1. Introduction

The pyro-process was under development jointly with national laboratories (KAERI, INL, LANL, ANL) of Korea and USA. Safeguards is one of the major R&D areas to be established for international credibility on the pyroprocess [1]. For the pyro-process, spent fuel is inserted into the head-end process and several fission products are eliminated in the main process. Therefore, as the process goes further, the material does not represent spent fuel property and has no same characteristics anymore. At each process [1] in the pyro-process, the material composition and radiation background are much different. TRU (transuranium) materials are produced and accumulated at the final stage [1]. The final products involve enriched plutonium isotopes. Theoretically, at the final product, uranium does not exist. However, in actual process, uranium might be expected to be involved in the TRU, even though a content of ²³⁵U is relatively small.

Safeguards system for the pyro-process was not fully setup and technologies to assay fissile content nondestructively were necessary to be developed [2, 3]. Therefore, new concept for safeguards system and technology development is required for the process as an evaluation of fissile materials in a manner for continuous of knowledge (COK). Monitoring system based on artificial intelligence (AI) is also helpful and under development. Non-destructive technology has been developed as well with Los Alamos National Laboratory (LANL). Therefore, the combination of monitoring system and nondestructive technique will increase a safeguardability in nuclear material management for the process.

The basic methodology to obtain fissile material at each material balanced area was determined [1]. Chemical analysis was decided to achieve low statistical error in fissile assay. However, many samplings and preparing time are required at each key measurement point. Therefore, as a supplementary way, many non-destructive technologies [2, 3] were reviewed and examined to support the chemical analysis in cost and time saving. The several non-destructive assay (NDA) measurement were applied based on the characteristics of nuclear materials in each material balance area. In the pyro-process, non-destructive technique is currently a big issue for large scale fissile assay. For application of non-destructive technique in the pyro-process, there are several conditions; direct measurement from each fissile material can be obtained and uranium signal must be discriminated from that of plutonium in the mixture. The detection signal must be independent from burnup history and cooling.

The neutron transmission technique was performed for fissile assay on spent fuel including assembly type, low limit detection, source neutron generation and neutron filters [4]. In the paper, the possibility of isotopic fissile assay was additionally examined for the process products, TRU-RE (rare earth), U-TRU mixture, and waste [1, 5]. The linearity in measurement was evaluated as well. The advantage of the transmission technique is that the measured signal can be discriminated between uranium and plutonium and the direct reaction with fissile isotopes is possible [4, 6, 7]. Neutron resonance technique can distinguish uranium signal from that of plutonium without help of burnup code. Uranium and plutonium have their own resonance structures. For the TRU-RE product, highly enriched plutonium is involved with other actinides and rare earth fission products. The enriched plutonium concentration must be accounted for the control of the nuclear material in the pyro-process. For actual pyro-process, the simulation was also done for the sample of uranium added TRU-RE material. The simulation was performed on different TRU sample density, plutonium content change, and uranium content variation as well. In addition, the fissile assay was simulated for hull waste. The hull has same property of spent fuel, but the content of fissile material is relatively small. The resonance energies for ²³⁵U and ²³⁹Pu were evaluated and determined. The measurement in resonances for uranium and plutonium has a direct correlation with the content change of the uranium and plutonium material. The linearity was also evaluated at the resonance energies for ²³⁵U and ²³⁹Pu. If the technique is working properly, it will contribute to the safeguardability of pyro- process system.

2. Neutron Transmission Measurement

TRU material has dominant resonances in resolved energy region. Neutron resonance measurement technique uses transmitted neutron signal on fissile materials. Particularly, uranium and plutonium isotopes have their prominent neutron resonance energies from keV to eV [6, 7, 8, 9, 10]. Therefore, their resonance energies can be used for obtaining transmitted neutron signal for uranium and plutonium. Generally, the transmitted signal is simply expressed as [4]

where I_o is an incident neutron flux on sample, I is a transmitted neutron flux, σ_t is a total reaction cross section, and d is a thickness of sample. The transmitted rate is simply expressed like

The transmitted ratio is a function of sample thickness and total cross section. Fig. 1 shows the schematic view of neutron transmission and measurement on sample. The transmitted neutron is detected at thermal neutron detector. For TRU application, spontaneous fission neutron emitted mainly by ²⁴⁴Cm is major background, however, it is discriminated at the measurement by energy difference.

Fig. 1

Neutron detection configuration for neutron transmission measurement.

The detection signal in the simulation was obtained by using the MCNP code [11]. The detected signal is simply expressed as

where φ is a source neutron arriving at detector, σ_nonabs is a non-absorption cross section, A is a detector area, and ɛ is a detector efficiency. The non-absorbed neutron traverses sample and is measured at a detector. The detected signal is classified by a neutron energy for uranium and plutonium. In the simulation, cell flux tally (F4) in the detector area was used and the ENDF/B7 cross section library was adopted in the reaction. The moderated neutron source was used using filter [4]. In the detection simulation, the relative error about 5% was obtained.

3. Simulation and Analysis

3.1 TRU-RE

At the final product, a highly enriched plutonium is expected to be produced with enriched minor actinides as well [1]. In order to analyze the content of highly enriched ²³⁹Pu in the product, the possibility of neutron resonance technique was simulated and examined on the high sample density and thickness. The sample density was changed until 20 g·cc⁻¹ and the sample size was changed from 0.1 cm to 3 cm in thickness. Highly enriched plutonium in TRU might have different characteristics in neutron reaction for resonance energy region. At relatively thick sample, neutron transmission could drastically decrease at low energy area when sample density increase. Such a decreased signal might give nonlinear response between detection and content of fissile.

Fig. 2 shows the neutron transmitted signal by changing sample thickness from 3 cm to 0.1 cm at resonance energies with different sample density. At thick sample, more neutron absorption was shown when sample density increases. However, the resonance structures were shown at all thicknesses. As the sample thickness decreased, the difference in detection in between densities was reduced at all energies. From the analysis, at 0.1 cm sample thickness, the result shows that the detection difference in between densities was relatively consistent at all energies.

Fig. 2

Energy dependent neutron transmission at different sample thickness (0.1–3 cm).

From the spectrum analysis with respect to thickness and density, several resonance energies were selected to examine the detection linearity for highly enriched TRU material, especially for ²³⁹Pu. The transmission rate was obtained with respect to the change of sample density, at the selected resonance energies for ²³⁹Pu. Fig. 3 shows the transmission rate on 0.5 cm and 0.1 cm sample thickness, relative thin sample target. At 0.5 cm thickness, unfortunately, the transmission rate does not represent linear response in density increase. However, at 0.1 cm thickness, relatively improved transmission rate was obtained at the selected resonance energy. As the thickness decreased, the enhanced transmission rate was obtained. Therefore, from the results, for the analysis of highly enriched fissile material using neutron resonance, sample density and thickness are important factors. Thin target is recommended for highly enriched TRU-RE material assay.

Fig. 3

Neutron transmission rate due to density change.

3.2 U add in TRU-RE

At the final product, theoretically, TRU is the major produced material with several existing fission products [1]. However, small amount of uranium might remain in the product. Therefore, uranium was added in the TRU sample and the possibility of neutron transmission application was simulated for isotopic assay of uranium and plutonium. The content of total uranium was changed in 5–30% and the content of ²³⁵U was obtained based on spent fuel property, 0.7% [12]. Table 1 represents the material composition in the simulation. The composition of TRU-RE was considered as a constant. The energy dependent detected signal was analyzed. The signal discrimination was also examined on ²³⁵U and ²³⁹Pu. In the TRU product, 5% addition of uranium is relatively very small for total TRU amount.

Table 1

Material composition of U added TRU-RE

3.2.1 Density Variation

The detection contribution by ²³⁵U is expected to be small. Fig. 4 shows the transmitted detection signal for uranium and plutonium isotope, as a reference (1 w/o enriched case). From the figure, the resonance energies were evaluated and the energies were selected for uranium and plutonium. The many resonance energies were well separated for ²³⁵U and ²³⁹Pu. However, some resonance energies for ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu were very closely located. Therefore, the closely located resonance energies could give an influence in transmission measurement. As an example, around 6.5 eV, ²³⁵U, ²³⁸U, and ²⁴¹Pu have close resonances. The exclusion of closely located resonances will help on the better analysis for highly enriched plutonium.

Fig. 4

Neutron transmitted signal for uranium and plutonium isotopes (1 w/o content).

Based on the TRU-RE material composition in Table 1, the transmitted measurement was simulated on the variation of uranium content in 5−30% and density in 10−18 g·cc⁻¹. The characteristics of neutron reaction were analyzed in low uranium and highly enriched plutonium. The content of ²³⁹Pu was chosen at 30 and 38%. Figs. 5, 6, and 7 represent the transmitted signals at the selected ²³⁹Pu content, when uranium is inserted into the TRU-RE sample at the selected density of 18.9, 15, and 10 g·cc⁻¹. The simulation showed well defined resonances for uranium and plutonium and the prominent resonances energies were selected for ²³⁵U and ²³⁹Pu. When uranium was added, the transmission rate decreased by density increase.

Fig. 5

Transmitted signal in uranium added TRU-RE sample (density=18.9 g·cc⁻¹).

Fig. 6

Transmitted signal in uranium added TRU-RE sample (density=15.0 g·cc⁻¹).

Fig. 7

Transmitted signal in uranium added TRU-RE sample (density=10.0 g·cc⁻¹).

Fig. 8 shows the resonance properties at the energies (7.8 eV, 12.4 eV, 22.2 eV, 23.4 eV) for ²³⁵U and ²³⁹Pu. In the simulation, the density was fixed at 18.9 g·cc−1and the content of ²³⁹Pu was 38%. As the uranium added into TRURE product, the measured signal decreased at the ²³⁵U resonances, however, the signal at the ²³⁹Pu resonances was not changed.

Fig. 8

Transmitted signal at the selected ²³⁵U and ²³⁹Pu resonances (²³⁹Pu=38%).

3.2.2 Linearity Analysis

Figs. 9, 10, and 11 show the neutron transmission rate at the resonance energies of ²³⁵U and ²³⁹Pu in the case of 30% and 38% of ²³⁹Pu when uranium addition was from 5 to 30%. The transmission rate for ²³⁹Pu was constant at each figure. From the results, for the highly enriched TRU-RE, the linearity was obtained at 23.4 eV and 39.4 eV for ²³⁵U and at 7.8 eV, 14.6 eV, and 22.2 eV for ²³⁹Pu.

Fig. 9

Transmission rate at resonances for TRU-RE-U sample (density=18.9 g·cc⁻¹).

Fig. 10

Transmission rate at resonances for TRU-RE-U sample (density=15.0 g·cc⁻¹).

Fig. 11

Transmission rate at resonances for TRU-RE-U sample (density=10.0 g·cc⁻¹).

3.3 Sensitivity in ²³⁵U and ²³⁹Pu

The sensitivity was examined for various types of fissile materials for actual application. The technical capability was evaluated by changing uranium and plutonium content equally from low to high enrichment. In the simulation, the sample thickness was fixed at 0.5 cm and the content of ²³⁵U and ²³⁹Pu was changed together in 0.5–20%. Fig. 12 shows the transmission and absorption rate at the selected resonance energies when the density is changed from 1.0 to 15 g·cc⁻¹. As sample density increases, non-linear rate is obtained.

Fig. 12

Linearity in the equal change of ²³⁵U and ²³⁹Pu at different sample density (1.0–15 g·cc⁻¹ change).

4. Hull Waste

The material in hull has same property as spent fuel [1, 13]. There are intense radiation backgrounds which have a restriction on direct assay of isotopic fissile. Plutonium has more production at pellet surface by capture reaction of thermalized neutron in coolant. Hull is classified as a waste in the process [5, 13]. The content assay of fissile materials in hull is also important to balance fissile materials in the process. For hull measurement, ²⁴⁴Cm emits intense spontaneous fission neutron as a background. Generally, a measured signal discrimination is required for direct assay of ²³⁵U and ²³⁹Pu.

In the simulation, the sample composition was decided; isotopes of uranium, TRU, and several fission products (Gd, Sm, Dy, Nd, Pr, Ce, La, Y) [12]. The fissile content in the sample was fixed same as in the spent fuel, 4.5% I.E. and 50 GWd·MTU−1 burnup [12]. The sample volume (1×1×1 cm) was selected and the detector size was 1 inch in diameter with 1 inch long. The schematic view of the neutron source, sample location, and detection system was shown in Fig. 1. The simulation was performed on the sample density change, from 0.1 to 12 g·cc⁻¹. The fissile content was assumed to be uniformly distributed in the sample volume.

Fig. 13 shows the measured signal with respect to neutron energies at different sample density. As shown in Fig. 13, the resonance structure was well shown for uranium and plutonium. Above 1 g·cc⁻¹ in density, all resonance structures are well defined in all neutron energies. However, below 0.5 g·cc⁻¹, the resonances are not well represented in all energies because of less neutron absorption. Therefore, the transmission depends on the sample density in hull waste.

Fig. 13

Transmission property in hull with respect to the density change.

Fig. 14 shows the neutron transmission rate in hull with respect to the density change. The results showed that the linear response was obtained at the energy of 26.2 eV and 32 eV for ²³⁹Pu and ²³⁵U in all densities. Non-linear response was also shown at other energies. Therefore, for uranium and plutonium in hull waste, the determined assay energies are applicable in content assay, even though fissile content variation occurs.

Fig. 14

Transmission rate in hull at the selected resonance energies with respect to the density change.

5. Results and Discussion

The simulation and evaluation were performed on the different types of fissile materials. The relationship between fissile content and measured signal was obtained when the sample density, thickness, and enrichment were changed. In general, a difficulty was encountered in direct measurement of fissile isotopes for spent fuel. Many non-destructive techniques analyze isotopic fissile content by indirect way using burnup code [3]. In the pyro-process using spent fuel, many different compositions of fissile materials are produced at each stage. ²³⁵U and ²³⁹Pu are key fissile material to be accounted for the safeguards purpose. Neutron transmission technique has advantage to discern detection signal for uranium and plutonium in mixture. Uranium and plutonium isotopes have distinguished resonance properties.

Basically, chemical analysis was adopted to account for fissile material content, however, non-destructive methodology might help for cost effectiveness and time saving in fissile analysis. In the several sensitivity simulations, the prominent resonance energies were determined for fissile assay, especially for ²³⁵U and ²³⁹Pu. The linearity in the fissile measurement was obtained. From the results, the sample density, content, and thickness are an important factors in the neutron transmission measurement. Therefore, for highly enriched uranium and plutonium, sample preparation is required to obtain linear detection response. Finally, neutron resonance technique is very direct and powerful to assay isotopic fissile materials for TRU product in the pyro-process. An accurate measurement of plutonium content in the pyro-processing will contribute to international safeguards for facilities re-using fissile.