Tidal Current Energy

The Pebble Bed Modular Reactor
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decisions or on active electrical and mechanical systems starting up or otherwise
operating correctly.
There are, of course, many other facets to radiation and nuclear safety that
have had to be addressed. For example, are radioactive effluents during normal
operation significant? What about routine radiation exposure of operating
staff? If, for any reason, a major pipe into the reactor pressure vessel breaks, can
sufficient air subsequently enter the vessel to cause the hot graphite to burn? If
there were such a break, would the building withstand the pressure surge due
to escaping helium? How much radioactive material would then be sent into the
atmosphere? Will the reinforced concrete building withstand a large earthquake
or a major aircraft crash?
Work is underway to satisfy the South African National Nuclear Regulator
and its international nuclear safety consultants in all such respects. The PBMR
designers are confident that in none of these or any other credible events will
protective actions be necessary to safeguard the public beyond 400 m from the
reactor. More detailed information concerning HTR safety can be obtained from
Ref. [7] .
Finally, there is the important and emotive issue of the ultimate disposal of
spent fuel spheres. It is increasingly acknowledged that it is unacceptably wasteful
to dispose of spent fuel from current light water reactors without ‘ reprocessing ’ it
to recover useful plutonium and residual uranium. Spent PBMR fuel, however,
will contain so little residual fissile or fertile material that reprocessing appears
unlikely unless thorium is incorporated into the fuel to breed uranium-233. This
was an objective in the German THTR design. The spent fuel will therefore probably
be disposed of 50 or more years after decommissioning, suitably immobilized
along with other high-level radioactive waste, in deep geological repositories.
The design of the fuel spheres will make this a more straightforward task for
PBMR fuel than for other spent fuel. As discussed, the spheres consist of minute
uranium dioxide kernels coated with graphite, embedded in graphite spheres
and surrounded by a 5-mm-thick shell of uranium-free graphite. Graphite is a
more durable form of carbon than is coal and coal deposits remain undisturbed
in the earth’s crust for many millions of years. So therefore should spent PBMR
fuel spheres, long after vessels that contain them have corroded away.
The total volume of spent spheres is, however, great relative to that of compacted
spent fuel from light water reactors. The cost, in particular, of transporting
spent fuel spheres to a repository will therefore also be relatively great.
Notwithstanding the durability underground of used spheres, it may therefore
be deemed appropriate to separate the coated particles from the graphite matrix.
Ways of doing so and even of recycling the graphite are being researched. If
such a process proves technically and economically feasible, the waste to be disposed
of will be reduced to around 5 % of the volume of the spheres themselves.
Because of the ceramic shell surrounding each uranium kernel, vitrified or otherwise
suitably packaged waste in this form will also have very high resistance
to possible groundwater corrosion. Further information concerning radioactive
waste can be obtained from Ref. [8] .