Search - OECD Nuclear Energy Agency

70 MW. The beryllium matrix has 79 cylindrical holes in a hexagonal lattice of 96.44 mm pitch at the
reactor midplane: there are 64 standard channels (84.2 mm diameter), 10 small peripheral channels
(50 mm diameter) and 5 large channels (200 mm diameter). All channels can receive fuel elements,
control rods, beryllium plugs or experiments, which allows a great flexibility of operation.
Each fuel element has a 762 mm active fuel length. The presently used 6n-G fuel elements
(Figure 1, right hand side) contain, when fresh, 400 g 235 U in the form of UAl x (1.3 g U/cm 3 ) + 3.8 g
boron (B 4 C) + 1.4 g samarium (Sm 2 O 3 ). The reactor core is loaded with 10 to 13 kg 235 U (30 to 40 fuel
elements, not all fresh). The concentration at discharge of the fuel elements is about 50% of the initial
fissile content value. The present nominal heat flux at the hot spot is 470 W/cm 2 , the maximum value
allowed for nominal cooling conditions (probable onset of nucleate boiling) is 600 W/cm 2 . Typical
neutron fluxes are (in the reactor hot spot plane):
• Thermal conventional neutron flux : v 0 n = v 0 ∫ 0
0.5 eV
n (E) dE:
2 to 4 10 14 n/cm 2 s in the reactor core
2 to 9 10 14 n/cm 2 s in the reflector and core flux trap (H1).
• Fast neutron flux : Φ >0.1 MeV = ∫ 0.1
∞MeV (G(
4 to 7 10 14 n/cm 2 s in the reactor core.
2.2 MYRRHA, a multipurpose ADS for R&D
MYRRHA, in its present development stage, is described in another paper of this conference [5].
It is based on the coupling of a proton cyclotron with a liquid Pb-Bi windowless spallation target,
surrounded by a sub-critical neutron multiplying medium in a pool type configuration [6]. Ion Beam
Applications (IBA), a world leader in accelerator technology, is in charge of the design of the
accelerator. The accelerator parameters presently considered are 5 mA continuous current at 350 MeV
energy. The proton beam will impinge on the spallation target from the top. The spallation target
circuit is separated from the core coolant as a result of the windowless design presently favoured to
avoid window overheating and embrittlement and loss of energy. To meet the goals of material
studies, fuel behaviour studies, radioisotope production, transmutation of MAs and LLFPs, the
MYRRHA facility should include two spectral zones: a fast neutron spectrum zone and a thermal
spectrum one.
The core pool contains the fast spectrum core zone, cooled with liquid Pb-Bi, and several islands
housing thermal spectrum regions located in in-pile sections (IPSs) at the periphery of the fast core. In
its present design phase, the fast core is fuelled with typical fast reactor fuel pins (triangular pitch:
10.2 mm) with an active length of 600 mm arranged in hexagonal assemblies with 122 mm pitch. The
central hexagon position is left free for housing the spallation module. The fast sub-critical core of
MYRRHA is made of 18 MOX fuel assemblies of which 12 have a Pu content of 30% and 6 a Pu
content of 20%: a horizontal cross-section is given in Figure 2 (the circumscribing circle has a
diameter of 610 mm).
The design of MYRRHA needs to satisfy a number of specifications such as:
• Achievement of the neutron flux levels required by the different applications considered in
MYRRHA:
Φ >0.75 MeV = 1.0 10 15 n / cm²s at the locations for MA transmutation,
797