Monte Carlo simulation of the measurement by the 2E technique of the average prompt neutron multiplicity as a function of the mass of fragments from thermal neutron-induced fission of 239Pu

Using a Monte Carlo method, we simulate the measurement, by the 2E technique, of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu. The input data for the simulation, associated with the primary fragment mass (A), consist of the yield (Y), the distribution of the total kinetic energy characterized by its average ((TKE) ̅) and its standard deviation (σ_TKE), the average prompt neutron multiplicity (ν ̅_s, a sawtooth approach of experimental data), and the slope of neutron multiplicity against total kinetic energy (dν_s/d<TKE>). The output data, associated with the simulated as the fragment mass measured by the 2E technique (µ), consist of the yield (y), the distribution of the total kinetic energy characterized by its average ((tke) ̅) and its standard deviation (σ_tke), and the average prompt neutron multiplicity (ν ̅_µ). In the mass regions A≈115 and A>150, ν ̅_µ is higher than ν ̅_s. This result suggests that, in those mass regions, the 2E experimental values associated with the average neutron multiplicity are overestimated, referred to the corresponding to the primary fragments.

On the determination of neutron multiplication by the Rossi-alpha method

Introduction. This work contains the results of determining the prompt neutron multiplication factor in the subcritical state of a one-core BFS facility, obtained by the neutron coincidence method, for which the influence of the error in the βeff in determining the multiplication factor turned out to be insignificant. The core of the facility consisted of rods filled with pellets of metallic depleted uranium, 37% enriched uranium dioxide and 95% enriched plutonium, sodium, stainless steel and Al2O3. Stainless steel served as a reflector. Methods. In contrast to the inverse kinetics equation solving (IKES) method, which is convenient for determining reactor subcritical states, the neutron coincidence method practically does not depend on the error in the value of the effective fraction of delayed neutrons βeff. If in the IKES method the reactivity value is obtained in fractions of βeff, i.e., from the measurement of delayed neutrons, the neutron coincidence method is based on the direct measurement of the value (1 – kσp)2, where is the effective multiplication factor by prompt neutrons. The total multiplication factor is defined as keff = kσp + βeff. If, for example, keff ≈ 0.9 (which is typical for determining the fuel burnup campaign), then it is the error in determining kσp that is the main one in comparison with the error in βeff. Thus, a 10% error in βeff of 0.003–0.004 (typical for plutonium breeders) will make a contribution to the error 1 – keff equal to 1 – kσp + βeff ≈ 0.00035, i.e., approximately 0.35%, but not 10%, as in the IKES method. Rossi-alpha measurements were carried out using two 3He counters and a time analyzer. The measurement channel width Δt was 1.0 μs. From these measurements, the value of the prompt neutron multiplication factor was obtained. In this case, the space-isotope correlation factor for the medium with a source was calculated using the following values: Φ(x) – solutions of the inhomogeneous equation for the neutron flux and Φ+(x) – solutions of the ajoint inhomogeneous equation. Results. The authors also present a comparison of the results of the Rossi-alpha experiment and measurements of the BFS-73 subcritical facility by the standard IKES method in determining the multiplication factor value. The data of the IKES method differ insignificantly from the results of the Rossi-alpha method over the entire range of changes in the subcriticality with an increase in the subcriticality of the BFS-73 one-core facility. Conclusion. It was impossible to apply the neutron coincidence method to fast reactors; however, the method turned out to be quite workable on their models created at the BFS facility, which was successfully demonstrated in this study.

FAST ROSSI-ALPHA MEASUREMENTS OF PLUTONIUM USING ORGANIC SCINTILLATORS

In this work, Rossi-alpha measurements were simultaneously performed with a 3He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt% 239Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate keff = 0:624 and α = 52:3 ± 2:5 ns for the measured assembly. The organic scintillator system estimated α = 47:4 ± 2:0 ns, having a 9.37% error (though the 1.09 standard deviation confidence intervals overlapped). The 3He system estimated α = 37 μs. The known slowing down time of the 3He system is 35-40 μs, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 μs.

EXPERIMENTAL STUDY ON NEUTRON CORRELATION ANALYSIS FOR A SUBCRITICAL SYSTEM DRIVEN BY A PULSED SPALLATION NEUTRON SOURCE IN KUCA

The Feynman-α and the Rossi-α methods have been frequently employed to determine the subcriticality of subcritical reactor systems driven by Poisson source such as Am-Be neutron source. In actual accelerator-driven systems (ADS), a spallation device will be applied as an intense neutron source. This device will be probably operated in a pulse mode and it is impossible to apply a conventional analysis method to determine the subcriticality in any ADS. In previous theoretical studies, some advanced formulae of neutron correlation analysis for spallation neutron source have been presented. However, the experimental study has been hardly reported to date. The major objectives of this study are to examine experimentally an applicability of these complicated formulae to a subcritical reactor system driven by an actual pulsed spallation neutron source and to determine the prompt-neutron decay constant α of the system. To achieve these goals, we constructed an ADS core at the Kyoto University Critical Assembly (KUCA). The core was composed of highly-enriched uranium fuel assemblies surrounded by many polyethylene reflector assemblies. We carried out a series of the Feynman-α and the Rossi-α analyses for the system driven by pulsed spallation source. As a result, the prompt-neutron decay constants were experimentally obtained by using a fitting formula. The prompt-neutron decay constants determined by Feynman-α and Rossi-α analyses agrees with each other within a statistical error range of least-squares fitting.

USING PROMPT NEUTRON DECAY CONSTANT MEASUREMENTS TO OBTAIN ADDITIONAL KINETICS INFORMATION

Prompt neutron decay constant measurements are typically used to infer the prompt neutron lifetime in a chain-reacting system, but these measurements include much more information about the kinetics of the system. This work focuses specifically on the Rossi-α method of measuring the prompt neutron decay constant. The Rossi-α system of equations consists of five unknown parameters. With five unknowns and three equations, the system of equations solved by the Rossi-α measurement is under-determined. It has been postulated that a well posed problem can be formulated using independent measurement configurations on a single system. The external source strength, the effective delayed neutron fractions, and Diven’s parameter do not typically change for different configurations of a single experiment. If good detector geometry is chosen, the detector efficiency does not change for multiple measurements of a single experiment using Rossi-α measurements. By making these assumptions, there are only two unique variables. This knowledge can be used to solve for most of the unique parameters using two independent, unique Rossi-α measurements.