Engineering Work for the ESS Target Station
G. Hansen, M. Butzek, H. Glückler, R. Hanslik, H. Soltner, V. Soukhanov, H. Stelzer and J. Wolters
Central Department of Technology, ZAT, Forschungszentrum Jülich, D-52425 Jülich, Germany,
[email protected]
Abstract-The European Spallation Neutron Source (ESS) is the proposed next-generation neutron
source to be built in Europe with a 10 MW linear proton accelerator feeding two target stations, a long
pulsed and a short pulsed one. The current contribution will cover the engineering layout of the target
stations and some work concerning the termalhydraulics of the target and the layout of the moderators.
I. INTRODUCTION
The European Spallation Source (ESS) is the
proposed next-generation neutron source to be built in
Europe. ESS will use a long-pulse (LPTS) and a
short-pulse (SPTS) target station with a time-averaged
proton beam power of 5 MW per target station fed by a
10 MW linear proton beam accelerator. For both target
stations mercury was chosen to be the target material.
The mercury is circulated inside a closed process loop
to allow the removal of about 2.8 MW of heat
deposited inside each target by the spallation process.
To slow down the high-energy neutrons generated by
the spallation process moderators located close to the
targets will be used. Combinations of water at ambient
temperature and hydrogen at cryogenic temperature
will be used as moderator materials. The low-energy
neutrons will be guided through 22 neutron beam lines
to the neutron scattering instruments. Therefore, the
basic requirements for the engineering layout of the
ESS target stations will arise not only from users’
demands and safety aspects but also from the layout
and engineering of the target and the moderators.
Key features of the ESS target station (Fig. 1) are
the horizontal proton beam inlet and a flowing liquid
mercury target. The two moderators are located above
and below the target serving as many as 22 neutron
beam lines on both sides of the proton beam line axis.
A remote handling cell basically dedicated for target
maintenance is located in proton beam forward
direction right behind the target (Fig. 2). A movable
trolley containing all the components of the mercury
loop as well as parts of some other ancillary systems is
used to carry the target from operation position into the
hot cell. A controlled handling enclosure (high bay
area) equipped with a 100 t crane is foreseen to allow
vertical handling using shielded flasks. Cooling water
supply as well as additional auxiliary systems are
located in underground caverns.
II. TARGET STATION ENGINEERING
II. A. General Layout
Fig. 1: The ESS target station building
II. B. Enclosure System
Fig. 2: General Layout of the ESS target area
Multispectral moderators combined from a cold as
well as an ambient temperature part will be used
allowing neutron spectra from cold to thermal neutrons
together in one neutron beam line. In order to assure
this multispectral extraction, it is necessary to provide
space for neutron guides with a cross section of up to
230 mm in width and 170 mm in height starting as
close as 1.6 m from the moderator surface. Taking into
account a usable angle for the neutron beam lines of
about 120° per side of the target station, the requested
11 beam lines lead to an angular separation of 11° and
therefore a 307 mm spacing of the guides at 1.6 meter
radius. The engineering solution to the requirements
mentioned above is the use of rotating disk shutters
starting at 1.6 m from the moderators, carrying the
front most part of the neutron guide. Due to the size
limitations the design of single shutter housings was
abandoned. All the shutters are located inside the inner
enclosure vessel. (Fig. 3)
Fig. 3: Arrangement of shutters inside inner vessel
While other spallation neutron source projects in
the past intented to use fissile material inside a so
called “booster” target, the ESS target will be a
spallation only mercury target. Thus, there is no need
to copy the high-level safety philosophy applied for the
enclosure of nuclear fuel elements in research or even
power reactors. Nevertheless, due to its substantial
radiotoxic (105 TBq) and chemically toxic contents (15
t Hg) and due to the materials used for moderators,
potentially forming explosive mixtures, the target
station requires a system of different enclosures. 1
Taking into account the intentionally used
materials like mercury, light and heavy water, liquid
hydrogen and helium as well as expected activated
gases and dusts, the basic requirements on the
enclosure systems can be described as follows:
•
•
control oxygen content in the core region to avoid
explosive mixtures in case of a moderator vessel
failure.
avoid spreading of activated gases and dusts as
well as spilled activated mercury and water to
areas with frequent personel access during normal
and abnormal operation.
The most common concept to avoid spreading of
activated media is to enclose the specified region and
operate it at a slightly negative pressure with respect to
the surrounding atmosphere. Thus, in case of a sealing
failure oxygen from the surrounding atmosphere will
be sucked inside these enclosure. The Oxygen content
and thus the possibility of forming explosive mixtures
will be difficult to control. This leads to the need for a
full double enclosure of the core region using an
oxygen-free interstitial space. Taking into account that
about 50 components located inside this inner
enclosure have to be handled with frequencies up to 4
times per year, a consequent double enclosure towards
the handling areas will make handling and
maintenance nearly impossible.
Therefore, the following concept was chosen for the
ESS target enclosure. (Fig. 4,5). The enclosure system
of the target station consists of a series of at least three
physical barriers for Hg and two ones for media
containing tritium. The first barrier is represented by
the piping or vessel system of the fluid itself. The
cryogenic hydrogen piping will be equipped with a
triple containment to avoid pumping of air to the cold
surface in the case of a small leak of the outer shell.
The mercury target vessel will have a second
containment (return hull) in the area of proton beam
penetration. Due to the high radiation damage rate in
this region and the uncertainties concerning lifetime of
the target vessel, the return hull is not considered to be
a safety relevant barrier.
High Bay
Concrete bars coverd with poyester sheets
Handling
Plug
Proton
Beam
Line
Top Plate
Hot Cell
(ATM)
Inner Liner
(ATM - 4hPa)
(ATM
- 2hPa)
Protons
(ATM
- 2hPa)
Target
Trolley
(ATM
- 100 hPa)
Decont./
Transfer
Cell
center of the arrangement) and thus placing the shutters
inside the inner liner leads to a diameter of the inner
liner of up to 10 m. All the components inside the inner
liner will be replaceable either scheduled or in case of
component failure. This liner is surrounded by a
second, outer one. Both liners will be hermetically
welded with no opening to be sealed except for one
port for static pressure monitoring.
Outer Liner
Hg
H2O
D2O
He
Wall
H2 / CH4
Struct. material
Vacuum
Ventilated Air
Seal
Reflector Cooling
Shield Cooling
PBW Cooling
Waste
fluid
storage
(ATM - 2hPa)
Underground Cooling Plan Room
Fig. 4: Enclosure concept (longitudinal cut)
The inner liner is to be the second barrier. It is
operated at a slightly higher pressure compared to the
adjacent atmospheres using helium as inert gas.
Therefore, diffusion of oxygen into the inner liner and
thus formation of explosive mixtures even in the case
of a hydrogen triple-pipe rupture is impossible.
High Bay
Concrete bars coverd with poyester sheets
Safety
Expansion
Tank
Ambient
and Cold
Moderator
Pipe with Rupture Disk
Outside
Building
(ATM)
(ATM - 2hPa)
Instrument
Hall
to
instruments
(ATM
- 100 hPa)
(ATM)
Moderator
handling
door
Beam port plug
consisting of two
separat parts
Outer Liner
Hg
H2O
D2O
Instrument
Hall
He
H2 / CH4
Struct. material
Inner Liner
Vacuum
Ventilated Air
Seal
(ATM
- 100 hPa)
Wall
Fig. 5: Enclosure concept (90° cut)
The third barrier recommended for mercury is
represented by a second (outer) liner towards the
instruments hall and a controlled ventilated air system
covering the top plate as well as the proton beam line
area. Therefore, no activated media will leave the
building in an uncontrolled way. If found necessary the
high bay area as well as the instrument hall can be
operated at a pressure slightly lower than atmospheric
pressure outside the building. Anyway, these
enclosures are not considered safety relevant according
to the enclosure concept mentioned.
The key point of the concept is the liner system
surrounding the target – reflector – moderator
arrangement (Fig. 3, 6). The innermost liner is placed
not closer than 2.5 m from the center in order to limit
the radiation damage within 40 years of lifetime to an
acceptable value. The requirement to start the shutters
at not more than 1.6 m from the moderators (also
Fig. 6: Liner system including top plate
The horizontal beam port plugs allowing to guide
the beam to the instrument hall will use double seals
with an interstitial space to be monitored. These plugs
will also include the neutron beam windows made
from thin aluminum foils. Separation of the helium
atmosphere from the vacuum of the proton beam
transport line will be provided by a double walled,
water cooled window, which has to be exchanged a few
times per year due to radiation damage. This window
will be designed as a module including cooling
structure and metal inflatable seals. The exchange of
this module will be carried out from the top using a
dedicated opening in the top plate.
Beside the proton beam window, the components
to be accessed through the top plate will be the
in-shutter neutron guide inserts, the shutter drive
systems and last but not least the reflector unit. All the
necessary openings in the top plate will be closed using
double seals with interstitial space to be monitored.
With all the openings from the inner liner to the high
bay using double seals, migration of activated gases
and dusts to the ventilated air system can be avoided
during normal operation.
II. C. Target Shielding
Shielding is one of the major and cost driving parts
of the target station. For a sound conceptual design as
well as for a reliable cost estimate, a detailed
knowledge about all requirements is vital. The basic
function of this shielding system is to provide shelter
from various kinds of radiation, taking into account
staff within the target building, public, and
environment.
Shielding a spallation neutron source is more
difficult than shielding a fission reactor. Neutrons from
spallation process reach significantly higher energies
than fission neutrons. For a spallation neutron source,
high-energy cascade neutrons approaching the energy
of the incident proton beam of up to 1.3 GeV are
extremely penetrating as well as being ineffectual for
becoming useful neutrons. Well designed shielding is
needed to prevent high-energy neutrons from causing
excessive biological dose rates as well as unwanted
backgrounds in experiments.
The ESS target shield monolith will be designed,
constructed and operated in such a manner as to protect
the safety of staff, public and environment. 2
Results from dose calculation by Monte Carlo
method show that the dose in the shield is determined
by neutrons with energies higher than 100 MeV.
Reactions of these high-energy neutrons produce
secondary neutrons with energies down to thermal
energies.
The coupling computer code CASL (Computer
Aided Shield Layout) was used for the design of the
ESS shielding monolith. This method calculates the
dose rate behavior inside the shield for different polar
angle regimes related to the proton beam directions. 3
The basic geometry of ESS shielding monolith
consists of a cylinder from iron with an outer shell
from ordinary concrete. The radius of the iron shield is
650 cm. The thickness of the iron shield above the
target area will be 480 cm. Below the target area the
iron thickness can be reduced to 330 cm if a large
concrete base for foundation of the target station is
necessary (see Fig.7).4 The iron shield itself is
surrounded by 50 cm of ordinary concrete and is based
on a 4 m thick ordinary concrete layer.
Water cooled
shielding
Roof shield
Inner shiedling
Inner
shiedling
Concrete
Outer shielding
Fig. 7: Geometry of shielding monolith
•
•
•
•
The shielding monolith design consists of:
Outer shielding
Inner shielding containing the target-reflectormoderator assembly, the shutter array and the
water cooled shielding
Roof shield made of ordinary concrete
Outer ordinary concrete wall of 50 cm with
400 cm thick concrete base plate.
Outer shielding is located outside to the inner liner
and inside the outer liner (Fig. 6). The outer shielding
region will be filled with cast iron blocks using
recycling material with He in the interstitial spaces.
The shielding requirements are different in the
ESS target shielding areas. From the point of shielding
efficiency and taking into account a low cost target,
iron is a cheap material which meets the material
properties needed for shielding of high-energy
neutrons. Specially cast iron offers the most
advantages in view of design and manufacturing.
In our current concept we intend to use cast iron
with pre-irradiated recycling material for the outer
shielding. Using recycling of scrap from
decommissioning of nuclear installations it is possible
to subsidize the shielding costs. Manufacturing of
shielding blocks using recycling material from nuclear
decommissioning is provided by Siempelkamp GmbH
Krefeld, Germany.
The outer shield is composed of large blocks with
irregular shapes with a maximum weight of about 65
tons. Shielding blocks from outer shielding are not
removable. In addition to that, the machining
requirements are lower compared to blocks for the
inner shielding.
We intend to use flat blocks with large footprints
for better stability especially taking into account
seismic events. Only the lower side of blocks from the
lowest layer will be machined. Stability of the outer
bulk shield can be achieved by a fixed connection of
the lowest block layer with the basis plate. The next
layers of blocks will be successively stacked without
screws (Fig.8).
Each of the blocks in outer shielding will be
toothed in lateral direction, on the top and bottom faces
of blocks. In addition each layer of blocks will have an
offset position with respect to the adjacent layer.
(Fig. 9)
The maximum tolerances between the toothed
structure caused by casting tolerances of adjacent
blocks will be limited to not more than 11 mm. This
design will avoid long gaps between the stacked blocks
that may cause streaming. The overlapping of material
will be at minimum about 85% from the bulk material.
The Gaps between the blocks will be not filled.
Outer liner
Inner liner
Inner
Shield
Surface
cleanness
machinable
removable
material
Lower
inner
shielding
low
partly
after
accident
Recycling
cast iron,
20%quota
covered by
stainless
steel thin
skin
Water
cooled
inner
shielding
high
yes
yes
Stainless
steel
Upper
inner
shielding
high
yes
yes
Standard
modular
cast
iron
with
protected
surface
Base plate
Anchor bolt
Fig. 8: Blocks of outer shield 5
Tab.1: Requirements of ESS Target inner shielding
The inner shielding is located outside to the
reflector and inside the inner liner. By the high neutron
flux in this area, fluids, gases and impurities like
corrosion products become activated during operation.
Therefore, these blocks should be protected by surface
layer materials with a high corrosion resistance or the
shielding material itself should be non-corrosive.
Space for beam holes
Fig 9: Outer shielding block 5
The inner shielding is composed of various layers
of blocks (Fig.7) with different requirements. The
requirements for each shielding together with the
recommended materials are summarized in Tab. 1
For maintenance or replacement activities some
blocks from inner shielding will have to be removed.
All handling of hot components in this area must
supply local shielding or shielded flask and
contamination control.
II. D. Neutron Beam Shutter
The shutters allow the closing of the neutron beam
at any beam line for sample exchange while the source
is running. The dose rate at 6 m distance from
moderator will be reduced to less than 10 Sv/h when
the shutter is in the closed position. The shutters
provide adjustable support for neutron guide inserts.
While in the closed position, easy exchange of neutron
optical components is possible.
Each beam line will be equipped with a single
shutter. There are 22 beam lines per target station
located at both sides of proton beam axis (11 per side)
and distributed uniformly at a constant angular
distance of 11°. The distance from the moderator
center to the start of the inner most neutron guide
located inside the shutter will be as close as 1.6 m
(Fig. 3)
The current shutter design consists a of wheel of
2.8 m in diameter rotating about a horizontal axis at
right angles to the neutron beam direction. A rotation
of 90° moves the beam hole from horizontal (open) to
vertical (closed) position, taking about one minute. The
shutter wheel will be made from stainless steel. The
highly activated center section in the closed position
will be designed as a plug, forming a small removable
part which simplifies remote handling activities. The
shutter wheel will weigh about 16 tons. The shutter
disk provides space big enough to allow a guide insert
of 230 mm x 170 mm to float inside.
The shutter wheel will be supported by lubrication
free hybrid bearings designed to the lifetime of the
Outer insert
Inner insert
Fig. 10: Arrangement of shutters allowing small angle
between shutters and small distance between guide and
moderator
facility. The shaft of 150 mm in diameter is supported
in the lower non-removable inner shielding block
(Fig.10). The upper shielding block as well as the
shutter wheel itself will be removable.
All the shutters are located in the helium
atmosphere of the main containment vessel, thus
eliminating the need for additional neutron beam
windows on the shutters themselves. The concept
without a separate atmosphere for each shutter allows
minimum distance of adjacent guides. Removal of the
heat deposited in the shutters is achieved by cooling
the surrounding shielding and by relying on conduction
through the helium and radiation to the cooled
shielding.
Each beam line will be equipped with either a
neutron guide or a collimator. Inserts containing either
a guide or a collimator will be chosen according to the
instruments needs at each beam line. Outer dimensions
of all inserts will be the same. The beam line within the
shielding monolith will consist of two inserts: (Fig. 11)
•
•
inner insert in the shutter with a length of 2.8 m
outer insert located in the outer shielding with a
length 0.8 m – 3.0 m depending on the varying
thickness of the outer shielding.
Alignment requirements for the guides are quite
high. The max. allowed horizontal and vertical offset
between inner and outer insert is 1 mm. The max.
allowed angular displacement from horizontal
direction is 0.1° (Fig. 12).
Guides will be pre-aligned within the inner and
outer inserts. Once installed, alignment of the guides
will be carried out by aligning the inserts only. The
adjustment of the insert will be accomplished in two
Fig: 11: Insert installation
steps. At first adjustment of outer inserts and at second
adjustment of inserts inside the shutter wheel.
P re adjustable
suppo rts
P re ad justed
supporting surface
Fig: 12: Adjustment of outer guide insert
Supports suitable to align the outer position of the
inner inserts will be attached to the outer inserts. These
supports will be pre-adjusted with respect to the outer
guide position, during assembly and manufacturing of
the outer inserts. Outer inserts will be mounted as one
unit in a pipe welded to the outer as well as to the inner
liner forming a hermetic separation from the outer
shielding atmosphere. The exchange of the outer guide
insert takes place from the instrument hall.
The inner guide insert will float inside the shutter
wheel. The horizontal adjustment will be provided by
means of a pre-adjusted support in the shutter wheel.
This support will be adjusted during the first assembly
of the shutter in order to care for manufacturing
tolerances within the shutter wheel. It is not supposed
to be changed later on. Thus, one side the insert will
rest on the in-wheel support and on the opposite side on
the supporting surface from outer guide insert. The
vertical adjustment will be carried out by means of an
adjustable stop for the shutter wheel. (Fig.13)
a shielded container after removing the top plate and
part of the upper shielding blocks. The frequency of
repair is expected to be very low.
II. E. Moderator Systems
Adjustable stop
Adjustable wheel
support
Fig: 13: Adjustment of inner guide insert
A shutter wheel rotation of 90° moves the inner
guide inserts from the horizontal to the vertical
position. This allows the exchange (through access
ports from top plate) the inner guide inserts (Fig.14)
without removing the shutter wheel itself.
The neutron guides located in the shutter wheels as
well as stainless steel shutter plug will be removed
vertically into a shielded flask. This flask will be
designed as universal handling flask, which can be also
used for other modules.
Neutron guides
Shutter wheel
Wheel support
Fig: 14: Inner guide exchange
Only for the repair of the bearings of the shutter
wheel the wheel itself has to be removed vertically into
Both target stations will have both thermal and
cold cryogenic moderators. The cryogenic moderators
employed in both target stations will be supercritical
hydrogen at about 25 K. The thermal moderators will
be water moderators at ambient temperature. Due to
the fact that the average beam power for each target
will not exceed 5 MW, the same estimated cooling
capacity of 7.5 kW will be assumed for any of the
individual cold moderators. The cooling power is
generated in a cryo-plant working with helium 6,7. The
cold helium, produced by expanding it in turbines, is
used to cool the hydrogen circuit including the
moderator chambers. A vacuum containment is
provided to insulate the cold sections of the helium and
hydrogen circuits. A blanket with a higher pressure
helium atmosphere is surrounding all the hydrogen
carrying parts (“triple containment”). Thus, any
leakage in the vacuum containment can be detected
because helium will penetrate the leak. Additionally,
the higher pressure helium atmosphere prevents other
gases from penetrating the vacuum walls through such
leaks. This is important for safety reasons: if gases
other than helium or hydrogen get in contact with the
hydrogen pipes at 25 K, they will freeze immediately;
then they are hardly to detect because they will not
produce any pressure increase. But solidified gases
may generate dangerous mixtures during warming up
and if a hydrogen leak occurs. The helium blanket
prevents this.
The supply systems for the two moderators in both
target stations will be completely independent of each
other, but as identical as possible. Thus, it will be
possible to operate the individual moderators at
different temperatures.
The moderators are placed horizontally into their
positions, one above and one below the target.
Therefore a plug-in module including the moderators,
the necessary service pipes and the shielding has been
designed (Fig. 15,16).
Fig. 15: Horizontal moderator plug in target station
Fig. 17: Design of the coupled cold moderator
including the inner vessel for supercritical H2 flow
(first containment), enclosed by a vacuum chamber
(second containment), again enclosed by a helium gap
(third containment)
III. THERMALHYDRAULICS
III. A. Computational Fluid Dynamics
Fig. 16: Moderators above and below the target
Different moderator configurations for the target
stations are provided. There will be bi-spectral,
side-to-side moderators (coupled ambient H2O and
supercritical H2), a back-to-back moderator (decoupled
ambient H2O and supercritical H2) and a supercritical
H2 moderator. For the decoupled moderators a
Gd-layer will be integrated between the cold and
thermal moderator. All the service pipes for the
moderators will be integrated with the horizontal
moderator plug-in module (Fig. 15) 8.
To ensure a good neutronic performance of the
moderator the only material used of the moderator
vessel, the insulating vacuum chamber and the casing
for the helium gap is aluminum alloy AlMg3
(Aluminum Association alloy code 5754), i.e. an
aluminum including about 3 mass-% of Mg. The
moderator vessels have an inner volume of 1 dm3 (liter).
Fig. 17 shows the design of the coupled cold moderator
as an example.
Beside solving the pressure pulse problem (cp.
Chapter III. C.) thermalhydraulics is one of the main
aspects regarding the target design. The work in this
field is done in close cooperation with the company
NRG (Nuclear Research and Consultancy Group) in
Petten, Netherlands.
The work on thermalhydraulics concentrated on
two main aspects, namely the assessment of available
CFD (Computational Fluid Dynamics) codes for heavy
liquid metal flows and improvement of the flow field
inside of the target with respect to removal of the
deposited heat, cooling of the target beam window and
prevent accumulation of bubbles used for pressure
pulse mitigation within the front part of the target.
III. A. 1. Validation of CFD models
Validation of the applied CFD models has taken
place within the European ASCHLIM (Assessment of
Computational Fluid Dynamics Codes for Heavy
Liquid Metals) project 9. In this project one benchmark
was directly related to the ESS project, namely the
benchmark on the so called ‘ESS Mercury Target
Model Experiment’ 10. The aim of the ESS mercury
target model experiment was to study the heat transfer
between a heated surface and a mercury flow, in a flow
configuration typical for the ESS spallation target.
III. A. 2. Flow field improvement for the ESS target
∅
∅
A series of numerical simulations were performed
for the test section of the ‘ESS Mercury Target Model
Experiment’ 11. The geometry of the test section and
the corresponding CFD model is shown in Fig. 18.
Calculations were performed with different turbulence
models and different near-wall treatments. The
suitability of wall functions and the eddy diffusivity
concept with constant turbulent Prandtl numbers was
investigated.
For the ESS target calculations were performed in
order to improve the flow field inside the target. One of
the most critical criteria for the flow field optimization
was the secure removal of the total heat deposited in
the liquid mercury by the proton beam and prevention
of mercury evaporation, respectively.
The CFD model used for the calculations at FZJ
was provided by NRG and is shown in Fig. 20. One
central inlet pipe is connected to the inlet manifold,
where the mercury flow branches into two side inlet
ducts and a central bottom inlet duct. The three inlet
flows are flowing together again at the target window,
where a good window cooling has to be achieved.
If no additional orifices are used for the three inlet
ducts, 39 % of the total mass flow will go through each
side duct and 22 % of the total mass flow will go
through the bottom inlet duct. This flow configuration
is the so called ‘reference case’. For this reference case
the flow paths colored by the mercury temperature are
shown in Fig. 21. The maximum mercury temperature
is about 274 °C and occurs in the outlet region of the
target.
Fig. 18: Geometry and CFD model of the test section
of the ‘ESS Mercury Target Model Experiment’
For the RANS (Reynolds-Averaged Navier
Stokes) approach a good agreement between
experiment and calculation is achieved for higher flow
rates than in the ESS target. If the boundary layers are
fully resolved or adopted then thermal wall functions
are used for the calculations (Fig. 19). Moreover, it was
shown that a constant turbulent Prandtl number with
the standard value of 0.9 is suitable for calculations at
higher flow rates.
Fig. 20: CFD mesh of the ESS mercury target,
provided by NRG, Petten
6
q = 1.5 l/s
5
∆ T [K]
4
3
2
experiment
1
CFD calculation
0
1
6
11
16
21
HETSS
Fig. 19: Comparison of measured and calculated
temperature increments along the heated wall of the
test section.
Fig. 21: Flow pattern in the ESS mercury target for the
reference case
In additional calculations the effect of the flow
distribution was studied. In Fig. 22 the maximum
mercury temperature is shown as a function of the
normalized mass flow rate through each side duct. It
was shown that an increased mass flow through the
bottom duct will have a significant negative effect on
the maximum mercury temperature in the target, while
the maximum mercury temperature will stay at an
acceptable level for an increased flow rate through the
side ducts (Fig. 22). Nevertheless, a minimum flow
rate through the bottom duct is necessary to shift the
stagnation point away from the most heated window
zone.
inlet channel, no water flow through the bottom
channel) in the central horizontal plane. The colour
indicates the flow velocity of water (red: high velocity,
blue: low velocity). The dark-blue colour marks
regions which cannot be analyzed by PIV due to
structure material blocking the free view to the water
flow.
maximum mercury temperature [K] .
800
750
700
650
reference case
600
design limit
550
500
0
10
20
30
40
50
mass flow through each side duct [% of total mass flow]
Fig. 22: Maximum liquid mercury temperature
depending on the mass flow rate distribution
III. B. Experiments
First measurements on the fluid dynamics in a
reference target model were performed using a
plexi-glass model filled with water. The plexi-glass
model has the same geometric size as the reference
target filled with mercury. The fluid dynamics inside
the target model was investigated with a laser based
optical method. (particle imaging velocimetry PIV).
This method allows the simultaneous measurement of
flow fields. Details about these technique can be found
elsewhere 12.
The goal of the measurements was to study the
flow field distribution near the entrance region in the
target. Due to technical reasons the total mass flow of
water through the target was limited to 30 m3/h with an
inlet water pressure of 2 bar absolute. The flow field
can be influenced by changing the flows through the
inlet water channels located at the left and right side
and at the bottom of the target. A parameter variation
for both symmetric (flows through the side channels
are equal) and asymmetric flows was performed.
Fig. 23 shows as an example the flow field for
symmetric inlet flows (15 m3/h from the left and right
Fig. 23: Flow distribution inside the target shown from
above.
A detailed analysis of the experimental findings
and a comparison between experiment and results
obtained from numerical calculation by finite element
simulations will be published in the near future 13.
III. C. Pressure Wave Mitigation
During each proton pulse the deposition of a huge
amount of energy within about 1 µs inside the target
volume leads to a quasi instantaneous temperature rise
∆T of the mercury and a pressure pulse with
amplitudes of more than 100 MPa. Because these
cyclic pressures are generated only a few centimeters
behind the target window, they pose a severe problem
for the structural integrity of the material. Even worse,
however, after about 40 µs a zone of negative pressure
follows, which leads to cavitation. It has been shown
by mechanical tests in Japan and with proton beams in
the US, that this effect erodes the inside of the target
window within such a short time, that impractically
short operation times would result. While on the one
hand harder materials may be developed and employed
to reduce the effects of cavitation, another approach is
to reduce the generated pressure via the increase of the
compressibility of the liquid by the admixture of gas
bubbles.
From theoretical considerations with the specific
ESS conditions as input parameters we infer that a void
fraction of about 0.6 % of bubbles in the 20 µm range
should be sufficient to drastically dampen the negative
effects of the pressure pulse 14.
As a result from CFD (computational fluid
dynamics) calculations, we learned that bubbles
smaller than about 50 µm in diameter will be able to
follow the flow in a mercury loop. Only these bubbles
will be able to travel from an upstream nozzle to any
place inside the loop and to the target in particular and
dampen the pressure waves there.
Our attempts at generating sub-mm small bubbles
in a static fashion by employing small bore orifices
submerged in liquid metal had been proven
unsuccessful. Therefore, for beam tests at the Weapons
Neutron Research facility at Los Alamos National lab
in 2002, we had employed a patented orifice which had
been developed for the generation of monodispersive
gas bubbles in non-metallic liquids. A picture of such a
device is shown in Fig.24.
Fig. 25: Bubble size spectrum of a bubble population in
GIT generated with the device depicted in Fig. 24.
With the bubbly liquid mercury generated with
this device, it was possible to reduce the erosion of the
inner surface of target flanges during irradiation by
about a factor of 4 as compared to flanges immersed in
the pure mercury but with otherwise the same
irradiation conditions.
The inside faces two corresponding flanges are
shown in Fig. 26.
Fig. 26:. Surfaces of flanges immersed in a Hg – He
mixture (left) and in pure mercury (right).
Fig. 24: Nozzle used for the generation of gas bubbles
in liquid metals
Both liquid and gas are forced simultaneously
thorough small bores on the front of this nozzle. The
gas ligament is periodically pinched off by the instable
liquid flow, thus generating small gas bubbles.
(Courtesy A.Ganán Calvo, Universidad de Sevilla,
Spain, 15)
Using an acoustic bubble spectrometer (ABS)
developed by the American company Dynaflow 16, we
were able to prove in a similar liquid metal (GIT:
gallium–indium–tin) that monodispersive gas bubbles
had indeed been generated by this device. Fig. 25
shows a corresponding bubble size spectrum
monitored by the spectrometer which is dominated by
monodispersive bubbles with radii of about 270 µm.
The left flange shows a considerably lower level
of damage, although for geometrical reasons it had
been subject to a higher proton flux.
These initial results on pressure pulse damping by
bubbles will be substantiated during tests at the
mercury loop at the Institute of Physics in Riga, Latvia,
in 2003. There, we will investigate the performance of
four different nozzle types, experience the acoustic
behavior of a volume of mercury filled with helium
bubbles of a few tens of microns, and monitor the
damping of electromagnetically induced pressure
pulses by the two phase liquid.
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