The general approach to undertaking a risk assessment is well described in International Standard IEC/ISO 31010, which also provides considerable information about risk assessment methods. It notes, however: “The standard does not provide specific criteria for identifying the need for risk assessment, nor does it specify the type of risk analysis method that is required for a particular application.”
A risk assessment of mine closure scenarios entails issues that do not normally arise in a “conventional” risk assessment, including:
- The long period of post-closure over which the probabilities of events giving rise to a risk may occur.
- The absence of any significant record of performance of post-closure mine facilities and hence the essential impossibility of establishing valid numerical probabilities of long-term, post-closure performance.
- The fact that there is a body of knowledge and practice for long-term mine closure based on the prescribed standard of closure for 1,000 years to the extent reasonably achievable and at any rate for 200 years. (This is the U.S. Department of Energy’s Uranium Mill Tailings Remedial Action (UMTRA) Project criterion.)
In the very long term, all closed mine facilities will respond to the inevitable forces of nature as would any other geomorphic form in the environment.
The report Cold Regions Cover System Design Technical Guidance Document (MEND 2012) recommends that a cover system be designed using a minimum 100-year design life. The document adds: “However, the design itself must be evaluated on a site-specific basis using a risk-based approach.” The document recommends use of a Failure Modes and Effects Analysis (FMEA) approach. Note that FMEA approach is but one of many risk-based approaches noted by ISO 31000.
MEND (2012) makes no mention of whether surveillance and maintenance may be considered in assessing cover performance over 100 years. In private discussion with the authors, we are informed that this issue was not considered by them. The authors do, however, concede in private discussion that it is reasonable to assume active surveillance and maintenance over 100 years. Once you do that it kind of reduces the risk assessment to a trivial and standard every day exercise.
At the recent cold covers workshop the authors of the MEND document talked about assessing the performance of the closure covers for 1,000 years. I asked them if that could include consideration of surveillance and maintenance, and again they confessed they had not considered the issue.
They did admit that probably one should do two or more entirely separate risk assessments: one for 100 years with OMS; and one for 1,000 years with no OMS. I have just done this and the results are very different.
The report De-Licensing of Oil Sands Tailings Dams Technical Guidance Document (Oil Sands Tailings Dam Committee, 2014) describes a risk assessment process for de-licensing of an oil sands tailings dam. The document does not consider risk assessment for various post-closure periods. In private conversation with the authors of the De-licensing Technical Guidance Document, they admit that the issue of different post-closure performance periods did not arise and was not considered.
The specificity of the approach to risk assessment incorporated into the De-licensing Technical Guidance Document has been criticized as too “specific” and indeed potentially misleading. In our opinion, there simply is not data to support the numerical approach of the De-licensing Technical Guidance Document and hence a simpler approach is used in this document.
With regard to the very long term nature and performance of the de-licensed oil sands tailings facility, the De-licensing Technical Guidance Document states:
“The objectives for the closure of an inactive oil sands tailings facility, and its transformation into a solid earthern structure that qualifies for de-licensing as a dam are: the resulting structure is considered to have a physical performance that, as a minimum, can be managed as a solid earthern structure (or conventional waste dump) but that ideally is compatible with the performance of the natural landforms in the region.”
Not sure what it means to be compatible with the performance of natural landforms. Did they actually mean comparable? Or did they actually mean consistent—which might imply the same as. As a lawyer opposing the system, I would argue that in this ghastly written sentence, compatible means the same as. When I tackled one of the authors on this point he replied: not really. Like Alice it means whatever I take it to mean. What a risk!