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Sensitivity analysis of the CP-1 model

​​In order to assess the reliability of the simulation results obtained for the CP-1 model, we have performed an extensive sensitivity analysis, covering both the effect of the nuclear data library and the impact of the technological uncertainties. Furthermore, we have quantified the temperature effect.

Publié le 22 novembre 2023

​Sensitivity to nuclear data

We have examined the sensitivity of the CP-1 model to nuclear data. Results are recalled in the table below.​
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The main effects due to the nuclear data and the corresponding impact on the reactivity, as computed by TRIPOLI-4.

We observe that the fact of artificially suppressing the thermal scattering law in graphite from the nuclear data​ library (ie, assuming the free gas model for graphite, instead of S(alpha,beta) laws) has a pronounced effect on reactivity, due to the fact that we change the physics of the neutron slowing down of neutrons.
On the contrary, the use of probability tables for the unresolved resonance range has a small effect on reactivity.
We have also taken into account photo-nuclear reactions (in coupled neutron-photon simulations): the overall effect on reactivity is negligible. 
Finally, we have replaced the ENDF/B-VIII.0 library (used in all our simulations) by the JEFF-3.3 library: the effect on reactivity is about 150 pcm.

Sensitivity to technological data

In order to substantiate our analysis, we have also somputed the sensitivity coefficients corresponding to variations in the density of the main materials used in CP-1, encompassing the fuel, the moderator and other structural materials (graphite in the pier and wood in the scaffold and in the filler). The methods used in TRIPOLI64 to compute first-order sensitivities is based on the IFP technique. The results are reported in the following table.​

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Sensitivity coefficients for density variations of several materials of CP-1, including fuel, moderator, and structures.

Temperature effect

During the operation of CP-1, the temperature effect was measured by opening a window and letting the pile cool down by about 10 degrees. The adjustment of the control rod position, based on the calibration of the control rod worth that had been estimated close to the critical point, gave an estimate of about -3 pcm/K. We have computed the temperature effect with TRIPOLI-4 and we have obtained the values reported in the following table. The overall effect for the whole reactor is in good agreement with the experimental measurement.​

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​Temperature effect for the pile components and for the whole reactor, as estimated by TRIPOLI-4.