Newly released documents expose Japanese Nuclear Safety Organization feared aftershocks at nuclear plant could damage spent fuel rods

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Japan feared three months after the Fukushima nuclear power plant was hit by a tsunami that aftershocks could further damage one of its fuel storage pools, causing rods inside to melt and spew radiation within hours, according to a newly released document.

Despite publicly reassuring that there was no danger to those living around the plant, secretly the experts at NISA were struggling to predict what could happen in the dangerously overstocked fuel ponds.

The agency was working on the calculations just as TEPCO was saying the nuclear fuel in three reactors at the plant was “slightly damaged.”

There are fuel ponds near the top of all six reactor buildings at the plant, adjacent to the top of each reactor so that the fuel can be unloaded under water, when the top is off the reactor pressure vessel and it is flooded. The ponds hold some fresh fuel and some used fuel, pending its transfer to the central used/spent fuel storage on site. (There is some dry storage on site to extend the plant’s capacity.)

Two of the reactor unit ponds (2 & 4) were unusually full even before unit 4 core was unloaded in November, since there was little spare space (only for 465 assemblies) in the central fuel storage pond on site.  Thus there was a lot more fuel in the reactor ponds with correspondingly high heat loads and cooling requirements than might have been the case.

At the time of the accident, in addition to a large number of used fuel assemblies (783 used out of 1331 total), unit 4’s pond also held a full core load of 548 fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November.


The temperature of these ponds is normally low, around 30°C when circulation is maintained with the Fuel Pool Circulation and Clean-up (FCP) system, but they are designed to be safe at about 85°C in the absence of pumped circulation (and presumably with moderate fuel load). They are about 12 metres deep, so the fuel is normally covered by 7 metres of water.

Unit 2, 3 & 4 ponds are about 12 x 10 metres, with 1240, 1220 and 1590 assemblies capacity respectively (unit 1 is about 12 x 7 m, 900 assemblies).

Unit 4 pond contains a total 1331 used assemblies (783 plus full fuel load of 548), giving it a heat load of about 3 MW thermal, according to France’s IRSN, which in that case could lead to 115 cubic metres of water boiling off per day, or about one tenth of its volume.

Unit 3’s pool contains 514 fuel assemblies, unit 1 has 292 and unit 2 has 587, giving it a heat load of 1 MW.

NISA Report Continued

The report shows that the pool remained vulnerable for nearly four months until its operator completed reinforcement work in July. Tokyo Electric Power Co had said before then that the building could withstand major aftershocks without reinforcement, but made repairs after acknowledging structural damage and water leaks from the pool area.

The Japan Nuclear Energy Safety Organization said it carried out a simulation that showed some 1,500 mostly used fuel rods at the plant’s No. 4 reactor building could start breaking in two hours if aftershocks further damaged the pool and caused cooling water to escape. The fuel rods could start melting within eight hours, the organization said in a report dated June 30 and published Friday.

In the report, the government-funded JNES said a loss of pool water due to additional cracks from aftershocks could cause the fuel rods to overheat. Their casings could break and start spewing radiation in about 2 hours. Fuel pellets inside each rod could start melting within 7.7 hours at about 2,800 Celsius (5,000 Fahrenheit), it said.

Accident sequence following earthquake

Unit 1 Unit 2 Unit 3
Loss of AC power + 51 min + 51 min + 51 min
Loss of cooling + 1 hour + 70 hours + 36 hours
Water level down to top of fuel + 3 hours + 74 hours + 37 hours
Core damage starts + 5 hours + 87 hours + 62 hours
Fire pumps with fresh water + 15 hours + 42 hours
Hydrogen explosion + 24 hours
service floor
+ 87 hours
suppression chamber
+ 68 hours
service floor
Fire pumps with seawater + 28 hours + 78 hours + 46 hours
Off-site electrical supply + 10 days
Fresh water cooling + 12-13 days

The simulation was based on a scenario that cooling water was lost in the Unit 4 spent fuel storage pool, located on the top floor of the building. The Unit 4 pool was considered high risk as it contained more fuel than the other three pools, as it also stored fuel rods that had been moved from the unit’s reactor core, which was being fitted with new parts.

NISA also concerned about meltdown and ‘China Syndrome’

The Nuclear and Industrial Safety Agency secretly calculated the possibility of a worst-case meltdown at Tokyo Electric Power Co.’s Fukushima No. 1 power plant, it has been learned.

The trial calculations were made under the premise that the nuclear fuel at the plant’s No. 1 to No. 3 reactors would melt down entirely, developing into a so-called China syndrome, the worst-possible scenario.

The trial calculations were carried out first on March 25, two weeks after the March 11 accident, followed by further calculations on April 6, 7 and 13.

On April 18 the agency acknowledged the reactors’ fuel had begun to melt down, but TEPCO failed to acknowledge the possibility of a meltdown until April 20.

When the JNES calculations were being conducted, both the agency–the nuclear safety watchdog of the Economy, Trade and Industry Ministry–and TEPCO publicly said the nuclear fuel in the reactors was “slightly damaged.”

The calculations indicated that if cooling water could not be injected, erosion could continue for more than 10 days, badly damaging the three-meter thick concrete walls of the Nos. 2 and 3 reactors’ containment vessels, the JNES said.

The erosion of the bottom of the No. 1 reactor’s containment vessel bottom would possibly stop after the vessel wall was eroded to a depth of 1.8 meters in eight days, according to the calculations.

It said the calculation results were conveyed to the agency, but it is unknown whether the information was shared with the Cabinet Office or other government organizations.

Source:, via Nuclear News | What The Physics?

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