Accident Sequence of Unit 2
After the SBO, RCIC of Unit 2 was functioning. However, until about 10:00 on March 14, more than 3 h earlier than the RCIC stop time, the water level of the RPV was decreasing and the pressure of RPV was increasing. It is believed that this trend shows that RCIC was losing its core-cooling capability. At the central control room, operators tried to rapidly depressurize RPV by opening the safety relief valves, and it was only possible after 5 to 6 h because of the delay for preparing batteries. The reason of this delay was the immediate chaos caused by the hydrogen explosion at Unit 3, resulting in much time needed to prepare the large amount of batteries. When the depressurization was initiated by opening the safety valves, the RPV water level was almost half of the core. The water level then reached to the bottom of the core, resulting in the loss of water in the core region.
As shown in Fig. 3.5, the increase of D/W pressure initiated after about 19:00 on March 14, and it showed almost the same value as the RPV pressure, which indicates the occurrence of the RPV failure. After the RPV pressure and D/W pressure increased with values similar to each other, they remained at the high values of 0.6 to 0.7 MPa for more than 7 h, which is much higher than the design pressure (0.427 MPa). Around this time, FPs in the RPV were released into D/W, which is indicated by the rapidly increased dose rate in D/W (Fig. 3.5). Through the flanges, hatches, airlocks, and penetrations with gaskets degraded by the high temperature and high pressure of the containment, hydrogen and volatile fission products, such as iodine, and cesium, were released to the reactor building, a similar phenomenon as the hydrogen explosion process in Units 1 and 3. It is believed that the opening of the blowout panel on the roof of the reactor building caused by the impact of the hydrogen explosion of Unit 3 prevented the occurrence of the hydrogen explosion of Unit 2.
The pressures of D/W and RPV both rapidly decreased from 0.65 MPa from about 07:00 to 11:00 on March 15. Especially, the pressure of D/W decreased to atmospheric pressure. This rapid decrease indicates that a relatively large failure occurred in D/W. It is therefore shows that a large amount of gases including a high level of radioactive materials in D/W was released into the reactor building in a short period because of the loss of containment leak-tightness. Also, sharp synchronized peaks were observed for both RPV and D/W pressures after the immediate decrease of D/W pressure. RPV pressure decreased and then rapidly increased to 0.65 MPa, which is almost before the rapid decrease, and then it rapidly decreased. The peak value of D/W pressure is about half of the RPV pressure spike, but they are almost synchronized and the shape is similar. This rapid increase and decrease of RPV pressure shows that the large amount of steam was generated in a very short period in RPV, and it was released to the D/W side and then released to reactor building through failure location of D/W. It is considered that the large amount of steam generated was the result of direct contact of core melt relocated to the lower plenum of RPV with the remaining water there. It is assumed that a new rupture of relatively large size was formed at the lower head of the RPV. It should be noted that containment venting was tried three times to depressurize the containment, but all attempts were unsuccessful.