An evaluation of scenarios for a radioactive repository using GoldSim

  Posted by        Jason Lillywhite

Safely isolating high-level radioactive waste for millennia requires more than just deep geological disposal; it relies on a "multi-barrier" approach. This involves several layers of protection designed to contain the hazardous waste. Key components are the "engineered barriers," which are man-made structures placed within the repository. These can include the waste's immediate disposal container, the surrounding structure often called a silo, and robust materials like concrete used extensively within the repository environment. These work together with the natural geological barrier (the surrounding rock) to prevent or slow the release of radioactive materials.

Today, I want to showcase a study evaluating scenarios for a radioactive waste repository using GoldSim. This study was presented at the recent GoldSim 2024 User Conference. It looks at the long-term durability of concrete and how its deterioration might affect the overall safety of these facilities over vast timescales.

This study, by researchers at Chosun University, explores how dynamic, probabilistic simulation can help predict the performance of these engineered barriers. Ensuring the long-term safety of high-level radioactive waste disposal requires understanding complex phenomena, and GoldSim provides a powerful way to evaluate potential scenarios.

Here's a visual representation of the kind of post-closure scenario they're modeling, showing how groundwater might interact with the repository barriers:

Figure 1: Disposal facility post-closure performance modeling evaluation scenario

Read on to explore how they combined experimental data with GoldSim modeling to assess the impact of concrete degradation.

The Challenge: Concrete Isn't Forever

As mentioned, concrete plays an important role as an engineered barrier. But over hundreds or even thousands of years, it doesn't stay pristine. Groundwater interaction, chemical reactions (like sulfate attack), and other factors can cause it to degrade. If that barrier weakens, there's a risk of radioactive material eventually making its way through the sequence of barriers (container, silo, concrete, surrounding rock) and into the environment.

Predicting exactly how this plays out over such incredibly long timescales (potentially hundreds of thousands to a million years) is tough.

Why GoldSim?

The researchers behind this study, Hyun Jin Yu, Ki Tae Yang, and Jong Soon Song from Chosun University used GoldSim for modeling this system. For those unfamiliar, GoldSim is dynamic simulation software that can perform Monte Carlo simulation with discrete and continuous simulation methods, facilitating the simulation of environmental and engineered systems under uncertainty. It has a solid track record in this field, famously used for the Yucca Mountain repository project in the US (NRC, 2004). It's particularly good at handling the complexities of mass transfer, nuclide migration, and groundwater flow in these kinds of systems. Plus, it's been used plenty of times before to specifically look at concrete deterioration (Brown et al, 2011).

Experiments and Modeling

Before building the GoldSim model, the team did some hands-on lab work. For this preliminary research, they took 20-year-old waste concrete specimens and gave them an accelerated aging process to observe deterioration. This involved immersing the specimens in a water tank for 365 days containing groundwater from the KAERI Underground Research Tunnel (KURT) in Korea mixed with sulfate. The KURT facility provides access to deep groundwater in geological conditions relevant to radioactive waste disposal research. To monitor the chemical and physical changes, samples were analyzed using SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive X-ray Spectroscopy) every 30 days. The data obtained from these experiments were then used to inform the GoldSim modeling.

Here's a peek at their experimental setup and one of the microscopic views:   

Concrete deterioration evaluation experiment

SEM analysis photo of 100 micrometers, 10 micrometers freshwater concrete specimen for 120 days using KURT groundwater and sodium sulfate.

The data gathered from these experiments (like the chemical composition changes shown in Table 2) fed directly into their GoldSim model.

Table 2 - KURT EDS analysis of concrete deterioration using groundwater and sodium sulphate.

	C	O	Mg	Al	Si	S	K	Ca	Fe	Cu	Na	Ti	Mn
30d	7.9	23.4	0.6	1.5	27.7	0.86	0.46	32.3	1.76	3.58
60d	4.85	34.8	0.41	0.93	3.84	12.7	0.78	0.37	38.5		2.94
90d	3.55	28.6	0.94	3.44	12.3	1.22	0.39	46.3	3.29
120d	3.5	33.7	4.71	5.38	8.74	0.91	0.22	39.6	1.08	0.54		1.26	0.38
150d	8.42	34.6	0.79	2.78	8.41	0.59	1.45	38.3	3.63	1.03
180d	4.62	26.1	0.41	1.91	11.7	1.54	0.33	50.9	1.2	1.26
210d	3.41	20.3		9.44	33.4		22.3	2.88	6.17	2.17
240d	3.91	29.3	0.8	2.16	16.1	0.57		44.6	1.29	1.26
270d	5.1	27.4	4.23	0.3	13.3		37.1	2.08	8.23	0.7	1.7
300d	3.3	32.2	0.51	1.58	14.8	0.58		45.6	0.91	0.55
330d	4.05	31.4	0.35	2.35	13.6	0.94	1.02	43.9	0.8	1.38	0.29
365d	5.2	29.5	0.9	2.32	12.9	2.13		43.3	2.45	1.05		2.45

They started with a conceptual sketch (always a good idea) mapping out how groundwater might flow through the different barriers (environment, concrete, silo) and eventually reach the ecosystem (and potentially humans).

GoldSim modelling preliminary sketch

Then, they built the model in GoldSim, simulating the key components:

• The waste source and inventory (using Source elements)
• The silo (modeled as mixing cells to simulate diffusion)
• The concrete barrier deterioration itself (using stochastic elements for random duration)
• The waste disposal cell, considering factors like porosity and partition coefficients

Here are a couple of glimpses into the GoldSim interface for this model:

Modelling concrete deterioration scenarios with GoldSim

Silo Modeling

What Did They Find?

The model simulated a scenario where radioactive waste leaks through the disposal container, the silo, the concrete barrier, and a natural barrier into the ecosystem. By using parameters and insights informed by the experimental concrete deterioration data (including the chemical species measurements), the results showed how this degradation influences the release of radioactive material over time. For example, the result below from their study tracks the modeled mass of different chemical species within the silo content over a simulated million years. This model is able to forecast concentration of radionuclides reaching the environment over time.

Mass in Silo Content

Mass in Concrete

This study provides a good look at how you can pair experimental work with simulation tools like GoldSim to analyze complex, long-term safety questions. What stands out is that effective modeling isn't just about running the software; it requires digging into the actual processes involved, like how concrete degrades over time. These authors put in the effort to do that, using real test data from their lab experiments to build their GoldSim model. This approach of understanding the physical processes and grounding the simulation with experimental results is really useful when trying to make sense of these challenging, long-term scenarios for radioactive waste repositories.

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References

Based on the paper: "An evaluation of scenarios for a radioactive repository using GoldSim" by Hyun Jin Yu, Ki Tae Yang, and Jong Soon Song, Chosun University. Acknowledging the Human Resources Development Project for HLW Management hosted by KORAD.

U.S. Nuclear Regulatory Commission (NRC). (2004). 2004 GoldSim User Conference Project Number 06002.01.351. (Accession No. ML043020546). Retrieved from https://www.nrc.gov/docs/ML0430/ML043020546.pdf

Brown, K. G., Smith, F., & Flach, G. (2011). GoldSim Dynamic-Link Library (DLL) Interface for Cementitious Barriers Partnership (CBP) Code Integration. Paper 11444 presented at the Waste Management Symposia 2011 (WMSym 2011). Retrieved from https://archivedproceedings.econference.io/wmsym/2011/papers/11444.pdf

Kim, G.Y., Koh, Y.K., Bae, D.S. and Kim, C.S. (2004) Mineralogical characteristics of fracture-filling minerals from the deep borehole in the Yuseong area for the radioactive waste disposal project. J. Miner. Soc. Korea, v.17(1), p.99-144.