Unit 1 Reactor
Details of the core
F. Tanabe has estimated that the core contained the following materials:
Cooling problems and first radioactivity release
On 11 March at 14:46 JST, Unit 1 scrammed successfully in response to the earthquake though evacuated workers reported violent shaking and burst pipes within the reactor building. At 15:37 all generated electrical power was lost following the tsunami leaving only emergency batteries, able to run some of the monitoring and control systems. It was later learned that Unit 1's batteries were damaged and unavailable following the tsunami. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for Units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems." The alert was temporarily cleared when water level monitoring was restored for Unit 1 but it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.
After the loss of site power and reactor shutdown, Unit 1 was initially cooled using the isolation condenser system. About 10 minutes after the earthquake, TEPCO operators removed both of Unit 1's isolation condensers from service, and instead chose to activate the HPCI (High Pressure Coolant Injection) systems to cool the reactor and the core spray system was activated at 15:07 to cool the suppression pool. The core spray system was disabled with AC power loss at 15:37 (The tsunami) and the HPCI system failed following DC power loss.
Operators were unable to restart the isolation condensers for an extended period of time after the tsunami (greater than 30 minutes). After that, the isolation condensers were operated intermittently, for unknown reasons. The isolation condensers were designed to successfully cool Unit 1 for at least 8 hours, and it is unknown how effective they were. After that, refill would have been required to the isolation condenser tanks which are under atmospheric pressure (low pumping head requirements). By design, isolation condensers would have removed the heat from the reactor transferring it out of the primary containment and into the atmosphere, but with limited and non-existent operation, core and containment cooling was not successful.
The isolation condensor apparently did not work. On 27 February 2012 NISA ordered TEPCO to reveal – before 12 March 2012 – why the layout of the isolation condensor was changed. In the papers of the original application (dated from July 1966) for reactor 1 the tubes were separated from each other, But in the building-papers – submitted October 1967 – the two tubes were connected with each other outside the reactor vessel. Without mentioning the change to NISA, all of this in violation with all legal procedures.
By midnight water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases. In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for Unit 1 and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Chief Cabinet Secretary Yukio Edano stated later in the morning the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST, the pressure inside Reactor 1 was reported to be 2.1 times normal levels, 820 kPa.Isolation cooling ceased to operate between midnight and 11:00 JST 12 March, at which point TEPCO started relieving pressure and injecting water. One employee working inside Unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.
Rising heat within the containment area led to increasing pressure. Electricity was needed for both the cooling water pumps and ventilation fans used to drive gases through heat exchangers within the containment. Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off but this also means cooling water is being lost and must be replaced. If there was no damage to the fuel elements, water inside the reactor should be only slightly radioactive.
In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." Dose rates recorded on the main gate rose from 69 nGy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 0.3855 mSv/h at 10:30 JST. At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near Reactor 1, which indicated some of the core's fuel had been damaged. Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred. Radiation levels at the site boundary exceeded the regulatory limits.
On 14 March, radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 0.751 mSv/h on one location and at 02:40 an intensity of 0.650 mSv/h at another location on the premises. On 16 March, the maximum readings peaked at 10.850 mSv/h.
At 07:00 JST on 12 March, Prime Minister Naoto Kan asked Daiichi director Masao Yoshida why his workers were not opening the valves to release rising steam pressure within the reactor. Yoshida answered that they could not open the electrical valves because of the power failure and the radiation was too high to send workers to manually open the valves. Nevertheless, with the pressure and temperatures continuing to rise, at 09:15, TEPCO sent workers to begin manually opening the valves. The high radiation slowed the work and the valves were not opened until 14:30.
At 15:36 JST on 12 March, there was an explosion in the reactor building at Unit 1. The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. The roof collapsed, covering the floor and some machinery on the south side. The walls were relatively intact compared to later explosions at Units 3 and 4.Video of the explosion shows that it was primarily directed sideways.
The roof of the building was designed to provide ordinary weather protection for the areas inside, not to withstand the high pressure of an explosion. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel. The secondary containment includes the top floor with water-filled pools for storing fresh or irradiated fuel and for storage of irradiated tools and structures.
Experts soon agreed that the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels exposing zircaloy structures/fuel assembly cladding, which then reacted with steam and produced hydrogen, with the hydrogen subsequently vented into the containment building.
Officials indicated that reactor primary containment had remained intact and that there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. The report of the fact-finding commission states that "There is a possibility that the bottom of the RPV was damaged and some of the fuel might have dropped and accumulated on the D/W floor (lower pedestal)." The Fukushima prefectural government reported radiation dose rates at the plant reaching 1.015 mSv/h. The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but the level fell below their guidance for emergency situations.
Seawater used for cooling
At 20:05 JST on 12 March, the Japanese government ordered seawater to be injected into Unit 1 in a new effort to cool the reactor core. The treatment had been held as a last resort since it ruins the reactor. TEPCO started seawater cooling at 20:20, adding boric acid as a neutron absorber to prevent a criticality accident. The water would take five to ten hours to fill the reactor core, after which the reactor would cool down in around ten days. The injection of seawater into the reactor pressure vessel was done by fire department trucks. At 01:10 JST on 14 March, injection of seawater was halted for two hours because all available water in the plant pools had run out (similarly, feed to Unit 3 was halted). NISA news reports stated 70% of the fuel rods had been damaged when uncovered.
On 18 March, a new electrical distribution panel was installed in an office adjacent to Unit 1 to supply power via Unit 2 when it was reconnected to the transmission grid two days later. On 21 March, injection of seawater continued, as did repairs to the control instrumentation. On 23 March, it became possible to inject water into the reactor using the feed water system rather than the fire trucks, raising the flow rate from 2 to 18 m3/h (later reduced to 11m3/h, and even further to reduce the build up of contaminated water); on 24 March, lighting was restored to the central operating room.
As of 24 March, the spent fuel pool was "thought to be fully or partially exposed", according to CNN. Pressure in the reactor had increased owing to the seawater injection, resulting in steam being vented, later alleviated by reducing the water flow. Temperature increases were also reportedly temporary. TEPCO condensed some of the steam to water in the spent fuel pool.
It was estimated that as much as 26 tonnes of sea salt may have accumulated in reactor Unit 1 and twice that amount in Units 2 and 3. As salt clogs cooling pipes and erodes zirconium oxide layer of the fuel rods, switching to the use of freshwater for cooling was a high priority.
The use of seawater has the potential to make the uranium chemistry more complex; in pure water the hydrogen peroxide formed by the radiolysis of water can react with uranium dioxide to form a solid peroxide mineral known as studtite. According to Navrotsky et al. this mineral has been found in the fuel storage pond at the plutonium production site atHanford. Navrotsky et al. report that when alkali metal ions are present, uranium can form nanoparticles (U60 clusters) which may be more mobile than the solid studtite. A review of the research done at the University of Notre Dame on the subject of nanoscale actinyl clusters was published in 2010.
By 24 March, electrical power (initially from temporary sources, but off-site power used from 3 April) was being restored to parts of the unit, with the Main Control Room lighting being restored.
On 25 March, fresh water became available again to be added to the reactor instead of salt water, while work continued to repair the unit's cooling systems. A volume of 1890 m3 (500,000 USgal) of fresh water was brought to the plant by a barge provided by the US Navy. On 29 March, the fire trucks which had been used to inject water into the reactor were replaced by electrical pumps.
On 28 March, pumping began to remove water contaminated with radioactive 137Cs and 131I from basement areas, storing it in the condenser system. By 29 March, pumping was halted because condensate reservoirs were almost full and plans were being considered to transfer water to the suppression pool surge tanks.
On 7 April, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions. The injection has been ongoing since then and has been repeated on the other units at Fukushima. On 7 April, before a large aftershock, temperatures in the reactor core unexpectedly "surged in temperature to 260 °C"; the cause was unknown, but the temperature dropped to 246 °C by 8 April. On 27 April, TEPCO revised its estimate of damaged fuel in Unit 1 from 55% to 70%.
On 17 April, remote control robot was used to enter the Reactor Building and performed a series of inspections, which confirmed on 29 April that there was no significant water leakage coming from the containment vessel.
On 23 and 26 April, concerns that Unit 1 fuel rods may be exposed to air caused TEPCO to consider filling the "containment vessel with water to cool the reactor" despite concerns for building integrity. Efforts were slowed by Unit 1 radiation measurements "as high as 1,120 mSv/hr". On 13 May, TEPCO announced it would proceed with a plan to fill the containment vessel despite the possibility of holes caused by melting fuel elements in the pressure vessel. TEPCO had expected to increase the amount of water pumped to Unit 1 to compensate for any leakage from the holes but decided on 15 May to abandon the plan after finding the Unit 1 basement was already half flooded.
On 5 May, ventilation systems were installed in the Reactor Building, to clean the highly radioactive air encapsulated in it.
On 12 May, the water level gauge for the reactor was calibrated, and it was subsequently identified that the water level was lower than previously thought (as the water level went off the lower side of the gauge).
On 13 May, preparatory work started on the installation of the Reactor Building covers. Construction work started on 28 June.
On 20 May, staff entered the Reactor Building, confirming reactor water level and radioactivity.
Since 2 July, the reactor has been cooled using fresh water from the on-site water treatment plant.
On 21 August, TEPCO reported that all of the temperature sensors of Unit 1 were recording temperatures less than 100 degrees Celsius on Friday 19 August. Once other goals are met, Unit 1 will have achieved cold shutdown state.
On 28 October, TEPCO reported the completion of cover construction at reactor building of Unit 1 of Fukushima Daiichi nuclear power station.
On 19 January 2012 the interior of the primary containment vessel of reactor 2 was inspected, by TEPCO for the first time after the accident, with an industrial endoscope. With this device photos were taken and the temperature was measured at this spot and from the cooling-water inside, in an attempt to calibrate the existing temperature-measurements that could have an error margin of 20°C (36°F). The procedure lasted 70 minutes. The photos showed parts of the walls and pipes inside the containment vessel, but they were unclear and blurred, most likely due to water vapors and the radiation inside. According to TEPCO the photos showed no serious damage. The temperature measured inside was44.7 °C (112.5 °F) and did not differ much from the 42.6 °C (108.7 °F) measured outside the vessel.
Possibility of criticality
Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 to 16 March raised the possibility that nuclear chain reactions could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi. 16 March reports that fuel rods in the spent fuel pool at Unit 4 could have been exposed to air appeared to indicate that uncontrolled fission may have occurred in that fuel pool. Later reports of exceptionally high iodine-134 levels appeared to confirm this theory because very high levels of iodine-134 would be indicative of criticality. The same report also showed high measurements of chlorine-38, which some nuclear experts used to calculate that self-propagating fission must be occurring in Unit 1. Despite TEPCO suggesting the iodine-134 report was inaccurate, the IAEA appeared to accept the chlorine-based analysis as a valid theory suggesting criticality when it stated at a press conference that "melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions". TEPCO confirmed its concern about the accuracy of the high iodine and chlorine report by formally retracting the report on 21 April,which eliminated both the exceptionally high iodine-134 and chlorine-38 levels as proof of criticality. TEPCO did not appear to comment on the criticality concern when withdrawing its report, but the IAEA has not withdrawn its comments, and some off-site experts find the currently measured iodine-134 levels higher than expected.
On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's containment vessel. The utility said that fuel rods of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly. According to a Japanese press report, there are holes in the base of the pressure vessel – these holes were meant for the control-rods. After the fuel had melted, it produced holes in the bottom of the RPV and then escaped into the containment vessel. In November 2011 TEPCO did not know the shape or porosity of the fuel mass, which is at the bottom of the containment vessel. As a result it is impossible to know exactly how far the fuel mass would have eroded the concrete floor, but TEPCO estimate that no more than 70 cm of a 7.6 meter concrete slab was eroded away by the hot fuel. The production of heat and steam in unit 1 has decreased, as suggested by both radioactive decay calculations and photographic evidence (same source from TEPCO).
TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours. In less than 16 hours, the reactor core melted and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in the environment. In June the Japanese government confirmed that Unit 1 reactor vessel containment was breached, and pumped cooling water continues to leak months after the disaster.
Spent fuel pool of reactor 1
From 31 March, additional sea water was added to the spent fuel pool, initially by using a concrete pump. Fresh water was used from 14 May. By 29 May water was able to be injected using a temporary pump and the Spent Fuel Pool Cooling (FPC) line.
On 10 August the spent fuel pool was switched from the water-injection system – that functioned some 5 months – to a circulatory cooling system. For the first time since the 11 March disaster, all four damaged reactors at the plant were using circulatory cooling systems with heat-exchangers.