Abstract
This thesis addresses the coolability of porous debris
beds in the context of severe accident management of
nuclear power reactors. In a hypothetical severe accident
at a Nordic-type boiling water reactor, the lower drywell
of the containment is flooded, for the purpose of cooling
the core melt discharged from the reactor pressure vessel
in a water pool. The melt is fragmented and solidified in
the pool, ultimately forming a porous debris bed that
generates decay heat. The properties of the bed determine
the limiting value for the heat flux that can be removed
from the debris to the
surrounding water without the risk of re-melting.
The coolability of porous debris beds has been
investigated experimentally by measuring the dryout power
in electrically heated test beds that have different
geometries. The geometries represent the debris bed
shapes that may form in an accident scenario. The focus
is especially on heap-like, realistic geometries which
facilitate the multi-dimensional infiltration (flooding)
of coolant into the bed. Spherical and irregular
particles have been used to simulate the debris. The
experiments have been modeled using 2D and 3D simulation
codes applicable to fluid flow and heat transfer in
porous media. Based on the experimental and simulation
results, an interpretation of the dryout behavior in
complex debris bed geometries is presented, and the
validity of the codes and models for dryout predictions
is evaluated.
According to the experimental and simulation results, the
coolability of the debris bed depends on both the
flooding mode and the height of the bed. In the
experiments, it was found that multi-dimensional flooding
increases the dryout heat flux and coolability in a
heap-shaped debris bed by 47-58% compared to the dryout
heat flux of a classical, top-flooded bed of the same
height. However, heap-like beds are higher than flat,
top-flooded beds, which results in the formation of
larger steam flux at the top of the bed. This counteracts
the effect of the multi-dimensional flooding. Based on
the measured dryout heat fluxes, the maximum height of a
heap-like bed can only be about 1.5 times the height of a
top-flooded, cylindrical bed in order to preserve the
direct benefit from the multi-dimensional flooding.
In addition, studies were conducted to evaluate the
hydrodynamically representative effective particle
diameter, which is applied in simulation models to
describe debris beds that consist of irregular particles
with considerable size variation. The results suggest
that the effective diameter is small, closest to the mean
diameter based on the number or length of particles.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 23 Oct 2015 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-8344-7 |
Electronic ISBNs | 978-951-38-8345-4 |
Publication status | Published - 2015 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- nuclear energy
- severe accident
- corium coolability
- debris bed
- twophase flow
- thermal-hydraulic experiment
- porous medium
- numerical modeling