Criticality Safety in Geological Disposal RSC Radiochemistry Group, 2 December 2009, Manchester Presented by Peter Wood, RWMD
Contents NDA & Geological Disposal Facility (GDF) Fissile Material Inventory Approach to Criticality Safety Chemistry Aspects of Criticality Safety
The NDA Non-Departmental Public Body since April 2005 Remit to clean up the civil public sector nuclear legacy Sites and facilities built from 1940 s onwards Responsible for 20 former UKAEA and BNFL sites & integrated waste strategy Safe and secure storage of radioactive waste Planning and implementing geological disposal
Geological disposal facility
Fissile inventory Volume m 3 Pu-239 t U-235 t U-238 t ILW 364,000 5.4 9.6 1,730 HLW 1,400 0.09 0.01 2 SF 11,200 23.4 29 6,260 Pu 3,300 66 0.1 0.01 U 80,000 0 487 159,000
Criticality: fission and chain reaction neutron 235 U 236 U prompt neutrons γ emission Fission fragment delayed neutrons
Criticality Safety Protection against the consequences of an inadvertent nuclear chain reaction, preferably by prevention of the chain reaction (ANSI/ANS-8.1 )
Consequences of criticality Fission products Heat Radiation and radiolysis, including gases Pressure Cracking Impact on facility performance
Contributions to safety (1) Detailed knowledge of the inventory of wastes and materials in ILW, the fissile material is mostly mixed with a large excess uranium-238 small amounts of ILW will contain separated plutonium and High Enriched Uranium (HEU), mixed with other waste materials. For pure materials such as plutonium and uranium we can design a wasteform that is subcritical Most spent fuel is removed from nuclear reactors because a large proportion of the fissile content has been used
Contributions to safety (2) Control packaging Specify and ensure control of all waste package contents For spent fuel, the design is already fixed so we would design a package to be subcritical; this might include using materials that absorb neutrons to prevent criticality In all cases we aim to design packages that are robust to operational faults.
Contributions to safety (3) A criticality post-closure would be a low probability event ILW packages may be well spread out: 15 tonne fissile material at a low average concentration within 1,000,000 tonne of waste, packaging and backfill materials. Much ILW encapsulated in cement, surrounded by a cement backfill that will maintain physical and chemical barriers to relocation of waste materials. For pure materials, we would design a waste form that is stable for long times and would only slowly release fissile material For spent fuel we would use a package and emplacement design to maintain sub-critical conditions over long timescales.
Chemistry Aspects of Criticality Safety Oklo natural reactors Scenarios: migration accumulation
Oklo - Location
Oklo - Geology Operated 2 billion years ago, U 3.68% enriched
Reactor formation - 1 U initially uniform in igneous rock - Life on earth photosynthesis: CO 2 O 2 - ~2bn yrs ago Fe oxidised first red beds - U has similar redox potential to Fe U(VI) formation allows dissolution and transport in water river delta - reducing conditions - precipitation in sandstone U bearing layer covered by marine deposit (organic-rich material) hydrocarbon formation - U concentration in ore layer 0.2 0.4% average
Reactor formation - 2 Uplift of granite west of ore-bearing rock -fracturing - circulate oxidising water - mobilise U U precipitated at reducing front at caused by organic material - concentrated U layer at boundary of organic layer - U concentration in reactors 15 60%
Typical conditions Lenticular shape ~ 10m x 10m x 1m thick Contain tonne quantities of FM Surrounded by clay - desilification by hot water from reactors Graphite found in some zones - liquefaction & solidification of organic material Reactor end states well preserved - only low-grade regional metamorphic changes over last billion years Concentration of 235 U in U: 3.68% 2 billion years ago 0.725% today As low as 0.265% in some reactors - most FM burned up
Lifetime of reactors FP inventory - burnup > deduced from reduction in 235 U - significant in-growth of 235 U from decay of 239 Pu reactors operated for ~ 100,000s of years Timescale and tonne quantities of FM suggest powers ~10s kw
Temperature of reactors UO 2 in form of uraninite in some zones - forms between 300-350 C Xe isotopic data 183 C (boiling point of I) < T < 452 C (melting point of Te) ( 131 I is 131 Xe precursor & 132 Te is 132 Xe precursor) Isotopics of Xe held in U-free La-Ce-Sr-Ca aluminous hydroxy phosphate in zone 13 grows quickly at 270-300 C trapping of FPs including Xe precursors
Pu-239 in backfill: criticality map 10000 Critical Mass, +ve coefficient 1000 Total Temperature coefficient = 0 Critical Mass, -ve coefficient Critical mass (kg of oxide) 100 10 14 kg/m3 20 kg/m 3 1 1 10 100 1000 10000 0.1 Concentration (kg m -3 )
ILW vault scale scenarios Systematic analysis identified four potential mechanisms for transport and accumulation dissolved colloids particulates slumping Probability of each mechanism leading to a criticality assessed to be low but cannot be ruled out
Accumulation mechanisms Chemical conditions in facility are influenced by backfill or buffer Little potential for dramatic changes as in Oklo Other processes may tend to accumulate, e.g. sorption, but would affect many materials Pessimistic assumptions and extremes of probability distributions can result in accumulation
Ore bodies How much uranium is in ore-bodies? 5E6 t economically viable 35E6 t mineral resource 1E14 t in earths crust (25 km thick) 1E10 t in oceans
Consequence model: assumed migration rates
Concluding remarks From knowledge of inventory, can design packages that are subcritical and robust to faults Packaging and backfill/buffer limits potential for migration and accumulation of fissile material Research is in progress, including modelling of Oklo, with aim of confirming and extending earlier conclusion that criticality would be a low probability, low consequence event.
Contact details Dr Peter Wood Criticality Research Manager Radioactive Waste Management Directorate NDA Harwell Curie Avenue, Harwell Didcot Oxfordshire OX11 0RH T +44 (0)1925 802929 E peter.wood@nda.gov.uk W www.nda.gov.uk