UMYU Scientifica

A periodical of the Faculty of Natural and Applied Sciences, UMYU, Katsina

ISSN: 2955 – 1145 (print); 2955 – 1153 (online)

ORIGINAL RESEARCH ARTICLE

THERMAL NEUTRON FLUX ESTIMATION IN THE INNER AND OUTER IRRADIATION CHANNELS OF NIRR-1 LEU CORE

Anas M. S¹ and Muhammad Taha Umar¹

¹Physics Department Ahmadu Bello University, Zaria-Nigeria

Abstract

This research work focused on the determination of the effective cross-section and thermal neutron flux of the inner (B2) and outer (B4) irradiation channels of the Nigeria Research Reactor-1 (NIRR-1). The effective cross-section for the inner (B2) and outer (B4) irradiation channels was found to be 1.85×10^-22cm² and 1.28×10^-22cm²respectively. This shows that B2 has a higher effective cross section than B4 which means that B2 will have more particle collision and produce more energy than B4. The thermal neutron flux for inner (B2) and outer (B4) irradiation channels were found to be \((4.78 \pm 0.22) \times \ 10^{11}\ \ n/{cm}^{2}s\) and \((6.86 \pm 1.86) \times 10^{11}\ \ n/{cm}^{2}s)\) respectively. This shows that the outer (B4) channel is more thermalized than the inner (B2) irradiation channel because the more fission, the more the thermal neutron flux produce. This signifies that B4 will produce more heat and energy than B2; meanwhile, B2 absorbed and scatter more neutrons by the materials in the reactor than B4 because B2 has lower neutron flux than B4. The results can help provide information about the reactor's operation, especially in neutron activation analysis and fuel management decisions to enhance its performance, safety, and efficiency.

Keywords: LEU, NIRR-1, High purity Germanium detector (HPGe), flux monitors, calibration sources.

INTRODUCTION

Thermal neutron flux refers to the number of thermal neutrons passing through a given area in a given time. It relates to the rate of nuclear reaction in a reactor, which determines how much heat and energy a reactor can produce, and the reactor's power output. The higher the thermal neutron flux, the more heat and energy the reactor can produce. The equivalent 2200m/s thermal flux (Φ_th) in which a monitor sample such as Au is irradiated can be calculated.

\(\Phi_{th} = \frac{R_{s} - F_{cd}R_{s,cd}}{g\sigma_{th}G_{th}}\)…………………………………………….(1.0)

Where R_s and R_{s, cd} are the reaction rate per atom of bare and Cd-covered isotope irradiation, g is the correction for departure from 1/v cross-section behavior, G_th the shielding factor for thermal neutrons, σ_th the thermal neutron cross-section and F_cd the cadmium correction factor (Karandag et al. 2003).

Thermal neutrons have the lowest energy and are in thermal equilibrium with the surroundings. Their energies are around 0.025eV, corresponding to the thermal motion of atoms at room temperature. They are essential in maintaining nuclear fission reactions. Epithermal neutrons have energies above thermal neutrons but below the fast neutrons (0.025eV -0.1MeV. They can contribute to certain nuclear reactions. Fast neutrons have higher energies, typically above 0.1MeV. They can cause damage to materials and contribute to certain nuclear reactions ( Karandag et al. 2003).

Nigeria Research Reactor -1 (NIRR-1) is a nuclear research reactor located at the Centre for Energy Research and Training (CERT), Ahmadu Bello University Zaria. The reactor is a miniature neutron source reactor (MNSR), a type of light water reactor designed by China’s Institute of Atomic Energy. It can produce a steady thermal power of 31kw, which became critical in 2004 with high enriched uranium (HEU), and is the only research reactor currently operating in Nigeria (Anas et al., 2023; Jonah et al., 2005).

Figure 1: A layout of NIRR 1 core Configuration showing the irradiation Channels

NIRR-1 was converted to a low-enriched uranium (LEU) core in 2018. The conversion might have brought about changes in the neutron flux parameters on which Neutron Activation Analysis (NAA) protocols were based. Since the primary function of NIRR-1 is NAA, there is a need to determine the thermal neutron flux in the irradiation channels. These necessitate the proper determination of neutron flux in the irradiation channel of the NIRR-1. The core physics parameters of the old HEU core and New LEU core are presented in Table 1.

Table 1: comparism of HEU and LEU cores of NIRR-1

The NIRR-1 achieved its first criticality with the LEU fuel 2018; the neutron spectrum parameters were also determined, and the NAA facilities were standardized for optimal utilization (Anas et al., 2023a and 2023b).

NIRR-1 has been converted from HEU to LEU, which might have brought about changes in the neutron flux parameters on which protocols for NAA were based. These necessitate the proper determination of neutron flux in the irradiation channel of the NIRR-1. This study aims to determine the thermal neutron flux in the irradiation channel of the NIRR-1 LEU core.

MATERIALS AND METHODS

Materials

The materials used for this research work are NIRR-1, High purity Germanium detector (HPGe), ¹⁹⁷Au flux monitor, and calibration sources.

Methods

Cadmium-ratio (Cd-Ratio) Multi-monitor method was used to determine the neutron Spectrum Parameters. Cd-ratios of four neutron flux monitors were used to calculate the neutron flux ratio “f” and Epithermal flux shaping factor “α” values in the irradiation channels. The flux monitor was clean with ethanol weighed and packed in a stack inside a cleaned polyethylene capsule for the bare irradiation, while a second set was encapsulated inside a 1mm thick cadmium box for the Cd-covered irradiation channel at a thermal power level of 17kW which corresponds to preset neutron flux value of 5.0e¹¹ncm^-2s^-1. The same is done for the outer irradiation channel at the same preset neutron flux. The irradiation protocol was carried in order to induce measurable activities of at least 10,000 counts in the flux monitors (Anas et al., 2023).

The α and f values were determined iteratively using the “Solver” utility in EXCEL to solve the equations for the Au monitor.

The Neutron flux of an ideal reactor is expected to be stable during the operation. In view of this, the thermal neutron flux will be determined for the NIRR-1 new LEU core for two irradiation channels in order to establish the flux stability using Equation 2.1 (De Corte et al., 1981).

\(\varphi_{th} = \frac{N_{p}M}{wN_{a}\gamma\theta\varepsilon_{p}}\frac{1}{(1 - e^{- \lambda t_{i}})e^{- \lambda t_{d}}}\frac{\lambda}{(1 - e^{- \lambda t_{m}})c\sigma_{eff}}\) (2.1)

where: N_p the net number of counts under the full-energy peak during counting time, t_m,

w is the weight of irradiated element,

\(S = 1 - e^{- \lambda t_{irr}}\) is the saturation factor, \(D = e^{- \lambda t_{d}}\) is the decay factor with t_d being the decay time, \(C = (1 - e^{- \lambda t_{m}})\) is the measurement factor correcting for decay during the measurement time, t_m, M is the atomic weight, λ; is the decay constant, \(\theta\); is the isotopic abundance, N_a; is the Avogadro’s number, \(\gamma\); is the absolute gamma-ray emission probability,

\(\varepsilon_{p}\); is the full energy peak detection efficiency, c is the concentration of the analyte and

σ_eff; is the effective neutron cross section in cm² as defined by (De Corte et. al., 1981) and presented in Equation 3.2:

\(\sigma_{eff} = \sigma_{0}\left( 1 + \frac{Q_{0}(\alpha)}{f} \right)\) (2.2)

where Q_o(α) is given as

\(Qo(\alpha) = \frac{Qo - 0.429}{Er\hat{}(\alpha)} + \frac{0.429}{(2a + 1)Ecd}\) (2.3)

where Q₀ = I₀/σ₀ is the ratio of resonance integral to thermal cross-section,

α is a measure of the non-ideal epithermal neutron flux distribution, and

f is the thermal to epithermal neutron flux ratio (De Corte et. al., 1981).

The description of the flux monitors and their nuclear data used for the determination of Epithermal flux shaping factor “α” and flux ratio “f” is given in Table 2 and 3

Table 2: Description of neutron monitoring foils used in this work.

Goldman et al., 2005

Table 3: Nuclear data characteristics of the neutron monitoring reactions

The result for “\(\alpha\)” and “f” were achieved by plotting a graph of Equation 3.1 as presented in Figure 2:

\(\log\frac{{\bar{E}}_{r,i}^{- \alpha}}{\left( F_{Cd}.R_{Cd,i} - 1 \right)Q_{O,i}(\alpha)G_{e,i}/G_{th,i}}\) versus \(\log E_{r,i}\) 3.1

where: \({\bar{E}}_{r,i}\) is the effective resonance energy of the i^th monitor

F_Cd is the Cd-transmission factor for epithermal neutrons

G_e,i is the epithermal neutron self-shielding factor for the i^th monitor

G_th,i is the thermal neutron self-shielding factor for the i^th monitor

R_Cd,i is the ratio of the specific activity of the i^th monitor irradiated without the Cd (A_sp,bare) to that with the Cd cover (A_{sp, Cd})

RESULTS AND DISCUSSION

Flux parameters show the nature of neutron spectrum and distribution within an irradiation channel of a research reactor. Because of this, ⁹⁴Zr(n,γ)⁹⁵Zr, ¹⁹⁷Au(n,γ)¹⁹⁸Au, ⁶⁸Zn(n,γ) ⁶⁹Zn, ⁶⁴Zn(n,γ)⁶⁵Zn and Mn⁵⁵(n,γ)Mn⁵⁶ foils were used to determine the flux parameters “α” value and flux ratio “f”). All the nuclear data were adopted from De Corte 2003, as presented in (table 3). The ratio of the activity of the foil irradiated without cadmium cover to activity of the foil irradiated with cadmium cover called (Rcd) for channel B2 and B4 were presented in tables 4 and 5.

Table 4: showing activities irradiated foils and there Rcd at channel B2

Table 5: showing activities irradiated foils and there Rcd at channel B4

The value of the epithermal flux-shaping factor (α) as one of the important characteristics of each irradiation channel of a research reactor such as NIRR-1 was determined from a suitable α-monitors Au¹⁹⁷(n,γ)Au¹⁹⁸, Zr⁹⁴(n,γ)Zr⁹⁵ and Zn⁶⁴(n,γ)Zn⁶⁵ (table 4 to 5).

Figure 2: Cadmium ratio multi-monitor plot for (outer irradiation channel B4)

The results of the effective cross-section of the Au monitor and thermal neutron flux for the inner B2 and outer B4 irradiation channel of NIRR 1 obtained were presented in Table 6.

Table 6: Calculated effective cross-section of Au monitor and thermal neutron flux for inner and outer irradiation channel of NIRR 1 LEU core.

The effective cross-sections for inner and outer irradiation channels were 1.85×10^-22cm² and 1.28×10^-22cm², respectively. This signifies that B4 has a higher effective cross-section than B2, which means that more particles will collide and more energy will be produced in B4 than B2, which also shows the effect of effective cross-section to the rate of nuclear reaction, which is, in turn, affects the energy output of the reactor.

Similarly, the thermal neutron flux, which induces fission reactions in the reactor’s fuel for the inner irradiation channel (B2) and outer irradiation channel (B4) were found to be \((4.78 \pm 0.22) \times \ 10^{11}\ \ n/{cm}^{2}s\) and \((6.86 \pm 1.86) \times 10^{11}\ \ n/{cm}^{2}s)\) respectively. This shows that the outer (B4) channel is more thermalized than the inner (B2) irradiation channel. This is because the more the fission the more the thermal neutron fluxes.

The effective cross-section affects the thermal neutron flux by affecting the rate at which thermal neutrons are lost in the reactor. The more neutrons that are absorbed or scattered by the materials in the reactor, the lower the neutron flux will be. This can impact the reactor’s performance, including the rate of fission and the amount of energy produced. From the values obtained for the thermal neutron flux, thus, B4 is better suited for high sensitive application of NAA than B2, which signifies that B4 will produce more heat and energy than B2. Meanwhile, B2 absorbed or scattered more neutrons by the materials in the reactor than B4 because B2 has lower neutron flux than B4.

CONCLUSION

This research determined the effective cross-section of the Au monitor and the thermal neutron flux for inner (B2) and outer (B4) irradiation channels. The values obtained for the effective cross-section for inner (B2) and outer (B4) irradiation channels are 185.1772×10^-24cm² and 128.9955×10^-24cm² respectively. This shows the possibility of having more collision and energy production in the LEU core of the NIRR-1. Similarly, the thermal neutron fluxes for the inner and outer irradiation channels were found to be \((4.78 \pm 0.22) \times \ 10^{11}\ \ n/{cm}^{2}s\) and \((6.86 \pm 1.86) \times 10^{11}\ \ n/{cm}^{2}s)\) respectively. This shows that B4 produces more heat and energy than B2. Also B4 has more number of fission than B2.

ACKNOWLEDGEMENT

The authors are grateful to the management of CERT's entire research team, Physics Department ABU Zaria, for their support and assistance in this work.

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