Tokyo Electric Power Company (Tepco) has warned:
The possibility of re-criticality is not zero.
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Confusing, conflicting and disturbing reports continue coming concerning the situation at the spent fuel pool at the Fukushima Daiichi reactor 4. A photograph published by the BBC shows that reactor 4 appears to be in far worse shape than reports had indicated. Moreover, the spent fuel pond at reactor 3 may also be having severe problems according to streaming reports on NHK news.
The building housing reactor 4 (left) appears severely damaged - despite the reactor itself being shut down
All of the fuel rods had been removed from reactor 4, which had been shut down for months for maintenance, yet the reactor building is badly damaged.
Reactors normally cool down rapidly within days of shut down as very short lived radioisotopes decay away. So how could reactors, shut down for months, be heating up, possibly causing fires and hydrogen explosions?
Cooler, older spent fuel rods in the fuel pool adjacent to the reactor are packed at a density designed for lower levels of heat production to save on costs and space. The water in spent fuel pools is treated with borate to adsorb neutrons and poison nuclear reactions that could cause an accidental, unintended criticality event in the spent fuel pool. Because untreated water acts as a moderator, (which slows neutrons, increasing the probability of induced fission) re-criticality is a theoretical possibility if borated water is lost and replaced with boron free water.
BBC reports:>Whatever the reason, the company admitted on Tuesday that water was below its normal level - and Japanese news sources reported that it was boiling. This is consistent with the idea that as in buildings 1, 2 and 3, hydrogen was released from a reaction between the zirconium fuel-rod cladding and steam, resulting in an explosion.
Except that in reactor 4 building, the rods were in the fuel pond, not in a reactor.
The government then ordered Tepco to put water back in the pool.
But either because of high radiation levels or broken pumps or some other reason, they could not - hence the (ed - aborted because it was unsafe) decision to drop water in from a helicopter, a technique that is used to fight forest fires.
The BBC report continues, explaining the TEPCO could be between a rock and a hard place if cooling water has boiled off of reactor 4's spent fuel pool. Because all of the fuel rods were offloaded into the pool and because the pool likely had more spent fuel from previous years, the fuel rods may be packed tightly enough to generate high heat levels when the cooling water is lost. Without the water, they may suffer a fire when the zircalloy cladding reaches over 2000 degrees F and spontaneously reacts with oxygen in the air. Water needs to be put back in the pool to stop the threat of fire. However, the large number of rods may be capable of going critical if boron free water is put into the pool to replace the lost cooling water.
The blog of the MIT Department of Nuclear Engineering students and faculty presents the more optimistic TEPCO report plus MIT's students' expert analysis:
Units 4-6: Flames at Unit 4 were reported to be the result of a pump fire, which caused a small explosion that damaged the roof of Unit 4 (See TEPCO’s press release on the most recent fire at http://www.tepco.co.jp/...) . Efforts at Units 4-6 are focused on supplying cooling water to the spent fuel storage pools. Temperatures in these pools began to rise in the days after the quake. At the time of the quake, only Unit 4’s core had been fully offloaded to the spent fuel pool for maintenance; roughly 1/3 of the cores of Units 5 and 6 had been offloaded. This explains in part why the temperature in Unit 4’s pool has risen faster than at the other reactors: it has a higher inventory, both in fuel volume and in heat load.
Outlook: The fuel within these pools needs to remain covered with cooling water in order to prevent the low levels of decay heat present from causing it to melt, and also in order to provide shielding. Boiling of the water results in reduction of the water level in the pools, so if/when the pools get hot enough for boiling to begin, water needs to be added to replace what boils off. The staff of Unit 4 plan to begin pumping water to the spent fuel pool from ground level as soon as radiation levels from Unit 3 are low enough for them to return. This pumping operation should be relatively easier than injection of cooling water into the reactor vessels at Units 1-3 because the pools are at atmospheric pressure.
Update from the comments, Lawrence Lewis, RE: the Guardian's live blog.
More on the comments on the Fukushima nuclear plant by Greg Jaczko, chairman of the US Nuclear Regulatory Commission, to a congressional committee in Washington:
We believe at this point that Unit 4 may have lost a significant inventory, if not lost all, of its water.... There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.
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Jaczko also said there was the possibility of a leak in the spent fuel pool in reactor No 3, "which could lead to a loss of water in that pool", as well as a falling water level in the spent fuel level at the No 2 reactor.
According to Reuters, Jaczko said radiation levels around the site could give emergency workers "lethal doses" of radiation, forcing them to stay away:
"We believe that around the reactor site there are high levels of radiation," Jaczko said. "It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."
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"No Data" for reactor 4 on the 16th is a bad sign. Something has apparently failed.
Cooling pond temperatures
As reported by the International Atomic Energy Agency:
Unit 4
14 March, 10:08 UTC: 84 ˚C
15 March, 10:00 UTC: 84 ˚C
16 March, 05:00 UTC: no data
Unit 5
14 March, 10:08 UTC: 59.7 ˚C
15 March, 10:00 UTC: 60.4 ˚C
16 March, 05:00 UTC: 62.7 ˚C
Unit 6
14 March, 10:08 UTC: 58.0 ˚C
15 March, 10:00 UTC: 58.5 ˚C
16 March, 05:00 UTC: 60.0 ˚C
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This MIT figure shows how decay heat from the recently shut down reactors drops rapidly over the first week as very short lived isotopes decay.
However, because there is a large inventory of radioactivity with half lives from 1 to 30 years, including the hard gamma emitting isotopes Co-60 and Cs-137, spent fuel packed into the spent fuel pool will not cool down as quickly. Note that heat production drops very slowly after 3 months of cooling. Another important factor after 3 months is that some of the very short lived radionuclides that decayed away in the first 3 months are neutron adsorbers which tend to poison nuclear reactions. Once those isotopes have decayed away it is easier to achieve re-criticality.
This table by the MIT nuclear engineering students and staff shows how the decline in decay heat levels off after the first 3 months.
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Tabulation of approximate decay heat for the Fukushima reactors from 1 second after the scram caused by the earthquake until 1 year after the event.
| Date/Time (Fukushima Time) |
Fukushima Daiichi-1 Decay Heat (MW) |
Fukushima Daiichi-2 & 3 Decay Heat (MW) |
Percent of Full Reactor Power |
| 3/11/11 2:46 PM |
92.0 |
156.8 |
6.60% |
| 3/11/11 2:47 PM |
44.7 |
76.2 |
3.21% |
| 3/11/11 2:48 PM |
36.9 |
62.8 |
2.64% |
| 3/11/11 2:50 PM |
31.4 |
53.5 |
2.25% |
| 3/11/11 3:00 PM |
24.1 |
41.0 |
1.73% |
| 3/11/11 3:30 PM |
19.1 |
32.5 |
1.37% |
| 3/11/11 8:00 PM |
12.8 |
21.9 |
0.92% |
| 3/12/11 8:00 AM |
10.1 |
17.3 |
0.73% |
| 3/12/11 8:00 PM |
9.1 |
15.5 |
0.65% |
| 3/13/11 |
8.5 |
14.5 |
0.61% |
| 3/14/11 |
7.8 |
13.2 |
0.56% |
| 3/16/11 |
6.9 |
11.8 |
0.50% |
| 3/20/11 |
6.1 |
10.5 |
0.44% |
| 4/1/11 |
5.2 |
8.8 |
0.37% |
| 7/1/11 |
3.7 |
6.3 |
0.26% |
| 10/1/11 |
3.3 |
5.6 |
0.23% |
| 3/11/12 |
2.9 |
5.0 |
0.21% |
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The heroic and brave nuclear workers are challenged with the task of refilling the spent fuel pools with borated water. Sea water or fresh water may not be adequate.
NHK is reporting that electric workers are rushing to restore power lines to the Fukishima nuclear plants so that they will have a stable source of power to restore cooling systems at the crippled power plants.
