Fukushima 329th - 336th day

Dear Colleagues: 329th - 336th day! 


I. NISA's intermediate reports Since October 17, NISA has been organizing a series of "hearings" from knowledgeable specialists, to investigate causes of the accident and to compile "lessons learned". So far, they held 8 of such meetings, where a bulk of viewgraphs reports have been released and discussed. NISA plans to implement the lessons leaned for early start of NPPs, although many of them have already in place. On February 16, NISA released the "intermediate reports" (only in Japanese). I plant to read the report in the near future and introduce some of the significant issues in this updates. In the following two sections, I introduced one of the documents I found interesting. In the coming April, remaining two units still on grids will soon be shutdown for scheduled maintenance. There are total of 54 NPPs in Japan, previously providing 30% of electric power. The resultant idling of practically all NPPs resulted in a significant increase in import of fossil energy resources. This induced an international trade deficit first time in the last 31 years for Japan. The government has not been able to provide permissions for restarting, since local governments of the NPP sites are very prudent, asking for a new safety review based on a new safety criteria. 


II. RELAP analysis of "short-termed station blackout." Among the bulk of report, I found an interesting viewgraph, which describes the RELAP analysis of the short-termed SBO, where the loss of off-site power is aggravated with further gradual depletion of batteries. It is a transition phase before going into the "long-tem SBO". In the latter, the damaged plant lost all measures of core cooling and should eventually goes into a melt-down. I am very much interested in the attached viewgraph (in Japanese), since their conclusion is much different from my vision of the current core state, based on my "back of an envelope" assessment. The analysis is about operation of IC (isolation condenser) for 1F1. This passive residual hear removal system should have successfully helped preventing the gross-core melt in my view. I believe 1F1 plant has not went so far into the "long-tem SBO". On page 6, their modeling is illustrated in a block diagram. I immediately noticed that the SRV (Safety an Release Valve) is not correctly configured in the modeling. The SRV is installed in the Main Steam Line, from which steam is released into the suppression pool, during the "feed and bleed" operation. The safety valve function works even without the battery power, since it is with a spring action. Perhaps because of this, on page 9, the rate of water level decrease is shown very fast, down to TAF in just 1.5 h after the arrival of tsunami. I estimated 5 hours or longer. In addition, their analysis does not incorporate the TEPCO's new findings described in their December 22 Internal Investigation Committee Report. The report indicates that an operator opened the MO-3A valve at 21:30 of March 11, to restart IC-A and confirmed release of steam from the secondary side of IC, indicating successful operation of the IC. This crucial operator action was motivated by knowing that IC should work, even without additional cooling water supply on the secondary side, as long as 10 hours. In addition, I began to suspect the turbine-driven HPCI may have worked, at least temporarily, although its operational status was not confirmed from the control panel. It is because the observed water level is one meter or so higher (TAF +200mm at 21:19 of March 11) than my estimation of the reactor water level, which assumes no additional water injection. My estimation simply follows the water level by calculating the amount of the reactor water released by evaporation. I introduced this issue more in detail in Earthquake (170, Dec 16-23). Although there are some questionable issues, I strongly request continuation of this kind of assessment, instead of SBO=core-melt type analyses. 


III. Update of the mysterious temperature rise in 1F2 I introduced this issue a week ago. Although this issue attracted a lot of media attention last week, it was concluded as a failure of the thermocouple (TE-2-3-69H1). The temperature plot was released as attached. TEPCO also quickly performed an environmental test using an insulation damaged thermocouple with cable, and found that high temperature readings are induced. Later, the reading showed below freezing temperature, indicating that such a failure is likely. However, TEPCP's conclusion does not dismiss a possibility that something did occur inside of the PCV locally, near the thermocouple. TEPCO put this sensor out of service for monitoring the core status. Well, let me stop here today! 
Genn Saji _______________________________________