2
TECHNICAL BACKGROUND
Current proposals for high-level waste handling in Germany
consider the disposal within underground rock salt forma-
tions as eligible. The POLLUX containers get filled with
used fuel rods and then transported underground via shaft
hoisting. They will be then loaded onto a rail bound trans-
port system and moved to the respective disposal drifts.
Past concepts proposed lowering the containers onto the
drift floor and then backfill the surrounding void with salt
grit slinger backfilling.
Slinger backfilling proved to be the only applicable
method that meets both the technical quality demands as
well as the safety requirements. This means, that high ini-
tial backfills densities of over 1.5 t/m3 can be reached even
though the operator is not in direct vicinity of the container
(Schaarschmidt, Paschke, Freyer, &Mischo, 2024).
Research has shown that there is a direct relationship
between backfill density and heat transmission from the
container over time (GRS, 2012). In fact, the emitted heat
from the container has a positive influence on the increase
of backfill density. Therefore, it is generally desired that
the heat emission from the container goes into the back-
fill. Since the backfill does have a lower density than the
surrounding host rock, its thermal conductivity is worse.
If the contact surface between the POLLUX and the host
rock is increased, the more likely it would be that the heat
gets transferred into the surrounding rock salt, ultimately
possibly leading to faster cooling of the container and less
heat accumulation around the container. Since the backfill
has an initial density and will only increase its density over
time due to salt creeping and convergence, the air trapped
inside partly acts as an isolating medium and keeping the
containers at a high temperature.
Figure 1 shows the relationship between the named
material characteristics in a 3D diagram and highlights the
interdependency between the thermal conductivity and
porosity, assuming a constant host rock temperature.
TEST DESIGN
In-Situ Testing
TU Bergakademie Freiberg operates an underground test
site in the active rock salt underground mine “Glückauf”
in the city of Sondershausen (Thuringia). In total, there
are 4 cross cuts at a depth of 500 m that have been used
for real-life sized backfill body testing in the past 8 years.
Initially, the research project series “GESAV” focused on
the development and practical testing of newly created
matrix-stabilized rock salt backfill. The material consists
of conventional rock salt backfill with salt binder addi-
tives that form polyhalite bridges between the grains
and stabilize the material as fast as 24 hours after build-
ing it in. In order to facilitate the sophistication due to an
increased level of gathered know-how and experience, the
follow-up project “SAVER” was kicked off. The main goal
of this project was to build two identical backfill bodies
with matrix-stabilized backfill and conventional moist rock
salt backfill and compare them over time with an imple-
mented sensor setup. One major task and difference to the
previous “GESAV” projects was mainly in the backfill body
design: TU Bergakademie Freiberg set the project goal to
develop a POLLUX dummy container with close to real-
life dimensions and backfill it with state-of-the-art methods
as depicted in German preliminary concepts. This means
that a container will be placed on each test drift floor and
then slinger-backfilled.
The container consists of steel-reinforced fiber-glass
pressure pipes which were closed with wood. With regards
to the hypothesis of increased contact surface, it was placed
in a trench within the test drift (Figure 2).
The trench placement does not just increase the contact
surface between the host rock and container but also gives
a stable foundation for the container. For the creation and
the cutting, an underground loader with an attached road
head was used. Practical testing showed high dependency
on skillset of the operator as well as on the road head diam-
eter. The annulus between the trench wall and container
was simply slinger backfilled later without any problems.
Figure 3 shows the fully emplaced and closed container
dummy. It is fitted with a pressure measurement plate on
its top, which is supposed to detect the vertical pressure
created by the gravitational weight of the backfill and pres-
sure from the surround host rock caused by convergence
and creeping.
Figure 1. Relation between thermal conductivity depending
on the porosity and the temperature (GRS, 2012)
TECHNICAL BACKGROUND
Current proposals for high-level waste handling in Germany
consider the disposal within underground rock salt forma-
tions as eligible. The POLLUX containers get filled with
used fuel rods and then transported underground via shaft
hoisting. They will be then loaded onto a rail bound trans-
port system and moved to the respective disposal drifts.
Past concepts proposed lowering the containers onto the
drift floor and then backfill the surrounding void with salt
grit slinger backfilling.
Slinger backfilling proved to be the only applicable
method that meets both the technical quality demands as
well as the safety requirements. This means, that high ini-
tial backfills densities of over 1.5 t/m3 can be reached even
though the operator is not in direct vicinity of the container
(Schaarschmidt, Paschke, Freyer, &Mischo, 2024).
Research has shown that there is a direct relationship
between backfill density and heat transmission from the
container over time (GRS, 2012). In fact, the emitted heat
from the container has a positive influence on the increase
of backfill density. Therefore, it is generally desired that
the heat emission from the container goes into the back-
fill. Since the backfill does have a lower density than the
surrounding host rock, its thermal conductivity is worse.
If the contact surface between the POLLUX and the host
rock is increased, the more likely it would be that the heat
gets transferred into the surrounding rock salt, ultimately
possibly leading to faster cooling of the container and less
heat accumulation around the container. Since the backfill
has an initial density and will only increase its density over
time due to salt creeping and convergence, the air trapped
inside partly acts as an isolating medium and keeping the
containers at a high temperature.
Figure 1 shows the relationship between the named
material characteristics in a 3D diagram and highlights the
interdependency between the thermal conductivity and
porosity, assuming a constant host rock temperature.
TEST DESIGN
In-Situ Testing
TU Bergakademie Freiberg operates an underground test
site in the active rock salt underground mine “Glückauf”
in the city of Sondershausen (Thuringia). In total, there
are 4 cross cuts at a depth of 500 m that have been used
for real-life sized backfill body testing in the past 8 years.
Initially, the research project series “GESAV” focused on
the development and practical testing of newly created
matrix-stabilized rock salt backfill. The material consists
of conventional rock salt backfill with salt binder addi-
tives that form polyhalite bridges between the grains
and stabilize the material as fast as 24 hours after build-
ing it in. In order to facilitate the sophistication due to an
increased level of gathered know-how and experience, the
follow-up project “SAVER” was kicked off. The main goal
of this project was to build two identical backfill bodies
with matrix-stabilized backfill and conventional moist rock
salt backfill and compare them over time with an imple-
mented sensor setup. One major task and difference to the
previous “GESAV” projects was mainly in the backfill body
design: TU Bergakademie Freiberg set the project goal to
develop a POLLUX dummy container with close to real-
life dimensions and backfill it with state-of-the-art methods
as depicted in German preliminary concepts. This means
that a container will be placed on each test drift floor and
then slinger-backfilled.
The container consists of steel-reinforced fiber-glass
pressure pipes which were closed with wood. With regards
to the hypothesis of increased contact surface, it was placed
in a trench within the test drift (Figure 2).
The trench placement does not just increase the contact
surface between the host rock and container but also gives
a stable foundation for the container. For the creation and
the cutting, an underground loader with an attached road
head was used. Practical testing showed high dependency
on skillset of the operator as well as on the road head diam-
eter. The annulus between the trench wall and container
was simply slinger backfilled later without any problems.
Figure 3 shows the fully emplaced and closed container
dummy. It is fitted with a pressure measurement plate on
its top, which is supposed to detect the vertical pressure
created by the gravitational weight of the backfill and pres-
sure from the surround host rock caused by convergence
and creeping.
Figure 1. Relation between thermal conductivity depending
on the porosity and the temperature (GRS, 2012)