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In a PWR, the reactor core fuel elements
require to be renewed from time to time
necessitating the removal and replacement of the
top lid of the reactor pressure vessel. This lid
is penetrated by a number of short steel tubes
fitted into holes cut in the lid and sealed by
welds on the inside of the lid. During operation
the reactor control rod drive machines are fixed
to flanges at the tops of these tubes. At a PWR
in Ohio, USA (Refs.15,16 and 18) from time to time
the tops of some tubes were found to be out of
alignment and had to be 'bent' back into shape.
This went on for several years until in 2003 one
of these tubes 'fell over'. By this time the full
thickness of the low alloy heat treated steel of
the top dome had been corroded away, leaving just
a few millimetres of internal stainless steel
'cladding' to carry the pressure load of the hot
reactor water. Reactor coolant had been leaking
continuously for several years, and the dissolved
boric acid had corroded the full eight inch
thickness of the steel lid away over an area of
about thirty square inches. A subsequent
assessment by the US nuclear regulators of the
likelihood of failure in these circumstances
(Refs.15 and 16) concluded reassuringly that the
safety of that reactor vessel against explosion
was not prejudiced. However, it is clear evidence
that unless a leak of reactor coolant was so
large that operation could not continue, then it
has been ignored (Ref.17).
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33. When it was realized that nuclear weapons
tests in the atmosphere had produced measurable
radioactive contamination of the entire
atmosphere of the northern hemisphere, the
decision was made to discontinue them. Weapons
tests continued, of course, but underground. A
kilometre or two down below the surface. And the
resulting additional discharges of radioactive
material into the atmosphere above ground were
thereby very much reduced.(ref.20) So that, in the light
of such knowledge, effective containment of a
reactor pressure vessel explosion is quite
clearly practicable, but is it reasonably
practicable? And there are doubtless other
practicable ways to contain the explosion of a
reactor vessel effectively, which might be proven
and evaluated?
View of a nuclear weapons test site.
The energy of a pressurized water reactor pressure vessel explosion is
miniscule by comparison with a nuclear weapon, and the peak pressure
exerted by the steam release or accompanying hydrogen explosion is also very
much less. The technicalities of the total prevention of leakage of any radioactive
material from a sub-terranean explosion are very well understood (references 20,
21 and 22). Therefore, the provision of an effective containment of a PWR, proof
against reactor pressure vessel explosion, is only a matter of economic decision.
If the safe power station is worth having, then it will be built. An effective underground containment for a nuclear power station offers the additional possibilities of maintaining the associated plant and equipment above ground free from radioactive material, and beyond the end of economic life, given suitable design, the parts of the plant which are radioactive can be left in situ for as long as necessary without technical difficulty or expenditure beyond maintainance.
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