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1. Introduction
In the EADF design [1,2], the primary-coolant flow is assured by natural convection and
enhanced by a particular system of cover gas injection into the bottom part of the riser. The aim of this
work is to present a simple model that has been recently developed at ENEA Casaccia to preliminarily
investigate the capability that the primary-coolant natural convection has to mitigate typical core
transients in ADS. The model, that allows to quickly evaluate the primary-coolant velocity variation
induced by the natural convection during the core transients, has been already implemented in the
Tieste-Minosse code [3,4] recently developed at ENEA to preliminarily investigate core transients in
solid fuelled ADS [5]. Results obtained by taking into account the primary-coolant velocity variations
evaluated by means of the simple natural convection model are also presented and compared with the
corresponding transient trends obtained by considering a constant primary-coolant flow. In particular,
the present paper results concern transients induced by: 1) the proton beam interruption [6-9] or short
duration beam trips [5,10]; 2) the proton beam jumps [5,11,12,13]; 3) the loss of the primary-flow due
to the failure of the active system of convection enhancement.
2. The simple model
We face the subject of the natural convection process, in the attempt to derive a simplified model
to be easily used in core dynamics codes for solid fuelled ADS. We aim to obtain a preliminary
evaluation of the natural convection impact on liquid metal cooled ADS dynamics. In order to make
this quick evaluation, we assume the possibility of reducing a three-dimensional configuration (the
actual plant) into a one-dimensional model. Practically, this assumption leads to neglect the recirculation
phenomena into the vessel pool.
Moreover, we will take into account the heat exchange from the fuel to the coolant into the core
and from the primary to the secondary loop coolant into the heat exchangers, but we will neglect the
heat exchange between the primary coolant and the loop walls. Finally, we will consider the coolant
movement, but we will neglect the heat conduction along the coolant.
To look for the hydraulic solutions of a single-phase fluid, flowing in a supposed onedimensional
loop, in the following we will indicate with:
ν the mean velocity of the primary-coolant
ρ the primary-coolant density
σ the generic section area of the primary loop
Q V
the volume flow rate≡ ρ σ
Q m
the mass flow rate ≡ Q σ = ρσ ν
V
G the specific flow rate ≡ Q m
/ σ.
As a consequence of the mass conservation, assuming a generic volume V inside two normal
sections of a stream-tube, we can write the following equation of continuity:
∂
∫ ρ ( t
∂ t
)
V
dV
=
Q
m
− Q ′
m
(1)
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