Role of Waste Acceptance Criteria (WAC)
The safety of a disposal facility, for both short- and long term management, not only depends on the quality of the design and construction of the disposal infrastructure, but also on the properties and characteristics of the waste.
Compliance with WAC ensures that the waste disposal facility will provide the required level of safety features during handling and disposal operations and also after disposal, during the period of surveillance of the disposal facility, and after the release from regulatory control.
Waste acceptance criteria are to assist waste producers (package requirements, waste processing) and facility operators in managing radioactive waste, Also they are essential to assure waste tracking. Finally, waste acceptance criteria can form a point of reference for communication/dialog between radioactive waste generators, waste processors, and operators of storage and disposal facilities as well as with regulators.
Safety principles leading to waste acceptance criteria
The fundamental safety objective is to protect people and the environment from harmful effects of ionizing radiation, without unduly limiting the use of technologies that may cause radiation exposure, according to the IAEA.
The strategy to achieve the fundamental safety objective with respect of radioactive waste disposal is to contain the waste and to isolate it from the biosphere, to the extent that this is necessary.
Protection of human beings and the environment requires the assessment of the radiological and chemical impact of the disposal facility and monitoring of facilities and of the environment;
With disposal of radioactive waste, safety has to be considered over different timescales, related to the half-life of the radionuclides present in the waste. Safety must be considered during the operational phase of the facility, during the period for which the disposal facility will remain under surveillance and maintained, and the period beyond, after the closure of the site, or post-closure.
In France, as in many other countries, the duration of the monitoring phase of surface disposal facilities is limited to 300 years.
The three life phases of a disposal facility
These three life phases are defined in IAEA Basic safety Rule (BSR) 1.2a :
- Operational phase including building of the final cap (70 years according the forecast inventory to reach the total capacity of 1 million m3 as defined in the creation act mentioned before).
- Monitoring phase (300 years corresponding to ten times of the Cesium-137 decay period) for decay of the low and middle activity beta and gamma radionuclides with small amount of long lived elements. This phase is set up to prevent intrusion and monitor the environment to verify the behaviour of the disposal different components.
- Post monitoring phase with an acceptable potential radiological impact for the population.
During the operational and monitoring phases, safety will rely on engineered components (called "barriers", and comprising the waste package, the disposal structure...), effectively containing the waste and isolating it from the biosphere. Those barriers are essentially man-made, located in an environment that protects them from aggression and would mitigate the results of a failure affecting multiple barriers. The performance of the barriers, in normal and incidental situations, is a key factor, as is the radiological contents of the waste packages. The external conditions should be selected or set to limit the risk of occurrence of incidents (low seismicity, no risk of flooding...), and to mitigate their effects, should it happen.
The barriers will remain effective until the end of the monitoring period, part because they were designed and built with this requested durability as an objective, and also because maintenance of the barriers, where possible, will be effective, during at least a part of the monitoring period.
During the post monitoring period, only passive measures are effective to bring the required level of safety. Only long-lived species will remain, and the objective is to limit and retard the amount of radionuclides that may migrate to the biosphere. 2 factors only contribute to this objective: the limitation of the inventory of long-lived radionuclides in the disposed waste, and the properties of the geological environment.