RRFM 2009 Transactions - European Nuclear Society

expansion and steam formation were the effective shutdown mechanisms. In the referred three
tests, the inserted reactivity was 2.63$, 2.72$ and 3.55$ respectively.
SIMMER-III simulations on the three disruptive tests have been performed. In order to well
model the heat transfer between pin surface and water, coolant channels have been
sub-divided into several meshes in the radial direction [8], [9]. The initial near zero-power state
was firstly well simulated in order to build up a proper model for the further simulations of the
transient tests. As can be seen from Figure 1, comparison between the simulated results and
the experimental data indicated that neutron flux profiles in the core have been well
represented.
Figure 1: Normalized axial neutron flux profiles at the initial state
The three power excursion tests with different periods showed similar transient developing
mode. In the tests, reactivity was inserted into the core through the ejection of the transient
rods which located in the core center. In SIMMER-III simulations, inserted reactivity was
defined directly with a certain ramp rate instead of the modelling of the transient rods’
movement. As will be presented in more detail in reference [8], SIMMER-III has well simulated
transients of the power, released energy as well as temperatures in all the three cases. Figure
2 shows that SIMMER-III simulated reactor power and released energy agree well with the
experimental results in the most disruptive case with a period of 3.2 ms, in which 3.55$
reactivity was inserted. Uncertainties in the modelling are still related to the feedback effects
from local boiling and plate expansion.
Figure 2: Reactor power and released energy in the case with period of 3.2 ms
2.2 Modelling of steam explosion with MC3D code
A part from SIMMER III work, the MC3D code is used to evaluate the loads induced by a steam
explosion that might follow a BORAX transient. The MC3D code is a modular numerical tool
devoted to multidimensional multiphase flow calculations with two main options (named
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