Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

068
progress report
2010
Figure 3.24 – Picture of the ITER Mock–up
assembled at FNG
Front casing
Front insulation
1st winding
2nd winding
Side and rear
casing
1st layer SB
1981 1982
Back of SB
2041 2047
CuCrZr
Rest of coil
Thermal shield IVVS
Side and rear insulation VV outer shell
VV inner shell
3rd layer
FW
Manifolds
First wall
Figure 3.25 – The radial section on the inboard
side of the latest ITER Alite model
Figure 3.26 – MCNP model (equatorial section
(z=0))
3.5 Neutronics
Neutronics shielding experiment on a mock–up of ITER:
dose measurement in the magnet coils
In the ITER design it is important to minimize the
uncertainty in the estimates of the nuclear loads; in
particular, the nuclear heating of the TF coils in the
inboard leg is the most critical. To this purpose, accurate
radiation transport calculations of the shielding is
requested. These calculations are very challenging, since the
radiation attenuation from the first wall to the TF coils can
be many orders of magnitude and, at the same time,
accuracy of the order of ±10% or better is required. The
calculation must be benchmarked as far as possible against
suitable experiments to attain the necessary validation of
nuclear data and codes used.
To check whether the present design calculations are able to
evaluate the shielding properties of the ITER shield at
inboard side with sufficient accuracy and reliability, an
experiment was realized at the Frascati Neutron Generator
(FNG) of ENEA–Frascati. The experiment was
commissioned by ITER IO.
In this experiment, a mock–up of ITER inboard shield,
vacuum vessel and TF coils was replicated and irradiated by
14–MeV neutrons (fig. 3.24). The mock–up also included
the borated steel plates presently foreseen by the design as
well as some pieces of the actual superconducting cables,
which represented the actual experimental region (TF coils).
The final mock–up dimensions and materials compositions
were based upon the dimensions and materials of the latest
version of the Alite Monte Carlo n–Particle (MCNP) model
of ITER (fig. 3.25).
The resulting nuclear heating in the TF coils was measured
by using state–of–the–art experimental techniques (high
sensitivity thermoluminescent dosimeters), and compared
with calculations performed with the MCNP code and the
ITER reference nuclear data library FENDL.–2.1. The
experiment also included the measurement of selected
reaction rates along the central axis of the mock–up as well
as in the coil region. These measured quantities were
compared with the results of calculations too.
A very detailed model of the experimental set–up was used (fig. 3.26). In this way, the highest level of accuracy
was attained in the ratio between the calculated and the experimental quantities (C/E ratio), thus providing
fundamental information about the dose absorbed by the superconducting inner coils. For example, for the
nuclear heating a slight overestimation is observed within the C/E error (±10%) in the coil region. This
accuracy in the C/E ratio was never reached in previous experiments.
Moreover, regarding the uncertainty margins in the FENDL–2.1/MCNP–5 prediction, it can be concluded
that:
• the fast neutron flux is calculated within an uncertainty margin of about ±15% in the ITER shielding
blanket and the magnet region.
• the thermal neutron flux is calculated within an uncertainty margin of about ±15% in the ITER shielding
blanket, vacuum vessel, up to the toroidal field coils.