Neutron absorbing bar damping device

A neutron absorbing bar for use in a PWR has an integrated damping device comprising a cluster of vertical absorbent rods fixed to the arms of a spider having a central pommel connectable to a drive mechanism. A damping device in the pommel includes a cylinder slidably receiving piston urged into a downward projecting position by springs contained in the cylinder. The cylinder and the piston are so formed that the leak flow at a cross-sectional area offered to the liquid driven out of the cylinder by the piston gradually decrease as the latter moves from an outermost position. The piston carries a hydromechanical damper damping the initial impact of the bar upon a scram.

BACKGROUND OF THE INVENTION 
1. Field of the Art 
The invention relates to neutron absorbing bars for liquid cooled nuclear 
reactors of the type having a cluster of vertical parallel neutron 
absorbing rods fixed to arms of a spider having a central pommel 
connectable to a vertical moving mechanism and a damping device in the 
pommel. It is particularly suitable for use in pressurized water cooled 
and moderated reactors. 
2. Prior Art 
Neutron absorbing bars for nuclear reactors include rods which contain a 
neutron poison for controlling the reactivity in the core of the reactor. 
They are inserted into the core to a variable degree. 
To cause an emergency shutdown of the reactor, all control bars are 
simultaneously lowered into the core by dropping them so that they enter 
the core under the action of their own weight. 
To damp the shock when the pommel abuts against the upper core plate of the 
reactor or against the upper end piece of a respective fuel assembly, the 
provision of a shock damper has already been proposed. A control bar 
described in European Patent No. 159 509 has a damping device consisting 
of a cylinder formed in the pommel and slidably receiving a piston urged 
downwardly by resilient means contained in the cylinder. From the moment 
when the piston abuts the upper core plate, continued downward movement is 
opposed by the compression of the resilient means and by the pressure loss 
undergone by the liquid which flows out of the cylinder between the wall 
thereof and the piston. Such a damping device has, however, only a limited 
effect: the damping effect due to the pressure loss does not change 
substantially during movement of the piston and only the increasing 
stiffness of the spring provides progresivity. Furthermore, the device in 
the pommel interferes with the flow of cooling liquid. 
Such shortcomings could be accepted for bars whose rods contain a neutron 
poison in coherent form and which do not require substantial cooling. It 
is no longer acceptable when the bars contain other compounds of limited 
resistance, and particularly when the rods of the bar contain a material 
used for varying the energy spectrum of the neutrons in the core. This 
material often consists of fertile material pellets (depleted uranium 
oxide, and/or thorium oxide for example) which do not withstand shocks. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a neutron absorbing bar having 
a damping device reducing the shock impressed to the bar when it engages 
the core plate and allowing satisfactory cooling, particularly of the 
rods, under all operating conditions. It is to be kept in mind that, 
contrary to the rods containing a poison having parasitic absorption, the 
rods containing fertile material must be cooled by a flow of cooling 
liquid. 
To this end, there is provided a bar of the above defined type wherein the 
cylinder and piston are formed so that the leak cross-sectional area 
offered to the liquid driven out of the cylinder by the piston decreases 
gradually during penetration of the latter from its position of maximum 
extension, and the piston has a hydromechanical damper for damping the 
impact when the bar initially contacts the upper core plate or fuel 
assembly for reducing the speed of the piston.

DESCRIPTION OF PREFERRED EMBODIMENT 
A neutron absorbing bar as described may be used in present day reactors as 
well as in spectrum shift reactors still in the design stage. It may, for 
example, be used in combination with a fuel assembly as disclosed in 
European Patent No. 159 509 already mentioned or with a fuel assembly as 
described in French Patent No. 84 19917. 
Referring to FIG. 1, the relative position of the pommel or hub 8 of an 
absorbant bar and of the elements which it contains is shown when the bar 
is separated from its drive shaft and rests on a bearing surface 10 which 
will be assumed to be the upper core plate of a reactor (but which could 
be the upper end nozzle of a fuel assembly). Pommel 8 and radial fins 12 
connected thereto constitute a unit generally called "spider". The arms 
12, formed as thin vertical vanes, carry vertical rods 14 which, in the 
position in which the pommel is shown, are completely engaged in guide 
tubes of one or more fuel assemblies. The drive shaft (not shown) has a 
conventional gripper whose fingers may be spread out for engagement into 
an upper internal recess 46 of the pommel. 
The damping device incorporated in pommel 8 may be considered as including 
three parts, namely a hydraulic brake, a damper or "dashpot" for 
attenuating the initial shock and an end-of-travel load absorbing coil 
spring. 
The hydraulic brake comprises a cylinder formed in the sleeve and closed at 
its upper end. A hollow piston 18 is slidably sealingly received in the 
bore 16 of the cylinder. 
The piston 18 has a transverse wall 19 supporting resilient return and 
damping means 20. As shown, the resilient means consist of two helical 
springs in series relation, having opposite winding directions to avoid 
rotational effects. The two springs 20 are guided by a central rod 22 
fixed to the bottom wall of the cylinder. 
The cylindrical wall of the cylinder is formed with openings 24 for 
throttling the water flow forced out of the cylinder by piston 18. The 
openings are spaced apart along the cylinder. They are distributed in a 
longitudinal plurality of sets (for example each of two holes) to balance 
the hydrodynamic transverse thrusts due to the water jets which during 
movement of the piston are forced out of the cylinder. The number of sets 
will depend on the desired progressivity, taking into account the 
difference in conditions when the coolant is cold and when hot. In 
practice, sixteen sets will generally be sufficient. 
Piston 18 advantageously has a downwardly directed radial shoulder 26 
between a portion which has a sliding fit in the cylinder and a portion 
which has an annular clearance. The shoulder 26 is at such a distance from 
the lower end of the piston that the clearance communicates with the lower 
sets of openings 24 even when the bar is completely inserted in the core 
(FIG. 1), for providing a cooling water flow. 
The extent of downward travel of piston 18 is limited by a stop ring 28 
housed in an internal groove of the cylinder. As shown in FIG. 1, the 
pommel has at its lower part a slot 30 for easier access to the stop ring 
28. The ring may be welded in position. 
The purpose of the shock damper is to attenuate the shock of piston 18 upon 
bar fall. The damper has a plunger 32 slidable in a blind bore formed in 
the piston below the dividing wall 19. A reset spring 36, of low stiffness 
as compared with spring 20, biases the plunger 32 downwardly against a 
stop ring 38. A restricted hole (or holes) 34 formed in the wall of the 
piston opposes a calibrated head loss to flow of liquid driven out by 
plunger 32 upon impact. A single hole has been shown in FIG. 1, but in 
general several holes will be provided with such a spacing that the impact 
speed of piston 18, when the plunger 32 is completely retracted, is 
reduced to as low a value as possible. 
Finally, an end-of-travel spring 40 is retained between the bottom of the 
cylinder and a flanged thimble 42 on which the resilient means 20 also 
rest. The flanged thimble 42 has a longitudinal size such that the piston 
18 comes in abutment thereagainst at the end of travel of the hydraulic 
brake. The compression of spring 20 absorbs the residual momentum of the 
bar after hydraulic braking. One or more openings 44 may be provided in 
the cylinder for allowing the liquid to flow out of the cylinder during 
the upward movement of flanged thimble 42. 
When the pommel bears on the core-plate, as shown in FIG. 1, the plunger 32 
is completely retracted in the piston. The latter projects by a slight 
amount, retained by the compression force exerted by the end-of-travel 
spring 40. 
The device operates as follows: 
As long as the bar is connected to its drive shaft, plunger 32 is held down 
in abutment against ring 38 by spring 36. The shoulder 26 of piston 18 is 
held in abutment against the stop ring 28 by springs 20. The springs 20 
are prestressed such that the piston 18 remains in abutment against ring 
28 despite inertial forces generated by the step-by-step control of the 
bar drive mechanism, which frequently causes accelerations reaching 15 g. 
It will generally be sufficient for the spring 20 to have a prestressing 
at rest of about 20 daN, if the weight of piston 18 is low enough. 
Finally, the end-of-travel spring 40 is completely relaxed. 
During a first phase of operation, only the shock damper of the dashpot 
acts: from the time that plunger 32 comes into contact with core plate 10 
(FIG. 2A), the plunger is moved into the piston 18 and drives liquid 
through the openings 34. At the end of the first phase (FIG. 2B) the 
piston 18 comes into contact with plate 10. 
During the second phase, piston 18 moves along the cylinder, compresses 
springs 20 and drives out water from the cylinder through the openings 24 
(not shown in FIGS. 2A-2E) which oppose a pressure drop which increases as 
the piston moves (FIG. 2C). 
The second operating phase ends when piston 18 comes into abutment against 
flange 42 (FIG. 2D) and begins to compress the end-of-travel spring 40. 
Continued penetration of piston 18 causes spring 40 to compress until 
complete damping is obtained (FIG. 2E). 
The openings 24 may all be located above the arms in which case they may be 
drilled after the arms have been screwed to the cylinder.