedwater system, as shown in Fig. 2.10. Water from the CS system is directly sent to the core and water from the feedwater system is sent to the lower plenum via the outer side of the core shroud. The reactor level is confirmed to be below TAF-5 m, based on the calibrated results of the water level indicators, that is, no sufficient water exists in the core region.

The status of Unit 1's core was estimated based on the above facts and aforementioned examination results, and is illustrated in Fig. 2.10. As can be seen in the figure, most of the molten fuel generated by the accident fell down to the lower plenum below the reactor pressure vessel and only a little fuel remains in the original core location. Most debris, which had fallen to the lower plenum, is believed to have reached the PCV pedestal. It is estimated that, after causing core-concrete interactions, the debris was cooled by injected water, decrease of its decay heat terminated the core-concrete interactions, and it now remains in the PCV.

At the in-containment investigation in October 2012, the level of residual water in the D/W was checked by cameras. It was about 2.8 m above the D/W floor (as of October 10, 2012).

Fig. 2.10 Estimated conditions of the core and PCV of Unit 1

Concerning the status in the S/C, the nitrogen gas injection experiment in September 2012 demonstrated a mechanism that Kr-85 and hydrogen generated at an early stage of the accident had remained in the upper space of the S/C and they were discharged to the D/W via vacuum breakers when the S/C water level was pushed down. This means that the S/C is currently filled with water.

The location of liquid phase leakage was confirmed at the D/W bottom and vacuum breaker valve line due to the following evidence:

• Water flow from suction drainpipe which exhausted accumulated water to outside the D/W in November 2013.

• Water flow from vacuum breaker valve line connected for reducing the pressure

difference between S/C and D/W in May 2014.

Unit 2

Water is being injected to Unit 2 from the CS and feedwater system, as shown in Fig. 2.11. Water from the CS system is directly sent to the core and water from the feedwater system is sent to the lower plenum via the outer side of the core shroud. Based on water filling to the condensing chamber on reference water level side piping shown by the water level indicators, the reactor water level is estimated to be below TAF-5 m, meaning there is not sufficient water for covering the core.

The estimated situation of the Unit 2 core, based on the above facts and aforementioned examination results, is illustrated in Fig. 2.11. As can been seen in the figure, part of the melted fuel generated in the accident fell down to the lower plenum below the reactor pressure vessel or to the PCV pedestal. Some of the fuel may remain in the original core location.

Fig. 2.11 Estimated conditions of the core and PCV of Unit 2

At the in-containment investigation in March 2013, the level of residual water in the D/W was checked by cameras. It was about 60 cm above the D/W floor (as of March 26, 2013).

The nitrogen gas injection experiment to the S/C conducted in May 2013 showed the S/C pressure of 3 kPag (as of May 14, 2013). This meant the S/C water level was at around the nitrogen gas injection inlet (O.P. 3,780 mm), because a certain water head should appear if the S/C was close to being full. When considered together with the low water level in the D/W, the water injected to the reactor is estimated to have flowed into the S/C via the vent lines from the D/W and leaked out to the reactor building from the bottom of the S/C, i.e., the current S/C water level can be estimated to be about the same level as the residual water level in the torus room.

The water leak paths from the S/C have not been located yet. But at least no leakage was confirmed at the S/C manholes, etc., when, for the internal investigation in the torus room in April 2012, robots accessed the corridor for visual checks; or at the lower ends of the vent tube, when they were checked at the internal investigation of the torus room in December 2012 and March 2013. Due to no damage at S/C top and low water level of D/W, leakage location of PCV is assumed to be at the S/C bottom.

Unit 3

Water is being injected to Unit 3 from the CS and feedwater system, as shown in Fig. 2.12. Water from the CS system is directly sent to the core and water from the feedwater system is sent to the lower plenum via the outer side of the core shroud. The reactor temperature was lowered to 70 °C as of November 11, 2011, which had been achieved by water injection from the CS system conducted from September 1, 2011 and the fuel debris in the CS water injection path, i.e., in the core, could be cooled.

The estimated situation of the Unit 3 core based on the above facts and aforementioned examination results is illustrated in Fig. 2.12. As can been seen in the figure, part of the melted fuel generated in the accident fell down to the lower plenum below the reactor pressure vessel or to the PCV pedestal. Some of the fuel may remain in the original core location.

No measured values are available so far concerning the D/W water level. But it could be estimated to be about 5.5–7.5 m above the fl by converting the S/C pressure to water head. The S/C pressure was obtained from its existing pressure indicators, not calibrated since the accident, so they are not highly accurate but they could be reliable as a trend to a certain extent because they have followed the pressure changes according to the water injection. In addition, leakage from around the expansion joint of PCV penetration of the main steam line D was confi The elevation of this leakage is the same as the presumed water level inside the PCV, so most of the leakage from the PCV is assumed to be from this location.

Fig. 2.12 Estimated conditions of the core and PCV for Unit 3

 
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