Sunday, July 23, 2017

How Much Fuel Was in Fukushima Daiichi Unit 3 Reactor?

I've followed the Fukushima Daiichi disaster so closely for so long that I was surprised to discover this morning that I really don't know exactly how much fuel was in reactor 3.

I have the data on fuel contained in the spent fuel pools, but NOT the data on fuel in the REACTOR CORES.  

Below find the spent fuel pool data:
TEPCO. Integrity Inspection of Dry Storage Casks and Spent Fuels at Fukushima Daiichi Nuclear Power Station (16 November 2010),
[PARAPHRASING] The total spent uranium fuel inventory at Daiichi in March 2010 was reported as 1,760 tons. The 2010 report asserts that approximately 700 spent fuel assemblies are generated every year. The report specifies that Daiichi’s 3,450 assemblies are stored in each of the six reactor’s spent fuel pools. The common spent fuel pool contains 6291 assemblies. The amount of MOX fuel stored at the plant has not been reported.

SO, 700 spent fuel assemblies were generated at Daiichi each year. There were 6 reactors at Daiichi, although not all were operational at time of earthquake. Assuming they were operational, can we infer that each reactor core contained approximately 100 assemblies? 

That estimate is supported by the US Nuclear Regulatory Commission's information on fuel assemblies found here, which suggests a reactor may contain up to 200 assemblies, with each assembly containing 200 or more rods:
US NRC (2017, April 10). Fuel assembly (fuel bundle, fuel element).

A structured group of fuel rods (long, slender, metal tubes containing pellets of fissionable material, which provide fuel for nuclear reactors). Depending on the design, each reactor vessel may have dozens of fuel assemblies (also known as fuel bundles), each of which may contain 200 or more fuel rods.
The website titled Nuclear Power Net, described as non-profit and founded by nuclear engineers, suggests a typical reactor  contains 157 fuel assemblies but those are composed of over 45,000 fuel rods (which is more than 200 in each assembly):
Nuclear Power Net (no date).

An 1100 MWe (3300 MWth) nuclear core may contain 157 fuel assemblies composed of over 45,000 fuel rods and some 15 million fuel pellets. Generally, a common fuel assembly contain energy for approximately 4 years of operation at full power. Once loaded, fuel stays in the core for 4 years depending on the design of the operating cycle. During these 4 years the reactor core have to be refueled. 
During refueling, every 12 to 18 months, some of the fuel – usually one third or one quarter of the core – is removed to spent fuel pool, while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. The removed fuel (one third or one quarter of the core, i.e. 40 assemblies) has to be replaced by a fresh fuel assemblies.
The variation in data is puzzling but I think we can move forward assuming that there were approximately 100 fuel assemblies in each reactor.

A typical fuel assembly for a Pressured Water Reactor (PWR) "stands between four and five metres high, is about 20 cm across and weighs about half a tonne" according to the World Nuclear Association (

USING THESE DATA POINTS, I can infer that Unit 3's reactor core contained approximately 50-75 tons of fuel. Does that sound roughly correct? A ton is 2,000 pounds so 50 tons of fuel is 100,000 pounds. For metric users, that converts into 45359.237 kilograms.

Why is it so important that I determine how much fuel was in reactor 3?

The reason lies in the media representations of TEPCO's robotic probe, Little Sunfish, which allegedly has (likely) located unit 3's  missing reactor fuel, illustrated in the media as "rocks on the floor":

Kohei Tomida (July 23, 2017). Melted nuke fuel images show struggle facing Fukushima plant. The Asahi Shimbun, 
In particular, what is believed to be nuclear fuel debris is scattered in the form of rocks in the area directly beneath the pressure vessel. 

My point is not to deny that these found rocks might be nuclear fuel debris. Rather, my point is that these shards of melted fuel and debris can hardly be construed as representing the entirety of the fuel.

Put otherwise, there is no compelling evidence that TEPCO has located the majority of fuel that was contained in Unit 3.

I decided to search my notes, books, and the web validated statistics on how much fuel each reactor contained. I was struck by how little information is available about the missing fuel and the sanitized accounts of what happened at unit 3, the one running MOX fuel.

Basically, at Daiichi Unit 3 TEPCO has misplaced 50 plus tons of reactor fuel that was "enriched" with plutonium and now were are being encouraged to think that shards of melted-fuel debris represent the entirety of the missing reactor core....a core that weighed more than a school bus....

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