Fuel Cycle

2 x 500 MW ThorCon power plants with Can and fuelsalt service ship

Each fission island has two Cans in silos. One generates power. After 4 years of exposure to neutron flux the graphite moderator expands and contracts, ending its service life. The fuelsalt is then pumped to the twin Can which is then used to generate power. The used Can remains in its protective silo as radioactive fission products decay and the Can cools. The CanShip visits ThorCon power plants regularly to remove used Cans and replace them.

After 8 years the fuelsalt becomes saturated with trivalent heavy metals and fission products, so it needs to be refreshed. The used fuelsalt is stored in a fuel cask in a passively cooled secure silo where fission products decay. During this cool down period, the old fuelsalt is as well protected as the salt in the operating Can. By the time we pump the old fuelsalt to a shipping cask, its decay heat will be down to 80 kW, 0.25% of the original.The visiting CanShip exchanges casks of used and fresh fuelsalt. The CanShip delivers Cans to a centralized recycling facility (CRF) and fuel casks to a separate fuel handling facility (FHF). 

The fuelsalt going both ways will be unattractive weapons material. The uranium will be fully denatured. The returning plutonium will be reactor grade. More importantly, it will be mixed with 50 times as much neutron absorbing thorium. Such a mixture cannot go critical. To produce even a weak fizzle weapon, the plutonium must be separated from the thorium. This is an extremely difficult process requiring a Thorex plant. Currently, no such plant exists. ThorCon’s fuelsalt will be more proliferation resistant than the MOX fuel which is regularly shipped from France to Japan.

Five kilograms of uranium fuel per day

ThorCon is a thorium converter. The initial fuel charge is largely thorium. During the eight year fuel cycle, a portion of the fertile thorium is converted to fissile U-233 which then becomes part of the fuel. Each ThorCon will require 5.3 kg of 19.7% enriched uranium and 9.0 kg of thorium per day, on average.

ThorCon clobbers coal on fuel cost. An extreme lower bound on coal fuel cost is 2 cents per kWh. Even at $90/kg U3O8, about double the current yellowcake spot price, ThorCon’s fuel cost is less than 0.6 cents per kWh.

Even on a once-through basis, ThorCon is uranium efficient. Averaged over 8 years, we annually feed 1,930 kg of 19.7% enriched uranium derived from 72,500 kg of natural mined uranium. This equates to 145 tonnes of natural uranium per full power GW-year compared to about 250 tonnes for a standard light water reactor (LWR).

After 8 years ThorCon will have been fed 3 tonnes of fissile U-235 fuel, but its “spent” fuel will still contain 1 tonne of fissile fuels U-233 (408 kg) and U-235 (624 kg). ThorCon’s net consumption of fissile uranium is less than half that of a LWR, due to higher thermal efficiency, removal of Xe-135, and U-233 production from thorium.

In the future, re-enriching this back to 19.7% would take about 48 SWU per kg U-235. At competitive enrichment costs of $50/SWU, $2400 for 1 kg of U-235 is cheap. Such future re-enrichment would cut ThorCon’s uranium requirements by a third.

Uranium supply

The World Nuclear Association reckons current uranium reserves are 5.9 million tonnes at $130 per kg uranium and 7.6 million tonnes at $260. If, for sake of argument, we assume 3 million tonnes were available to ThorCon and no re-enrichment, then we have 21,000 GW-years of uranium. If we start turning out 100 one GW ThorCons per year, then at year 20 we will have used up our 3 million tonnes. At this point, 2100 one GW ThorCons will be producing about half the world’s electricity while generating no SO2, no NOx, no ash, and nil CO2.  ThorCon will have been spectacularly successful.

This fleet will also be eating into the remaining reserves at the rate of 300,000 tonnes per year. Of course, this is 20 years from now. Future improvements, including re-enriching, will halve this burn rate. But even so, if reserves were static, we’d run out of uranium in another 30 years, about 50 years from now.

But reserves will not be static. If there is any shortage of uranium the price of uranium will go up and new reserves will be developed. Known low grade uranium sources such as phosphate deposits, enrichment tailings, and coal ash will be exploited. In commodities, the rule of thumb is a doubling in real price increase reserves ten-fold. Miners don’t produce reserves until they have an economic motive to so. For example, the US has been operating successfully at an oil Reserves to Production ratio (R/P) less than 10 for 50 years. Uranium’s current 90:1 R/P is an anomaly. Most commodities operate at much lower R/P, usually with no long-term increase in real price.

Advances in exploration and extraction technology always seem to outpace the predictions. For example, the sea contains about 4.6 billion tonnes of uranium. River flows add about 32,000 tonnes of uranium to the ocean each year. Solar powered evaporation then increases the concentration of uranium in sea-water. The uranium concentration is still a very low 3 ppb. But activated polymers are being developed which have a remarkable ability to pull uranium out of the water. Currently, Japanese and US Pacific NW National Lab researchers are claiming seawater extraction costs of about $600 per kilogram of uranium.  Even using this expensive uranium source, ThorCon’s generated electricity cost would increase less than 1 cent/kWh.

PNNL seawater uranium absorption fibers

ThorCon can accept a ten-fold increase in the real price of uranium, and still beat coal. One way or another, such a price increase will result in a massive increase in reserves. And that massive increase will carry us to 2100 by which time we can confidently expect order of magnitude improvements in our ability to extract nuclear power from uranium and thorium.

The problem is not what happens 50 to a 100 years from now. The problem is what happens in the next 20 years. That’s the problem that ThorCon focuses on.

Annual 500 MW ThorCon fuel cycle flows in tonnes, averaged over 8 years.

ThorCon’s once-through fuel cycle flows are the black numbers. Since ThorCon’s spent fuelsalt after 8 years contains uranium 8.6% enriched in U235 and U233, simply storing it is uneconomic. The green numbers illustrate how in the future the uranium in the spent fuel can be separated by fluoride volatility, a process used every day in uranium enrichment, and then re-enriched back to 19.7%. This re-enrichment takes only 4.5 SWU/kg  of product and reduces ThorCon’s requirement for mined uranium by about one-third.

A beautifully small problem

ThorCon will send one cask to storage every four years. Each 3 m diameter cask results from 2 GW-years of electric power generation. At 5 m spacing 800 GW-years of casks would occupy one hectare.  If Indonesia tripled its electricity consumption to 87 GW using only ThorCon power, she would have to devote a hectare of space every 9 years to cask storage.

In contrast, a typical 1 GWe coal plant produces roughly 300,000 tons of solid waste annually. This ash is much lighter than ThorCon solid waste, roughly 1000 kg/m3 in bulk form. In terms of volume a coal plant produces over 100,000 times more solid waste per kWh than ThorCon.

All the waste from 450 MW’s for 28 years. If coal plant ash for the same energy was placed on this pad, it would be a column 2000 m high.

Each ThorCon will produce 4,000,000,000 kWh of non-intermittent, dispatchable, pollution-free, CO2-free electricity each year. That is enough power to support 500,000 people at better than European standards. The remarkable fact is that a ThorCon does this while producing waste flows that are measured in a few tons per year.