Radiological Safety Review of Siting and Layout Aspects of the ESS Technical Design Report, ESSdoc-274, April 23, 2013
Produced under contract for the Swedish Radiation Safety Authority: SSM 2012-5686
P. Wright, Radiation Protection Adviser and Radioactive Waste Adviser: UK HSE Certificate
00000397
14-09-2013
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Contents
Page
R1 Background
3
Introduction
Scope
R2 Observations on Siting
4
R3 Observations on Layout
6
R4 Qualitative Assessment of Hazards and Risks
9
R5 Conclusions and Recommendations
11
R6 References
12
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R1 Background
Introduction
This review report's purpose is to provide support for SSM in the licensing process of ESS
and will help to form the basis for authorization decisions. If a license is given it may include
specific requirements as license conditions which are deemed necessary to supplement the
existing legal regulations so that a resulting satisfactory radiation safety is obtained from an
authority perspective. The requirements shall be based on identified deficiencies in the
radiation safety assessment process. This review supports the aims of the SSM and by
clearly identifying specific radiological risk areas in the layout and siting concepts.
The background is that SSM has received an application for a license under the Radiation
Protection Act for a unique facility. SSM and Sweden have no experience with comparable
facilities. There are no current legal requirements adapted to this type of facility in Sweden.
ESS will become unique worldwide as an accelerator-based neutron spallation source with
such a high beam power. There are some similar but smaller facilities regarding beam power
with slightly differences in designs in the world. It is important for SSM to capitalize on existing
relevant experience in the licensing process of the ESS.
The facility will house a large amount of radioactive material that pose a significant risk if it is
released into the environment and can cause dose to humans. Several aspects relate to
potential emissions during normal operation and accidents of all kinds including malicious
acts. Waste management during operation and future decommissioning are also important
considerations. At the initial stage, questions about the design, layout and location of the
facility in the proposed region and accessibility of adequate waste repository are of primary
interest.
Facility design with successive barriers and defines in depth are important details to estimate
the protection against unacceptable emissions and exposures. SSM's believes that strategies
and requirements can in some respects be designed in accordance to what applies to nuclear
facilities and power plants. The level of requirements should however be deemed to be a
graded approach taking the current risk into account. Aspects that all necessary operation
and maintenance work must be executable within the limits imposed by the dose to personnel
also provide implications for the design, layout and location of the facility.
This approach to report review has been made to enable timely feedback to the SSM and to
assist with the process.
Scope
This review covers the siting and layout of the ESS and is primarily based on the technical
design report of the ESS project. It considers the potential impacts of radiological risks to
operators, members of the public and the environment.
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Comments on the technical design report are given only in selected sections (TDR reference
headings in black italics) relating to siting and layout with comments in blue italics. Contents
headings of this report are prefixed as Rxx.
R2 Observations on Siting
The proximity of the ESS site to nearby occupied areas places increased importance on the
analysis of the significance of potential radiological hazards to people and biota from;
radiation skyshine, airborne radioactive discharges (gases and dusts), radioactive liquid
effluent (cooling water and sewage) discharges, spills and flooding, activated ground water
and soil erosion, contaminated storm water run-off, water run-off from fires involving
radioactive contamination, theft or loss of radioactive material and transport of radioactive
packages including radioactive waste or scrap.
The risks to critical groups and biota from these hazards at the proposed site location should
be evaluated and conform to those risk levels given in the ESS General Safety Objectives-as
a minimum requirement. Although this has been done to some extent with emphasis on
airborne hazards (Ene, ref.1 ) and a basic Environmental Impact Assessment (EIA) has been
produced in Swedish for public information: a comprehensive environmental impact
assessment covering a range of critical groups, with references collated in one document
could be made that addresses all the hazards given in the preceding paragraph. For example,
a critical group identified in ESS-1241368, 2012 and the TDR at Ostra Torn is at 330 m from
the target station yet the dose to this group is not explicitly given in the TDR and the reasons
for choosing the critical group at Vastra Odarslov in preference to those at Ostra Torn are not
explained.
Other considerations such as the use of a societal risk as distinct from individual risk may be
helpful for comparison with standards of other countries. This may also involve the use of
such concepts as qualitative risk. But an explicit evaluation of risk to large communities such
as those at Lund should be made. It could also be expected that such considerations would
extend to the the regional future land use plans lodged with the Brunnshog district County
Administrative Board. One of these plans is for a Science Village between ESS and MAX IV.
It could be expected that the above evaluations would cover; construction, commissioning,
operation, maintenance, modification/extension and decommissioning of the ESS project.
Security of radioactive materials has become a more prominent issue in the last decade
especially with regard to high activity sealed sources (HASS) and theft of radioactive scrap
metals. This aspect could also be given more attention when dealing with the analysis of
radiological risks associated with siting and access.
TDR, 7.2 Location and conditions at the site
The distance from the nearest area of high population density, Lund, is about 2 km with
smaller populations scattered around the site at much closer distances. All of these distances
are not so large that proximity cannot be discounted as a major factor in the analysis of the
consequences of operational or accidental release of radioactivity to the environment. Indeed,
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studies have been undertaken (refs. 2, 3, 4, 5) to show that the risks are low but these rely
heavily on a hypothetical release term that largely discounts the production of aerosols and
releases from building locations other than the main ventilation stack. It may therefore be
prudent to carry out a stress test on the accidental release scenario.
If the results of any stress test on the accidental release scenario is positive it will be a good
recommendation for the low radiological impact of accidental releases from the facility.
However an important factor in this assessment would be the magnitude of the risks
associated with the release of radioactive aerosols.
Although the risk of contamination of property from accidents at the site may be shown to be
very low and individual risk likewise, international experience with this scenario has shown
that the consequences of loss of property or economic output is a very significant issue.
Therefore it could be expected that any risk assessment would include consideration of nonqualitative aspects of this type of accident.
Care should be taken when using international accident data since solid spallation
contamination sources have an intrinsically lower release fraction of radio-toxic and volatile
nuclides compared to nuclear reactors.
Operational releases of radioactive gases could also be subject to a review. It seems unusual
for this type of facility not to have significant releases of short-lived gaseous nuclides such as
Ar41, N13 or C11 (TDR Section 10.4.2 and Ene, ref. 1) when compared to other much lower
powered facilities. However, uncertainties concerning the ventilation systems operating
modes could be a factor in this assessment.
It should be noted that while other international proton spallation facilities such as SNS and
ISIS have developed from sites or campuses with long histories of involvement with
radiological hazards with the attendant personnel knowledge, expertise, demonstrable safety
and regulatory compliance: this may not initially apply to the same extent at the ESS location.
Therefore consideration of the impact of the available levels of personnel safety culture,
experience, training and skills when undertaking risk assessments should be a prominent
factor. In practical terms for example, a probability of failure of safety critical procedures such
as 10-2 for challenges to controls of hazards could be considered.
TDR, 7.2.5 Ground conditions
It has been noted (TDR Section 10.5) that environmental samples such as; foodstuffs,
surface pond water, groundwater, well water, soil and rock would be taken and analysed to
provide background data for future environmental monitoring programmes. This would also
include the installation of permanent water and soil sampling bore holes at suitable locations
around the facility. The bore holes should be at a depth so as not to provide flow paths
through any aquifer barriers. The number of boreholes would be of the order of at least ten
and should be sited to collect water from the watershed plumes.
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Measurement of the elemental concentrations of soil and water samples could be carried out
to assist with activation modeling. It could also be advantageous to carry out trial activation of
some samples, including shielding materials, to validate any models.
The closeness of the ground water level (3-5 m) to the ESS ground level could place
increased focus on activation caused by beam losses in the accelerator and below the target
station monolith. The accelerator building will have a floor below ground level and the target
monolith will have penetrating neutron fluences. A check on the assessment of this aspect
could be indicated. It is noted that consideration of groundwater migration was carried out
(TDR, 10.5.2) but the conclusion was not definitive. There is also some doubt about the
calculated level of tritium migration.
There appears to be no consideration of radon in the TDR. Radon levels on the soil pore air
(ref.6) could be significant- up to 40 kBq/m 3 in the western part of the site. How these levels
translate to exposure during construction, operationally in buildings and from ventilation
discharge activities is not clear. It may also be possible that construction could change the
porosity of the soils and generate elevated levels of natural radon progeny in dusts but the
contribution of these to risks is not known.
The relevant SSM high radon limit for soils is quoted in the reference document as 50
kBq/m3. If this reduces to a lower level in the near future, with anticipated changes to radon
dose risk factors (times 2) recommended by the ICRP (ref. 7), there may be an impact on
occupational exposure risks. Depending on the results of further assessment and
requirements of SSM, engineered abatement of radon in ESS buildings may need to be
considered.
R3 Observations on Layout
This section only covers broad aspects of the buildings and the layout of their systems since
detailed information in the TDR is not generally available.
TDR, 7.3 Logistics, earthworks and buildings
The general layout of the buildings on site is good, although it is not clear in the diagram
(Figure 7.13) how radioactive material generated in the facility will be transported to and from
the radioactive waste store. These routes could be shown on a separate diagram and include
off-site transport. The on-site routes for intermediate level waste should be separate from the
main traffic flows or at least have provision for exclusive use when needed.
It is not clear whether the SSM Road Transport Regulations for radioactive materials are to
apply for transport on the site. If this is the case then a good level of safety will apply and
flasks, vehicles and personnel may have to be licensed to move radioactive packages on site.
Provisions for exemption from road transport regulations exist within the European
Agreement Concerning the International Carriage of Dangerous Goods by Road (ADR 2013,
Subpara. 1.8.3.2(b)). Such provisions are commonly adopted by national competent
authorities.
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A documented Quality Management System and emergency response plan for transport
accidents would also be needed.
Site emergency evacuation routes for fires involving radioactive contamination could be
defined to help with internal road planning.
TDR,7.3.2 Earthworks
Storm water and run-off drainage systems should be designed so that run-off from
contaminated water used in fire extinguishment or contamination spills can be dammed and
recovered before entering the drainage company systems.
TDR, 7.3.3 Buildings
Fresh air intakes and make up air for the working environment should be located so that
direct, re-entry of radioactive airborne discharges from the facility is not possible.
Operation of ventilation systems for radioactive environments within the facility should be so
as to remain operational initially during a fire to remove smoke and maintain contamination
containment. The removal of smoke is important so that fire fighting personnel can locate
missing persons and the seat of the fire. Exceptions to this general advice could be for
flammable and explosive cryogenic gases such as hydrogen or methane.
TDR, 7.3.4 Accelerator buildings
Concrete shielding for the accelerator and target station can become activated and present
long term occupational and decommissioning radiation or contamination hazards. A study of
the type of aggregate to be used in the concrete could be made to determine whether
aggregate with low atomic number elements is preferable in some locations.
In some areas “water proof” concrete can be coated in strippable paint or water impervious
membranes used to provide a barrier to tritiated water adsorbtion or ingress of water from the
accelerator mound soil. This may also be important for future decommissioning where the
large bulk of contaminated material can cause difficulties with disposal and high costs.
The location of the radiation office at the front end of the building could be too remote from
the more active parts of the facility and thus affect response times to incidents or accidents
involving radiation or contamination hazards.
TDR, 7.3.5 Target building
The roof of the large void space between the target monolith and the experimental halls could
be a weak point for neutron leakage affecting the occupational background radiation level in
the hall. The shielding design for this could be checked.
Although it is expected that mechanical and civil engineering will be to a high standard,
special attention could be given to the long term stability of the monolith foundations and
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mechanical alignment. Experience with another facility has shown that slight tilting of massive
shield structures can occur and coupled with tight clearances, motion of shield assemblies
has been inhibited or beam alignment has been affected. The high levels of activation
associated with these structures can involve significant doses in recovery work.
TDR, 7.3.7 Central laboratory, DMSC, offices and auxiliary buildings
Consideration could be given to locating the main ventilation stack at the front end of the
building to gain the most advantage from the distance between more occupied buildings at
on-site and off-site locations. The impact of turbulence caused by the wind energy fan on the
stack discharge plume could also be considered.
There does not appear to be explicit provision for storage of radioactive spare components.
These items could be expected to be stored in the radioactive waste facility since they form
part of the radioactive materials stream that, if not used within a time agreed with the SSM,
will ultimately be declared as waste.
TDR, 7.4.2 Low and extra low voltage systems
Special lighting could be considered for areas with prompt radiation hazards such as the
accelerator tunnel.
TDR, 7.5.1 Cooling water system
There does not appear to be any explicit provision in the cooling water system design for
leakage water containment, collection, characterisation for radioactive contamination and
processing. A typical facility would have floor drainage channels, collection sumps, bunds and
possibly delay tanks in all water cooled areas where radioactive contamination is possible.
TDR, 7.5.4 Cooling water interfaces
The apparent lack of a second heat exchange interface between active cooling circuits such
as the target and the district heating line (TDR, Figure 7.31) has been discussed in a previous
report on the CDR. Primary pipe lines may need to have double containment to provide
protection from leaks of radioactive water.
TDR, 7.6.1 Compressed air and gas systems
Provision of separate compressed breathing air supplies for personal protective equipment
such as air fed suits could be considered.
TDR, 7.6.2 Heating and ventilation
Diagrams of the active ventilation systems for the accelerator and target could be useful. It
may be difficult for the accelerator tunnel ventilation plant to provide sufficient protection
against loss of negative pressure differential across the accelerator/outer atmosphere
interface during periods when the plant is not on and the accelerator is operating. This has
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been discussed in a previous report on the CDR.
There could be strong operational pressures for continuous operation of the accelerator
ventilation plant when the accelerator is running and these may force a change in use of the
lower, workplace environmental conditioning fan speed mode to maintain flow. The
consequences of this could be the continuous release of activated air from the stack.
In some experimental beam lines activation of sample gases or beam line air may be possible
and provision for active ventilation of these areas could be considered in the design.
TDR, 10.5.3 Accidental release scenarios
Paragraph one, penultimate sentence, the volatile release fraction is given as 0.5% but table
10.18 shows 0.05%.
There is also an apparent inconsistency between tables 10.17 and 10.18 where the Xe121
dose component is zero in table 10.17 but 1.4 x 10 -7 Sv in table 10.18. They should be equal
since the tritium component is equal in both tables.
It is also not clear why the Xe121 dose component in the TDR for table 10.17 is zero while it
is 140 µSv in both tables, 7 and 8 of the referenced report; ESS-0001898.
R4 Qualitative Assessment of Hazards and Risks
The observations in the preceding sections were prioritised in terms of perceived hazard and
risk significance and the following two aspects have been chosen for further qualitative
evaluation in this section.
Aspects chosen from the observations section are: operational and accidental release
scenarios.
Operational Release Scenarios (TDR, 7.2 Location and conditions at the site)
It seems unusual for this type of facility not to have significant releases of short-lived gaseous
nuclides such as Ar41, O15, N13 or C11 (TDR Section 10.4.2 and Ene, ref. 1) when
compared to other, lower powered facilities.
The discharge levels for three spallation facilities shown in table 1 suggest that, rounded to
one significant figure, the calculated short-lived nuclide releases for the ESS are not
comparable when scaled with measured or modeled releases from other spallation sources.
Such discrepancies should be carefully analysed and explained because of the impact on
doses to the public and the environment. Uncertainties concerning the ventilation systems
operating modes for all facilities could be a major factor in this assessment.
In the case of SNS the accelerator tunnel is not ventilated during accelerator operation but for
ISIS all systems are continuously ventilated to; maintain a negative pressure differential,
condition the air temperature and to reduce corrosion from the buildup of moisture in some
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areas. The ESS calculations assume continuous ventilation to remove moisture and heat
(TDR, 10.4.2) only as a worst case example.
For a hypothetical example, if the ESS release of short-lived gases is factored, by an average
scaling factor of 300 derived from the table, the dose component to the critical group living at
Vastra Odarslov, 660 m from the stack would be 90 μSv/y. Allowing for a reduction by a factor
of two for a longer turnover rate, the dose would be 45 μSv/y.
The calculated tritium releases for the ESS are also not fully comparable when scaled with
SNS but do scale with ISIS. The main source of the HTO discharge in the case of SNS is the
Hot off-Gas exhaust and there is a relatively small fraction from the accelerator tunnel. The
difference between ESS and SNS may be due to the relatively higher release of tritium from
the SNS liquid mercury target via the hot off-gas system.
For the hypothetical example and excluding the SNS case because of the liquid mercury
target, if the ESS release of tritium is factored, by an average scaling factor of 1.5 derived
from the table, the tritium dose component to the critical group living 660 m from the stack
would be low at 0.048 μSv/y.
Totals of short-lived nuclides and tritium doses would thus be for the hypothetical case 45.05
μSv/y which is close to the GSO limit of 50 μSv/y total (and above the limit of 10 μSv/y from a
single source) with not much margin for variations such as for example; the critical group
location from the stack release point, the design stack height, plant operating conditions etc.
A critical group, identified in ESS-1241368, 2012 and the TDR, at Ostra Torn is at 330 m from
the target station yet this group is not included in the TDR operational dose assessment. The
reasons for this could be explained.
Table 1, Comparative Site Airborne Discharge Levels For Various Facilities
Site
Short-lived nuclides Tritium as HTO
(TBq/y)+
(GBq/y)
Beam power
(MW)
Power scale factors
(Short-lived/MW, HTO/MW)
SNS** 400
30000
1
400, 30000
ISIS
300
0.2
200, 2000
1
30
ESS* 7
6000
5
1,
1000
**Abated model, accelerator only, SNS 102010203-ES0001-R00, 2008
* Calculated TDR, all sources, one day turnover; Section10.5.1
1 Measured
+Note: the short-lived nuclide breakdown fractions for SNS, ISIS, CERN and ESS accelerator
tunnels are not all similar; as shown in table 2. The most significant difference in terms of the
effect on total dose is that from Ar41 and this could, if assumed true, increase the plume
dose.
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Table 2, Comparative Short-Lived Nuclide Fractions For Accelerator Tunnels at Various Facilities
Site/Nuclide
SNS*%
ISIS***%
ESS**%
CERN+%
C11
57
7.6
38
55
N13
38
73
39
12
O15
<1
2.4
22
-
Ar41
4.3
16
1
22
* Abated model
** One day turn-over model
***Model
+ Measured; Patterson and Thomas, Accelerator Health Physics, Academic Press, 1973
Accidental Release Scenarios (TDR, 10.5.3)
Paragraph one, penultimate sentence, the volatile release fraction is given as 0.5% but table
10.18 shows 0.05%. It is not clear how the scale factors for components of dose operate for
the different fractions of release given in the tables.
The description of accident scenarios for the target DBA including a “defined hypothetical
accident” could perhaps be given in more detail either in the TDR or elsewhere. Several
documents have been reviewed but an analysis to a level such as that of the LANSCE
publication; Bounding Radionuclide Inventory and Accident Consequence …. LA-UR-1110388, 2007 could not be found. In particular, it could be prudent to firmly establish the basis
on which the assumptions of no aerosol releases are made. For example, loss of coolant
accidents with tungsten-steam reactions producing highly radioactive tungsten oxide or
tungstic acid have been considered in studies of solid target alternatives for SNS. These were
assumed released by mechanical containment failures and/or pressure relief devices.
The estimated doses for various fractions of accidental volatile release are only given for the
critical group at Vastra Odarslov, 660 m from the stack and do not include doses to the critical
group at Ostra Torn, 330 m from the target station.
Estimated doses from accidents to critical groups at the ESS site, Max IV, Lund and other
communities could also be made to enable members of the public and authorities (SSM) to
compare and evaluate risks.
R5 Conclusions
The application for approval could be based on a more comprehensive, radiological
environmental impact assessment (EIA). The treatments given in the TDR and elsewhere
(e.g. in the Preliminary Safety Assessment Report) are in themselves good but could be up
dated and presented in a more wide ranging, single document for public information that
additionally includes all relevant papers.
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An environmental monitoring program could be included in the EIA to show due consideration
of all of the issues identified including the commitment of resources and presented in a form
that can be easily understood by the public. All conservative assumptions should be
transparently stated.
Software based models using codes such as MCNP can be inaccurate and in some cases
give levels greater than a factor of 3 higher or lower than actual measured dose results. When
combined with other variables such as tunnel geometry, changes to plant operating or beam
loss conditions considerable errors in dose estimation can occur. It is therefore advised that
operational releases of airborne radioactivity should be subject to a comparative analysis
including uncertainties to resolve apparent differences between other similar facilities and to
provide an estimation of the upper level for doses to members of the public.
The atmospheric discharge models and dose assessments could be peer reviewed to ensure
consistency and that the omission of aerosols from the release inventory is justified.
Since the discharge calculations depend very much on the ventilation system and its mode of
operation this aspect should be well determined. The proposal not to discharge continuously
from the accelerator tunnel should be justified since it is not easy to see how a pressure
differential with respect to ambient conditions will be maintained to prevent leakage of
radioactive air from the tunnel near ground level.
R6 References
1
2
3
4
5
6
7
Ene,Environmental Impact Analysis, ESS-0001898 Rev 1, 14/12/2012
Health Risk Assessment ESS-003789 Rev. 1(4)
Studsvik, Environmental Dose Assessment, ESS Part 2 N-13/021
Summary of Analysed Events ESS-0001878,
Studsvik, Assessment of Radiological Impact of Unplanned Events at ESS, N-13/183
SWENCO FFNS, Report on Site Selection, 2002
J Vaillant et.al.,Management of radon: a review of ICRP recommendations, J.
Radiol.Prot. 32 (2012) R1-R12
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