No abstract

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by 90Sr and 137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:• Low hydraulic conductivity compared to the host rock;• Good swelling properties to seal construction-caused joints and rock fractures;• Plasticity to absorb rock movements and to distribute pressure homogeneously;• Good retention of radionuclides; and• Stability over a period of at least 106 years.Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.

Kurzfassung In dieser Ubersich tsarbeit wird zunachst iiber die verschiedenen Methoden zur Untersuchung fester Korrosionsprodukte orientiert. Anschliefiend werden die Grundlagen zur Konstruktion von Stabilitatsdiagrammen vermittelt. Der spezielle Teil behandelt die Korrosionsprodukte von Mg, Zn, Cd, Pb und Cu. Er enthalt Angaben iiber Gittertyp, Loslichkeit, natiirliches Vorkommen und iiber die Mikromorphologie der Verbindungen. Es werden nur Korrosionsprodukte beriicksichtigt, die sich an der Atmosphare, in natiirlichen Wassern oder im Erdboden bilden.

Zusammenfassung: Die chemischen Reaktionen an der Grenzfliiche Oxid/L6sung -Protolyse, Adsorption von Anionen und Kationen -lassen sich zu einem Koordinationsmodell der Grenzfl~che zusammenfassen und mit Massenwirkungsausdr/icken quantitativ beschreiben. Mit Hilfe dieses Modells gelingt es, die Aufl6sungskinetik oxidischer Festphasen unter einem einheitlichen Gesichtspunkt darzustellen: In Gegenwart adsorbierbarer Anionen verlaufen solche Reaktionen fiber Oberfl~chenkomplexe, und die Aufl6sungsgeschwindigkeit R l~iflt sich beschreiben mit6/A ist der Oberfl~ichenbedeckungsgrad mit einem anionischen Komplex. Das Koordinationsmodell wird auf~erdem auf die Adsorption anionischer Inhibitoren angewendet.Abstract: Chemical processes at the hydrous oxide-solution interface -proton transfer, specific adsorption of cations and anions -can be interpreted in terms of a surface coordination model and quantified in terms of mass law equlibrium constants. With the help of this model the kinetics of dissolution of solid oxide phases can be formulated in a unifying way : In the presence of specifically adsorbable (surface-coordinating) anions the dissolution rate R depends critically on the relative concentration of surface complexeswhere 0 A is the relatlve surface coverage with an anionic surface complex. The coordination model is also used to interpret the adsorption of anionic inhibitiors.

The production of hydrogen from the anaerobic corrosion of iron or steel is an important issue in low/intermediate level nuclear waste repositories where large quantities of iron and steel (e.g. as drums and reinforcing steel) accompany the waste.Most of the iron in intermediate level repositories is in a cemen-titious environment. A review of the literature on the corrosion of iron and steel at high pH values, in particular in cementitious environments, points to hydrogen evolution rates between 22 and 220 mmol(H2)m−2a−1. There is some indication that the rates might be lower but for normal engineering applications there has been no practical need to demonstrate this, and hence a lower rate cannot be assumed on current evidence.In the present work a volumetric method was used to measure hydrogen evolution rates over several thousand hours under conditions relevant to intermediate level waste repositories. The sensitivity of this method (0.4 mmol(H2)m−2a−1) is sufficient to detect hydrogen evolution rates lower than those predicted for iron and steel in concrete.Hydrogen evolution rates in highly alkaline cement pore water were below 0.4 mmol m−2a−1 and almost 1 mmol m−2a−1 for a pore water representative of an aged cement; no decrease was observed even after 12000 h. In general hydrogen evolution rates in alkaline media were observed to take several thousand hours before approaching a constant rate.