WM2015 Development Backfill LILW HLW 15374

Development of a Backfill Material for LILW Disposal Galleries 

in the current Belgian Geological Disposal Concept 

Repository design consisting of a category B waste section and a category C waste section. 

Engelhardt/Haverkamp 

Engelhardt, H.-J.; Haverkamp, B.; von Borstel, L.E.; Van Marcke, Ph. & Coppens, E. 

ONDRAF/NIRAS DBE Technology GmbH – Backfill – Project & ONDRAF/NIRAS 

Tests B 31/07/2014 & C Techno (MPA – Meeting Braunschweig) 

Category WM2015 Brussels, -B 15374 & 21/02/2013 C 

1

It is planned to produce the backfill above ground and 

to pump it into the galleries. 

Objectives of the backfill measure 

1) filling the void space in the disposal galleries 

2) limiting the amount of free water in the repository 

3) providing stability to the galleries 

4) providing a chemical environment consistent with the design concept 

4a) no disturbance of the corrosion-protective environment 

4b) no formation of mobile radionuclide complexes 

5) no damage of the gallery walls 

6) limited strength to enable the retrieval of waste packages 



ONDRAF/NIRAS DBE Technology GmbH – Backfill – Project & ONDRAF/NIRAS 

Tests B 31/07/2014 & C Techno (MPA – Meeting Braunschweig) 

Category WM2015 Brussels, -B 15374 & 21/02/2013 C 

2

Backfill requirements 

Hydraulic transport of the backfill. 

1) filling the void space in the disposal galleries 

2) limiting the amount of free water in the repository 

3) providing stability to the galleries 

→ Negligible amount of bleed water 

(development of a water layer on the surface of a suspension) 

separation of water and suspension during the pump process 

and in the galleries 

→ Segregation resistance 

(capacity of a suspension to preserve a homogeneous 

grain size distribution) 

→ High flowability and good spreading behaviour 

during a time span of approximately 5 hours. 



ONDRAF/NIRAS DBE Technology GmbH – Backfill Project – Project & ONDRAF/NIRAS 

B Tests & C B 31/07/2014 Techno & C Techno (MPA – Meeting Braunschweig) 

Category Brussels, WM2015 -B 07. 15374 & 21/02/2013 November C 2014 

3

Backfill requirements 

Objectives of the backfill measure 

4) providing a chemical environment consistent with the design concept 

→ Limitation of soluble chlorides, sulphur-containing substances, organic materials. 

The chemical requirements influence the selection of the ingredients (no fly ash, 

no burnt slates, no slag containing materials, e.g. CEM III-cement, blast-furnace slag, etc.) 

5) no damage of the gallery walls 

→ Limitation of the autogenous swelling or shrinkage of the backfill. 

6) limited strength to enable the retrieval of waste packages 

→ Limitation of the compressive strength. 

A high porosity of the backfill (diffusion and flow of gases) and 

a high thermal conductivity (galleries with heat-generating waste) are favourable. 






4

Initial series of investigation 

CEM II/A-LL 32.5 + calcareous powder + river sand + water 

CEM II/A-LL 32.5 + barite powder + silica fume + river sand + water 

CEM II/A-LL 32.5 + silica fume + river sand + water 

Step 1) Determination of the composition of non-bleeding, segregation resistant 

mixtures. 

Step 2) Comparison of the mixtures. 

Mixtures with silica fume show 

the best flow and spreading behaviour, low thixotropy 

and the highest total porosity. 

Results of laboratory bleed water tests which were carried out 

to identify the border between bleeding and non-bleeding mixtures. 






5

Second series of investigation (specification of a reference mixture) 

It is assumed that mixtures with a mini-slump of more than 200 mm (during a mix 

time of 5 hours) have a sufficient flowability. 

According to the intersection points of the mixture trend lines and the “200 mm-slump-line” results that 

flowable mixtures have contents of 

> 11.4 wt.-% cement, > 11.9 wt.-% silica fume, < 33.5 wt.-% sand, and > 43.5 wt.-% water. 

This approach results in a minimum water-cement-ratio of 3.8. 






6

Second series of investigation - specification of a reference mixture 

> 11.4 wt.-% cement, > 11.9 wt.-% silica fume, < 33.5 wt.-% sand, > 43.5 wt.-% water, water-cement-ratio > 3.8. 

water-cement-ratio > 2.8 → compressive strength < 10 MPa 

→ water-cement-ratio of the reference backfill = 4.25. 






7

CEM II/A-LL 32,5 

Bei der Verfüllung von Strecken mit wärmeentwickelnden Abfallen teilweiser 

Austausch des Silikastaubs gegen Eisenoxide. 






8

Simulation of the backfill process with Plexiglas tubes 

Stationary Zyklos 100 L pan mixer 

High-shear / high-speed hand mixer 

Screw pump and flexible pipeline (1 inch) with pressure sensors 






9


Flow rate between 3 litres and 5 litres per minute (mix temperature 23 °C). 

Tube 1: Hollow cylinder (length 2 m, diameter 0.5 m) 

Failure of the high-shear mixer 

during the first 2 mix batches 

→ Increase of silica fume content 

→ Development of bleed water and 

a wavy surface (ripple marks) 

The bleed water and air left the tube. 

The tube was completely filled with the 

exception of some air bubbles. 






10


Tube 2: Cylinder (length 2 m, diameter 0.5 m) with two supercontainer models 

Negligible amount of bleed water (water film). 

The tube was completely filled without 

a problem. 






11


Temperature development 

Temperature increase of 15 K (corrected value 21 K). 

Good agreement with the calculated value (24 K). 

Low stresses during the warming phase 

(material creep). 

Possibility of thermal induced 

crack formation during cooling 

(category B) due to the length 

of the backfill sections (30 m). 






12


Autogenous shrinkage < 70 mm/30 m (length of the backfill section) or 

< 2 mm/0.8 m (maximum distance supercontainer-tunnel liner) according to the 

calculated chemical shrinkage of the backfill material. 

A laboratory test showed comparable values. 

The backfill in the Plexiglas tubes showed no shrinkage cracks. 

It is assumed that shrinkage cracks will occur more or less perpendicular to the 

axis of the disposal galleries. 

Higher risk of shrinkage cracks in the galleries with category B waste 

(no thermal expansion of the backfill due to decay heat). 






13

Evaluation of the backfill injection 






14

Evaluation of the backfill injection 

The evaluations come to the conclusion that standard equipment, such as tanks, pumps, pipelines, 

pig trap stations, and damper can be used for the installation of the backfill system. 






15

The laboratory experiments illustrate the feasibility to backfill the disposal 

galleries according the requirements. 

Possible optimizations: 

Category B: 

Use of air-entraining agents. 

→ Increase of gas-filled pore space (storage volume for gas). 

→ The capillary-breaking pores reduce the pore saturation 

with Boom Clay pore solution. 

Category C: Addition of iron oxides (pigments) ± increase of the sand content 1 ± 

superplasticizers 

→ Increase of the thermal conductivity 

Index 1 : cf. Xu, Y. & Chung, D.D.L. (2000) Effect of sand addition on the specific heat and thermal conductivity of cement. Cement 

and Concrete Research, 30: 59-61. 






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ONDRAF/NIRAS – Backfill Project – Project B Tests & C B 31/07/2014 Techno & C Techno (MPA – Meeting Braunschweig) 

Category Brussels, B 07. & 21/02/2013 November C 2014 

17

WM2015 Development Backfill LILW HLW 15374

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