The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

Repository Engineering
All repositories for SNF disposal plan to use long-lived waste package designed to last thousands
to hundreds of thousands of years. Because radioactivity decays with time (Figure
5A.1) the waste package can provide a significant barrier to radionuclide release when the
hazard is the greatest and enable the wastes to be retrieved if a problem is found in the performance
of the repository.
SNF and HLW generate significant decay heat that could raise repository temperatures sufficiently
to accelerate degradation of the waste, waste package and geology with degradation
of repository performance. This has several implications.
p Repository temperatures are controlled by limiting the decay heat per waste package
(that is, the quantity of SNF and HLW in each package) and spreading out the waste
packages over a large underground area. Decay heat is conducted from the waste through
the waste package and the rock ultimately to the earth’s surface.
p Repository cost and size is partly dependent upon total SNF and HLW decay heat. The
greater the total heat load, the larger the number of waste packages and the more tunnels
that must be constructed to spread the SNF and HLW over a large area. For the proposed
YM repository, about 40 tons of SNF can be emplaced per acre of repository space. This
implies about 50 acres of repository are needed to dispose of a year’s production of SNF
in the U.S (~2000 tons).
p Repository programs have adopted a policy of storing SNF and HLW for 40 to 60 years
before disposal to reduce decay heat and thus reduce repository costs and performance
uncertainties. Engineering tradeoffs determine storage times. After the first decade, the
fission products strontium-90 ( 90 Sr) and cesium-137 ( 137 Cs) produce most of the decay
heat. These radionuclides have 30-year half-lives and thus the radioactivity and decay
heat drops in half every 30 years. After ~50 years, the transuranic isotope 241 Am becomes
the dominant source of decay heat in SNF with a half life of 470 years before decaying to
Neptunium-237 ( 237 Np). The decay heat per fuel assembly initially decreases at a rapid
rate and then slows down when the decay heat is controlled by 241 Am. This occurs 40 to
60 years after SNF discharge—the technical basis for the storage time.
Repository capacities are not limited by waste volume or mass. For some wastes there are
incentives to increase waste volumes to improve repository performance. For example, liquid
HLW is converted into HLW glass for disposal. 11 This increases waste volumes by a
factor of three to four compared to calcining the waste; but, glass is a superior waste form
with lower handling risks and is less soluble in water—resulting in better repository performance.
Economics is not a major repository engineering constraint. The cost of geological disposal
of HLW and SNF is a small fraction of the cost of nuclear electricity—typically a few percent
of the cost of electricity.
160 MIT STudy on The FuTure oF nuclear Fuel cycle