Fukushima: Background on Fuel Ponds
(updated September 2012)
Used fuel needs to be cooled and shielded. This is initially by water, in ponds. After about three years under water, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained.
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.)
The intention is to ship used fuel from the plant periodically for recycling. Tepco and JAPC are building a Recyclable Fuel Storage Centre in Mutsu, due to operate from mid 2012 with 5000 t capacity. The JPY 100 billion facility will provide interim storage for up to 50 years before used fuel is reprocessed at Rokkasho. NISA approved this in August 2010. Until the Mutsu storage is finished and operational in 2012 there has been a build-up of used fuel at reactor sites. The Rokkasho plant has been much delayed, and is now expected in commercial operation in October 2012. There is some storage capacity there, though this may be full.
At the time of the accident, in addition to a large number of used fuel assemblies, 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. Other estimates put the heat load at 2 MW. 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. There is no MOX fuel in any of the ponds. Unit 4 pond also has 204 fresh fuel assemblies which were ready for loading. In 2012 some of these were removed and checked, and found to be undamaged.
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.
Moving the used fuel from reactor ponds to central storage involves loading it under water into casks which are lowered down and trucked the short distance (see RH side of cutaway diagram above). It requires access from the service floor and the use of cranes which were damaged in the hydrogen explosions.
The central fuel storage on site near unit 4 has a pond about 12 x 29 metres, 11 m deep, with capacity of 3828 m3 and able to hold 6840 fuel assemblies. In March 2011 it held 6375 assemblies, and was not damaged in the accident. Its building is about 55 x 73 m. Due to the fuel here being older, it has very low decay heat. As well as this pond, there are 408 used fuel assemblies in dry cask storage - utilized since 1995 for used fuel no longer needing much cooling.
See also comments in Reactor Background regarding oxidation of zirconium cladding.
A further concern raised during the accident was regarding criticality in the spent fuel ponds. Studies of safety and security of spent fuel storage have noted this possibility but not analysed it, pointing out that no previous criticality accidents have resulted in significant radioactive releases outside the plants, since the criticality itself immediately disperses the source material.