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1. Introduction
New reactors using the uranium-thorium fuel cycle are under studies in order to provide safer and
cleaner nuclear energy as highly radiotoxic actinide waste (Pu, Am and Cm isotopes) will be produced
in lower quantities than the currently used uranium fuelled reactors. Moreover, further developments
of this thorium based cycle rely on nuclear data libraries of the quality achieved for the uranium ones.
The primary reaction of importance using the thorium cycle is the one producing 233 U from
neutron capture on 232 Th, the net production of 233 U is controlled by the 27 days half-life of 233 Pa: a
fertile nucleus ( 232 Th) is transformed into a fissile nucleus ( 233 U) after neutron capture and 2 successive
β decays.
+n β β
232 Th
233 Th
233 Pa
(22 min) (27 days)
233 Pa as a precursor of 233 U may capture neutrons and could lead to a reactivity decrease of the
reactor, conversely and after a shut down, the build up of 233 U increases this reactivity; this so called
Protactinium effect should add severe requests on the 233 Pa and 233 U inventories for reactivity control.
There is no equivalent effect in the well studied 238 U- 239 Pu fuel cycle as the equivalent intermediate
isotope 239 Np has a relatively shorter half-life (2.35 days).
Furthermore, neither the neutron capture cross-section nor the neutron induced fission
cross-section have been measured for 233 Pa up to now. The reason of this absence is the short decay
half life of the 233 Pa nucleus (27 days) that leads to an extreme activity of 7.10 8 bq µg -1 s-1. Due to this
high radioactivity, there is no technique presently available to measure directly the 233 Pa(n,f) reaction
cross-section.
The particular aim of this work is to provide data for the neutron induced fission of 233 Pa in the
fast neutron energy range from 0.5 to 10 MeV. To overcome the problem of the induced radioactive
233 Pa, we have used the transfer reaction 232 Th( 3 He,p) 234 Pa that leads to the desired 234 Pa nucleus as it
should be observed in the 233 Pa(n,f) reaction. Several years ago, this method has been used
successfully to estimate the neutron-induced fission of short-lived targets like 231 Th (25.6 h), 233 Th
(22.1 min) etc.
233 U
2. Theory
Transfer reaction measurements give access to the fission probability Pf(E * ) as a function of
excitation energy. The equation relating the two quantities can be written as:
Pf(E
exc
where α(E exc
,J,π) is the relative population of spin states (J, π).
⎛
⎞
⎜
Γ (E
* π ⎟
⎜
⎟
∑ α π F
,J, )
)= ( E
⎜ exc
, J, )
⎟
(1)
Jπ
⎜
∑Γ
i
(E
*
,J, π)
⎟
⎝
i
⎠
Neutron induced fission measurements give access to the fission cross-section σ F (En) as a
function of the incident neutron energy that is relating to the excitation energy as:
E exc
≈ Sn + En (2)
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