Patent Number: 
Section: description

The present description of the method of the invention is made in its application to the measurement of the lowering of a control cluster into a pressurized water nuclear reactor. The lowering channel is made up of guide tubes, a cluster guide, the heat sleeve, an adapter, the mechanism casing and the rod sheath. Step no 1 of the method of the invention is a measuring and recording step. It takes place at the time of start-up of the reactor, that is to say before use of the control rods. The equipment is assumed to be perfectly new and unused, with no opposing force that is not provided by the mechanism disturbing the functioning of the control cluster. This measurement is therefore a reference measurement. In this case a rod position indicator (RPI) is used. More exactly, it is used here to measure the instantaneous lowering velocity of the assembly in relation to time, that is to say V1=f1(t). Step no 2 is a calculation step on the basis of the measurement previously made under step 1. It consists of calculating distance of travel with integrated change in velocity of the object before the onset of friction, V1(t), for the purpose of obtaining the change in this velocity in relation to the distance of travel d of the mobile assembly:             d      1        ⁢          (      t      )        =            ∫              u        =        0            t        ⁢                  V        ⁢                  (          u          )                    ⁢              xe2x80x83            ⁢              ⅆ        u             From this equation the formula giving the velocity in relation to travel can be deduced: V1=g1(d). The two following steps consist of conducting steps no 1 and no 2 but after a certain operating time of the nuclear reactor, when undesired opposing friction forces occur which influence the time and lowering velocity of the mobile control rod assembly. Therefore, step no 3, using the rod position indicator RPI, consists of measuring and recording the deteriorated instantaneous velocity V2 in relation to time of the lowering of the mobile assembly. Step 4 then consists of calculating distance of travel with integrated velocity change of the mobile assembly using the measurement made of instantaneous velocity V2(t). The distance of travel can then be obtained, by integration:             d      2        ⁢          (      t      )        =            ∫              u        =        0            t        ⁢                  V        2            ⁢              u        ⁢                  (                      xe2x80x83                    ⁢                      ⅆ            u                    )                     From this, the velocity of the mobile assembly can be deduced in relation to distance of travel after the onset of friction forces, that is to say: V2=g2(d) With step 5 it is possible to obtain the difference in lowering velocity of the mobile assembly before and after the onset of additional friction forces. All that is needed is to calculate the velocity difference V3=(V1xe2x88x92V2)=(g1xe2x88x92g2)(d)=g3(d) in relation to distance of travel d. To make this calculation, the reactor must be under the same operating conditions. Having regard to the fact that the behaviour of the measuring instrument is not fully controlled, in this case the rod position indicator RPI, since it is installed in a non-accessible containment where no human operation is possible, it was decided only to use this indicator after calculating the difference at step 5. It can indeed be considered that this measuring instrument may behave abnormally and give deformed measurement signals. Particular allusion is made here to data transmission problems which are relatively constant when the rod position indicator is installed. These problems are due in particular to pressure and temperature. On the other hand, it is considered that this deformation peculiar to this measuring instrument is always of the same order. Therefore, by only using the difference in measurements made before and after the onset of friction forces, any operating default of the RPI is overcome and only the variation in measurements made with this instrument is taken into account. Consequently, in accordance with step 6, the basic magnitude of the lowering velocity of the mobile assembly is calculated using a predetermined calculation code which takes into account known thermohydraulic, mechanical and dimensional conditions before the start-up of the nuclear reactor. Evidently this calculation code does not take into consideration friction forces occurring after start-up of the reactor, which cannot be predicted. This calculation code therefore gives the theoretical lowering velocity of the mobile assembly under non-deteriorated conditions. With the code it is therefore possible to obtain the change in theoretical velocity of the mobile assembly V4=g4(d). From this, the sum of normal forces is deduced: xe2x80x83Mxcex31=xcexa3normal forces Step 7 consists of taking into account the variations in velocity measured during the first steps and incorporating these in the result calculated during the previous step. This amounts to subtracting from the theoretical velocity, in relation to travel, the difference calculated using the measurements: V5=(g4xe2x88x92g3)(d)=g5(d). From this is deduced the sum of outside forces: Mxcex35=xcexa3normal forces+xcexa3additional friction forces Step 8, the last step, consists of deducing from the above the outside forces Foutside in relation to distance of travel f(d). For this purpose the fundamental equation of dynamics is used. Using the equation of the balance of forces: Mxcex34=xcexa3normal forces xcexa3normal forces=assembly-related forces, sheath related forces, guide related forces, other forces. Each force F is a function dependent upon velocity V, upon distance of travel d, upon system geometry, upon temperature xcex8 and other parameters, in which: M(xcex35xe2x88x92xcex34)=xcexa3additional friction forces The curves shown in FIGS. 2, 3 and 4 help to better understand the approach of the method according to the invention. In FIG. 2, time is shown along the X-axis while velocity and distance of travel are both on the Y-coordinate. If only a slight variation is observed between the two curves representing the reference distance of travel denoted d1 and deteriorated distance of travel d2, the variations in velocity are more significant. In respect of the latter, it is observed that the two measured velocities V1 and V2 are greater than the calculated velocity V4 and the velocity V5 obtained at the end of the method. Evidently, it is ascertained that the deteriorated velocity V2 is slower than the reference velocity V1. In addition, it is found that this difference between V1 and V2 is transferred to V4 and V5. Moreover, this velocity difference V3 between V1 and V2 appears in the graph in FIG. 3 in which the same remarks apply. In FIG. 4, which shows the change in friction forces (Y-axis) in relation to distance of travel (X-axis), only the two curves having the greatest variations are to be taken into consideration. This figure shows the results of the method of the invention, that is to say the additional friction forces calculated during the last step of the method of the invention, step 8, and these same additional friction forces when filtered. The two other curves concern measured forces.