GEOLOGICAL SOCIETY OF THE PHILIPPINES
Hyperalkaline Natural Analogue Potential in the Philippines: Implications for Nuclear Waste Disposal Studies
Nuclear waste disposal is one of the most controversial aspects of nuclear energy. Multi-engineered barriers are key to nuclear waste safety and bentonite is one of the most safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. The choice of bentonite results from its favourable properties (including plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides) and its stability in relevant geological environments (see Alexander & McKinley, 1999, for details). However, bentonite is unstable at high pH (e.g. Metcalfe & Walker, 2004). Due to the fact that cementitious materials react with groundwater to produce initial leachates with pH >13 (later falling to around pH 12.5), this led to some repository designs (e.g. Nagra, 2002), that specifically exclude the use of concrete in any sensitive areas containing bentonite. To build a robust safety case, it is important to have supporting natural analogue data to confirm understanding – and validate models – of the long-termed performance of bentonite.
As a result of a review of the available literature and geological investigation (McKinley et al., 2008; Yamakawa et al., 2008), several sites in the Philippines were selected as particularly promising for this purpose; preliminary field investigations confirmed the presence of hyperalkaline springs with a pH of 10-12. Indeed, the studies indicated that on-going serpentinisation in the target area in the Zambales ophiolite (northwest Luzon) may have produced more pervasive high pH groundwater than any other location worldwide. Bentonite is mined in the vicinity and smectite layers are common, occurring in layers many hundreds of metres thick around local volcanic plugs. Bentonite/zeolite layers act regionally as aquitards, isolating flow of deeper high pH waters from surface runoff. Several sites are now under active investigation in the search for evidence of hyperalkaline groundwater/bentonite interaction and this programme has been developed into the International Philippines Hyperalkaline Analogue Project (IPHAP).
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