Nuclear fuel structures OK for disposal

21 October 2011

European scientists studying the geological disposal of high level radioactive waste have concluded that the process is "an effective isolation barrier for tens of thousands to millions of years."


Writing for the June e-news bulletin of the European Nuclear Society, the conclusion comes from researchers at Laboratoire Subatech in France and SCK-CEN in Belgium and is based on the results of two EU-funded waste research projects they coordinated.
Field work for the Glamor project was carried out between 2002 and 2006 and coordinated by SCK-CEN. It was followed by the Micado project, which ran from 2007 to 2010 and was coordinated by Subatech. Both studies assessed the uncertainties in existing models predicting the dissolution rates of nuclear waste from the open and closed fuel cycle over geological time periods.
The Glamor project involved scientists from Belgium and France, who assessed uncertainties of models about the dissolution of vitrified waste glass in pure water without additional materials, such as bentonite or metallic corrosion products.

It concluded that high leve l waste blocks could be expected to retain integrity for over 100,000 years. It noted that the final dissolution rate can be as much as 10,000 times less than initial rates – a key factor to ensuring block longevity. However, remaining uncertainties over the mechanisms that affect dissolution rates provided the basis for subsequent model development.
Micado brought together the expertise of waste management agencies, regulatory technical support bodies and research organisations from Belgium, France, Germany, Spain, Sweden, Switzerland and the UK. They undertook uncertainty analyses about how radionuclides would be released from used fuel structures – cladding and glass – over different time intervals: for the first few thousand years, when a container was intact; and for the next few hundreds of thousands of years, when a container had corroded and released a lot of hydrogen; and very long subsequent time periods, where there was less hydrogen, but containers were even less likely to be dependable.
The project concluded that the dissolution rate of vitrified glass containers (from reprocessing) and fuel rod cladding was slow; between 0.02 and 5 micrograms per metre square per day in in the latter two time periods. This meant that the largest contribution to dose would come from the radionuclides in non-confined zones of the fuel, rather than for instance the fuel pellet, and that "the resulting lifetime of the spent fuel [glass and cladding] is significantly longer than 106 years."
Both studies were praised as helping to achieve consensus in a number of critical issues. They have also identified further areas for research.
Geological waste disposal is universally accepted by experts as the safest and most feasible method for the disposal of high level waste. It should be noted that the radioactivity of reprocessed fuel will drop beneath that of the ore the uranium was originally extracted from within 10,000 years, and that most volatiles decay much sooner than this. The times listed here do not even account for the time radionuclides must then take to resurface from the hundreds of metres underground they would be buried.
By Keith Nuthall
for World Nuclear News