Fukushima Daiichi Reactor Recovery Measures in Detail
Fukushima Accident 2011: Appendix 1
30 September 2011
The following material was part of the Fukushima Accident paper until early June 2011, when a better understanding of what had happened inside the reactors emerged. The paper was then streamlined accordingly, and this material is retained as an Appendix for reference.
On Sunday 13th, seawater injection to unit 1's Primary Containment started. Pressure in unit 1 RPV dropped to below 500 kPa after the explosion and has remained 300-500 kPa since then. Containment pressure was about 800 kPa until the explosion, but then dropped and on 19th was low for four days, but jumped up on 23-24th and has been consistently about 200 kPa below RPV pressure from 25th. Water injection has been via the feedwater line. On 6 April Tepco started injecting nitrogen into the containment vessel on account of suspected hydrogen levels there. This raised the CV pressure slightly to 195 kPa for about ten days. Early in May Tepco announced that it proposed to raise the water level in the containment vessel to above the level of the top of the fuel, but reconsidered this when it appeared that much of the fuel was in the bottom of the RPV anyway.
An ambient air filtration system has been installed in unit 1 reactor building to lower the levels of airborne radioactivity and enable access so that a heat exchanger can be connected into the RHR circuit, enabling better heat removal than currently. Workers were able to gain access from 9 May and over next few days were able to calibrate gauges which showed that the water level in the RPV was below the fuel level and the fuel was presumed to have melted and dropped to the bottom. It is evidently being cooled adequately by about 4 m3 of water per hour injected to RPV outside the shroud. The basement of the reactor building has about four metres depth of contaminated water in it.
Seawater injection to the containment was started on 15th via the fire-fighting line. RPV pressure was very high from mid 14th and drywell pressure reached 650 kPa (gauge), well above design base maximum of 380 kPa(g), so that at 6.14 am on Tuesday 15th, unit 2 apparently ruptured its pressure suppression chamber under the actual reactor, releasing significant radioactivity and dropping the drywell pressure inside. At 10.30 am Tepco was told to inject water into the pressure vessel and to vent the containment, and these activities continued. Since about 17 March RPV pressure has been atmospheric, and drywell pressure about 200 kPa (100 above atmospheric). On 1 April Tepco discovered a crack in the wall of a 2m deep services pit which was leaking highly-contaminated water, apparently from the reactor itself, to the sea. The company eventually plugged it early on 6 April after some 4.7 PBq of radioactivity had been released.
Within the RPV the fuel is presumed to have melted and dropped to the bottom, where it is evidently being cooled adequately by about 4 m3 of water per hour injected to the RPV outside the shroud.
In the reactor 3 itself, RPV water levels were up and down over 13 March but settled at a low level by midday on 14th, with core half uncovered. Water injection, including seawater, was initially via the fire-fighting line but changed over to feedwater line in mid May. Early on Wednesday 16th there was a major release of smoke and/or steam from the top of the building. Because of the possibility that the Primary Containment of Unit 3 was damaged, the operators evacuated from the central control room of Unit 3 & 4 (a shared facility) at 10:45 am, but they returned to the control room and restarted the operation for water injection 45 minutes later. Pressure then built up to 320 kPa in the containment but no further venting was required.
Since at least 16 March, pressure vessel damage has been suspected, and some leakage is apparently confirmed by radioactivity levels in the building. Also since 20 March the reactor pressure and drywell pressure decreased and then remained stable, leading Tepco to believe that “the reactor pressure vessel is not seriously damaged.” By the end of March internal pressure was little more than atmospheric, and by 10 April water temperature was below boiling point. However, whereas cooling of units 1 & 2 is achieved with about 4 m3/hr, unit 3 requires about 9 m3/hr. This is attributed to the melted fuel probably being retained on the core support plate within the shroud, so that water injected outside the shroud does not reach it effectively. Consideration is being given to changing the injection to the core spray system.
Units 5 & 6:
On 18 March, Tepco made holes in each of the superstructure roofs of units 5 & 6 to allow ventilation of any hydrogen, though pond temperatures had reached only about 69°C. On 19th the residual heat removal pumps for units 5 & 6 ponds were restarted as power was restored, and temperatures then declined.
In the last ten years many national regulators have required licensees to go well beyond the original design basis in ensuring the safety of nuclear plants in a variety of accident and assault scenarios. The original 1960s GE Mark I containment design in 32 reactors around the world (23 in USA) has been significantly modified in the light of operational experience and studies on accident scenarios. In particular, the design was modified from 1975 through to the 1980s to address improvements in the technology and changing regulatory requirements, and a number of severe accident countermeasures were undertaken in the 1990s. These included retrofitting containment venting systems and installing air-cooled diesel generators at Daiichi 2, 4 & 6 - the last being the only one to survive the tsunami. Apparently all engineering modifications and upgrades recommended were carried out by Tepco, as they were in the USA, including fitting hardened vents from the suppression chamber, though this measure was not mandated by NISA as it was by US NRC. All of the changes required by regulatory authorities were implemented. JAPC's Tsuruga 1 (431 MWe, 1970) is reported to be the only Japanese BWR without retrofitted hardened vent.
Certainly regarding direct seismic vulnerability a lot had been done as mentioned above in Earthquake background. Also as a result of tsunami analysis in recent decades, the design basis height was increased from 3.1 to 5.7 m and Tepco moved pump motors and other equipment higher accordingly. But turbine halls were not waterproofed here as at Hamaoka, and generators and electrical switchgear remained vulnerable to the 15-metre tsunami.