The present invention relates to a beam pipe in an accelerator-driven
nuclear system, for guiding an accelerated particle beam onto a nuclear fuel target
through a vessel filled at least partially with a coolant. It relates in particular
to such a beam pipe with an integrated safety function.
Calculations of severe cooling disturbances (such as e.g. "Loss-of-Flow"
and "Loss-of-Heat Sink" accidents) and also reactivity accidents in a liquid metal
cooled accelerator-driven nuclear (subcritical) system (ADS) show that the switching
off of the accelerator or the interruption of the proton beam are the only means
of reducing the power to safe levels.
For switching off the accelerator, active and also complex passive
systems have been suggested. Active systems will e.g. rely on temperature measurements
leading to alarms in the control room when too high values are detected. Complex
passive system will e.g. rely on a computer logic that will lead to an automatic
shut-off of the accelerator when a certain percentage of thermocouple readings
are too high.
There is still a need for simple and easily understandable safety
devices for interrupting or switching off the proton beam, in order to further
reduce the probability of serious accidents and to make people feel more confident
about safety of accelerator driven systems.
In the publication "Conceptual Design of a Fast Neutron Operated Energy
Amplifier", CERN/AT/95-44 (ET), C. Rubbia et al describe a beam pipe with an integrated,
entirely passive, safety function. The beam pipe includes an emergency beam dump
volume with an overflow device located slightly higher than the normal reactor
coolant level. If the reactor coolant level rises due to the thermal expansion
of the overheated coolant, the coolant flows through the overflow device into the
beam dump volume. The proton beam is interrupted if enough coolant overflows into
the beam dump volume.
A major drawback of this safety system is that its reliability may
be impaired by too many parameters. The rate of thermal expansion of the coolant
is driven by the reactor power. But the reactor power is decreasing with the rising
coolant level in the emergency beam dump volume, so that the thermal expansion
of the coolant and consequently the overflow is gradually decreasing. Whether
the overflow stops before the power is down to a safe level or whether a further
heating up of the system and a second overflow event will occur, has to be determined
by complicated coupled reactor core and primary vessel thermo-hydraulics, together
with a detailed neutronics calculation including a determination of the neutron
source due to the spallation of coolant by the proton beam. In other words, exact
working predictions of the above safety system require sophisticated calculations.
Furthermore, the overflow rate of the coolant into the beam dump volume is dependent
on the height of the coolant in the primary vessel and on the mean temperature
increase of the total coolant volume. Assuming that the coolant heats up uniformly
and that the vessel does not heat up and expand, one can calculate for a 25 m high
coolant column a total thermal expansion of 28 cm per 100°C for lead and 36 cm
for lead/bismuth. More recent designs use however a reactor vessel that is only
half as high. The total coolant expansion would then only be 14 cm for lead and
18 cm for lead/bismuth per 100°C. In an accident leading to a very slow heating
up of the coolant, the diameter of the reactor vessel would expand and the total
axial coolant expansion would even be considerably less than the values given
above. Whereas in a relatively fast heating up of the reactor core, with a time
constant shorter than the round-trip time of the coolant, or in a "Loss-of-Flow"
condition with the primary heat exchangers still removing heat at the top of the
reactor vessel, a small amount of the coolant in the vicinity of the core may be
considerably overheated, while the mean temperature of the total coolant volume
does not vary much. Such a local overheating may lead to core damages before the
coolant level rises significantly. Also in case of a vessel leak, the above described
emergency beam dump volume would lose its function. The coolant level in the primary
vessel would drop and the overflow device would no longer work. Similarly, in case
of a loss of vacuum in the beam pipe, the emergency beam dump volume would no longer
Object of the invention
The technical problem underlying the present invention is to provide
a beam pipe for an accelerator-driven nuclear reactor with an improved integrated
safety function. This problem is solved by a beam pipe as claimed in claim 1.
General description of the invention
A beam pipe in accordance with the invention includes a temperature
triggered flooding device for flooding the interior of the beam pipe with the coolant
in case of overheating. This flooding device includes, below the normal coolant
level in the vessel, a communication with the interior of the beam pipe. In an
emergency situation leading to an abnormal temperature increase, the temperature
triggered flooding device opens its submerged communication and the coolant fills
the interior of the beam pipe in accordance with the principle of communicating
vessels. In case of equal pressures in the vessel and in the beam pipe, the coolant
will rise in the interior of the beam pipe up to the level of the coolant in the
primary vessel. In most cases the pressure inside the beam pipe will be lower than
the pressure in the primary vessel, so that the final coolant level inside the
beam pipe will even be higher than the coolant level in the primary vessel. In
any case, if the flooding device is triggered by an abnormal temperature increase,
a coolant column will reliably build up in the beam pipe, creating a new target
for the accelerated particle beam at the top of the coolant column, i.e. several
meters above the reactor core. It will be appreciated that the beam pipe of the
present invention provides a reliable safety function that is significantly affected
neither by fluctuations of the coolant level in the reactor vessel, nor by the
build-up of a counter-pressure inside the beam pipe. Finally, if the flooding device
was erroneously activated during normal reactor operation, the accelerator driven
reactor would be switched off and there would only be an economic penalty to replace
or clean the beam pipe.
It will also be appreciated that if the flooding communication is
located at a significant distance below the normal coolant level, the flooding
device of the present invention will still reliably work if the reactor coolant
level drops significantly, e.g. due to a leak in the reactor vessel. It will also
be appreciated that the hydrostatic pressure urging the coolant through the communication
into the interior of the beam pipe increases with the coolant height above the
In a preferred embodiment of the beam pipe, the temperature triggered
flooding device includes temperature triggering means located at a short distance
from the tip end of the beam pipe, i.e. at a short distance from the nuclear fuel
target. It follows that the flooding device will be responsive to a temperature
increase close to the nuclear fuel, so that it will respond promptly both to a
slow and to a fast heating up of the reactor core.
The reliability of the integrated safety function of the beam pipe
may be further increased by providing several temperature triggered flooding devices
located at different distances from the tip end of the beam pipe.
The flooding device preferably includes an entirely passive temperature
triggering device, such as for example a fuse or a bimetallic release device.
This temperature triggering device triggers the opening of a flooding gate in the
flooding communication. If triggered by its temperature triggering device this
gate may for example open under the action of a spring and/or the action of the
hydrostatic pressure produced by the coolant column in the primary vessel.
In a preferred, because very simple embodiment of the invention, the
temperature triggered flooding device includes a melt-rupture disc, which is sealing
the flooding communication between the vessel and the interior of the beam pipe.
(It is pointed out that such a "melt-rupture disc" may be a body having any suitable
form for sealing the flooding communication. It must not necessarily have the geometric
form of a disc.) The material of this melt-rupture disc may be chosen so that it
will soften (and subsequently rupture) or melt at a predetermined temperature,
opening the flooding communication between the vessel and the interior of the beam
pipe for the coolant. According to a preferred solution, the melt-rupture disc
is sealingly fixed into a mating opening by a solder material that is chosen so
as to free the melt-rupture disc at a predetermined temperature and to open thereby
the flooding communication for the coolant.
In a preferred embodiment, stop means are provided for preventing
the melt-rupture disc from being pushed through its opening from the interior to
the exterior of the beam pipe. In other words, the stop means assure that the disc
will be pushed inwardly into the beam pipe and will not fall into the reactor
vessel, where its retrieval would be more complicated. The stop means may include
for example a tapered mounting hole and/or a tapered melt-rupture disc.
When there is a vacuum in the beam pipe, the coolant flooding the
beam pipe in an emergency case will rise above the level of the coolant in the
reactor vessel. This part of the beam pipe would not be cooled by the coolant circulating
in the reactor vessel and could be overheated if the proton beam were not switched
off. (It will be appreciated in this context, that a rupture of the beam pipe would
not be a major safety problem, because the coolant column will still protect the
core from the proton beam.) To improve the conditions for cooling of the upper
part of the beam pipe in case of flooding, the present invention suggests to include
at least one melt-rupture disc above the normal coolant level, for equalising the
coolant level inside and outside the beam pipe after the coolant has flooded the
interior of the beam pipe through a submerged flooding device. Preferably there
shall be a series of melt-rupture discs at different distances above the normal
coolant level. These secondary melt-rupture discs could have a higher rupture temperature
than the melt-rupture disc(s) below the normal coolant level in the primary vessel.
Furthermore, these secondary melt-rupture discs should preferably be allowed to
fall in both directions -i.e. into the beam pipe or into the primary vessel. The
rupture of one or several of these discs, due to the heat generation of the impinging
proton beam, should get the coolant level in the beam pipe down to the coolant
level in the reactor vessel.
According to a further aspect of the present invention, the beam pipe
has -in the region where the coolant level is likely to establish itself inside
the beam pipe after flooding - a beam pipe portion having a significantly increased
cross-section. This pipe portion with an increased cross section will help to avoid
major damages to the beam pipe, when the proton beam impinges on the surface of
the coolant column inside the beam pipe. Major damages to the beam pipe could indeed
make its withdrawal difficult. A partial melting or explosion of the beam pipe
would also generate steel debris falling in the reactor vessel. The efficiency
of the above described beam pipe portion with an increased cross-section is advantageously
increased by providing cooling fins at its outside.
Detailed description with respect to the figures
The present invention will now be described, by way of example, with
reference to the accompanying drawings, in which:
- Figure 1:
- is a schematic section through an accelerator-driven system with a beam pipe
in accordance with the present invention;
- Figure 2:
- is a three-dimensional view of the lower end of the beam pipe of Figure 1;
- Figure 3:
- is a three-dimensional cutout of a beam pipe in accordance with the present
- Figure 4:
- is a cross-section through the lower end of the beam pipe of Figure 1 and the
cutout shown in Figure 3.
Figure 1 schematically shows a typical layout of a liquid metal cooled
Accelerator-Driven (subcritical) System (ADS). This system is contained in a main
vertical silo 10 which may have a height from 10 to 30 m. Reference number 12
indicates a primary vessel, which is filled with a heavy metal coolant 14, such
as for example lead or a mixture lead/bismuth. The normal coolant level in the
primary vessel 12 is indicated by reference number 16. The primary vessel 12 is
contained in a containment vessel 18, which has among others the function to collect
an overflow of the coolant 14 at the top of the primary vessel 12 (see overflow
path indicated by reference number 20). An air cooling system comprising cold air
downcomers 22 and hot air risers 24 is located between the outer wall of the containment
vessel 18 and the inner wall of the main silo 10.
Reference number 26 globally indicates the core, comprising a nuclear
fuel region, a spallation region and a plenum region. In the core region 26 the
coolant 14 is heated up. The hot coolant 14 is then rising through a central chimney
34, which is separated from the rest of the primary vessel 12 by a thermal insulating
wall 38, to the top end of the primary vessel 12. Here it passes through heat exchangers
38', 38'', which cool down the coolant 14 and create a cold, descending coolant
stream 40 to the bottom end of the primary vessel 12, where the core 26 is located.
Reference numbers 42', 42'' indicate the secondary cooling circuits of the heat
exchangers 38', 38''.
The nuclear system is driven by a proton beam 44 which is guided by
a beam pipe 46 through the coolant 14 into the lead or lead/bismuth spallation
region 30 of the core 26. Such a beam pipe 46 may have a length of more than 30
m for an internal diameter of about 20 cm.
Figure 2 represents an enlarged view of the tip end 48 of the beam
pipe 46. This tip end 48 is closed by a tungsten "window" 50, so that a relatively
high vacuum can be maintained in the interior of the beam pipe 46. But it has also
be suggested to use a beam pipe with a windowless tip end, in which a fast-moving
lead/bismuth flow at the bottom of the pipe creates a pressure of a fraction of
an atmosphere inside the beam pipe. Such or other embodiments of the tip end of
the beam pipe 46 can of course also be used in combination with the present invention.
It will be noticed that the preferred beam pipe 46, of which details
are shown in Figures 2 to 4, is a double-walled or jacketed pipe, comprising an
inner pipe 52 for guiding the proton beam and an outer jacket 54. The jacket 54
is delimiting an annular gap 56 around the inner pipe 52 for receiving either a
special cooling circuit or an insulation material.
In accordance with the invention a melt-rupture disc 60 is provided
at a short distance from the tip end 48 of the beam (see Figure 2), i.e. well under
the normal coolant level 16 in the primary vessel 12. The distance between the
melt-rupture disc 60 and the core 26 will preferably be chosen great enough to
avoid a high neutron flux and small enough to respond quickly to a heating up
of the reactor core 26.
The melt-rupture disc 60 is sealing a passage or communication 62
extending from the primary vessel 12 into the interior 64 of the beam pipe (see
Figure 4). In the preferred embodiment this passage 62 consists of a tube 66 bridging
the annular gap 56 between jacket 54 and inner pipe 52. The melt-rupture disc 60
is sealingly soldered into a mating opening 68, wherein the solder material 70
is chosen so as to soften or melt at a predetermined temperature, thereby freeing
the melt-rupture disc 60.
When the coolant 14 heats up in an emergency situation, the soldering
material 70 around the melt-rupture disc 60 will consequently soften or melt.
The considerable hydrostatic pressure acting from the coolant-side on the melt-rupture
disc 60 will then push the latter into the interior of the beam pipe. After the
melt-rupture disc 60 has been pushed inward, the beam pipe 46 will be flooded by
the coolant 14 through the submerged passage 62. The coolant 14 will rise within
the interior 64 of the beam pipe 46 up to the coolant level 16 in the primary vessel
12. If there is a vacuum within the interior 64 of the beam pipe 46, the final
coolant level inside the beam pipe 46 will even be higher than the coolant level
16 in the primary vessel 12. In any case, a considerable liquid metal column will
block the proton beam 44 from the core 26.
Attention will have to be paid to the choice of solder material 70.
It should maintain its integrity at operating temperatures and should preferably
become soft at 100-200°C above it. For a lead/bismuth coolant and a coolant temperature
of 500°C at the core outlet, a silver solder (including also copper, zinc and
possibly cadmium) could e.g. be used. In order to avoid a reaction with the coolant,
the solder material 70 should be covered by a protective coating (e.g. a ceramic
coating), so as to prevent a direct contact between the solder material 70 and
the coolant 14.
In the embodiment shown in Figure 4, the melt-rupture disc 60 and
its opening 68 are tapered from the interior to the exterior, so that the melt-rupture
disc 60 has to fall inside the beam pipe 46 and may not be pushed through its
opening 68 in the primary vessel 12. Furthermore, in order to make the retrieval
of a released melt-rupture disc easier, it can be provided with an arrester chain
or wire 72 attached e.g. to the beam pipe 64.
The reliability of the safety function of the beam pipe 46 may be
further increased by providing a series of melt-rupture discs located at different
distances from the tip end of the beam pipe. Figure 3 shows one of such melt-rupture
discs 60 located between the tip end and the normal coolant level 16. The more
melt-rupture discs 60 will open in an emergency case, the more rapidly the interior
of the beam pipe 46 will be flooded.
It has already been said that in case of a vacuum in beam pipe 46,
the coolant 14 flooding the interior 64 of the beam pipe 46 will rise above the
level of the coolant in the primary vessel 12. It follows that the coolant column
in the beam pipe will form a target surface for the proton beam above the level
of the coolant in the primary vessel 12. The beam pipe shown in Figure 1 has in
this region a beam pipe portion or chamber 80 with a significantly increased cross-section
-by comparison with the main body of the beam pipe extending from the chamber
80 towards the tip end 48. It should be noted that the chamber 80 extends below
the normal coolant level 16, so that its outside wall is cooled by the coolant
14 circulating in the primary vessel 12. In order to improve the efficiency of
the cooling of chamber 80, this outside wall of the chamber 80 is advantageously
provided with cooling fins (not shown), which extend into the coolant 14.
Alternatively to the chamber 80 or even in combination with the chamber
80, one can install one or several secondary melt-rupture discs (not shown) in
the beam pipe 46, starting from the normal coolant level 16 up to the level to
which the coolant 14 will rise in the beam pipe 46 because of a vacuum therein.
These secondary melt-rupture discs should have a solder material with a higher
melting point than that used in the submerged part of the beam pipe 46 and they
should be preferably allowed to fall in both directions -i.e. either into the beam
pipe or into the primary vessel 12. The rupture of several of these secondary melt-rupture
discs, due to heat generation of the proton beam (which may generate more than
10 MW) impinging on the top surface of the coolant column in the beam pipe 46,
will lower the coolant level in the beam pipe 46 to the coolant level in the primary
vessel 12 and thereby improve evacuation of the heat generated by the proton beam,
which is impinging on the coolant column in the beam pipe 46.
It should be appreciated that the beam pipe described above provides
a safety function in an accelerator driven system that has following advantages:
- a) It works in a fully passive way.
- b) It is simple and easy to understand.
- c) It leads to a quick filling of the beam pipe 46 with liquid metal coolant
at least up to the coolant level in the primary vessel 12, thus maximizing the
blocking of the proton beam.
- d) Its response is keyed to the temperature increase close to the top of the
core. Therefore it will respond promptly both to a slow and a fast heating up of
the reactor core.
- e) It would still reliably work if the reactor coolant level were low, e.g.
due to a leak in the reactor vessel.
- f) It would still reliably work if a counter-pressure were to build up in the
- g) If the melt-rupture disc was erroneously activated during normal reactor
operation (e.g. through an unforeseen degradation of the solder), the ADS would
be switched off and there would only be an economic penalty to replace or clean
the beam pipe.