atw - International Journal for Nuclear Power | 05.2021

atw Vol. 66 (2021) | Issue 5 ı September
DECOMMISSIONING AND WASTE MANAGEMENT 44
Material
Bulk modulus
(GPa)
Shear modulus
(GPa)
Density
(kg·m -3 )
Elastic modulus
(GPa)
1200 meters below surface with
different anisotropic stress ratios
(minimum to maximum principal
stresses) up to 1:3. SSiC canister and
foundation are modelled as elastic
material. The falling rock blocks are
simulated either as elastic material or
as assembly of distinct blocks with
calibrated elasto-plastic parameters,
which allows to consider the rock
disintegration during the impact.
Damping was not applied for the
dynamic simulations because data are
not available, but viscous boundary
conditions were applied to avoid
unrealistic reflections at the lower
bottom of the ground. This makes the
simulations once again conservative.
The interface stiffnesses at the contact
between the colliding parts (e.g.
between canister and rock block or
ground) are adjusted in such a way,
that impact induced stresses reach
maximum values (corresponding lab
data are not available). So far not
explicitly otherwise mentioned the
model parameters given in Table 3 are
applied.
µ jkn
(GPa·m -1 )
jks
(GPa·m -1 )
Rock & Foundation 40 29 2500 70 0.21 100 100
Buffer 1 0.5 2000 1.35 0.27 100 100
SSiC 200 180 3100 415 0.15 100 100
Clay-stone 40 18 2500 47 0.30 100 100
Clay-stone
jkn
TPa·m -1
75
| Tab. 3.
Matrix and interface contact parameters.
jks
TPa·m -1
25
jcoh
MPa
40
jtens
MPa
10
jfric
°
0
res_jcoh
MPa
0
jkn = jks = 440 TPa·m -1 for dynamic loading to avoid unrealistic penetration
| Fig. 3.
Left: Cross section of cylindrical canisters (see also Table 4); middle: Model set-up for canister completely
imbedded inside the host rock incl. buffer; right: Impact constellations of canisters after free fall.
Canister a/mm b/mm c/mm d/mm e/mm f/mm
HTR (5 pebbles) 62 305 92 335 15 15
CANDU 102 510 142 550 20 20
PWR/BWR 400 4930 470 5000 35 35
Vitrified waste 450 1350 500 1400 25 25
| Tab. 4.
Dimensions of canisters (see also Figure 3).
| Fig. 4.
Maximum tensile stress inside unprotected canister for different inclination
angles of VW (up) and HTR (middle) canister (see Table 5).
res_jtens
MPa
0
res_jfric
°
27
Four different canister types are
considered as given by Figure 3 (left)
and Table 4. The considered waste
canisters are hollow cylinders sealed
at one, respectively the two ends using
the technique of Rapid Sinter Bonding
(RSB) as proposed by Knorr and
Kerber [7].
Static loading scenarios
This loading case considers anisotropic
earth pressure on completely in
a rock mass embedded VW and HTR
canisters (Figure 3 (middle)). Table 5
lists all the considered earth pressure
constellations in terms of principal
stresses (X and Y (both horizontal)
and Z (vertical): 1:1:1, 2:1:1, 3:1:1,
2:2:1, 3:2:1, 3:3:1. These constellations
cover all typical stress states
existing in potential host rocks. The
considered maximum principal stress
ratio is 3:1. The angle between canister
axis and Z-axis is set to 0 °, 30 °,
60 ° and 90 °, respectively. Figure 4
documents the maximum induced
tensile stresses inside the SSiC canisters.
The considered depth is 1200 m,
and the vertical earth pressure will
be about 30 MPa. Overall, in any
case the maximum tensile stress in
the unprotected canister does not
exceed 50 MPa which is significantly
below the tensile strength of SSiC,
which is 150 MPa. Maximum tensile
stress is mainly distributed around
the two lids of the canister as shown
in Figure 5. The average stress in
the VW canister is bigger than that
in the HTR canister. The thickness of
the canister is a controlling factor.
For the VW canister, the thickness/
height ratio is 1/56, while the thickness/height
ratio for the HTR canister
is 1/22.
Exemplary, for the HTR canister
some selected simulations with a
buffer sealing (200 mm thick) were
performed. As Figure 6 documents,
such a clay (bentonite) buffer can
significantly reduce maximum tensile
stresses.
Decommissioning and Waste Management
SSiC Nuclear Waste Canisters: Stability Considerations During Static and Dynamic Impact ı Yanan Zhao, Heinz Konietzky, Juergen Knorr and Albert Kerber