Fukushima watch 2011-04-22

Tepco is talking about plans to cool units 1 and 3 of Fukushima 1 via the containment vessel. The water level inside the usually dry portions of the containment (called the dry well, D/W) will be raised high enough to flood the exterior of the reactor pressure vessel (RPV) up to the top of the fuel rods. Normally only the inside of the suppression chamber (S/C, a circular pipe running around the bottom of the RPV) is filled with water. Tepco thinks the reinforced concrete of the dry well is sturdy enough to take the weight of the water and is now seeking the approval of the nuclear regulators.

The US NRC has previously voiced concerns about the safety of the containment in earthquakes when filled with large amounts of water:

When flooding containment, consider the implications of water weight on seismic capabilities of containment.

Unit 2 is not included in the plan because its suppression chamber appears to have suffered damage in an explosion on March 15 and has probably been leaking since then. Most of the highly radioactive water in a trench under the turbine hall is assumed to have leaked there from unit 2. It is not clear if or how those leaks can be plugged.

Tepco has requested French nuclear company AREVA (which is also its supplier for nuclear fuel) to set up a plant for decontaminating radioactive water at the reactor site. This would make it less risky to move the water to the nuclear waste plant in Rokkasho village, Aomori prefecture or for recycling it as cooling water.

Yesterday I experienced my first earthquake since returning to Tokyo. It was a M6.1 quake centered under Chiba, some 60 km from Tokyo. During the night I was woken up by a M5.6 quake that hit Fukushima.

UPDATE 1: Tepco will be short of 8500 MW this summer

Tepco’s maximum supply capacity today is 38.4 GW (one Gigawatt is 1,000 Megawatt or 1,000,000 Kilowatt). The forecast maximum demand today is 34 GW. Hence there will be no rolling blackouts today. For comparison, last year’s peak demand around this time was about 44 GW.

Tepco expects power demand to stay moderate in the near future, due to mild temperatures and the Golden Week holidays in May, rising to about 38 GW by the end of May. By then it hopes to have restored enough supply capacity for a celing of 39 to 42 GW. Tepco expects to have 46.5 GW online this summer versus a demand peak of 55 GW. Last year’s exceptionally hot summer even saw a peak load of 59.9 GW.

Tepco’s customers will largely have to give up on air-conditioning this summer or periodically lose all electric power. Perhaps after Cool Biz it is time to go one better and recommend Bermuda shorts.

UPDATE 2: Rising pressure in unit

Pressure in the reactor pressure vessel (RPV) of Fukushima 1 unit 1 has gradually increasing since March 24, when it was below 0.5 MPa (about 5 bar). On April 19 according to a NISA report it was at 1.141 MPa (about 11 bar) according to sensor B, while sensor A indicates 0.524 MPa. It is not clear how much the nitrogen being injected is responsible for the rise in pressure and how much may be due to a buildup of hydrogen, but the pressure buildup started two weeks before the nitrogen injection, which was started in response to the rising pressure.

NISA status reports:

Update 3: Spent fuel pools comparison – unit 2 and unit 4

A week ago I wrote here about the results of radioisotope testing on water in the spent fuel pool of unit 4 of Fukushima 1.

There results for the unit 4 spent fuel pool were:

  • Caesium 137: 93,000 Bq/l (half life: 30 years)
  • Caesium 134: 88,000 Bq/l (half life: 2 years)
  • Iodine 131: 220,000 Bq/l (half life: 8 days)

For comparison, here are the figures from an analysis of water in the skimmer surge tank of unit 2, taken on April 16. The skimmer surge tank is an overflow container adjacent to the spent fuel pool into which water can splash or overflow from the main pool. Tepco tested it to get ideas for installing a heat exchanger and decontamination unit for the pool.

Here are the unit 2 skimmer surge tank figures:

  • Caesium 137: 150,000,000 Bq/l (half life: 30 years)
  • Caesium 134: 160,000,000 Bq/l (half life: 2 years)
  • Caesium 136: 4,000,000 Bq/l (half life: 13 days)
  • Iodine 131: 4,100,000 Bq/l (half life: 8 days)

The small container of sampled water radiated at 3.5 mSv/h at the surface, which is roughly 100,000 times the natural background radiation level.

The caesium radioactivity of the skimmer surge tank water in unit 2 is about 1000 times higher than water in the spent fuel pool of unit 4. However the shortlived iodine is only about 20 times more active. This could suggest the radioactivity is from damaged fuel rods in the spent fuel pool and not the adjacent damaged unit 2 reactor core. Spent fuel was unloaded into the unit 4 spent fuel pool more recently than into the unit 2 spent fuel pool, hence it will have a higher Iodine-131 to caesium ratio. The higher caesium activity in unit 2 suggests more damage to fuel rods, but in the case of iodine this is partly counterbalanced by a longer nuclear decay since the last unload from the core into the pool compared to unit 4.

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