Page 4 - AsiaElec Week 33 2021

AsiaElec COMMENTARY AsiaElec
China is about to disrupt the nuclear power industry worldwide
ASIA
CHINA is one month away from the startup of a new design of nuclear reactor that uses molten thorium fluoride as its fuel source in place of enriched uranium. An experimental 2-MW reactor is in the final stage of pre-start tests at Wuwei in Gansu Province, about 800 km north of Chengdu. Plans are advanced to use the test data to build a 100-MW plant for startup in 2023.
The Wuwei project is not a bolt from the blue. China has had some 1,000 people working on thorium-based reactors for two decades. The thorium fuel cycle itself is not new (it’s been the- orised since the beginning of nuclear power in 1945) and works on the principle that (stable) thorium can be turned into (fissile) uranium 233 when it is bombarded with slow neutrons. A working molten salt reactor wraps a blanket of thorium fluoride salt around the outside of the reactor core, where spare neutrons transform it into uranium 233 (via protactinium – an inter- mediate element). The U233 is then fed into the core as fuel.
Engineering is the challenge, not physics
The engineering and chemistry required to run a continuous process have been much more chal- lenging than the nuclear physics. Thorium and uranium fluoride salts flow through the reactor at 800 degrees Centigrade and are highly corro- sive, requiring the development of steels, pumps, valves and pipes that can withstand the molten salt environment. It has also been necessary to develop control and monitoring systems that can withstand the nuclear flux inside the reactor vessel.
Further challenges arise outside the reactor. The thorium fuel cycle generates fission products that must be removed from the molten salt mix continuously, meaning that a working reactor must be attached to a chemical separation plant also capable of handling hot corrosive molten salt. This plant extracts and holds protactinium which, after a short period (about a month), fis- sions into uranium 233, which is then reinjected into the reactor as its fuel supply.
Molten salt beats PWR several ways
Thorium molten salt reactors (MSRs) offer very large advantages over the universally accepted pressurised water reactors (PWRs) being built today (some 70 reactors are in build or on order around the world at present). Principally, the reactor operates at a pressure of only 1.5
atmospheres. This removes the need to build expensive high-pressure containment systems and also dispenses with the need for expensive multiple-redundant water circulation pumps. Lower pressures mean that a whole raft of sup- porting systems can be down-scaled and made cheaper. On the revenue side, MSRs operate at about 500 degrees centigrade hotter than PWRs, which means that thermal conversion from reactor power to turbine power rises from 35% for a PWR to nearly 50% for an MSR. Produc- tion-scale MSRs are expected to have a capital cost of about half that of a PWR – $5,000 per KW of electrical capacity rather than $10,000, and financial models suggest that the fuel cost of produced power will start at $30 per MWh and should fall to about $10 per MWh.
The absence of high pressure water and steam also makes MSRs safer. If a salt reactor overheats it melts a “freeze plug” below the reac- tor core, which allows the molten salt to flow into a cool-pan where it cools, freezes and stops fissioning. There are no pumps to fail, and no reactions between water coolant and zirconium fuel rods to generate explosive hydrogen to breach containment (at Fukushima the tsunami overwhelmed the circulating pumps, allowing the reactor to overheat and generate a hydrogen explosion).
Thorium thorium everywhere
It is not just capital costs that are lower. MSR use unprocessed thorium, compared with the enriched uranium required by PWRs. Fuel preparation is both simpler and cheaper. Fuel does not have to be encapsulated in expensive fuel rods (and equally, the costly process of removing and reprocessing used rods is also avoided). Thorium also does not have to be enriched. Some five times more abundant than uranium, the US has a 65,000-tonne ore deposit in Montana, while India counts some 800,000 tonnes in its proven reserves. 500 tonnes of tho- rium fissioning would generate the US’ entire energy demand for a year. Combining lower capital costs and low fuel costs Chinese sources suggest that thorium MSRs offer the potential to produce carbon-free power at a cost of $20 per MWh – lower than the most efficient gas, wind and even coal-sourced power.
Finally, and as a bonus, the Thorium fuel cycle produces no plutonium, removing a sig- nificant weapons proliferation risk.
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