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3. Irradiation targets and irradiation positions in BR2 and MYRRHA
All irradiation targets considered (for the irradiations both in BR2 and in MYRRHA) were
assumed to have an outer diameter of 8.36 mm and to be clad with HT-9 steel with outer diameter
9.5 mm. The target compositions are indicated in Table 2. The targets were given an active length of
200 mm, positioned symmetrically with respect to the maximum axial flux level of BR2 and
MYRRHA.
Table 2. Composition of the various targets
MA or LLFP Chemical form of the target Density
(%TD)
237 Np
77.9 wt% 241 Am
+22.1 wt% 243 Am
99 Tc
129 I
20 vol% NpO 2 + 40 vol% MgAl 2 O 4 + 40 vol% Al
20 vol% Am 2 O 3 + 40 vol% MgAl 2 O 4 + 40 vol% Al
Tc-metal
NaI
90
90
75
70
The BR2 irradiation targets were assumed to be introduced into a loop consisting of concentric
aluminium tubes with in between cooling water circulation. The outer diameter of the outer tube was
25.4 mm, allowing the loop to be substituted for the central aluminium plug (also with diameter
25.4 mm) of a standard BR2 fuel element such as shown in Figure 1, right hand side. As irradiation
position in BR2 the high-flux channel B180 (see Figure 1, left hand side) was selected.
For the irradiations in MYRRHA, the targets with their cladding were assumed to be introduced
into the irradiation space between the pressure tube surrounding the spallation source and the six
inner hexagonal assemblies of the fast sub-critical core (see Figure 2), at a radial distance of 59 mm
from the MYRRHA main axis.
4. Calculated transmutation yields
The transmutation rates of the MAs and of the LLFPs, both in BR2 and in MYRRHA, were
calculated with the aid of the Monte Carlo code MCNP-4B [10]. The way BR2 was modelled in great
detail is explained in [11,12,13] (e.g. the meat and the cladding of each of the 6 × 3 fuel plates of each
of the fuel elements of the BR2 loading is considered as a separate zone). Also for MYRRHA the
calculations were performed in great detail: each of the 2286 fuel pins (with for each: fuel, gap and
cladding described separately) was modelled. The neutron cross-section library used was ENDF/B-VI,
except for the nuclides not present in ENDF/B-VI; for these ENDF/B-V was used.
In Table 3 the fission and “disappearance” reaction rates are indicated for the various nuclides
considered in the present study. The values are averaged over the target volumes. By “disappearance”
is meant the sum of all nuclear reactions leading to the removal of the nuclide considered from its
(A,Z) position in the table of isotopes. The “disappearance” reaction is hence, practically, the sum of
the processes 16 (= all (n,2n)), 17 (= (n,3n)), 18 (= total fission) and 101 (= neutron disappearance,
viz mainly (Q n,p), (n,d), (n,t), (n,He-3) and (Q ZKHUHWKHQXPEHUVUHIHUWRWKH07QXPEHUVLQ
the ENDF/B format. It should be noted that in the present calculations performed for MYRRHA the
very high energy part of the spallation neutron spectrum (above 20 MeV) is not taken into account.
800