By VOJIN JOKSIMOVICH, PhD

Modern Tokyo Times

Subsequent to recent publication of my article Fukushima Nuclear Accident: Tsunami Induced but Man-Made Disaster, some stunning new information became available. I am primarily referring to the report published by TEPCO on May 15: Reactor Core Status of Fukushima Daiichi Power Station Unit 1 as well as the May 17 and May 19 articles published by the New York Times (NYT) titled respectively In Japan Reactor Failings, Danger Signs for the U.S. and U.S. Was Warned on Vents Before Failure at Japan Plant. The material presented in these new sources provides insights, which substantially alter understanding and analyses of the Fukushima Daiichi Unit 1-4 accidents in particularly with regard to Unit 1 in which the core melt as well as the hydrogen explosion took place first. TEPCO was careful to characterize the analyses presented as provisional with further info to come. Reanalysis by this author will wait until further info becomes available.

TEPCO May 15 Report Highlights

The most stunning insight is that the data taken indicate a distinct possibility that the key plant components (pipes, valves, etc) have been compromised by the quake itself before the tsunami waves arrived. All the statements made thus far, including those by the Nuclear Industry Safety Agency (NISA), as well the analyses performed by the reputable entities such as the French AREVA postulated that the plants withstood shaking but not the tsunami.

According to the May 16 Kyodo news report the plant personnel entered the Unit #1 reactor building (RB) during the night to hear their dosimeter alarms go off seconds later indicating radiation level about 300 milliSieverts/hr. The building was filled with highly radioactive steam so the decision was made for the workers to evacuate. Seismic damage offers a plausible explanation for an “early” core-melt with associated release of radioactivity to the containment.

Following the installation and calibration of two new water level gauges, water level readings point out that most of Unit #1 fuel had melted earlier than previously thought. Readings showed the water level lower than 5 meters below the top of the active fuel (TAF). Given the active fuel length is about 4 meters; the water level is lower than the original bottom active fuel (BAF) level. For weeks it was reported that the lower half of the fuel was flooded. TEPCO has only recently revised the fuel damage estimate from 70% to 55%. So called corium –consisting of damaged nuclear fuel, control rods, structural materials from affected parts of the reactor– dropped into the RPV lower plenum where it was cooled adequately, as the RPV temperature has remained between 10-120 degrees C.

The TEPCO analyses (water level vs. time) indicate that the water level dropped to the TAF level approximately in 3 hrs after the reactor scram (~ 18:00 on March 11)  and to the BAF level in 4.5 hours (~19:30). The chart shows that the fresh water injection started on 5:50 on March 12 and ended at 14:50. Not clear where this water cane from. Injection of the seawater started at 20:20. There is also a chart of maximum core temperature vs. time, which shows that the core temperatures started increasing after the water dropped below the BAF level and reached the fuel melting temperature of 2800 degrees C at ~20:00. In about 16 hrs after the scram (~6:50 March 12) most of the core dropped into the RPV bottom plenum.

If this seismic damage assumption holds, it would have a profound effect on other nuclear utilities in Japan such as the Chubu Electric, which has recently shutdown Hamaoka units 4 and 5 at the request of the Prime Minister because they are located right on the top of a fault line. The utility plans to restart those units after the tsunami defenses have been beefed up.

Olympic Pool-sized Pond

Given that the water injection rates reached tons/hr, some time in April this writer intuitively became concerned. The pumped water must have been going elsewhere rather than into the RPV only. Then TEPCO announced drywell flooding with enough water the raise the level to above TAF. It still didn’t make sense. After pumping 10,000 cubic meter of water, the water level in 7,400 cubic meter drywell was below half, not even reaching BAF. On May 15 TECO discovered an Olympic swimming pool-sized pond of 4.2 meters of highly radioactive water in the basement of the Unit #1 reactor building (RB). TEPCO explanation is that the RPV developed leaks attributable due to contained corium, which subsequently damaged the CV. Flooding of the drywell was abandoned. An unmanned robot on the first floor of the RB detected maximum radiation levels of 2,000 milliSv/hr, the highest measured level since March 11.

Isolation Condenser Manually Valved off

Japan Tines reported on May 17: Worker error may have led to meltdown. TEPCO announced on May 16 that the operators might have valved off the Isolation Condenser (IC) 10 minutes after the quake (~15:00) because the RPV pressure dropped from 70 to 45 atmospheres causing a concern. The IC system is designed to provide core cooling even if all AC power is lost. It features only in the Unit #1 design, not in other units. Had the system worked as designed more time would have been available before initiation of the meltdown. Records show that at 18:10 the valve was open, closed again at 18:25, open again at 21:30. The IC finally failed at 01:48 on March 12 presumably because the batteries ran out… There was no explanation was this on/off valve operation. Original March 12 TEPCO press release stated that the reactor was cooled by the IC. This writer relied on the AREVA analyses based conceivably on bad data.

Unit #4 Explosion Attributable to Unit #3

In my above referenced publication I expressed skepticism that the Unit  #4 explosion was caused by hydrogen. TEPCO now thinks that hydrogen leaked out from unit #3 to cause unit #4 explosions. Hydrogen released from unit #3 flowed through a gas-treatment line and entered unit #4 because of a breakdown of valves. Hydrogen leaked from ducts in the second, third and fourth floors of the RB and ignited a massive explosion in unit #4. Sharing of equipment between adjacent units is a common feature of the Fukushima plant designs.

Failed Containment Venting

Unfiltered containment venting through the service area causing hydrogen explosions rather then filtered venting through the exhaust stack was a mystery to this writer. The NYT articles provided an explanation. The venting system was designed to be operated from the control room with safeguards using switch under lock and a key. However, the system relied on the same power source as the core cooling systems. In addition, the quake might have damaged the valves. Of course, the valves were designed to be operated manually as well. However, the plant personnel found the radiation levels near the vent way too high.

The NRC claimed that similar BWRs in the U.S. were safe because the venting system designed in the 1980s was hardened. TEPCO, however, said that Daiichi plants installed the same expensive and time-consuming retrofitting vents in the 1998-2001 timeframe. This is yet another example of lack of diversity in power supplies.

At Unit #2 operators tried to open the valves but the reactor pressure did not come down suggesting that venting may not have been successful. At Unit #3, the operators tried seven times to open the valve manually but it kept closing. The consequences of failed venting were disastrous.  

Writer has the PhD in nuclear engineering and is a retired nuclear safety specialist

 Dr. Vojin Joksimovich is a highly claimed specialist.