RD&D-Programme 2004 - SKB

Extractants consisting solely of carbon, hydrogen, oxygen and nitrogen are therefore being
sought. These extractants can be burned completely, whereby the radionuclides are trapped in
filters. The volume of radioactive waste can thereby be reduced considerably compared with
current technology. The current research is largely focused on this type of extractant.
The research on aqueous partitioning processes has made some progress in the past few years,
and there is good hope of being able to develop a process that recovers over 99.9 percent of
both the uranium and all transuranics from spent fuel. The main problems are the cost and the
irradiation stability of the organic molecules.
However, once a process has been defined that works well on a small scale, extrapolation to an
industrial scale is no great problem. The technical components needed for such a process are
tried and tested in other applications.
The limited stability of organic molecules under the influence of the strong ionizing radiation
from the high-level waste was a factor contributing to the early search for non-aqueous
partitioning processes. The research soon came to be focused on pyrochemical processes.
Such a process was built at the EBR-II breeder reactor in the USA in the 1950s. This reactor
had metallic fuel and was cooled by liquid sodium. The reprocessing of the fuel was done by
controlled oxidative slagging after the fuel had been melted. The purified fuel was then recast to
new fuel rods which were returned to the reactor. Another, more promising method was fluoride
volatilization, which was tested in the MSRE project with fuel in the form of molten salts. This
method was further developed for light-water reactor fuel, but encountered great difficulties in
the early 1970s. The difficulties included keeping process losses down, control of criticality,
handling of small amounts of water vapour and oxygen, and corrosion.
Due to the technical problems, development of pyrochemical methods declined greatly during
the 1970s and 1980s. Interest in these methods was, however, renewed when interest in P&T
increased in the early 1990s. This research increased above all in Japan, but later also in
the USA, Russia and Europe. The pyrochemical methods are considered to be of interest in
particular for reprocessing or partitioning of fuel with a high level of radiation, for example
fuel with mainly americium, curium and plutonium with a large fraction of heavy isotopes.
The development of pyrochemical partitioning processes also requires technical development of
equipment suitable for industrial application. Material and corrosion problems must be solved
as well as technical and radiological safety issues. The road to industrial application is probably
longer for the pyrochemical processes than for the aqueous processes.
Current research on transmutation
The radionuclides that are primarily of interest for transmutation are the transuranics. These
elements are most suitably transmuted by neutron-induced fission. Many of these nuclides
are, however, only fissionable with fast neutrons. Two types of facilities are being considered
to achieve a sufficiently strong flux of fast neutrons: fast reactors with self-sustaining chain
reactions and accelerator-driven subcritical systems (ADS). A number of fast reactors were
constructed in several countries during the period 1950–1985. Most of them had liquid sodium
as a coolant. The biggest was the French Superphénix, an electricity-generating plant with
1,200 MWe of power. Only a few of these fast breeder reactors are still in operation, however.
Interest in accelerator-driven systems increased in the 1990s, and currently there is considerable
interest in research on such systems. In Sweden nearly all transmutation research is focused on
ADS. An ADS consists of the following main components:
• A proton accelerator that can provide a strong current (from a few up to several tens of
milliamperes) of protons with up to about 1 GeV of energy per proton.
• A spallation source where the proton current hits heavy atomic nuclei (lead, bismuth,
tungsten, tantalum etc) and creates via spallation a neutron flux of several tens of neutrons
per impinging proton.
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