In the first week of the crisis, Fukushima Daiichi Reactor 1 is considered to have had the most severe situation of the three reactors which ultimately suffered meltdowns. To date, none of TEPCO’s simulated analysis of the disaster sequences have been able to simulate the actual events which lead to the meltdowns.
In all three units, the nuclear fuel rods are believed to have melted through the pressure vessels, and accumulated in the outer primary containments, but the sequence at Unit 1 worsened at a relatively early stage after the tsunami struck the plant on March 11th.
A few words on venting
It has long been known within the industry, that the Mark I PCV is small and that if steam is leaked into the PCV due to incidents such as broken pipes, the pressure rises quickly, which creates a problem easily.
PCV venting is a serious emergency operation which operators are authorized to employ in hopes of stemming increasing pressure by allowing PCV pressure to vent into the atmosphere. If the core has been damaged, implementing this measure will temporarily discharge radioactive materials from the PCV.
There are two pipes used to vent the PCV, one from the Dry-Well (D/W) and the other from the Suppression Chamber (S/C (wet-well)), and each have large and small valves that operate by air (AO valve). In some reactor designs at the point where the two pipes converge, there is a motor operated valve (MO valve), a rupture disk that ruptures under a given amount of pressure, and then connecting to the stack.
When venting the PCV, fundamentally venting of the S/C is given priority, because the vented gases are passed through the water of the S/C which traps a portion of the discharged radioactive materials, as if passing them through filter. There is another type of vent, which comes involves the D/W, which is called a “dry vent” as materials are not pushed through the water in the S/C.
Accident Sequence at Unit 1 – March 11th – March 13th
Within 4 hours of the earthquake, the reactor was already sustaining damage, and for the rest of the first week of the disaster, workers were unable to inject water into Unit 1 longer than at Units 2 and 3. By 15:50 on March 11th, the reactor water level in Unit 1 could not be determined by workers, and they also could not confirm that coolant water was being injected into the reactor.
Late on the night of March 11th, after 21:30, workers noted that the radiation dose in the reactor building was increasing, and within an hour had also increased in the turbine building. The radiation levels continued to rise, forcing operators don protective gear, and to abandon the Unit 1 control room for the Unit 2 side.
March 12th, 2011
By Midnight, during the late-night hours early on March 12th, the pressure levels in the Primary Containment Vessel were nearly exceeding the maximum design pressure of 528 kPa. Within two hours, the radiation levels in the reactor building were over 300 millisieverts per hour, and when the airlock was opened to the reactor building, white pillars of steam billowed out.
Just 15 hours after the onset of the disaster, the Reactor Pressure Vessel (RPV) in Reactor 1 is assumed to have broken as the reactor pressure rapidly decreased despite no actions were taken to initiate the decrease, simultaneously, the pressure in the PCV increased. During this process, part of the melted nuclear fuel was reduced to particles as it escaped from the RPV into the PCV.
The explosion scattered debris across the site and damaged critical emergency equipment set up to keep coolant water injecting into the reactor, and the radiation levels further delayed workers being able to repair the water injection system.
March 13th, 2011
The next day, on March 13th, workers were forced to increase the amount of water injected into the reactor, which is considered to be due to expanding leakage from the PCV.
As the melted fuel relocates into the Primary Containment Vessel (PCV), it begins attacking the concrete of the floor of the pedestal and the drywell, in a process known as core-concrete interaction. Only 3 hours later (18 hours after the onset) the PCV started leaking, and within 7 hours (25 hours after the onset) of the containment vessel leaking, the reactor building exploded.
The pool of coolant water, which TEPCO was continually injecting tons of water per hour to maintain would not stop the fiery attack on the concrete at the bottom of the melted nuclear fuel, and every day the corium continued eroding away the containment vessel itself.
In May 2011, TEPCO conducted an analysis using the ‘MAAP’ simulation software, and formulated a hypothesis of the present conditions inside the reactor based on water injection records, temperature and water level data. The results show that in Unit 1, most fuels were damaged and melted down to the bottom of the PCV through the RPV, where some of them are submerged, thought some may still be part exposed. For example, it has been noted in TEPCO investigations that the feedwater nozzle temperature is higher than expected, and this has been considered to likely be the effect of superheated steam generated from where some parts of the fuels are exposed in the RPV.
Inside of the containment, RCW piping is installed underneath the RPV to cool down the drain pit under the pedestal which is meant to catch melted fuel. It is assumed that fuel dropped in the drain pit damaged the RCW piping and high contaminated water or steam moved to RCW secondary piping as high radiation levels have been measured near the RCW heat exchange equipment area.
Some of the fuel debris would have accumulated in a sump cavity which is nearly 5 feet wide and just under 4 feet deep. In November 2011, TEPCO’s analysis of Unit 1 estimated that nearly 3 feet of melted nuclear fuel was contained in the cavity, and was relentlessly assaulting the 40 inches of concrete between it and the outer steel shell of the PCV.
In their analysis, TEPCO estimated that just 8 months after the disaster the fuel would have eroded at least 25 of the 40 inches of concrete at the bottom of the reactor containment vessel, nearly three times deeper erosion than was estimated at Unit 2 (5 inches) and Unit 3 (8 inches)at the time. 
Extreme temperatures still found in Reactor Building months later
Over the last year and a half since the disaster, the temperatures inside of the reactor building have continued to flux, despite attempts by workers to maintain cooling trends. In June 2011, when work began investigating inside of the reactor building, workers found steam ejecting from underneath the first floor, but had dramatically decreased by October 2011.
The temperatures inside of Reactor 1’s RPV have remained relatively high at several points in the upper areas, while temperatures at the bottom of the RPV decreased. As of August 2011, the temperature at the bottom of the RPV became lower than saturation temperature, and the lowering temperature trend continued thereon.
In late October 2011, the temperature in both the RPV and PCV decreased rapidly as water injection volume increased, but after workers increased the water injection volume, the temperature of the Suppression Chamber pool increased until it exceeded the temperatures of both the RPV and the PCV. Workers hypothesized that one complication may be that melted nuclear fuel remained in various places around the reactor side piping.
Comparing details with current investigation findings
These details are not only interesting by themselves, but also when considering the fact that during the investigation yesterday, TEPCO found that at the surface of the water, the radiation level fell to as low as 0.5 sievert per hour, which TEPCO experts have not been able to explain. If the fuel which was melted down to the bottom of the vessel was not fully immersed in the water, the temperature inside the vessel should have been higher, and the radiation levels also much higher than were measured this time.
Workers are still forced to continually ensure that enough coolant water is injected into the three crippled reactors to keep the melted fuel inside cool, but a large amount of the coolant water is known to be leaking out because significant increase of reactor water level has not been shown despite constant water injection has continued for the last 19 months.
Tokyo Electric Power Company says its latest endoscopic probe gave evidence that the depth water level in the reactor’s containment vessel may be around 2.8 meters (8 ½ feet) . TEPCO says it detected lethal levels of radiation, some over 11 sieverts per hour in some parts of the containment vessel, which is high enough to kill in 40 minutes of exposure.
The containment atmosphere was filled with steam, and corrosion had affected much of the scaffolding inside of the containment vessel. The footage showed a bolt about 10 mm in diameter on a foothold, and washers, and a 30-centimeter long metal rod lying on the scaffold. Ono said the bolt is unlikely to have dropped from a large object because the size of the bolt is small. The utility does plan to take a sample of water from inside the reactor Friday, the first such attempt since the crisis erupted in March last year.
TEPCO is still unable to locate the melted nuclear fuel which escaped the reactor pressure vessel, admitted TEPCO Spokesman Masayuki Ono, adding that the investigation may not be able to reach an immediate answer, but stressed that TEPCO doesn’t expect to change its plan for decommissioning the reactor.
Source: 47 News Japan
Source: JiJi Press
Source: The Japan Times
 TEPCO Fukushima Nuclear Accident Investigation Report
 TEPCO – Fukushima Daiichi Reactor 1 Event Log
 TEPCO – Outline of analysis results of Unit 1 (PCV Pressure)
 TEPCO – Unit 1 Outline of analysis results (Reactor water level)
 TEPCO – Estimated based on the Actual Temperatures around RPV
 TEPCO – Contamination of Unit 1 RCW Heat Exchange Equipment Area
 TEPCO – Analysis Result – Unit 1
 TEPCO – Analysis Result – Unit 2
TEPCO Analysis Result – Unit 3
 TEPCO – Cooling State of Unit 1 – (Steam Generated)
 TEPCO – Knowledge acquired from behavior of temperature and pressure at Unit 1