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TitleCivil Engineering Design for Decommissioning of Nuclear Installations
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LanguageEnglish
File Size7.4 MB
Total Pages107
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Page 1

The Commission of the European Communities

CIVIL ENGINEERING DESIGN FOR
DECOMMISSIONING OF

NUCLEAR INSTALLATIONS

Page 2

This Report was prepared as part of the European Atomic Energy
Community's cost sharing research programme on
"Decommissioning of Nuclear Power Plants", Contract
No. DE-G-002-UK.

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47

48 Section through loop pipe penetration of typical PWR
pressure vessel.

49 Section through detector slot of typical PWR pressure
vessel.

50 Plan showing detector slots of typical PWR pressure
vessel.

51 Plan on I.S.I. gallery of typical PWR.
52 Plan on fuelling pool of typical PWR.
53 Section showing possible location for water tanks in PWR

primary shield wall.
54 Plan showing possible location for water tanks in PWR

primary shield wall.

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48

TABLE 1

STRUCTURE

Type 1 (No planes
of weakness)

Type 2, with all
reinforcement
outside planes of
weakness zone

RESULTS OF ULTIMATE LOAD ANALYSES OF
STRUCTURES HAVING PLANES OF WEAKNESS

VESSEL PRESSURE LOAD
AT ULTIMATE LOAD FACTOR
(p.s.i)

1632 2.534

1624 2.521

Type 2, with haunch 1631 2.533
reinforcement
increased by 20%

FAILURE
CRITERIA

Longitudinal
crack width

Lo ng it ud ina1
crack width

Longitudinal
crack width

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(d) Improvement in the neutron shielding properties of concrete to
reduce the volume of activated concrete and surface dose rates.

Activation of the concrete in a PCRV is caused by production of
radioactive isotopes due to capture of neutrons by the nucleii of
certain elements present in the concrete. Before capture, fast
neutrons must be slowed down to thermal levels. Hydrogen is very
effective at slowing down neutrons over a wide range of energies.
By increasing the amount of hydrogen in concrete, the neutron
attenuating properties of the concrete can be improved and two
methods of increasing the proportion of hydrogen in hardened
concrete are:

(1) Increasing the amount of free and chemically bound water in
the hardened cement paste.

(2) Using hydrous aggregates.

Method (1) could involve increasing the fineness of the cement
and increasing the cement and water content of the mix combined
with stringent curing procedures during construction. Migration
of free water through concrete is very slow, even under PCRV
operating temperatures. Naturally occurring aggregates with a
high hydrogen content include limonite, geophite and serpentine.

The adoption of any of these measures would have to be preceded
by trials to establish the effects on workability, heat of
hydration during construction, creep and shrinkage properties and
long term strength and durability of the resulting concrete.

Boron is very effective in capturing thermal neutrons and does
not produce gamma-emitting isotopes. The addition of boron to
concrete will reduce dose rates received by decommissioning
workers from harmful gamma radiation. Some of the more readily
available boron containing additives are soluble in water and
have a deleterious effect on the concrete setting process. As a
result, borated concretes used in practice in reactor
construction in the U. S.A. and Japan have typically had boron
contents of less than 1% by weight. However, Japanese
researchers have succeeded in producing concrete mixes wi th a
boron content of up to 6.4% using non-soluble boron frits. The
presence of these appears to retard slightly the early gain of
strength, but 28 day strengths similar to or even greater than
comparable mixes without boron are achieved, and all of the trial
mixes reported had 28 day strengths of greater than the 40 N/mm2
commonly specified for PCRVs. Incorporation of boron in the PCRV
concrete may also affect reactor operating characteristics.

(e) Use of an alternative material to mild steel for the liner.

Discussions with liner design engineers have revealed that
alternative materials to steel for liners have been investigated
in the past. One of the important properties required by
material used on heavy civil engineering construction sites is
durability against damage from both normal site operations and
misuse of equipment such as dropped scaffold tubes, reinforcing
rods and the like. Recent tests on high density polyethylene
sheet have shown that this material can be easily penetrated by
dropped objects and, although easily repaired, the integrity of

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the liner would depend on all such damage being identified before
the liner is built in or covered up. Plastic materials used in
industry for containment liners have good chemical resistance and
elasticity properties but are not able to withstand the high
temperatures normally found in a reactor. The long term effect
of radiation on some plastics, with resulting embrittlement, is
also a problem to be taken into account.

Stainless steet is used in nuclear installations as an
impermeable containment membrane but this material is normally
used where corrosive conditions prevail and Where ease of
decontamination is required. Compared with mild steel, stainless
steel is considerably more expensive and exhibits a higher level
of harmful radioactive isotopes, such as nickel, when subjected
to radiation. All enquiries to date have failed to identify a
better material than mild steel for the liner.

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