High-purity niobium for neutron activation detectors

High-purity niobium has successfully been used as neutron detector material in advanced neutron dosimetry. Reliable and accurate neutron dosimetry measurements are needed to control the neutron load of the reactor pressure vessel (RPV) during its operation. Neutrons-irradiation of the RPV material causes an embrittlement of the ferritic steel. The effect is highly safety-relevant and must, thus, be monitored. It is standard to use a set of several materials with different neutron activation or spallation reactions. An cases of the niobium detectors the nuclear reaction one makes use of 93Nb(n,n`)93mNb reaction. It is specially favourable because the energy dependence of this reaction is similar to the energy dependence of the damage function of the RPV material. Furthermore, the half-life is long (16.1 years) and results in a nearly constant weighting over the reactor cycle. Therefore, the decay correction is only small in comparison with other detectors.
Niobium of low purity is not appropriate for this purpose. Impurities can affect additional activation reactions. Especially detrimental is tantalum. The neutron capture of 181Ta leads to 182Ta resulting in high energy gamma lines (1121,1122 keV) and added excitation of the detected 16 and 18 keV niobium lines. The correction of these effects is difficult and yields additional errors.
The requirements of the purity are strict: a good niobium detector must not have a Ta concentration of more than 1 ppm.
The niobium was produced by a technology which consists of an electrolytic refining, melting processes and mechanical treatment. In the cathodic niobium (electrolysis conditions: T = 750°C, j=0.4 mA/mm²) the so-called "problematic element" Ta could not be detected by instrumental neutron-activation analysis (cTa <<1 at.ppm).
With the niobium produced in this way, the neutron fluence at the VVER-type RPVs of the nuclear power plants in Greifswald (Germany), Rovno, and Balakovo (Russia) were determined. Int ...