Movement detection system

In a system for detecting abnormal movement of a part of a gas-cooled nuclear reactor, for example abnormal tilting of a boiler unit caused by failure of a boiler support, a tracer material is released into the gas coolant path in response to the movement. The releasing of the material may be effected by causing a gas canister to be pierced as a result of the movement. The presence of the tracer material in the coolant is detected by the coolant monitoring system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a movement detection system for use in detecting 
movement of a body, such as abnormal movement resulting from failure of a 
member supporting the body. For example, the system may be used for 
detecting abnormal movement of a boiler unit in a nuclear reactor, caused 
by failure of a boiler support. 
2. Description of Related Art 
In the construction of advanced gas-cooled reactors (AGRs) a number of 
vertical boiler units are mounted around the reactor core and within the 
concrete pressure vessel. Heat is extracted from the core by passing 
carbon dioxide coolant gas over the fuel, and is transferred from the 
carbon dioxide coolant to the water in each boiler unit. The steam thereby 
produced drives a turbine to which is coupled a generator. 
Each boiler unit is supported on horizontal beams, and these beams are 
subject to corrosion or erosion by the hot gas which flows around them. 
Eventually a support beam may fail, resulting in an abnormal movement of 
the boiler unit. 
It is clearly necessary to provide a warning of such failure, and this has 
previously been effected by a number of different methods. In one such 
method, mechanical markers have been attached to the boiler units or their 
supports, so that a visual indication of abnormal boiler unit movement is 
given. However, such devices can be viewed only by inservice examination 
equipment during reactor shutdown. In an alternative method, cables which 
connect reactor temperature monitoring thermocouples to their associated 
indicators are located in such a manner that they will become severed in 
the event of abnormal boiler unit movement. This severing will be apparent 
from the lack of temperature indication. Such a method allows immediate 
detection of the abnormal movement without waiting for reactor shutdown. 
However, the method is not wholly reliable, since it is not possible to 
ensure that any undesirable movement of a boiler unit will necessarily 
give rise to severing of a cable. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved system for 
detection of movement within a nuclear reactor. 
According to the invention there is provided a movement detection system 
for detecting abnormal movement of part of a nuclear reactor which reactor 
has a path therein for gaseous coolant, the system comprising means 
operable to release a tracer material into the coolant in response to said 
movement; and means to monitor the content of the coolant to detect the 
presence of the released material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, an advanced gas-cooled reactor (AGR) 
includes a concrete pressure vessel 1 in which the reactor core 2 is 
mounted. The core is contained within a domed steel shell 3 which acts as 
a gas baffle. Vertical boiler units, such as the boiler units 4 and 5, are 
mounted around the core. There may be, for example, twelve boiler units 
arranged in four groups of three units, each group of units together 
constituting a boiler. The boiler units are contained in an annulus 
between the gas baffle 3 and the inner surface of the pressure vessel 1. 
The boiler units are mounted on horizontal beams, such as beams 6 and 7, 
which are supported at their ends by the pressure vessel 1 and the gas 
baffle 3. 
Carbon dioxide coolant is circulated through the core to cool the fuel 
elements, and the carbon dioxide heated thereby passes downwards through 
the boilers, so that steam is generated for driving a turbine-generator 
unit (not shown). 
Four quadrant plenum chambers, of which only two chambers 8 and 9 are 
shown, are provided beneath the respective boilers. Gas circulators, of 
which only two circulators 10 and 11 are shown, are provided to circulate 
the carbon dioxide coolant. The gas is drawn from the plenum chamber at 
the bottom of each boiler by the respective circulator, and is discharged 
into a sub-core plenum 12. The gas flow then splits, part enters the 
bottom of the fuel channels directly and the remainder passes firstly up 
an annular passage 13 between the outside of the core 2 and the inner 
surface of the gas baffle 3. This latter flow, after entering the 
hemispherical space 14 within the gas baffle 3 and above the core 2, 
passes down through the core structure through special passages until, at 
the bottom of the core, it combines with the above-mentioned direct flow. 
The combined flow then passes up the fuel channels and passes via tubes or 
pipes 15 through the hemispherical space 12 of the gas baffle to enter an 
upper plenum 16. The hot gas is drawn down through the boilers and back to 
the circulators. 
The carbon dioxide is continuously monitored and processed to maintain the 
correct coolant composition. For example, oxygen and methane are injected 
into the coolant by monitoring and processing equipment at each 
circulator, such as equipments 17 and 18. 
The existence of the coolant monitoring equipment makes it possible to 
provide a simple boiler movement detection system in accordance with the 
present invention. 
Each boiler unit has vertical stiffeners. Stiffeners 19 and 20 of two 
adjacent boiler units are shown in FIGS. 2 and 3. A release device 21 is 
attached to the stiffener 19 by means of a clamp 22. The device comprises 
a cylindrical housing 23 having a hemispherical bottom, into which housing 
a capsule 24 containing a tracer material, such as argon or neon gas is 
inserted At the side of the housing 23 opposite to the clamp is attached a 
horizontal cylinder 25 in which a plunger 26 is housed. The plunger has a 
sharp end 27 which is normally just clear of the capsule. The plunger is 
held in that position by a shear pin 28, which passes through the walls of 
the cylinder 25 and through a hole in the plunger 26. The plunger has a 
head 29 which is normally spaced, say, 10 mm from the stiffener 20 of the 
adjacent boiler unit. 
In use of the system, if a support beam of the boiler unit having the 
stiffener 19 collapses, such that the boiler unit tilts so that the 
stiffener 19 moves towards the stiffener 20 of the adjacent boiler unit, 
the plunger head 29 of the plunger 26 will come into contact with the 
stiffener 20 with sufficient force to break the shear pin 28. The sharp 
end 27 of the plunger will pierce the capsule 24, which will therefore 
release the tracer material. The release device 21 is located in the path 
of part of the carbon dioxide coolant flow, and the tracer material will 
therefore be released into the coolant. The presence of the foreign tracer 
material in the coolant will be very quickly detected by the coolant 
monitoring and processing system 17 or 18, or by a burst fuel-can 
detection equipment (not shown) which is part of a conventional AGR. 
It will be apparent that if the boiler unit associated with the stiffener 
20 tilts so that the stiffener 20 moves into contact with the plunger 26, 
the capsule 24 will be pierced and will similarly provide a warning of 
abnormal boiler unit movement. 
Instead of, or in addition to, piercing the shell of the capsule 24 the pl 
of the release device might pierce a barrier which separates two 
non-volatile materials, which, when mixed, react to produce a volatile 
product which then escapes into the coolant path and is detected. In some 
types of reactor, other than the AGR, the materials might be a metal and 
an acid. For example, zinc and sulphuric acid might be used to produce 
hydrogen as the tracer material. 
It will be realized that the use of the present movement detection system 
in a nuclear reactor is very advantageous. Firstly, the detection system 
makes use of the coolant monitoring equipment which is already provided. 
The invention requires the addition of a simple tracer material release 
device which is cheap to produce, and many such devices can be located at 
suitable points around the reactor. The devices can be added to the 
reactor at any suitable time. The system gives an immediate indication 
that there has been an abnormal boiler unit movement, without awaiting 
shutdown and without relying on cable-severing techniques. Once a reactor 
has gone critical, work inside the reactor involving shutdown is 
time-consuming and involves considerable loss of electrical output. The 
release devices can be installed very rapidly. In a preferred system using 
an inert gas such as neon or argon, a gas chromatograph would preferably 
be used for gas analysis This could be easily installed as an addition to 
the coolant monitoring equipment already provided, and the additional cost 
would be negligible. 
Although the movement detection system is described above in relation to 
boiler units in a nuclear reactor, a system which involves a release 
device and means to detect the presence of released material in accordance 
with the invention may be used for monitoring abnormal movement of any 
part of a gas-cooled reactor.