Antinuclear

Dispelling Claim 4: Thorium reactors are safer than  conventional uranium reactors  Thorium ‒ a better fuel for nuclear technology? Nuclear Monitor,   by Dr. Rainer Moormann  1 March 2018

The fission of U-233 results in roughly the same amounts

of the safety-relevant nuclides iodine-131, caesium-137

and strontium-90 as that of U-235. Also, the decay heat is

virtually the same. The differences in produced actinides (see

next claim) are of secondary importance for the risk during

operation or in an accident. In this perspective, thorium use

does not deliver any recognisable safety advantages.

Of greater safety relevance is the fact that uranium-233

fission produces 60% less so-called delayed neutrons than

U-235 fission. Delayed neutrons are not directly created

during the fission of uranium, but from some short-lived

decay products. Only due to the existence of delayed

neutrons, a nuclear reactor can be controlled, and the

bigger their share (for instance 0.6% with U-235), the

larger is the criticality range in which controllability is given

(this is called delayed criticality). Above this controllable

area (prompt criticality) a nuclear power excursion can

happen, like during the Chernobyl accident. The fact that

the delayed super-critical range is with U-233 considerably

smaller than with U-235, is from a safety point of view an

important technical disadvantage of thorium use.

During the design of thermal molten salt reactors (breeders),

the conclusion was that the use of thorium brings problems

with criticality safety that do not appear with classical

uranium use in this type of reactors. For that reason, it was

necessary to turn the attention to fast reactors for the use

of thorium in molten salt reactors. Although this conclusion

cannot be generalised, it shows that the use of thorium can

lead to increased safety problems.

As mentioned, a serious safety problem is the necessity to

restart breeder and reprocessing technology with thorium.

Thorium is often advertised in relation to the development

of so-called advanced reactors (Generation IV). The

safety advantages attributed to thorium in this context are

mostly, however, not germane to thorium (the fuel) but

rather due to the reactor concept. Whether or not these

advanced reactor concepts bring overall increased safety

falls outside the scope of this article, but that is certainly

not a question with a clear “yes” as the answer.