Fukushima disaster: filling spent fuel ponds

Paul Boughton
The focus at the stricken Fukushima Daiichi nuclear power plant has turned to refilling with water spent fuel ponds in a bid to prevent dangerous radioactive releases, says IHS Energy Asia Pacific analyst Tom Grieder

The last reported temperature level in the spent fuel pond at Unit 4 was 84 degrees Celsius, although increases in temperature up to 1,000 degrees Celsius could cause the fuel rods' zirconium cladding to catch fire, leading to large, unchecked releases of radioactive isotopes. In view of these dangers, an expansion of the evacuation zone imposed by the Japanese government would be a prudent move.

On the sixth day of Japan's nuclear nightmare, the focus has turned to refilling spent fuel ponds at Units 4, 5, and 6 to prevent dangerous radiation releases. A fire was already reported in the spent fuel pond area at Unit 4 on 15 March, which is believed to have been the primary cause of increasing radiation levels recorded around the site—which reached 400 millisieverts per hour at one point. Unlike the reactors, which are better protected against radioactive releases by primary containment vessels, these spent fuel ponds are comparatively vulnerable.

In a status report from 9.00 am today, the Japan Atomic Industry Forum (JAIF) has conceded that in Unit 4 water levels are low and that damage to fuel rods is already suspected. The extent of damage to the spent fuel may be affected by how long the fire burned for on 15 March. Based on reports from the Nuclear Industrial Safety Agency (NISA) the fire was reported at 9:38 am and 'extinguished spontaneously' at 11.00 am on Tuesday (15 March). However, a fire was reported earlier on the same day at 5.45 am, but could not be confirmed. This suggests that the fire may have been burning at a low level for a number of preceding hours before it was confirmed.

Another factor which will determine the threat of radiation releases is the density of the fuel racks in the pond. There has been a shift in the nuclear industry over preceding decades towards using high-density racks in a closed structure in spent fuel ponds, which is partially the result of a lack of spent fuel storage space. High-density racks mean that more radioactive material is stored in the pool, and so the potential dangers from releases of material are higher. If the fuel in the ponds has been discharged from the reactor relatively recently, then the risk of a fire is higher. According to the Institute of Natural Resources and Security Studies, fuel discharged for one month can ignite in less than two hours after water is lost from a pool, while fuel discharged for three months from the reactor may ignite in three hours.

There are a number of hurdles workers will face in filling up the spent fuel ponds. Given that fire and fuel rod exposure have already occurred, and given the lack of containment vessels, radiation levels will probably be among the highest in Fukushima Daiichi in the spent fuel area of Unit 4. These radiation levels will impede cooling efforts, as workers will be unable to stay near the ponds without suffering high and health damaging radiation doses. The strategy of using a helicopter to spray the spent fuel pond at Unit 4 is perhaps an acknowledgement of this radiation risk. However, the idea of spraying water onto a spent fuel pond would not be feasible if the zirconium cladding around the spent fuel rods was to ignite. Instead of helping to fill the pool, the water would then feed a reaction which would generate flammable hydrogen.

Furthermore, as in most nuclear power plants (NPPs) with boiling water reactors (BWRs), the spent fuel ponds are located high up in the reactor building—which raises accessibility issues. According to the latest JAIF status report, Unit 4 has sustained severe damage to reactor building integrity. This appears to be confirmed by recent images taken by satellites. World Nuclear News reports the damage appears to have been the result of a build-up of hydrogen in the spent fuel pond, which suggests the area around the pool itself would be most badly affected. However, the blast at Unit 3 on 13 March may also have contributed to the damage at Unit 4, suggesting other parts of the building may also be affected. Damage to Unit 4 could potentially cause debris to fall into the spent fuel pond, distorting rack structures, which would block air flows and potentially lead to temperature increases and ignition. There is also a more remote danger that the fuel ponds themselves might crack, which would severely undermine refilling efforts. However, cracking the spent fuel ponds would probably take a large hydrogen explosion, as they are usually made from reinforced concrete walls four to five feet thick and sometimes have stainless steel liners.

Outlook and implications

To refill the ponds, water might have to be pumped from the ground—a complex operation to arrange in view of radiation levels in the unit, facility damage, and no onsite electricity for water pumps. The fires so far appear to have occurred as a result of hydrogen build-up. The latest available data from the facility suggests the spent fuel pond at Unit 4 was at 84˚C. If the temperature of the spent fuel pond increases to around 1,000˚C as a result of exposure to air, the zirconium cladding around the spent fuel might catch fire. This scenario would mean large radiation releases would severely hit workers. The fire would be extremely hard to extinguish and would risk spreading to adjoining units, causing hydrogen explosions. The Nuclear Regulatory Commission (NRC) estimates that such a fire would lead to large releases of the dangerous cesium 137 isotope, which is very radioactive. Such releases, combined with an unfortunate change in wind direction, could blow the material towards densely populated areas. To protect against this risk, the Japanese government should immediately extend the evacuation area around Fukushima Daiichi.

The problems at the spent fuel ponds, however, should not detract from the ongoing struggle to cool reactors at Units 1–3. These reactors remain in much the same condition as yesterday, according to JAIF status reports. However, seawater pumping appears unable to significantly lower the reactor core temperatures, possibly because not all the water is flowing into the reactor cores. Media sources suggest the Tokyo Electric Power Co. (TEPCO) is working to restore electricity transmission lines, which could improve the reliability of power supplies needed for water pumps, reducing interruptions as batteries run flat. However, ongoing containment venting continues to raise risks of hydrogen explosions, which could prompt radiation releases given already damaged containment structures at Unit 2 and Unit 3 and damaged core integrity.

The recent manpower increases at Fukushima are a positive sign, and they were desperately needed to prevent a "cascading scenario"—where problems at some reactors lead to neglect or side impacts elsewhere, causing an escalation of the crisis. However, efforts to dump sea water by helicopter on Unit 3 are a desperate step and unlikely to provide the precision injection needed to cool the reactor. Running multiple pumping lines to the reactor cores might be more effective both in cooling reactors and in reducing interruption times when batteries go flat. Pursuing this strategy could involve further manpower at the site, putting more TEPCO employees in mortal danger.

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