The Harpur Hill Site : an assessment as an analogue to a cementitious GDF

This report provides a review of the Harpur Hill Site and whether or not it could be used to provide complementary information on understanding aspects of a cementitious geological disposal facility (GDF) for low and intermediate level radioactive waste (L/ILW) (hereafter simply referred to as ‘GDF’) and in a GDF for higher activity wastes where large amounts of structural concrete are utilized. The relevance to near surface disposal is not explicitly considered here, even though for some aspects the similarities between the site and such a facility may be greater. Leachate within and associated with an evolving GDF utilising cement based grouts/backfills for conditioning and backfilling of L/ILW will initially have a pH of more than 13 over the first 104 years post closure where sodium, potassium and calcium hydroxides (K-Na-Ca-OH) dominate the alkaline fluids derived from the cement. Demonstrated by on-site analysis, the alkalinity of the leachate at the Harpur Hill Site, which is derived from leaching of residual lime (CaO) in lime burning wastes, has a maximum pH of about 12.5. This is dominated by calcium hydroxide (Ca(OH)2). As a result, the Harpur Hill Site is not a suitable analogue of the early stage of evolution of a cementitious GDF for L/ILW, where highly alkaline K-Na-Ca-OH (ca pH 13) dominates the geochemical interactions. The pH 12.5 Ca(OH)2 dominated leachate present on parts of the site may, however, provide a suitable analogue for a more evolved (>105 years) cementitious GDF, and for the more distal parts of a leachate plume from a GDF, where K-Na-OH is no longer present, and help understand alkaline fluid interactions with clays.
While the deeper parts of the Harpur Hill Site are reducing, the near surface parts of the site are oxidizing. From soon after closure a GDF will be reducing and thus many of the processes known to be occurring at Harpur Hill, including metals corrosion and microbial degradation and resulting fate of cellulose degradation products, are not appropriate analogues for a cementitious GDF. This is because both the leachate chemistry and redox conditions are not relevant as they do not represent conditions within a GDF early post closure while the waste form evolves rapidly.
Because of these factors the site is unlikely to provide meaningful information on the expected evolution of a cementitious GDF that could be considered in a safety case and the site is not considered to be an adequate analogue for a cementitious GDF.