South Africa: Investigations at the Vaalputs near surface repository

Vaalputs is the national repository for the disposal of LILW in South Africa. The Koeberg nuclear power plant is currently the dominant waste generator disposing of waste at Vaalputs. Near surface trenches are utilized for the disposal of LILW (short lived). The LLW (short lived), contained in metal drums, and the intermediate level waste (short lived), contained in concrete containers, is disposed of in separate trenches. The disposal trenches at Vaalputs were designed and constructed to be 100 m long, 7.7 m deep and 20 m wide at the bottom and with sides that slope upwards at an angle of 80 degree. The relatively big trenches described above were designed in anticipation that the LILW from 4 nuclear power plants the size of Koeberg would eventually be disposed of at Vaalputs. Because the nuclear energy programme in South Africa did not develop as originally anticipated, together with strategic changes at Koeberg (e.g. waste minimization techniques), the volumes of radioactive waste shipped to Vaalputs for disposal did not materialize as initially expected. This resulted in the trenches being left open for extended periods of time, thereby causing prolonged exposure of the waste packages to adverse weather conditions. In May 1997 it was observed that some of the exposed concrete containers inside the disposal trench showed minor cracks and that material containing traces of 60Co and 137Cs was leaching from these cracks. Due to premature signs of deterioration of the exposed waste containers, the national regulatory body required that the existing trenches be backfilled and capped as soon as possible. At that stage, only 70% of the intermediate level waste trench and 30% of the LLW trench had been filled with waste packages. This requirement in itself implied a modification of licence requirements and operational procedures for the site. In addition, the national regulatory body required that in future the metal LLW drums be covered within one month and the concrete intermediate level waste containers within two months after being emplaced in the respective trenches. The premature closure of the trenches necessitated some changes to their original design. The main feature of the implemented changes entailed a concrete cut-off wall, the width of each trench, at the live end of the waste stacks. These walls effectively sealed off the waste stacks and provided a barrier against which backfill and capping could proceed. To prevent the difficulties encountered with the partial closure of waste filled sections of trenches, it is intended to utilize smaller trenches in the future to accommodate a pre-defined number of waste packages delivered in concentrated consignments, and backfilling and capping of the trenches as soon as possible after emplacement of the waste packages