Process and apparatus for the detection and prelocalization of a can fracture in a fuel assembly of a fast nuclear reactor

Process and apparatus for the detection and prelocalization of a can fracture in a fuel assembly of a fast nuclear reactor. The device comprises at least one detector for the neutrons emitted by the fission products transported in the outflow of cooling agent as the result of the fracture of a can, said detector being immersed within the vessel in the outflow of cooling agent in an area protected from neutrons emitted by the core by the volume of cooling agent and by structures immersed in the annular space between the core and the vessel.

BACKGROUND OF THE INVENTION 
The present invention relates to a process and to an apparatus permitting 
the detection and prelocalization of the fracture of a can in a fuel 
assembly of a fast nuclear reactor cooled by the circulation of a liquid 
metal, generally sodium, by counting the delayed neutrons emitted by the 
fission products escaping from the defective assembly and located by the 
liquid metal. 
Provisions have already been made for carrying out a measurement of delayed 
neutrons due to fission products released as a result of the fracture of a 
can by passing all or part of the cooling agent flow within the vessel as 
close as possible to a neutron detector, which is itself located 
externally of the latter. Neutrons emitted by fission products with a 
certain delay compared with the initial nuclear reaction in the core 
therefore have, at the time where they pass in the vicinity of the 
detector, a harder energy spectrum than that of the neutrons produced in 
the core. Thus, their detection can be ensured under more reliable and 
effective measuring conditions making it possible to carry out the 
necessary interventions on the defective assembly within a shorter period 
of time. 
BRIEF SUMMARY OF THE INVENTION 
The present invention relates to a process and to an apparatus which make 
it possible to considerably increase the sensitivity of detection of 
neutrons emitted by fission products. This sensitivity is in fact improved 
for the following reasons: 
In all directions the neutron detector is surrounded by the cooling agent 
carrying the fission products. Absorption of neutrons is decreased, more 
particularly due to passing through the wall of the vessel. The neutrons 
are measured throughout the spectrum and not only in the area of the 
neutrons thermalised by passing through retarding materials. 
To this end, the process comprises performing a direct measurement of the 
delayed neutrons in the outflow of cooling agent from the reactor within 
the actual vessel containing said cooling agent. 
For the performance of this process, the invention also relates to a 
special measuring device which comprises at least one detector for the 
neutrons emitted by the fission products transported in the outflow of 
cooling agent as the result of the fracture of a can, said detector being 
immersed within the vessel in the outflow of cooling agent in an area 
protected from neutrons emitted by the core by the volume of cooling agent 
and by structures immersed in the annular space between the core and the 
vessel. 
Preferably, the neutron detector comprises a U 235 fission chamber mounted 
within a gloove finger submerged in the cooling agent. 
In the conventional solution for a fast nuclear reactor, called the 
integrated solution where the reactor vessel contains in addition to the 
core primary exchangers and circulating pumps distributed around said 
core, the exchangers having a group of exchange tubes, a sheath 
surrounding said group and at least one ferrule arranged around ports for 
the entry of cooling agent into the sheath, the neutron detector is 
mounted between the ferrule of the exchanger and the inner wall of the 
vessel. 
In a preferred embodiment, each exchanger is associated within the vessel 
with three detectors regularly spaced around the exchanger on a circular 
arc. 
According to other variants, the detectors are placed in the upper portion 
of the group of tubes level with the cooling agent entry ports or are 
applied against the inner wall of the main vessel or are finally 
positioned above the entry ports.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the drawing, reference numeral 1 designates the vessel of a fast nuclear 
reactor, intended more particularly to contain a volume 2 of a liquid 
cooling metal, generally sodium. The base of vessel 1 supports a 
supporting structure 3 for a cross-member 4 on which rests the core 5 of 
the reactor. This core is completely immersed in sodium volume 2 and is 
surmounted by a core cover 6 carrying a series of measuring devices and 
instruments necessary for the operation of the installation. Core 5 is 
laterally surrounded by a protective structure 7, vessel 1 being separated 
into two portions by bend 9 of an inner vessel separating sodium volume 2 
into two zones. 
According to a conventional arrangement, the reactor has circulating pumps 
10 and heat exchangers 11 distributed around the core 5 in said vessel and 
permitting the sodium volume which leaves the core and which is collected 
in the inner vessel to traverse the exchangers and leave the same in the 
space between the two vessels where it is then taken up by pumps 10 which 
finally return it beneath cross-member 4 for a new passage through the 
core. 
Each exchanger 11 comprises a cylindrical sheath 12, having a vertical 
axis, containing a group of exchange tubes 13, said sheath 12 being itself 
surrounded in its upper portion by a ferrule 14. Sheath 12 has entry ports 
15, which permit the sodium leaving the core in the direction of the 
arrows to penetrate the exchanger in order to come into contact with the 
group of exchange tubes 13, whereby, once cooled this sodium leaves the 
exchanger via ports 16 located beneath the bend 9 of the inner vessel. 
According to the invention, the delayed neutrons emitted by the fission 
product entrained by the sodium circulating in the vessel as a result of a 
fracture of the can of a fuel assembly in the core are measured by means 
of at least one detector 17 mounted within tubes 18 in the form of a 
gloove finger submerged below the level of the sodium. These detectors 17 
are arranged in a region of vessel 1 where they are protected from the 
direct radiation of neutrons from the core. This, the gloove fingers 18 
containing said detectors can be placed either directly against the inner 
wall of the vessel 1 or within the sheath 12 of the exchanger among the 
group 13 of exchange tubes, or against the ferrule 14 or above ports 15, 
said different assembly arrangements can be adopted among others in view 
of the fact that they utilise at least partly the protection given by the 
actual exchanger relative to the neutrons from the core. Moreover, the 
device according to the invention can have one or more detectors suitably 
distributed around the exchanger. Thus, it is possible to arrange around 
each exchanger 11 three detectors such as 17 regularly spaced about the 
exchanger on a circular arc. Thus, a signal is obtained on the detectors 
which will vary as a function of the location of the can fracture in the 
core, thus permitting a prelocalization of the portion of said fracture. 
Advantageously, the detectors used are constituted by fission chambers, 
more particularly U 235 cylindrical chambers having a positive axial 
electrode and a uranium deposit of limited thickness, arranged against the 
inner wall of the detector chamber, the latter being filled by an 
atmosphere of neutral gas, for example argon or helium. 
The present invention is not limited to the embodiments described and 
represented hereinbefore and various modifications are possible thereto 
without passing beyond the scope of the invention.