Sur ce site

Sur le Web du CNRS

Accueil du site > ANGLAIS > Research > Back End of the Cycle and Nuclear Energy > Activities > Thorium cycle

Thorium Cycle

^{232}Th + n ->^{233} Th {\beta^-\over 22m} ->^{233}Pa {\beta^-\over 27j} ->^{233}U

The production of the fissile nucleus ^{233}U is controlled by its predecessor ^{233}Pa nucleus. The latter plays a major role in this chain due to its long half life (27 days). It has time to capture neutron, then to fission or emit gamma ray, many processes that can modify the quantity of ^{233}U present in the fuel and thus affect the neutron equilibrium in the reactor environment. Thus, the knowledge of fission and radiative capture reaction cross-sections is of primary importance. It is because of high radio-toxicity of ^{233}Pa and of the difficulties arising in its chemical separation that such measurements were never implemented.

The originality of our approach is in the use of transfer reaction to measure the fission probability for the nucleus (A) as a function of its excitation energy.

Concerning the ^{233}U nucleus which is the fissile nucleus in the chain, we re-measured the fission cross-section ^{233}U(n,f) in the range of fast neutrons and the experiment planned for 2007 will be concerned with simultaneous measurements of fission and radiative capture cross-sections for ^{233}U in the range of epithermal neutrons.

Study of ^{233}Pa nucleus

In order to determine the cross-sections for reactions ^{233}Pa(n,f) and ^{233}Pa(n,\gamma), the fission and radiative capture probabilities must be multiplied by the cross section σ_{NC} for the formation of the compound nucleus (A) formed through the reaction n+(A-1).

We have validated this procedure, in the case of fission, by comparing with measurements induced with neutrons.

In the first case, we were interested in the fission of the excited nucleus ^{234}Pa* formed in the reaction ^{233}Pa+n (En = 0.5-10 MeV). We formed the nucleus ^{234}Pa* in the transfer reaction ^{232}Th(^{3}He,p).
Then, we studied the radiative capture (n,γ) (En = 0-1 MeV) through the same transfer reaction.

Study of ^{233}U nucleus

The fission cross-section ^{233}U(n,f) was measured directly with the help of fast neutrons obtained from electrostatic accelerator of CENBG in the energy range of neutrons from 1 to 6 MeV. ( see fission ^{233}U(n,f)).

In the second case, we proposed to simultaneously measure radiative capture and fission cross-sections for ^{233}U nucleus in the region of epithermal neutrons.
The \alpha yield ratio of these cross-sections σ(capture)/ σ(fission) is an essential parameter in the calculation of regeneration power in the chain ^{232}Th- ^{233}U. Very recently, analyses of sensitivity and uncertainty showed evidence of a lack of reliability in the evaluations of this ratio \alpha which are based on old measurements attaining 25% dispersion.
(see alpha yield ratio)

Thorium cycle

More to learn on

Alpha yield ratio

The main difficulty in the measurement of the $^233$U $\alpha$ yield ratio ( $\alpha$ = σ(capture)/ σ(fission) is the separation of γ rays associated with the reaction $^233$U(n,γ) from the ones that come from fission fragments and which represent parasitic radiation. The prototype of an ionization chamber that would allow to identify fission fragments was constructed and tested on AIFIRA accelerator of CENBG. Detection efficiency for fission exceeds 90% and (...)

Read more