MEASUREMENTS AND CRITICAL SAFETY IN THE ALPHA WASTE INCINERATOR AT CEA VALDUC

The Valduc Research Center, which reports to the CEA’s Military applications Division, generates solid waste contaminated with alpha emitters in the operation of its installations. An incineration plant has been built to treat the contaminated waste. The selected process is the IRIS process, developed by the CEA’s Nuclear Energy Division in the Marcoule Research Center. It features two-step incineration and produces ash with good reduction factors, and in a quality allowing further treatments. The active waste incineration has operated for seven years. Criticality risk prevention is based on limiting the mass of active material undergoing treatment in the facility. A balance is compiled continuously by calculating the difference between the mass of active material entering the facility and the mass leaving it. These masses are evaluated by the activity measurements performed on qualified measurement circuits. The bags of waste to be incinerated are measured by gamma spectrometry by the producer of the waste. The bags are put in drum, and the drums are controlled by gamma spectrometry by the service in charge of analysis and measurement. If the two results are coherent, waste can be incinerated. Ashes are measured by gamma spectrometry. Due to measurement uncertainty, the balance must be zeroed periodically by cleaning and drainage of all the equipment and the absence of holdup in the components must be checked. The holdup measurement of the installation is made periodically, chiefly by gamma spectrometry, allowing overall or targeted measurements. For the post-combustion furnace, measurements are completed by passive neutron counting. Tests have been conducted, previous to beginning of the active incineration, with calibrated radioactive sources to qualify the systems for measuring holdup from outside the equipment. The visualization of hold up is also achieved by endoscopy. This paper presents the salient facts concerning the plutonium balance, the feedback from the operations of cleaning and measurement in active conditions during these seven years. Retention is weak and well identified and the total measured retention is in good accordance with the weight of plutonium loss in the process. Furthermore, the evolution of measured retention is in quite good accordance with evolution of plutonium losses.