atw 2018-09v3

atw Vol. 63 (2018) | Issue 8/9 ı August/September
474
AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP
corium resulting in a porous rubble
structure that mainly consists of fuel
and control rods. If this porous structure
enters in contact with the right
amount of water acting as moderator,
there is a potential for recriticality.
In order to avoid recriticality and its
adverse consequences, a criticality
evaluation of the debris bed needs to
be carried out.
The conditions of the debris bed
can be very diverse and strongly
depend on the accident scenario. The
criticality safety control of the fuel
debris is a challenge principally due to
the large uncertainty of the fuel debris
conditions (location, geometry, composition,
temperature, etc.). Severe
accident codes are able to simulate the
accident progression and can be used
to estimate the debris bed conditions,
however, an adequate observation,
sample taking and analysis of the real
fuel debris are crucial to perform an
accurate criticality evaluation.
Due to the high uncertainty of fuel
debris properties, it is necessary to
prepare a comprehensive and extensive
database, which embraces criticality
data of any possible debris bed.
The main factors on the criticality
evaluation of the fuel debris after a SA
are listed below:
• Total amount of corium
• Composition of corium
• Fuel debris geometry
• Coolant conditions
3 Calculation model
3.1 Geometrical model
of the debris bed
Figure 1 shows the conceptual
geometric model of the debris bed
for the Monte Carlo criticality calculations.
The innermost region of the
model represents the debris itself, as a
porous structure consisting of fuel
| | Fig. 1.
Geometric model of debris bed.
Parameter Range Boundary value
Particle size 1 to 14 mm 10.7 mm
Porosity 0.32 to 0.8 Optimum Porosity
Water void fraction 0 to 90 % 0
Fuel burnup 0 to 60 GWd/t HM
25.8 GWd/t HM
(accident conditions)
Debris bed size 10 to 200 cm 200 cm
Water boration 0 to 2,000 ppm B 0
| | Tab. 1.
Criticality parameters and ranges.
particles and water. For conservative
results, the shape of the debris was
spherically arranged minimizing the
neutron leakage and the critical mass.
Surrounding the fuel debris there is a
water reflector of effectively infinite
thickness (approx. 30 cm). Such configuration
was already used for a
criticality safety evaluation for the
TMI-2 safe fuel mass limit [5].
Debris beds comprise particles of
different shapes and sizes, which are
chaotically arranged in the space. In
order to reduce the computational
effort for the criticality calculations,
some simplifications have been
applied to model the porous structure
of the debris: the particles were
assumed to be spherical, all the particles
were assumed to have the same
size and the particles were assumed to
be regularly distributed in the space
following a Body Centered Cubic
(BCC) lattice [6].
3.2 Corium composition
In this study, the Unit 1 of Fukushima
Daiichi NPP was used as reference
[7, 8].
Conservatively, it was assumed
that there was nothing present in the
fuel debris but fuel pellets and water.
Thus, the negative reactivity effects
due to the possible presence of cladding,
fixed absorbers and structural
materials are ignored. As boundary
conditions, room temperature and a
fuel density of 10.4 g/cm 3 are considered.
ORIGEN 2.1 [9] was used to calculate
the radionuclide inventory for
different average burnups, from fresh
fuel up to a burnup of 60 GWd/t HM .
The average burnup in the reactor of
Unit 1 at the moment of the accident
was calculated to be 25.8 GWd/t HM
[8] and was used as reference model.
To perform the burnup calculations,
fresh fuel UO 2 with an initial enrichment
of 3.7 % wt 235 U was irradiated
considering a specific power of
20 MW/t HM in the reactor.
3.3 Coolant composition
Light water is used as moderator. The
density of the water (or void fraction)
was varied to analyse the influence on
the neutron multiplication factor.
Additionally, boron was added in
every scenario in order to know the
required concentration that guarantee
a subcritical condition of the
debris. Room temperature was considered
for all the calculations.
4 Criticality calculations
Criticality calculations have been
performed for multiple scenarios
using the calculation model described
before. Six parameters have been considered
for these calculations: particle
size, porosity, water void fraction, fuel
burnup, debris size and water boration.
The parameters and ranges of
variation are resumed in Table 1.
In order to analyse all the possible
dependencies between these parameters,
they all have been combined by
pairs, resulting in 15 possible combinations
or calculations sets. In each
calculation set, the paired parameters
have been varied over their whole
ranges, giving to the rest of parameters
a boundary value. The neutron multiplication
factor k eff was then calculated
for all the possible combinations. All
the boundary values have been chosen
to be conservative, except the burnup,
where the value at the moment of
AMNT 2018 | Young Scientists' Workshop
A Preliminary Conservative Criticality Assessment of Fukushima Unit 1 Debris Bed ı María Freiría López, Michael Buck and Jörg Starflinger