Health Physics & Environment
12.
H E A LT H P H Y S I C S & E N V I R O N M E N T
INTRODUCTION
In the siting of a nuclear power plant, three zones are defined for control of population. The innermost zone, called the Exclusion Zone
(EZ), surrounds the plant and defines an area directly under the control of the plant. The second zone, an annulus around the exclusion
zone defines the Sterilised Zone or the Low Population Zone, where the growth of population is limited by administrative control. The
outer-most zone defines the minimum distance to a high population centre This chapter talks about the evolution of exclusion zones,
siting practices and factors determining the extent of exclusion zone for the current and future nuclear facilities.
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12.1 E X C L U S I O N B O U N D A R Y F O R N U C L E A R
FACILITIES
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Evolution of the Exclusion Zone
The concept of the Exclusion zone originated in the USA in the
Being adjacent to the plant fence, the exclusion zone is the area
early 1950s, when there was an acute awareness of the
of greatest importance. It essentially defines a buffer zone where
potential effects of nuclear accidents on the surrounding
the public has no access. It also helps to define the fenced plant
population. This idea was mooted primarily to insulate the
area, the site area and the public area. A regulatory dose control
public from the harmful effects of low probability, high
structure can now be put in place so that planned releases from
consequence accidents.
the plant during normal operation and inadvertent releases during abnormal conditions can be controlled to within
The earliest attempt at sizing the EZ was made by the US
acceptable limits.
Safeguards Committee in USAEC Document WASH-3, wherein
exclusion distance was numerically specified as a circle of radius
An interesting use of the exclusion area is that several nuclear
R (miles) = 0.01 √P, where, P is reactor thermal power (kW).
power stations can share the same site and take support from
This formula obviously would not yield practical sizes for
the common auxiliary facilities, such as fuel fabrication plants,
medium-sized or large power reactors: for a typical
Fuel Reprocessing PSlants [FRPs], waste management facilities.
3000 MWt reactor, this formulation gives an exclusion radius
As many as four twin-unit stations have been co-located at sev-
of 17.3 miles (27.9 km).
eral places around the world. The Rawatbhata site in Rajasthan
is an example.
The US siting practice as embodied in 10 CFR 100 for the
determination of the exclusion boundary and the low population
zone around a reactor defines these radial
distances as follows:
An exclusion area of such size that an
individual located at any point on its boundary
for two hours immediately following onset of
the postulated fission product release would
not receive a total radiation dose to the whole
body in excess of 25 rem or a total radiation
dose in excess of 300 rem to the thyroid
from iodine exposure.
A low population zone of such size that an
individual located at any point on its outer
boundary who is exposed to the radioactive
cloud resulting from the postulated fission
product release (during the entire period of
its passage) would not receive a total radiation
dose to the whole body in excess of 25 rem or
a total radiation dose in excess of 300 rem to
the thyroid from iodine exposure.
Schematics of various zones
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The methodology for implementing this in the US Context is
On the lines of the above, all NPP sites in India have an exclusion
coded in the USAEC document TID-14844 . When implemented,
boundary of radius 1.6 km, except the Kaiga site, where the
the exclusion distances for most US reactors fall in the range
radius is 2.3 km. Where more than one twin station is sited, each
0.5–1.6 km.
must have an exclusion radius of 1.6 km.
.
Exclusion zone in the Indian Context
The Indian siting code defines the exclusion area as follows:
An exclusion area of at least 1.5 km radius around the plant shall
be established. This area shall be at the exclusive control of the
station and no public habitation shall be allowed in the area.
Under design basis accident conditions, a member of the public
shall not receive a dose equivalent more than 0.1 Sv for whole
body and 0.5 Sv for thyroid of children.
A Sterilised area upto 5 km around the plant shall be established
by administrative measures. In this area, natural growth is
permitted but planned expansion of activities which will lead to
an enhanced population growth are not allowed by administrative measures.
.
Siting Practice in Other Countries
The practice in some countries is illustrated in the table:
.
Factors Determining Exclusion Boundary
The factors determining the exclusion boundary are: reactor
type and power, engineered safety features, containment design
and characteristics of the site. The US code of practice assumes
a severe beyond design basis accident and does not give credit
to design features save the containment. In the Indian
regulatory practice, a design basis accident is considered at the
reactor site and the distance is found out at which both the
reference levels are met defines the exclusion zone. However, a
minimum exclusion radius of 1.5 km is always applicable.
Typical National Practice
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The first Indian power reactors were the two 210-MWe BWRs
the consequence could be limited to the fenced area of each
sited at Tarapur. A site radius of 1.6 km was selected. The
plant.
distance was believed to be conservative but to what extent
could not be ascertained. Recently an interesting application of
In view of the involatile waste forms and relatively small
the 10 CFR 100 methodology was made to the Tarapur BWRs.
inventories, the same can be said to be valid for waste
The results indicate an exclusion radius of about 950 m. An
management facilities at power reactor sites.
adaptation of the US methodology to Indian PHWRs is shown in
Tables. For 220 MWe double containment PHWRs, an exclusion
boundary of about 625 m is calclulated.
.
Exclusion Zone for Advanced Reactors
As stated earlier, an exclusion zone reflects the hazard to the
Similar results are also indicated from recent DBA calculations
public from the normal and abnormal operation of a power
made on the Indian PHWRs for submission to the AERB’s NPP
plant. It represents a passive safety area that cushions the
Zones Review Committee. An exclusion radius of about 800m
impact on public of abnormal operation of a nuclear plant. Since
was indicated. It is on the basis of these and other calculations,
releases for normal operation through the liquid and air routes
that the committee has recommended an exclusion zone of
cannot be eliminated altogether and similarly, since high
1.0 km for all future Indian reactors with the same reference
consequence accidents cannot be entirely ruled out, exclusion
levels.
zones can be optimized but perhaps not done away with
.
altogether for the present reactors.
Exclusion Zone for Other Nuclear Facilities
It is now accepted that while the second generation nuclear
Auxiliary nuclear facilities like fuel reprocessing plants are sited in
plants aimed for improvement of reactor safety through active
the Exclusion Zone of an NPP, thereby implying that they pose a
features and the third generation plants aim at the same through
relatively small hazard to the public which can be accommodated
passive safety features, the Fourth Generation reactors aim to
within the buffer zone. This has been recently confirmed for the
positively eliminate accidents through Safety-by-Design approach.
Tarapur site for two FRPs, one handling the irradiated PHWR fuel
The last approach can obviate the need for off-site emergency
and the other the high burnup AHWR fuel, when it was
preparedness, which in turn might make it possible to eliminate
demonstrated that for four Upper Limit accidents in the FRPs,
the need for exclusion zone in power plant siting.
N.B.:
Whole body dose controlling for Proposed Assumptions
Thyroid dose controlling for 10 CFR 100 Assumptions
Exclusion Boundary Distance (m) for Light
Water Reactors
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N.B.:
All exclusion distances above based on thyroid dose
which is controlling.
N.B.:
Partial double-containment exclusion distances controlled
by thyroid dose. Full double-containment exclusion
distances controlled by whole body dose
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Exclusion Boundary Distance (m) for
Single-Containment PHWRs
Exclusion Boundary Distance (m) for Double
Containment PHWRs