Thorium is a naturally-occurring chemical element with atomic number 90 which means there are 90 protons and 90 electrons in the atomic structure. The chemical symbol for thorium is Th
. Thorium was discovered in 1828 by norwegian mineralogist Morten Thrane Esmark. Joens Jakob Berzelius, the swedish chemist, named it after Thor
, the Norse god of thunder.
Thorium is a naturally-occurring element and it is estimated to be about three times more abundant than uranium. Thorium is commonly found in monazite sands (rare earth metals containing phosphate mineral).
Thorium has 6 naturally occurring isotopes. All of these isotopes are unstable (radioactive), but only 232Th is relatively stable with half-life of 14 billion years, which is comparable to the age of the Earth (~4.5×109 years). Isotope 232Th belongs to primordial nuclides and natural thorium consists primarily of isotope 232Th. Other isotopes (230Th, 229Th, 228Th, 234Th and 227Th) occur in nature as trace radioisotopes, which originate from decay of 232Th, 235U and 238U.
Fissile / Fertile Material Cross-sections. Comparison of total fission cross-sections.Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1
Source of data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1
, which alone makes up nearly all natural thorium, is the most common isotope of thorium in the nature. This isotope has the longest half-life
(1.4 x 1010
years) of all isotopes with more than 83 protons. In fact, its half-life is considerably longer than the age of earth. Therefore 232
Th belongs to primordial nuclides
232Th decays via alpha decay into 228Ra . 232Th occasionally decays by spontaneous fission with very low probability of 1.1 x 10-9 %.
232Th is a fertile isotope. 232Th is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand 232Th can be fissioned by fast neutron with energy higher than >1MeV. 232Th does not meet also alternative requirement to fissile materials. 232Th is not capable of sustaining a nuclear fission chain reaction, because too many of neutrons produced by fission of 232Th have lower energies than original neutron.
Isotope 232Th is key material in the thorium fuel cycle. Radiative capture of a neutron leads to the formation of fissile 233U. This process is called nuclear fuel breeding.
232Th is the predominant isotope of natural thorium. If this fertile material is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay) to 233U, that is very good fissile material. On the other hand proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.
Conversion Factor for Thorium Reactor
A quantity that characterizes this conversion of fertile into fissile material
is known as the conversion factor
. The conversion factor
is defined as the ratio of fissile material created
to fissile material consumed
either by fission or absorption. If the ratio is greater than one, it is often referred to as the breeding ratio
, for then the reactor is creating more fissile material than it is consuming.
When C is unity, one new atom is produced per one atom consumed. It seems fertile material can be converted in the reactor indefinitely without adding new fuel, but in real reactors the content of fertile thorium 232 also decreases and fission products with significant absorption cross-section accumulates in the fuel as fuel burnup increases.
The fertile-to-fissile conversion characteristics depend on the fuel cycle and the neutron energy spectrum. For a thermal neutron spectrum (E < 1 eV) and the uranium fuel cycle, fuel breeding (C>1) is not feasible although η for both isotopes is greater than 2. This is due to the fact η is not large enough to compensate for the neutron leakage and its parasitic capture.
For a thermal neutron spectrum (E < 1 eV) and the thorium fuel cycle, the situation is considerably better. Due to very low capture-to-fission ratio, the reproduction factor for uranium 233 is about η = 2.25. From this point of view is thorium fuel cycle is promising and a thermal reactor of this type could successfully be made to bred.