2011-03-22 – NRC – Total Core-Melt of Fukushima Daiichi Unit 1 on March 11 or 12, 2011


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Miroslav Gregoric
From: GREGORIC, Miroslav
Sent: Tuesday,22 March 2011 14:42
Cc: FLORY, Denis; ANDREW, Graham; NILSSON, Anita Birgitta; LYONS,
James E.; SUZUKI, Satoshi; MRABIT, Khammar; YLLERA, Javier; LIPAR,
Miroslav; CARUSO, Gustavo; HAHN, Pil-Soo; CZARWINSKI, Renate; VINCZE,
Pal; BUGLOVA, Elena; MARTINCIC, Rafael; Kryuchenkov,Vladimir
(V.Kryucbenkov(aiaea.org); COLGAN, Peter; COLGAN, Tony; DUSIC, Milorad;
WINTER, Denis Jacques
Subject: Core melt at Fukushima Unit 1 from 11 to 12 March 2011 JST
***NOT for distribution***
Importance: High
Dear colleagues
Please find attached calculations for Fukusbima Daiichi Unit 1 core
melt from basic principles. Of course with your input the
calculations could be improved.
Best regards
DK 1888 of 1892
Total Core-melt of Fukushima Daiichi Unit 1 on 11 or 12 March 2011
Basic hand held calculations by Miroslav Gregoric, checked by Vladimir Kryuchenkov on 22 March
2011. (Note: The excel sheet is attached. Of course the modelling by MELCOR or other severe
accident codes will give better results, but one cannot go against basic heat equations.)
Basic assumptions based on known reported data from TEPCO and NISA
1) At the earthquake, reactor scrammed from 1380 MWth on I I March at 14:46 SJT, and after station
blackout, main steam isolation valves closed. Reactor was cooled by injecting condensate water to
Reactor Pressure Vessel via diesel operated pump (via steam turbine – not confirmed), and by
releasing steam to containment suppression pool -wetwell. This went on almost an hour, when at
15:42 tsunami flooded both diesels and many electrical distributing equipment and washed away or
damaged condensate storage tanks. Yet NISA reported that water injection continued for almost
additional hour until 16:36 when water injection failed. By that time 198000 MJ of the residual heat
was generated. In order to cool the core also the accumulated heat in the core needed to be taken
away, but that was small compared to decay heat. Assumption is that all this heat was successfully
discharged to the wetwell, acting as the only hit sink, where the temperature and pressure increased.
At least 79 tons of condensate water was needed to be injected and boil off to take the heat away.
No measured pressures are available for this period.
2) After loss of water injection on 11 March at 16:36 there was no water flow to reactor for almost
28 hours, up to 12 March at 20:20 when sea water injection was established via fire pumps to
reactor. During this time 1000000 MJ (one million Mega Joule) of the residual heat was generated. In
order to cool the core at least 412 tons of water should be boiled off in the core, but this was not
available. The core dried and overheated. If average heat capacity of the core is 0.3 ki/kg/degC, and
if fuel in the core and core internals mass is 140 tons then it takes only about 42 MJ to heat the core
for one degree Celsius. To melt the core it should be heated first to the melting point(s) and then the
melting (phase transition) will consume additional 260 kJ/kg or 62000 MJ in total. The residual heat
generated in this period is much higher (ten times or more) than needed for heating up to melting
points and for melting. The available heat could heat up the core far above the melting points. The
only cooling during this time was heat irradiation to the reactor pressure vessel from the outer layers
of fuel elements.
On the 12 March at 0:49 (or 8 hr 13 minutes after loss of water injection) un unusual increase of PCV
pressure was detected (drywell). At that point the residual heat generated after loss of water
injection was 390000 MJ, which would need additional 156 tons of water to boil off, which was not
available and the core heated up above melting point. Before core melting Zirconium in the fuel
cladding starts oxidising and adding chemical reaction heat. This added additional heat and also a lot
of hydrogen, causing sudden increase of pressure in reactor pressure vessel, discharging hydrogen
through the relieve valves to the wetwell. We can assume that once the Zirconium started to oxidise,
very soon all fuel rods have broken to release all noble gasses and volatiles like Iodine and Cesium
into the reactor. Some of the iodine and Cesium could be trapped in the wetwell water, but not the
noble gases.
All of the above points to a conclusion that a substantial core melt in reactor unit 1 has
happened starting in the night from 11 to 12 March and going on up to the start of injection of
sea water on 12 Mach at 20:20. It is possible that the vessel has melted through already before
increase in PCV pressure on 12 March at 0:49 hours, 8 hr 13 minutes of no cooling, and molten
core has penetrated the drywell as no water was there.
DK 1889 of 1892
Page 2
3. Venting of the containment started on 12 March at 14:30, releasing mixture of water vapour,
hydrogen, most of noble gases in the core, Iodine, Cesium and all other volatile radionuclides. Release
point was not given, stack release was probably not successful as in less an hour later, at 15:36 a huge
hydrogen explosion blasted the top of reactor building I sideways and upwards. The explosion must
have damaged the operating floor where spent fuel pool is located, with the crane for spent fuel is
located (and maybe the crane for the reactor vessel).
The wind was on 11 and 12 March blowing to the Pacific during the containment venting and
explosion, so that all noble gases and volatile radionuclides of the first release were going towards
ocean. However sharp peaks should be observed on the monitoring stations inland, 3 km to the west,
mainly reading the cloud shine (to be checked with actual data).
DK 1890 of 1892