The document discusses a potential collaboration called HotBENT to conduct a joint experiment at the Grimsel Test Site to evaluate the behavior of clay buffers at temperatures of 150-200°C. The experiment would provide data on clay properties at high temperatures that are limited in current models. Partners like NAGRA are interested to justify higher design temperatures that reduce costs and to ensure most of the clay buffer remains intact. A proposed experiment involves placing bentonite in the old FEBEX tunnel at Grimsel up to 200°C to study moisture transport and swelling properties over 5 years.

1. Spent Fuel and Waste Science and Technology
Potential HotBENT Collaboration
Liange Zheng and Jens T. Birkholzer
LBNL, Berkeley, California 94720, USA
Las Vegas, NV.
May 23-25, 2017

2. Spent Fuel and
Waste Science and
Technology
HotBENT - Studying the effects of high
temperatures on clay buffers/near field
2
A planned collaboration project, led by NAGRA, to conduct a joint GTS
experiment integrated with lab and modeling studies to evaluate buffer
behavior at 150 oC to 200 oC
Other potential partners: NUMO, RWM, SURAO, GRS (plus SKB, ENRESA?)

3. Spent Fuel and
Waste Science and
Technology
 Due to the high temperature it is expected that the following physico-
chemical effects will occur:
– cementation possibly affecting mechanical properties
– illitization (under certain conditions, e.g. high potassium concentrations) affecting
mechanical properties
 Due to the strong thermal gradients:
– complex moisture transport process, including convection of vapor
– delayed saturation
– heterogeneous, time-dependent density distribution (differential swelling)
What to expect in the buffer for
T > 150oC?
 Numerical models developed, or being
under development, can be used to
simulate the thermal period of a
repository, but database for
T > 150 °C limited (laboratory) or non-
existent (large scale)
Pellet- based
buffer

4. Spent Fuel and
Waste Science and
Technology
NAGRA’s Interest in the Experiment
 A part of the bentonite buffer is exposed to temperatures higher than 100oC
in the current NAGRA designs
 Higher design temperature can achieve significant cost reductions
 NAGRA’s PA argumentation is that at least half of the bentonite needs to
remain intact. Ensuring that the largest part of the buffer remains intact
adds additional robustness as construction related voids might be present
reducing overall buffer related performance (e.g., overall swelling pressure).
 The higher and the more homogeneous the swelling pressure  the lower
the chance of microbial impacts at the canister surface  the easier the
argumentation related to the canister lifetime.
 NAGRA’s interest in HotBENT is to:
– Increase database of bentonite performance at temperatures higher than
150oC *
– Understand processes that can only be captured at the large scale, address
the up-scaling effects (at scales relevant to the repository design)
– complement/underpin results from lab studies, HE-E, FE

5. Spent Fuel and
Waste Science and
Technology
DOE’s Interest in the Experiment:
Direct Disposal of DPCs
5
Hardin, E.L., Repository Engineering
 Largest capacity: Magnastor
DPC system (37-PWR or equiv.)
 Thermal limits: 35.5 kW
storage/24 kW transport
 Fuel cool time >4 yr OoR
depending on burnup
Pictures and data
from NAC
International
website
31Mar2012

6. Spent Fuel and
Waste Science and
Technology
Proposed “HotBENT” in the FEBEX
Tunnel at Grimsel Test Site
 Old/former FEBEX-DP tunnel (70m, well characterized, multiple
boreholes,…)
 40 “Big Bags” and 10 pallets of bentonite blocks MX80 (FE-experiment)
 Auger machine (to be adjusted), currently at FMT

7. Spent Fuel and
Waste Science and
Technology
HotBENT Modular Design and
Schedule
2019 2020 2021 2022 2023 2024 2025
Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Excavation/
analysis/
modelling/
reporting
1- 2 yeras
Experiment
construction
7 month
Experiment runing/monitoring
5 years (-->2023)
2016 2017 2018
Conceptual design
and modelling
(budget)
Formalise
partici-
pation
6 month 2 month
Site and experiment
preparation
6- 8 month
Proposed Timeline

8. Spent Fuel and
Waste Science and
Technology
Scoping calculation for HotBENT (1)
Scoping calculation based on THMC model for FEBEX-DP
Spatial distribution of temperature and water saturation at the “hot” sections if we
sample at 5 and 10 years.
The time needed for bentonite to reach fully saturation,without artificial hydration?
0
10
20
30
40
50
60
70
80
0 1000 2000 3000 4000 5000 6000 7000 8000
Relativehumidity(%)
Time (day)
R = 0.52 m
WCSE2-03 WCSE2-04
WCSE1-03 WCSE1-04
FEBEX,100 °C HotBENT 200 °C
Bentonite near the heater remains fairly dry
for a long time whereas bentonite near the
granite because fully saturated rapidly just
like that in FEBEX test.
0
5
10
15
20
25
30
35
0.4 0.6 0.8 1 1.2
Watercontent(%)
Radial distance (m)
HotBENT, 200°C, 5.3 yrs
HotBENT, 200°C, 5 yrs
HotBENT, 200°C, 10 yrs
HotBENT, 200°C, 10.3 yrs
heater
granite
HotBENT undergoes strong re-
distribution of moisture during the
cooling period

9. Spent Fuel and
Waste Science and
Technology
Scoping calculation for HotBENT (2)
Speed up the process: artificial hydration for HotBENT
Artificial hydration from the bentonite-granite interface
only speed up hydration slightly.
heater
X
Z
0 0.45 m 1.135 m 50 m
bentonite granite
0.125 m
1.1E-6 kg/s
0
5
10
15
20
25
30
35
0.4 0.6 0.8 1 1.2
Watercontent(%)
Radial distance (m)
out ring hydration, 1 yr
out ring hydration 2 yrs
out ring hydration, 5.3 yrs
HotBENT, 200 °C, 5.3 yrs
heater
granite
Raise the temperature of FEBEX
mock-up test in Madrid?

10. Spent Fuel and
Waste Science and
Technology