Bentonite activity tested in ONKALO
Bentonite clay will soon be put to the test in the underground research facility of ONKALO. A bentonite test that will start in September 2011 will test how the barrier material used in the disposal facility functions in practice.
What is the purpose of the bentonite test? Pasi Rantamäki, Development Engineer at Posiva:
"The goal of the bentonite test is to obtain more information about the initial activity of the bentonite buffer and to apply this information on a larger scale. Another important goal is to learn to build and operate under the conditions at ONKALO. This is Posiva's first bentonite test that is more extensive than those conducted in a laboratory," says Pasi Rantamäki.
What is the test like?
"Two holes have been drilled in the rock at Olkiluoto's ONKALO research facility (PL1475). Bentonite buffer, with canisters of stainless steel inside them, will be installed in the holes. The hole diameter is 800 mm and the depth is three metres, which means that they are smaller than the intended deposition holes. The test is on a scale of 1/3. One of the holes will be irrigated, and the other one will not. The purpose of the irrigation is to investigate how water affects the bentonite. The holes will be sealed with leak-proof covers anchored to the rock.
The canister installed in the hole has heating resistors. They are to produce relatively as much heat as the actual fuel canisters would. The heating power of the test canister is 600 W. The temperature of the canister will be increased to 90 degrees. The temperature, humidity and pressure of the bentonite will be measured at the same time. "
When will results be available?
"One buffer hole will be irrigated and the other one will not. The irrigated hole will be monitored for two years and the other one for about five years. This means that the test will be completed around 2016. However, data will be immediately obtained once the test has started and the data will be monitored on a continuous basis."
Flexible bentonite will be used as a buffer material in the disposal
Bentonite is a very important material for the disposal of spent nuclear fuel: The bentonite buffer is one of the barriers according to the multi barrier principle of the disposal. It is a buffer material for isolating the copper canister (that contains fuel rods) from the rock surrounding the canister. A number of blocks of tightly compressed bentonite are installed as buffer material between the canisters and the rock in the deposition hole.
Bentonite is a naturally occurring type of clay that expands manifold when coming into contact with water and that, on the other hand, conducts almost no water at all. This means that expanding bentonite fills the space surrounding the disposal canisters. In this way, the bentonite prevents water from coming anywhere near the copper canisters. In the event of a possible capsule leak, it also stops the radioactive elements from coming into contact with the rock.
The bentonite buffer surrounding the canisters also protects them against mechanical wear, i.e., possible movements of the rock. Bentonite behaves somewhat like modelling clay: it buckles when necessary, but can also retrieve its shape because of its elasticity. Additionally, it fills up and quickly seals any fractures that might occur when the rock moves.
For more information about the activity of the bentonite buffer in the disposal, please visit Posiva's website.