Fukushima Dai-Ichi Initial Technical Lessons Learned

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    12 Nuclear Exchange, May 2011

    This article considers the post-earthquake, post-tsunami state-of-affairs

    and lessons learned (to date) from the ongoing incident at the Fukushima

    Dai-ichi nuclear power plant in Japan. Prepared for Nuclear Exchange in

    mid-April, it examines some of the technical factors which influenced

    events and presents some of the lessons learned at this early stage.

    Keywords: nuclear power plant, accident, meltdown, spent fuel pool, loss of off-site power, earthquake, tsunami

    By Akira T. Tokuhiro and Olumuyiwa Omotowa, Department of Mechanical Engineering, University of Idaho, USA

    The Japanese Fukushima Dai-ichi(D1) and Dai-ni (D2) nuclear powerstation with 4 GE-BWRs (4 x Mark

    I type) units (U1-4) at one site and 2BWR units (U5-6; 1 each, Mark I, MarkII) respectively co-located side-by-side onthe north-central eastern coast of Japanwithstood a 9.0 earthquake and a large-scale tsunami on March 11, 2011. All sixunits were constructed via a GE/Hitachi/Toshiba collaboration from 1967-1979.Two planned GE ABWRs due to beginconstruction in 2012 have recently beencancelled. In spite of the immediate shutdown of all units ( D1, U4 was shutdownat the time) based on ground-level accel-

    eration and decay heat cooling for some30-45 minutes, loss of off site powerby ingress of water into the earthquake-proof diesel generators pit, initiated anevent that can be broadly defined asloss-of-heat-sink, classified as a beyond

    design basis accident. Further, alongwith decay heat cooling of the reactorcore, all units faced additional, unantici-pated challenge of decay heat cooling ofspent fuel pool (SFP) situated above thereactor core in proximity of both the coreand containment building. In fact, forU1-4, the spent fuel pool is situated in alightly-structured confinement building.During the initial week, March 11-18,there were up to three larger (likely H2explosion) explosions, vapor/steam jetsand fires that further stressed the RPV,the containment and (weather) confine-ment buildings. One of the later explo-sions conceivably damaged the primary

    (coolant) containment and thus, waterfound in the adjacent basement of theturbine building pointed to high-levelsof radiation including fission products.Additional large volumes of contami-nated water were found in the U-shaped

    electrical conduit trenches off of U1-3and spreading into other areas suchas beneath the reactor site. Technicalspecifications for U1-6, focused onrelevant parameters at the time of (acci-dent) initiation, are given in Table1. Notethat the Table includes details aboutthe lateral acceleration threshold forshutdown and also the number of fuelassemblies in the SFP that contributedto the accident. The common SFP isalso included. Table 2 gives estimatesof the reactor core decay heat versustime after shutdown; thermal decay heatdetermines the cooling needed aftershutdown. A little known site map is

    given in Figure 4, along with before andafter (March 18) images in Figures 1 &2. The site map is particularly handy inconjunction with images by Cryptome(2011). Figures 3 and 5 respectivelyprovide qualitative schematics of the

    Fukushima Dai

    Initial technicallessons learned

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    Nuclear Exchange, May 2011 13

    Mark I plant with unique toroidal sup-pression pool and location of the SFP,and a sectional view, with several overallrelevant dimensions. Notable in Figure5 is the realization that there are flow

    paths of contaminated water from thecontainment into the basement of theturbine building and further into channels(trenches) beyond where it can leak intothe hydrosphere (ocean or water source).These are presented here to facilitateyour own assessment of the timelinesand news releases since March 11,2011. Regularly issued press releasesinclude the following: MEXT, JAIF, IAEA,NRC, TEPCO, Kyodo News Wire, TheJapan Times, Wikipedia and Der Spiegel(all since March 11, 2011). The mostviewed Internet-based news included(all Japanese): NHK-BS2, Fuji NewsNetwork, Tokyo Broadcasting System,

    Nikkei and Asahi News Network.Since event initiation the above newssources have reported a number of(likely) hydrogen explosions, venting ofradioactive steam plumes, and fires.Interestingly enough, the reported

    events leading to global radiation fearsmay have substantiated the authorswork (Tokuhiro); that is, that mass-mediadissemination of nuclear and radiationevents accentuates perception of risk bya factor of at least 1000-times.Under emergency response and (techni-cal) crisis management, the world haswitnessed helicopter drops of water,pumping of seawater into the contain-ment building, spray injection coolingof the SFP using a ~15m(50 foot) armaffixed to a large truck and recent pump-ing of clean water brought to the sitevia a large barge-based tank. Globalmass-media has reported the events

    non-stop, week after week. As a resultof the recovery effort, large volumes ofcontaminated (radioactivity-laden) waterhave collected in various channels andbasins and in early April were releasedinto the ocean. Detectable concentra-tions of radioactivity have been meas-ured in farm and sea products, such asgreen vegetables, beef and fish, withinthe 20km and 30km evacuation zones.Potable water was also contaminatedto varying degrees. Through all this,TEPCO, NISA and the Japanese govern-ment have been the face of the dailyreporting of the situation. It is evidentthat, as the situation persists, the public

    perception of risk and benefit of nuclearenergy is shifting toward the former.

    Lessons learnedFrom a technical perspective, up tothe end of the first week of April 2011,I believe that the global communityof nuclear energy professionals mayacknowledge the provisional list of les-sons learned. At this point, it is clear thatfull technical assessment of the extentof in-core, in-vessel, in-containment dam-age will not be forthcoming for at least5-years. Post-TMI images of the partiallymelted core became available in 1985,6 years after the accident (Wikipedia).

    Cleanup was completed in 1990. So,with multiple units with partial core and/or SFP damage, we can only project thatassessment may be in 2016 (5-years)and cleanup sometime between years,2017-2027. As for the potential impact

    on the global nuclear energy renais-sance, it seems self-evident that therewill be a 1-3 year slowdown; however,anything beyond this is highly specula-tive. Permit me to instead, list some ofthe lessons learned to date. These areclassified broadly into two general cate-gories, Design and Operational, withsecond classifier that is more specific asnoted. So, the lessons are:

    1 Design/R&D. Nuclear R&D institu-tions must consider alternatives tozirconium-based and zircaloy claddingso that the chemical reaction thatgenerates hydrogen is prevented.

    We (as an industry) need to acceler-ate development and deploymentof non-hydrogen producing claddingmaterials; that is, assuming that thecoolant/moderator/reflector remains(light) water. For GCR, water ingressmay initiate a reaction with graphite togenerate CO2 and H2.

    2 Design/R&D. Nuclear R&D institu-tions must consider degradationdynamics of cladded fuel and fuelassemblies in-core and in spent fuelpools. We need a better understand-ing of the potential fuel reconfigura-tion under, zircaloy-water chemicalreaction such that the cladding is lost

    and further, the fuel begins to melt.Unless the reconfigured core or SFPis known, one cannot adequatelyknow the availability of coolant flowpaths other than the fact that theentire spent fuel mass is submergedin coolant. Our predictive capabilityof partially (5% to 95%) melted ordegraded core (or SFP) configuration isinadequate.

    3 Design. It is clear that the spent fuelpool (SFP) cannot be in proximity ofthe reactor core, reactor pressure ves-sel or containment itself. The SFP, inits current form, is essentially an openvolume subcritical assembly that is not

    subject to design requirements gener-ally defining a reactor core. Yet, unlessthermohydraulic cooling is maintained,it is subject to similar consequencesas a reactor core without adequatecooling. Therefore, we need new pas-

    ichi;

    Figure 1 and 2. Aerial photos of the

    Fukushima plants before (A) and after (B) the

    damaging events.

    In (A) Units 1-4 from left to right,

    Units 5, 6 at top right.

    Photos by Cryptome (2011), original photo:

    Air Photo Service Co. Ltd. Japan

    A

    B

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    Nuclear Exchange, May 2011 15

    sive designs of the SFP, away fromthe actual plants reactor core.

    4 Design. (Thus) New standard anddesign requirements are needed forthe SFP. This is especially true forNPPs located in earthquake zones.The SFP should be reclassified as asubcritical assembly with potential forcriticality with no active/passive con-

    trol (rod or soluble poison) mecha-nism. This warrants engineered designof immersed, co-located control rodelements, including DBA and BDBAanalyses. In addition, in order to pre-vent loss-of-coolant, the SFP shouldhave a condensate cover in order tominimize loss-of-coolant due to slosh-ing of the free surface. Lastly, the SFPshould be some distance from thereactor plant.

    5 Design. In future designs of nuclearreactor systems, we need to recon-sider core configurations with lowaspect ratio rather than high aspectratio. Under decay heat natural circula-tion cooling (single-phase or underboiling), one may need to minimizethe coolant volume inventory thatis needed to maintain the core sub-

    merged in liquid coolant. This furtheridentifies a need to select a coolantthat is on-hand; that is, an intendedcoolant if it is a LWR and a second-ary coolant that may suffice for decayheat cooling. This obviously opensup a fundamental question on fluids/materials selection to serve as coolant,moderator and reflector in a nuclearreactor system. The overall maximumvertical height should be limited by the

    height of emergency responder cool-ing equipment such as a crane-basedfirehose or similar. In other words,it is not wise to build a structure sotall that firefighting equipment cannotadequate inject water or similar liquidcoolant.

    6 Design/Site Design Basis. Havingmultiple (reactor) units at one site

    needs critical review in terms of post-accident response and management.We must consider the energeticevents at one unit exacerbating thesituation (safe shutdown) at the other.For nuclear installations, we shouldreview the co-location of facilities witha potential for a criticality accident inone exacerbating access to anotherduring first response. Access is obvi-ously needed for cold shutdown ofthe intact unit, relative to the damagedunit, in a timely manner.

    7 Design/Site Design Basis. There isa need for standby back-up power,via diesel generator and also bat-tery power, at a minimal elevation(100feet/31m) above and remotelylocated. This is needed to offset lossof off-site power for plants subjectto environmental water ingress (fore-most tsunami). Spare battery powershould also be kept off-site and in aconfirmed charged state. There isalso a critical need for standby back-upcooling capability (gas or liquid); thatis, a liquid-to-air or liquid-to-liquid heatexchanger that can be installed in atimely manner if/when the primary/secondary cooling circuits are unavail-able.

    8 Design/Site Design Basis. Fornuclear power plants located in or

    near earthquake zones, we cannotexpect structural volumes and chan-nels to maintain structural integrity.We should also expect the immediateground underneath these structures tobe porous . It may liquify under earth-quake and thus become a path forcontaminants. Thus design of thesevolumes and channels should be suchthat they minimize connections toother (adjacent) volumes from which

    Figure 3. Boiling Water Reactor, Mark-I similar to D1, U1-5. D1, U6 is a Mark-II type.

    (Cf. www.nei.org)

    Locationname, -unit

    BWRtype Startconstruction/criticalityoperations

    PowerMwth/Mwe

    Design,peak gndaccel.(g)

    Reactor supplier/A&E/ Construction:GE-GeneralElectric,

    EB-Ebasoo,KA- Kajima,TO-Toshiba,Hi Hitachi

    Reactor FuelAssemblies,F/As

    SpentFuelPool,

    F/As

    Fueltype

    FreshFuel

    Fukushima 1-1 BWR-3; Mk-I 7/67;10/70; 3/71 1380/ 460 0.18 GE/EB/KA 400 292 LEU 100

    Fukushima 1-2 BWR-4; Mk-I 6/69;5/ 73;7/ 74 2381/784 0.45 GE/EB/KA 548 587 LEU/ 28

    Fukushima 1-3 BWR-4; Mk-I 12/70;9/ 74;3/ 76 2381/784 0.45 TO/TO/KA 548 514 LEU/

    MOX

    52

    Fukushima 1-4 BWR-4; Mk-I 2/73;1/78;10/78 2381/784 0.45 Hl/Hl/KA 0 1331 LEU 204

    Fukushima 1-5 BWR-4; Mk-I 5/ 72;8/77; 4/ 78 2381/784 0.45 TO/TO/KA 548 946 LEU 48

    Fukushima 1-6 BWR-5; Mk-II 10/73; 3/ 79; 10/79 3293/1100 0.45 GE/EB/KA 764 876 LEU 64

    Fukushima 1-7 ABWR Cancelled 04/2011 1380MWe NA Cancelled 04/2011 NA Cancelled

    04/2011

    LEU NA

    Fukushima 1-8 ABWR Cancelled 04/2011 1380MWe NA Cancelled 04/2011 NA Cancelled04/2011

    LEU NA

    Central SFP NA NA NA NA NA 6375

    Table 1. Summary of Fukushima NPP unit selected specifications

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    16 Nuclear Exchange, May 2011

    contaminated (liquid) effluents canflow.

    9 Design/Site Design Basis. Placelarger structures of hydrodynamicdesign toward the ocean and smallerinstallations in the wake regionbehind these larger structures. That is,in the case of Fukushima, the tsunamialways approaches from the ocean. It

    thus makes imminent sense to hydro-dynamically design these larger struc-tures with smaller structures in thewake region of the larger structures.Structures in the wake region willlikely have a higher probability of sus-taining less damage if place directly onocean-side.

    10 Design/Reactor Design Basis. Loss-of-offsite-power and only partial avail-ability of diesel generator-based and/orbattery-based backup power needs tobe analyzed as DBA or BDBA.

    11Design/(Gas-Cooled) Reactor DesignBasis. For loss-of-offsite-power andonly partial availability of generator-based and/or battery-based backuppower, any depressurization DBA orBDBA may require timely injection ofadditional coolant (gas), assuming thatwater is not an option.

    12Operational. If an in-containmentSFP is maintained, then the fuel trans-fer crane system must be designedso that it is available to remove thefuel during a post-accident phase or asecond means such as a robotic armneeds to be available.

    13Operational. We need to identify orreview key valves for emergency corecooling and review requirements thatthey are non-electrically functional.That is, these valves need a secondary

    means of open and closed status thatis remotely located.

    14Operational. Further, there is adefinite need for a backup (shielded)reactor plant and/or criticality possibleinstallation control center that is off-site (remote) so that the accidents canbe managed with partial to full extentof reactor plant or criticalitypossibleinstallation status (P, T, flowrates,valve status, tank fluid levels, radiationlevels). Sensor will need to be hard-ened to withstand energetic events.

    15Operational. There needs to be a vol-umetric guidance analysis for ultimate(decay heat) cooling contingency plansso that not only limitations on volumeare understood but also transfer of

    liquids from one volume to another.Spare tanks and water-filled tanks

    32) diesel generator building

    33) (AT-radioactive)waste storage building

    34) Radioactive waste(collection) transfer(removal) vessel inlet/

    outlet building(same as 3 but for U5,6)

    35) Exhaust stack (for U5,6)

    36) ( for U5,6) ultra high tensionopen/close place (likely the

    switching station for highvoltage). Same as 16

    37) storage building

    32

    33

    34

    35

    36

    37

    28) Spent fuel (transport or movement) storage equipment building29) Water inlet

    30) Water out ( for U5, 6)

    31) breakwater

    30

    31

    29 28

    25) (AT-sea) water intake26) Pure water tank

    27) Water processing room

    27 26

    25

    1 2 345

    1) Turbine building, ventilation system stack2) Incinerator building (AT-I think for materials that

    can be burned)

    3) (contaminated/radioactive) Waste transfer(removal) vessel inlet/outlet building

    4) (contaminated/radioactive) Waste collection(concentration) processing (disposal) building

    5) Water outlet for U1-4

    6) Site bunker building7) Suppression pool water surge

    (tank)8) coarse solid (contaminated/

    radioactive) waste processing

    building9) (operations assist, common) facility

    building. (likely the subcontractorsengineering building)

    10) (contaminated/radioactive) wasteX building

    11) (AT - for U3, 4) ultra high tensionopen/close place (AT-likely the

    busbar for high voltage). Same as 1612) Exhaust stack

    13) activated noble gas holdup

    equipment building14) Fukushima nuclear power

    technical training center15) Service hall ( visitors center)

    7

    8

    910

    6

    14

    13

    12

    11

    15

    16) (for U1,2) ultra high tension open/closeplace (switching station for for high voltage).

    Same as 11

    17) weather monitoring station/instrumentation)18) Settling (precipitation) basin

    19) Exhaust stack20) Filtration water tank

    21) Solid waste storage

    22) Business office building23) improvement construction office place24) Solid waste storage

    18

    17

    19

    21

    20

    16

    24

    2223

    Reactor Type BWR-3 BWR-4

    Thermal (MW) 1380(100%FP) 2381(100%FP)

    Power After Shutdown

    Time (seconds) Time (s,m,h,d,y) MW (thermal) % FP MW (thermal)

    1.00E-01 0.1 s 139.33 10.0961 240.39

    1.00E+00 1 s 86.86 6.2943 149.87

    1.00E+01 10 s 53.76 3.8956 92.75

    1.00E+02 100 s 32.87 2.3820 56.72

    1.00E+03 16.7 m 19.69 1.4271 33.98

    1.00E+04 2.8 h 11.38 0.8245 19.63

    1.00E+05 28 h 6.13 0.4445 10.58

    1.00E+06 11.6 d 2.84 0.2057 4.90

    1.00E+07 116 d 0.89 0.0641 1.53

    1.00E+08 3.2 y 0.12 0.0087 0.21

    1.00E+09 31.7 y 0.01 0.0006 0.02

    Table 2. Thermal power output of reactor core fuel after shutdown for selected times.

    Figure 4. Fukushima nuclear power station site map (original in Japanese)

    need to be kept on site as uptaketanks for runoff in case of addition ofcoolant during accident managementphases. Additiional means to produceboric acid need to be available off-site.Earthquake-proof diesel generatorhousing also needs to be water-proof.Remote diesel generators are alsoneeded with access to equally remotediesel fuel tanks (also see6).

    16Operational. Under emergency andcrisis management, wider accessroads are needed to and from NPPs.The access roads need to be clear ofdebris and of such width to accom-modate large-scale trucks needed as

    first response and during the recoveryphase. A way to access the plant via

    LEGEND:

    The buildings below are marked but NOT NUMBERED but on diagram

    T/B = turbine building; R/B = reactor building

    RWB = reactor waste building;CB = control room buildingU1, U2, U3, U4, U5, U6 correspond to Units 1-6

    SCALE is approximately as follows: 500m = 1.843inches

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    Nuclear Exchange, May 2011 17

    water calls for infrastructure (boats,water-containing barges, jet-skis etc)and is needed as part of a contingencyplan for those plants located near bod-ies of water.

    17Operational/Standardization. Color-coded major components so that incase of an accident such as that atFukushima NPP station, we will be

    able to quickly identify the major com-ponents from digital images. Nuclearplant site maps, such as Figure 1,should be downloadable or availableupon request in order to be used inaccident management.

    18Operational/International. An inter-national alliance of nuclear reactoraccident first responders and beyondfirst response, a crisis managementteam are needed. No such internation-al alliance is available at this time. Theglobal nuclear industry cannot wait 3weeks for international participation.

    19Operational/International. Weshould consider and work toward aninternational agreement on standardsfor regulated levels of radiation activityand exposure to the general public andseparately, to those workers respond-ing under emergency and extendedrecovery phases. We should also beconsistent in definition and practice ofimplementing evacuation zoning. Weshould also strongly encourage accept-ance and use of SI units for radiationactivity and exposure (consistent useof Sv/hr or mSv/hr)

    20Operational/International. In orderto promote and actively practice a trueGlobal Nuclear Energy Partnership(GNEP), I hereby propose that wedesignate the Fukushima NPP siteand its reactors as an internationalnuclear post-accident managementand decommissioning center (INPMD).The INPMD should be sanctioned bythe Japanese government and led bya new Japan-based commission withinternational lead members (for exam-ple, IAEA, U.S., France, Korea, China).The commission should however

    establish a consortium of membernations. The official language shouldbe English. The level of participationshould correspond to commitment ofequipment and experts. The lessonslearned and data generated in therecovery phases should be open toall. Each consortium member nationsshould also have risk communicationrepresentatives.

    References available on request.To contact the authors email:[email protected]

    Figure 5. Representative cross-sectional perspective of D1 Unit (not all details are shown)

    Reactor Building

    Turbine Building

    Central Control Room

    Pressure suppression pool

    Spent Fuel Pool

    Condenser

    Feed Water Tank

    Reactor Vessel

    Turbine generator

    Exhaust Tower

    Reactor