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1、Nuclear Engineering ReactorBWR and Fukushima NPP Accident 1.The accident process（after the earthquake）2.The accident process（after the tsunami）3.The accident process（Unit 1）4.The accident process（Unit 3）5.The accident process（Unit 2）6.DiscussionOverviewThe accident process（after the earthquake）March
2、 11,2011,about 14:46:magnitude 6-strong earthquake hitsUnits 1 through 3 were in normal operation,and 4 through 6 were in scheduled shutdown.Units 1 through 3 automatically SCRAMed,operators switched the electric power to the site to an external power sourceAt the same time of the switching,external
3、 power was lost and the emergency D/Gs kicked in automatically.Steel TowerEmergency D/GThe accident process（after the earthquake）March 11,about 14:50:Unit 2 RCIC started manuallythe operator manually started the RCIC with remote operation.RCIC automatically stopped because of high water level.Eleven
4、 minutes later,the operator restarted the RCIC.March 11,2011,14:52:Unit 1 IC started automaticallythe operator started and stopped the IC three times,controlling the reactor pressure at 6-7 MPa.March 11,15:05:Unit 3 RCIC started manuallyAt about 15:25,on detecting high reactor water level,the RCIC s
5、topped automatically.Conceptual sketch of the IC.Proceeded normally!March 11,about 15:27:first tsunami waveIt was low enough to not cause damage that led to the SBO.March 11,about 15:35:second tsunami waveThis wave caused the loss of almost all electric power sources.The plant response headquarters
6、were set inside the seismic isolation building in a room with no windows and no cameras to look outside.loss of AC power with all units and even DC power with Units 1,2,and 4.The accident process（after the tsunami）the starting point of this severe accident Elevations of Elevations of Fukushima-1Fuku
7、shima-1The accident process（after the tsunami）This picture shows the areas affected by the tsunami at the fukushima daiichi nuclear power plant(Blue:flooded area)Almost all of the plant were submerged福岛第一核电厂受海啸影响的情况福岛第一核电厂受海啸影响的情况Assumed highest tsunami water level O.P.+5.7mInundation heightapx.O.P.
8、+14-15mFukushima The accident process（after the tsunami）March 11,about 15:35:second tsunami waveThe states of water submergence were about the same for Units 1 through 3.Seawater entered the turbine buildings from the equipment hatch and ventilation holes to flood the basement and the concourse floo
9、r between the basement and first floor of the turbine,control,and reactor buildings.The basement of the turbine buildings contained the emergency generators and the regular and emergency switchboards.The control building basements contained the DC power systems(Units 1,2,and 4)The reactor building b
10、asements contained the pumps and other equipment for RCIC and HPCIThe accident process（after the tsunami）the sea water pumps fails,the nuclear rectors loses its heat sink to the oceanAll units that were up and running at the time of the accident had lost their ultimate heat sinks Locations of emerge
11、ncy D/Gs and their Locations of emergency D/Gs and their switchboardsswitchboardsThe accident process（after the tsunami）March 11,15:37-42:loss of all AC powerExcept for the Unit 6 air-cooled emergency D/G(6B),all AC power sources for Units 1 through 6 were lost.Units 1 and 2 lost their DC power as w
12、ell.Unit 3 had its DC power on the concourse level between the basement and first floor.The unit was covered with water,but managed to retain its function;Unit 3 allowed operation of its RCIC and HPCI for a few days.All M/C switchboards and most of the P/C switchboards were submerged and had lost th
13、eir functions.The description“the earthquake destroyed the external power source,and then the tsunami submerged the emergency D/G to cause the total loss of AC power”is wrong!The accident process（after the tsunami）SwitchboardsUnits 1-4,all the M/C switchboards and most P/C switchboards were sub merg
14、ed and had lost their functions.Even if the external power had reached,the state of loss of all AC power during initial stage of accident would have been the same.Unit 6,one D/G and its M/C were functional.Units 5 and 6 did not have to face SBO The description“the earthquake destroyed the external p
15、ower source,and then the tsunami submerged the emergency D/G to cause the total loss of AC power”is wrong!March 11,about 15:37:IC isolation valves closed with fail-safe functionUnit 1 lost all its electrical power including DC.The central control room was pitch dark.The most critical reactor water l
16、evel and pressure became unknown.IC was in normal operation with the operator turning it on and off.The loss of DC power,however,caused the fail-safe logic to send“close”signals to all four valves.The cooling function of the IC was almost completely lost.This design philosophy contradicts the requir
17、ement to“keep the valves open”on a severe accident.The accident process（Unit 1）The Unit 1 IC was designed that on failure(an abnormal condition),all flows were stopped to prevent possible radioactivity leakage IC in operationIC in operationMarch 11,about 15:37:IC isolation valves closed with fail-sa
18、fe functionNo one had ever experienced the IC in operation.Lack of education and training for the hypothetical situation of loss of DC power.The staff were not aware of the Unit 1 IC stoppage with the fail-safe function Unit 1 was in an abnormal situation with no water injected into the core until w
19、ater injection from a fire engine started at 4:00 the next morning.The accident process（Unit 1）Pig snoutPig snoutThe accident process（Unit 1）March 12,about 02:30:sudden rise of D/W pressureThe D/W pressure was reported to be at 0.84 MPaMarch 12,02:45:sudden RPV pressure dropthe RPV pressure was meas
20、ured at 0.8 MPa.That was a large drop from 6.9 MPa,there was a large leakage from the pressure vesselRPV pressure and D/W pressureRPV pressure and D/W pressureThe accident process（Unit 1）March 12,15:36:hydrogen explosionThe accident process（Unit 3）March 11,about 15:38:Unit 3,SBORCIC had started at 1
21、5:05 and stopped automatically with the“reactor water level high”signal at 15:25 before the loss of AC power.March 11,16:03:RCIC manually startedDC power had survived in Unit3,the central control room was able to operate the RCIC and other equipment as well as read parameters off the indicators.Marc
22、h,12,11:36:RCIC automatically stoppedThe RCIC stopped for some reason(still unknown)The operators looked for the cause and tried to restart it from the central control room,but did not succeed.The RCIC had two sources of water,the condensate storage tank(2500 tons)and S/C(3000 tons).The accident pro
23、cess（Unit 3）The accident process（Unit 3）March 12,12:35:HPCI automatically startedHPCI automatically started on detecting the lower water level in reactor.HPCI is driven by steam from the reactor and thus does not require AC power for running its pump.HPCI is the last resort of emergency core cooling
24、 that can inject water into RPV.HPCI water injected effectively cooled the reactor,and its pressure gradually dropped.At this time,however,the reactor water level had not been read,and the effect of water injection was not known.The accident process（Unit 3）March 13,02:42:HPCI manually stopped(1)Runn
25、ing HPCI at a rotation slower than its design speed may lead to damage the HPCI.(2)Opening the SPVs would lower the reactor pressure to allow low-pressure water injection.After stopping the HPCI,however,when the operator tried to activate the SRVs,they would not open because of low battery capacity.
26、The HPCI had successfully cooled the reactor to drop its pressure to 0.58 MPa.After stopping,however,the pressure quickly rose to 0.77 MPa at 03:00 on March 13th,and to 4.1 MPa at 03:44The accident process（Unit 3）March 13,about 03:00:alternative water injection from D/DFP was unsuccessfulThe D/DFP,w
27、hich is a low-pressure system,did not have enough discharge pressure to inject water.They depended on water injection from D/DFP and stopped the HPCI,but the reactor pressure increased to a high level that did not allow water injection from D/DFP.The operators tried to restart HPCI or RCIC,but faile
28、d due to the low battery.RPV pressure from March 12th to 13thRPV pressure from March 12th to 13thThe accident process（Unit 3）March 14,11:01:Unit 3 hydrogen explosionThe accident process（Unit 2）March 11,15:41:Unit-2 SBOThe central control room was pitch dark,and all the indicators were not functional
29、.Fortunately,the RCIC had been manually turned on,but its status from that point was unknown.March 12,01:00-02:55:RCIC confirmed to be operationalA worker confirmed the RCIC discharge pressure was slightly higher than the RPV pressure,values that show that the RCIC was running normal.March 12 about
30、04:30:RCIC water source switchedThe operator switched the water source to the S/C from the condensate storage tank.The RCIC had two water sources,the condensate storage tank and S/C(about 3000 tons for Units 2 and 3).Using S/C as the water source ensured plenty of cooling water;however,without a hea
31、t sink to dump the heat,circulating the water from the reactor to the RCIC and back to the reactor would keep the S/C temperature and pressure up.The accident process（Unit 2）March 14,13:25:RCIC judged to have lost its functionthe cooling water had been storing heat for over two days.the S/C at about
32、 12:30 was at an abnormal state of high temperature and pressure with temperature at about 149 and pressure at 0.49 MPa.March 14,about 15:00:D/W pressure droppedLeakage from the CV that had been subject to high temperature and pressure for an extended time.D/W pressure drop on March 14th.D/W pressur
33、e drop on March 14th.The accident process（Unit 2）March 14,about 16:00:vent valve did not openMarch 14,about 16:34:SRVs did not openMarch 15 about 00:16-01:11:highly dangerous state with high CV pressureThe CV had already started its leakage,and after about 20:00,large leakage from the RPV started as
34、 well.Thus the CV was leaking to the outside but at the same time had RPV leaking into it.water injection from the fire engine to the RPV with pressure above 1 MPa was impossible.The CV venting failed,and the D/W pressure rapidly went up around 23:00 on the 14th to reach 0.7 MPa in about 30 min and
35、stayed at this dangerous level,which could have resulted in CV explosion.RPV pressure transition on 14th and 15th,an indication of possible contact of melted fuel and water.DiscussionReasons why Unit 2 didnt explodeReasons why Unit 2 didnt explodeReasons why Unit 2 didnt explodeReasons why Unit 2 di
36、dnt explodeWhen the fifth floor of the reactor building of the adjacent Unit 1 exploded,the blowout panel of Unit 2 opened like a window,and hydrogen never accumulated in the top.What seemed like damage from an event turned around and saved the building.The blowout panel did not pop open with Units
37、1 and 3Unit 2.Steam was coming outUnit 2.Steam was coming outDiscussionReasons why Unit Reasons why Unit Reasons why Unit Reasons why Unit 4 4 4 4 explode explode explode explodeThe hydrogen that exploded in Unit 4,which had been shut down for periodic maintenance,must have entered the reactor build
38、ing of Unit 4 from the exhaust tower shared with Unit 3 when Unit 3 was vented.Unit 4Unit 4.Half oalf of the walls were gonethe walls were goneDiscussionThe hydrogen explosions spread the radioactive material?The hydrogen explosions spread the radioactive material?The hydrogen explosions spread the
39、radioactive material?The hydrogen explosions spread the radioactive material?In fact,however,radioactive material was not scattered by the hydrogen explosions,but rather leaked out because the containment vessels(CVs)lost their containment function due the high temperature and pressure caused by the
40、 meltdowns.Unit 2 that released the largest amount of radioactive material did not have a hydrogen explosion.DiscussionThe ICThe ICThe ICThe IC、RCIC and HPCI are passive safety technology?RCIC and HPCI are passive safety technology?RCIC and HPCI are passive safety technology?RCIC and HPCI are passiv
41、e safety technology?How the How the accident might accident might have been have been avoided?avoided?DiscussionSafety measures in place in other countries:(1)Emergency powerBrowns Ferry NPP(Mark-I)in the United States has a portable battery rack to serve as an 8 h emergency power supply to take rea
42、dings from instruments(2)Watertight doorsThis is another preparation at Browns Ferry.The emergency D/Gs are located in a room with watertight doors(3)Manual handle for IC valvesMilstone NPP has handles on the IC valves inside the CV and conducts training to open the valves from outside the CV.Fukush
43、ima-1 Unit 1 IC valves inside the CV could not be opened from the outside.How the accident might have been avoidedEmergency battery power unitEmergency battery power unitHow the accident might have been avoided(4)Snorkel Diablo Canyon NPP(PWR)has air inlets for seawater pumps at 13.5 m above sea lev
44、el using snorkels.(5)Vent filter Miihleberg has a filtering system on its vent lines that will filter out many of the radioactive materials by passing the vent gas though water mixed with special liquid ingredients.The liquid is injected by gravity so no electricity is needed.(6)Independent emergenc
45、y cooling system The Miihleberg NPP has developed,in addition to the GE-designed cooling system,an entirely independent emergency cooling system for each building.Inside the building is a watertight room with an emergency D/G and a switchboard.Snorkel vent holesSnorkel vent holesHow the accident mig
46、ht have been avoided(1)Providing Variety with Switchboard Installation(2)Preparation for Loss of DC Power(3)Providing Watertight Structures with Buildings(4)Storing Portable Air Compressors(5)Improving the Water Level Gauge(6)。Measures that were feasible:Measures that were feasible:Measures that were feasible:Measures that were feasible:？Thank you