The Future Of Fukushima Daiichi Spent Fuel

This little talked about set of challenges will play a considerable role in the decommissioning and safety of Fukushima Daiichi. We have collected up all of the bits of information about the spent fuel and the handling options to gain a better understanding of the challenge.

Spent Fuel:

Each fuel assembly weighs about .5 tons
Each fuel assembly contains 50 to 70 fuel rods, an 8×8 BWR fuel assembly contains 64 fuel rods plus 2 hollow tie rods.

Common pool inventory: 6375
1-4 sfp’s inventory: 3108
Total plant inventory (not including reactor cores):11417

• Unit 1 SFP: 292 irradiated (used) fuel assemblies and 100 unirradiated new unused assemblies
• Unit 2 SFP: 587 irradiated (used) fuel assemblies and 28 unirradiated new unused assemblies
• Unit 3 SFP: 514 irradiated (used) fuel assemblies and 52 unirradiated new unused assemblies
• Unit 4 SFP: 1331 irradiated (used) fuel assemblies and 204 unirradiated new unused assemblies
• Units 5 and 6 also contain spent fuel though these reactors did not sustain substantial damage and should be able to be removed by normal means when ready to do so. These two units are cooling their fuel pools through the original pool cooling system but both currently use improvised water intake pumps to bring in cooling loop water.
• Unit 5: 946 irradiated (used) and 48 unirradiated new unused assemblies
• Unit 6: 876 irradieated (used) and 64 unirradiated new unused assemblies

These numbers do not include the reactor core fuel from units 1-6. Units 1-3 melted down. The condition of the fuel in the reactors at units 5 and 6 is not known but is assumed to not be heavily damaged.

 

Fuel Handling

TEPCO intends to cover each reactor with some type of building or temporary shelter before removing fuel from the pools. This will allow any released radiation from the process to be mostly kept out of the environment. These shelters will also keep fuel handling equipment and workers from being exposed to the elements while work is conducted.

TEPCO plans a similar building for unit 3. It is unclear how they intend to remove fuel from unit 1. A tent structure was hastily put up without first removing the roof that fell onto the refueling floor.

The process for removing fuel under normal circumstances involves submerging a transfer cask into the spent fuel pool, then loading the fuel using the fuel handling machine (crane). Once the cask is filled, the lid is bolted on, water is then pumped out of the cask and the cask lid is typically seal welded. The fuel is dried by applying a vacuum to the canister and injecting helium. Some water can remain inside and the heat up of the fuel caused during the drying phase can cause some stress to the rods. Storage casks hold about 10-18 metric tons of used fuel, which is equivalent to about 24-32 pressurized water reactor (PWR) assemblies or 56-68 boiling water reactor (BWR) assemblies. Then the fuel can be moved to the common spent fuel pool where it is lowered into the pool, the lid removed and fuel is moved to common spent fuel storage. Fuel in the common pool can then be loaded into the dry storage cask by lowering the cask into the common spent fuel pool and loading the fuel into the cask. The lid is bolted on, water is then pumped out of the cask and the cask lid is typically seal welded. The fuel is dried by applying a vacuum to the canister and injecting helium.

Common Pool

The common pool is a large spent fuel pool facility located near unit 4. The building includes the large storage pool and also some cask handling room in the lower level. TEPCO’s plan is to first move fuel out of the spent fuel pools into this storage pool for further inspection and holding.

The total capacity of the common pool is 6840 assemblies, it currently has 6375 assemblies stored inside. The pool has 465 open slots. The fuel racks cover the entire span of the pool minus the corner cask holders. TEPCO states in their newest roadmap that they intended to use half of the pool for fuel being removed from the reactor spent fuel pools. TEPCO began work on the cask storage facility in June 2012 but has not explained any progress made to date. TEPCO has also not explained how or when they intend to offload almost half of the common pool inventory to make room for the incoming spent fuel.

 

Casks

Large steel casks can be used to store fuel after it reaches a certain age. 37 or 52 assemblies can fit in a cask depending on the design.

US research estimated the radiation at 2 meters away to be about 1 millirem/hour (.01 mSv/h) and 5 millirem/hour (.05 mSv/h) at contact with the closed cask.

A US nuclear operator estimated $120 million USD for a 40 cask concrete storage system. That is about $3 million per cask. For about 220 casks estimated at Fukushima it would cost $660 million USD, not including the ongoing maintenance. Casks are typically rated for 20 years of storage life.

The casks are cooled by convective cooling. Air is allowed to flow over the cask through vents in the storage facility to disperse heat. The steel, lead and concrete shell helps absorb gamma radiation. Polyethylene, concrete or boron impregnated metal helps to absorb neutrons. Criticality is controlled by the lattice structure of the holding basket and boron infused metal within the basket.

Most casks are considered to be only for transportation and short term storage.

Typical cask size

 

 

Existing Cask Storage

There is some existing cask storage on site at Daiichi. This was not intended for long term storage but as a system to send spent fuel off to the UK or France for reprocessing. The building holds casks near the sea front waiting to be loaded during the next delivery of reprocessed fuel and waste. The total capacity for the cask storage building is 20 casks. As of 2010 only 9 were in use with plans to fill and add 11 more casks. The casks used hold either 52 or 37 assemblies. About 408 assemblies are currently held in casks

The building was swamped by the tsunami but the casks and building were mostly unscathed. The crane and the overhead door were damaged by the water inflow.

cask storage building before the disaster.

 

TEPCO’s Future Cask Storage

TEPCO reports in September 2012 that they began work on this facility in June of 2012 to create a location for storing spent fuel at Fukushima Daiichi. TEPCO has not reported on the progress of this since Sept 2012 and did not document specific work finished at that time. The report included a drawing of a 48 cask facility. It is not clear if this was literal on TEPCO’s part or simply a vague artist rendering. With 11417 assemblies (maybe more) that need to be stored long term TEPCO will need about 220 casks, 240 if fuel from units 5 and 6 reactors can be offloaded.

TEPCO’s drawing of the spent fuel storage plan.

What will be needed, possibly 240 casks.

The land surrounding the plant has been steadily populated with tanks needed to handle the growing amount of contaminated water on site. It is not clear where they plan to put the needed casks.

 

Long Term Disposal

Japan’s plan before the disaster had hinged on the idea of reprocessing all of their fuel. This involves extracting the reusable materials from spent fuel but leaves a considerable amount of waste that still needs to be handled. The interim storage facility at Mutsu is still under construction but a considerable amount of casked fuel is already stored on site at the large reprocessing center at Rokkasho.

Rokkasho has some facilities already built on site and in operations, the actual reprocessing plant is years from completion. The Rokkasho and Mutsu facilities are both now in question. Japan is reconsidering their plan for reprocessing fuel as the future of nuclear power is debated. The local governments permission for the facilities was dependent on the large reprocessing facility bringing jobs and money to the area. Without the reprocessing plant the local governments do not want the waste storage in their area. Some have suggested sending back any spent fuel currently being stored at the facilities.

The planned fuel cycle that is now in question.

Without a long term storage facility the future of all the spent fuel at Fukushima Daiichi and the other reactors around Japan is not known. Some of Japan’s spent fuel had been sent to France and the UK for reprocessing into MOX fuel. The remaining waste needs to be returned to Japan. Both countries have sought assurances Japan would still accept their waste.

There was an old demand by the US that all Japanese spent fuel be sent to the US, over the years this demand has been dropped. The US has been unable to create their own long term waste storage facility. The US has had considerable experience dealing with nuclear waste. The original site from the Manhattan Project that built the first atomic bomb in Hanford WA still struggles to deal with pits of waste and aging tanks of mixed nuclear stew.

Japan’s unstable geology has made the idea of a deep ground long term repository unlikely. Most of the country is heavily populated and does not want a giant nuclear waste dump near their homes.

Canada and Australia were suggested by industry insiders as locations since both have large uranium mining operations. The uranium mining companies have no interest in becoming waste managers and obviously the local populations would never accept such an idea.

This has created an ethical and logistical dilemma, there is no appropriate facility due to natural factors or public resistance for a massive amount of nuclear waste than nobody wants.

Resources:

Britain, France seek Japan’s promise to accept nuclear waste
http://ajw.asahi.com/article/0311disaster/fukushima/AJ201209130090

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