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The latest technological advances in nuclear energy

Can thorium compete with uranium as a nuclear fuel?

Isabelle Dumé, Science journalist
On March 31st, 2022 |
4 min reading time
Key takeaways
  • Thorium is a metal that could be used in molten salt reactors; one of the next generations of nuclear power in which the reactor coolant and the fuel itself are a mixture of hot molten salts.
  • Th-232 is of interest for nuclear power generation because it can easily absorb neutrons and transform into Th-233. Th-233 can become protactinium-233, which in turn becomes a fissile and energy-producing isotope: U-233.
  • Thorium has many qualities but also many disadvantages: difficult to handle, fertile and non-fissile metal, higher risks.
  • But it produces less waste than plutonium or uranium and remains an attractive option for the future of nuclear energy.

The idea of using tho­ri­um as a nuclear fuel was large­ly aban­doned in the past because, tra­di­tion­al­ly, nuclear pow­er was linked to mil­i­tary nuclear research and devel­op­ment – and both ura­ni­um and plu­to­ni­um were used to make atom­ic bombs. For pow­er gen­er­a­tion, how­ev­er, tho­ri­um could have real advan­tages and sev­er­al coun­tries are invest­ing in this chem­i­cal ele­ment. The met­al could be used in molten salt reac­tors, one of the new gen­er­a­tion designs in which the reac­tor coolant and the fuel itself are a mix­ture of hot molten salts. These types of reac­tors can reach very high tem­per­a­tures, which great­ly increas­es the effi­cien­cy of elec­tric­i­ty production.

The prob­lem, how­ev­er, is that more than 400 nuclear pow­er plants in oper­a­tion around the world use main­ly ura­ni­um (U) as fuel. Although this ele­ment is abun­dant, less than 1% of the ura­ni­um on Earth is U‑235, the iso­tope of ura­ni­um that is fis­sile. The rest is U‑238. The U‑235 con­tained in ura­ni­um must there­fore be con­cen­trat­ed and then enriched in com­plex and expen­sive processes.

And that is not all: the fis­sion of U‑235 pro­duces high­ly radioac­tive waste that must be han­dled care­ful­ly and then stored in a safe place for extreme­ly long peri­ods of time. This waste also con­tains a type of plu­to­ni­um that can be used to make nuclear weapons.

Thorium reactors around the world

Chi­na has con­struct­ed an exper­i­men­tal tho­ri­um reac­tor at Wuwei, on the out­skirts of the Gobi Desert1. Tho­ri­um has been test­ed as a fuel in oth­er types of nuclear reac­tors in coun­tries includ­ing the US, Ger­many, the Nether­lands and the UK. It is also part of a nuclear pro­gramme in India because of the nat­ur­al abun­dance of the ele­ment in that coun­try. In France, stud­ies are being car­ried out by the CNRS, which is devel­op­ing a project called MSFR (for Molten Salt Fast Reac­tor), using tho­ri­um 2.

Four times more abun­dant than uranium

Tho­ri­um (Th) was dis­cov­ered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thun­der. It is a slight­ly radioac­tive met­al found in rocks and soils and is quite abun­dant in the Earth­’s crust. Indeed, its main iso­tope, Th-232, is about four times more abun­dant than U‑2383 and as abun­dant as lead. The amount found in the Unit­ed States, for exam­ple, could meet that coun­try’s ener­gy needs for a thou­sand years with­out the need for the enrich­ment required for ura­ni­um-based fuels.

The rare earth phos­phate min­er­al, mon­azite, con­tains the most tho­ri­um – up to about 12% tho­ri­um phos­phate 4 Mon­azite is found in igneous and oth­er rocks and the world’s mon­azite resources are esti­mat­ed at about 16 mil­lion tonnes, of which 12 Mt are found in heavy min­er­al sand deposits on the south and east coasts of India.

Th-232 is of inter­est for nuclear pow­er gen­er­a­tion because it can eas­i­ly absorb neu­trons and trans­forms into Th-233. This new iso­tope emits an elec­tron and an anti­neu­tri­no with­in min­utes to become pro­tac­tini­um-233 (Pa-233). This iso­tope, in turn, trans­forms into U‑233, which is an excel­lent fis­sile mate­r­i­al. Indeed, the fis­sion of a U‑233 nucle­us releas­es about the same amount of ener­gy (200 MeV) as that of U‑235.

The prob­lem of cooling

In con­ven­tion­al reac­tors, ura­ni­um is stored in sol­id fuel rods, which are cooled by huge amounts of water. With­out this cool­ing, the rods would melt, releas­ing dan­ger­ous radi­a­tion. The tho­ri­um would under­go its reac­tions in an entire­ly dif­fer­ent type of reac­tor, called a molten salt reac­tor (MSR), con­tain­ing a mix of flu­o­ride salts in which the nuclear fuel is melt­ed. This type of reac­tor does not need to be built near water­cours­es, as the molten salts them­selves serve as a coolant.

This means that the reac­tors can be installed far from coast­lines, in remote and even arid regions. These reac­tors can­not ‘melt­down’ in the con­ven­tion­al sense either and, in an emer­gency, the fuel can be quick­ly drained from the reac­tor. MSRs deploy­ing tho­ri­um are also safer because they oper­ate at pres­sures close to atmos­pher­ic pressure.

Like ura­ni­um, tho­ri­um absorbs neu­trons, as men­tioned, but unlike ura­ni­um, it does not release more neu­trons to per­pet­u­ate the nuclear chain reac­tion. This reac­tion starts when a ura­ni­um atom is hit by a neu­tron, releas­ing ener­gy that caus­es more neu­trons to be eject­ed from the ura­ni­um atoms, start­ing the cycle again. By reduc­ing the num­ber of neu­trons inject­ed into the fuel, it is the tho­ri­um itself that lim­its the rate of the nuclear reaction.

R&D investments needed

The use of tho­ri­um as a new pri­ma­ry ener­gy source has been an attrac­tive prospect for many years, but extract­ing its latent ener­gy val­ue in a cost-effec­tive way is a chal­lenge. The devel­op­ment of new tho­ri­um-fuelled nuclear pow­er plants will there­fore require sig­nif­i­cant research and devel­op­ment and test­ing – some­thing that may be dif­fi­cult to jus­ti­fy giv­en that ura­ni­um is rel­a­tive­ly cheap and abundant.

Anoth­er dis­ad­van­tage is that tho­ri­um is ‘fer­tile’ and non-fis­sile, so it can only be used as a fuel in com­bi­na­tion with a fis­sile mate­r­i­al, such as recy­cled plu­to­ni­um, as a con­duc­tor to main­tain a chain reac­tion (and thus a sup­ply of excess neutrons).

The U‑233 pro­duced at the end of the cycle is also dif­fi­cult to han­dle, as it con­tains traces of U‑232, which active­ly emits gam­ma radi­a­tion. While some researchers sup­port the use of tho­ri­um as a fuel because its waste is more dif­fi­cult to turn into atom­ic weapons than ura­ni­um, oth­ers argue that risks remain5.

On the bright side, there is less plu­to­ni­um pro­duced over­all dur­ing reac­tor oper­a­tion. Some sci­en­tists say that tho­ri­um reac­tors could even help deplete the tons of plu­to­ni­um we have cre­at­ed and stored since the 1950s.


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