Development of strategy for fuel-containing materials (FCM) retrieval and waste management
The problem of amount and condition of nuclear fuel in the destroyed Unit 4 of Chernobyl NPP is one of key one during determining the condition of nuclear, radiation and ecological safety of the Shelter. Nowadays, one can consider specified that more than 95% of fuel remains inside the Unit from initial loading.
According to scientists’ estimates, total amount of nuclear fission materials is about 200 tons. However, the researchers did not succeeded in obtaining the reliable information regarding many places within the Shelter where detectable amounts of fuel could be.
Currently, irradiated nuclear fuel is inside of Shelter Object in the following FCM modifications:
- solidified lava-like materials, which were generated during the active phase of the accident under the influence of high temperature fuel with structural materials of the Unit and spread along under-reactor premises;
- finely dispersed fuel (dust), the hot fuel particles, which are observed almost in all premises of the Unit and soil samples in near and long-distance areas of the facility;
- core fragments, the most part of which is thrown away by the explosion to upper floors of the Unit, particularly, to Central Hall;
- soluble compounding forms of uranium, plutonium and americium. The cause of such compounds occurring is a destruction of different modifications of fuel under influence of a number of factors, main of which is water penetrating through incompactness of the Shelter roof.
The evident changes of exterior view are taking place in the condition of lava-like FCM for last years, such as continuous destruction of their surface followed by significant dust formation. To solve FCM problem is a required condition for Shelter transformation into ecologically safe system.
FCM retrieval is mandatory as Ukrainian Legislation prohibits the disposal of long-lived and high-level waste (including FCM) in the storage facilities of any type, except for storage facilities located in stable geological formations. That’s why the Shelter Object could not be transformed into storage facility for FCM and long-lived waste. For the Shelter transformation into ecologically safe system, constructing New Safe Confinement or concreting existing SO is not enough. FCM and long-lived waste should be retrieved and disposed according to prescribed standards. Construction of new confinement without an intention of further retrieval of these materials is not problem solution, but its shift to next generations.
Therefore, under SIP performance according to tasks 14, 19 and 20, the following are envisaged: FCM characterization (determining their location, amount and features), creation of technical conditions for continuous monitoring, development of strategy for their retrieval and development of potential retrieval technology demonstrating an example of separate FCM accumulation.
Conduction of full-scale FCM characterization providing for sampling and creating hot cells is associated with high dose and labor costs, and it is inappropriate based on ALARA principles. Thus, the need to develop FCM behavior monitoring program prior to and during FCM retrieval was noted. Considering the fact that FCM retrieval is long period enough, FCM condition monitoring program should enable to control FCM condition and provide an ability to take necessary measures on limiting the unfavorable development of events.
With this purpose, a contract for development of FCM management control program was concluded with Institute for Safety Problems of NPPs of Ukrainian NAS (ISP NPPs) and Russian Scientific Center “Kurchatov Institute” in August 2003.
Based on studying long-term behavior of FCM, the specialists from RSC “Kurchatov Institute” and ISP NPPs defined the requirements to FCM behavior monitoring program. Provisional assessment of proposals from RSC KI and ISP NPPs shows that a number of FCM monitoring directions could be performed using subsystems of nuclear and radiation monitoring of IAMS together with SIDB. Appropriate conceptual technical decision was developed.
Using the document “Strategy of FCM Retrieval and RAW Management. Further Action Plan”, it is planned to develop and implement the FCM behavior monitoring program under SIP, and develop recommended preventive measures on mitigating unfavorable changes of FCM condition, practical implementation of which is not included into SIP.
Preliminary strategy of FCM management proposes to postpone the retrieval till the storage facility for final disposal of FCM is constructed, i.e. for several decades, meanwhile performing continuous monitoring of FCM condition. Considering this time allowance, development of previously provided prototype of FCM retrieval technology was recognized inappropriate to develop FCM Retrieval Strategy, as more advanced and safe technologies for high-level waste management could be developed, and technology prototype could be obsolete during this time. Therefore, the International Coordination Team of experts did not recommend continuing these works. One should take into account that required data on labor costs, dose rates and costs for development of FCM Retrieval Strategy could be obtained during early dismantling of unstable structures of SO.