Heating-Irradiation Experiments to Assess the System Bentonite-Carbon Steel in a Radioactive Waste Underground Storage Facility
Authors: Dies J. , Tarrasa F. , Miralles L. , De las Cuevas C. , Pueyo J. J. 
Published: April 2020

One of the principal components of the engineered barrier system of a deep radioactive waste disposal in granite is the buffer, made of compacted bentonite blocks. Since the beginning of the operation of the repository the buffer will be subjected to a gamma radiation and temperature fields. Also, in some tens of years bentonite will be fully saturated.

Cylindrical samples of saturated and compacted bentonite (200 mm in length and 50 mm in diameter) with embedded carbon steel discs, have been exposed to heat and radiation to assess their potential alteration under repository conditions. Two kinds of experiments were carried out. The first one was a heating experiment, called BIC-2A, in which the samples, bentonite and carbon steel discs, were subjected to a thermal gradient, ranging from 130°C to 90°C, during 120 days. The second one, named BIC-2B, was an irradiation and heating experiment, with the same time and temperature conditions than the BIC-2A, using a maximum dose rate of 3.5 kGy/h and a total absorbed dose of 10.5 MGy.

The irradiation system consists of a set of six canisters, which hold the samples, placed between two cooper plates. These plates were subjected to a constant thermal gradient produced by a heating system driven by a PID controller, and consequently, the canisters were subjected to the same thermal gradient. Thermoresistances were used to control and monitor the temperature, whose evolution was stored on a computer. The canisters and the copper plates were placed inside a container that holds the system.

High dose rates were achieved by placing the system inside the irradiation cell of an industrial irradiation facility, used to sterilise medical and pharmaceutical products with gamma rays produced by a 60Co source with a total activity of 300 000 Ci. Active and passive detectors were used to monitor dose and dose rate. An ionisation chamber was placed inside the container to obtain a continuous measure of the dose rate. The ionisation chamber was polarised and its signal measured by a picoammeter and the results stored on a computer. Moreover, several types of dosimeters were employed to measure the absorbed dose for several periods of time. Perspex, alanine and quartz dosemeters were used to measure low, intermediate and high doses.

Either for the heating or for the heating-irradiation experiments, two cylinders were replaced by two new ones after two months of experimental running. Consequently, eight canisters were obtained at the end of each experiment, four canisters run during four months, and four additional canisters for two months.

After basic manipulations, including a granulometrical separation, the bentonite samples were subjected to several mineralogical, chemical and physical analyses; namely, X-ray diffraction, X-ray fluorescence, Eh/pH measurements, and determinations of water content, cation exchange capacity, thermoluminescence response, electron spin resonance and oedometric tests. These analyses were carried out on a reference bentonite sample, which had not been heated nor irradiated, on the heated samples, and on the irradiated and heated samples, in order to discriminate which alterations are really due to what cause. The carbon steel discs, heated as well as irradiated and heated, were subjected to several metalographical analyses in order to assess the relevance of the corrosion processes that worked under these conditions.