Patent Number: 
Section: claims

1. A process for estimating the concentration (C) of a chemical element in the primary coolant of a nuclear reactor, the reactor comprising means for injecting a solution for diluting the concentration of said chemical element below a predetermined limit into the primary coolant, means for injecting a concentrated solution of said chemical element having a predetermined concentration (C*) into the primary coolant, and a sensor able to measure a quantity (Cm) representing the concentration of said chemical element in the primary coolant, wherein the process is an iterative process comprising repeatedly in each time step k:a stage of acquisition of a quantity (qdk) representing the injected flow of the dilution solution in step k, a quantity (qk) representative of the injected flow of concentrated solution in step k, and a quantity (Cmk) representing the concentration of said chemical element measured by the sensor in the primary coolant;a stage of calculating an estimated value (Cek+1) for the concentration of said chemical element in the primary coolant in step k+1 based on the representative quantities (qdk, qck, Cmk) acquired in step k. 2. A process according to claim 1, wherein the calculation stage is carried out with the help of Kalman equations. 3. A process according to claim 2, wherein the stage of calculation in step k is carried out by considering in the Kalman equations a state parameter x=ln(C) when the quantity (qk) representing the injected flow of concentrated solution is below a predetermined limit and a state parameter x=ln(C*−C) when the quantity (qk) representing the injected flow of the concentrated solution is above said predetermined limit. 4. A process according to claim 3, wherein the calculation stage in step k is carried out by considering in the Kalman equations a measured parameter y=ln(Cm) when the quantity (cck)representing the injected flow of concentrated solution is below a predetermined limit and a state parameter y=ln(C*−Cm) when the quantity (qk) representing the injected flow is above said predetermined limit. 5. A process according to claim 4, wherein the equations used in the calculation stage are:xk+1/k=xk /k+uk uk=−(Δtk/Vol) qdk when the quantity (qck)representing the injected flow of concentrated solution in step k is below a predetermined limituk=−(Δtk/Vol) qck when the quantity (qck) representing an injected flow of concentrated solution in step k is above a predetermined limitPk+1/k=Pk/k+W xk+1/k+1=xk+1/kKk+1(yk+1−xk+1/k)Pk+1/k+1=(1−Kk+1)Pk+1/k Kk+1=Pk+1/k/(Pk+1/k+V)where xk/k is the value of state parameter x in step k determined from the data available in step k, Δtk is the duration of time step k, Vol is the volume of the primary circuit, Pk/k is the variance for the state parameter x in step k determined from the data available in step k, and W and V are predetermined constants. 6. A process according to claim 5, wherein the V/W ratio lies between 100 and 10000. 7. A process according to claim 1, wherein it comprises an initialisation stage in the course of which an initial estimated value (Ce0) for the concentration of said chemical element in the primary coolant is calculated directly from the quantity (Cm0) representing the concentration of said chemical element measured by the sensor in the primary coolant. 8. A process according to claim 1, wherein the chemical element is boron or a boron compound. 9. A process according to claim 1, wherein the nuclear reactor is a pressurised water nuclear reactor.