Fusion Neutronics Experiments for Thorium Assemblies

Thorium is a fertile element that can be applied in the conceptual blanket design of a fusion-fission hybrid energy reactor, in which 232 Th is mainly used to breed 233 U by capture reaction. It is essential to validate 232 Th nuclear data by carrying out integral fusion neutronics experiments for macroscopic thorium assemblies. The thorium assemblies with a D-T fusion neutron source consist of a polyethylene shell, depleted uranium shell, and thorium oxide cylinder. The activation of γ -ray off-line method for determining the thorium reaction rates is developed. The 232 Th(n, γ ), 232 Th(n, f), and 232 Th(n, 2n) reaction rates in the assemblies are measured by using ThO 2 foils and an HPGe γ spectrometer. From 232 Th reaction rates, the fuel and neutron breeding properties of thorium under different neutron spectra are obtained and compared. The leakage neutron spectra from the ThO 2 cylinders are measured by a liquid scintillation detector. The experimental uncertainties are analyzed. The experiments are simulated by using the MC code with different evaluated data. The ratios of calculation to experimental values are analyzed.


Introduction
The fusion-fission hybrid energy reactor, consisting of a low-power magnetic confinement fusion assembly and a subcritical blanket, is one of the advanced reactors of applying fusion technology to solve the present energy crisis. Natural thorium contains one isotope 232 Th. Thorium is a fertile element that can be applied in the conceptual blanket design of a fusion-fission hybrid reactor [1,2]. The actual neutron spectrum in the subcritical blanket based on the Th/U fuel cycle is composed of fast and thermal spectra. The 232 Th capture cross section at fast neutron is slightly larger than that of 238 U, and 232 Th is more suitable to breed 233 U under fast spectrum. Since 232 Th capture cross section for thermal neutron is about 2.7 times larger than that of 238 U, the conversion rate in the Th/U fuel cycle is more than that in the U/Pu fuel cycle and the neutron economy of thorium is better. Moreover, the 233 U capture cross section for thermal neutron is smaller than that of 239 Pu and 233 U needs to absorb neutrons many times to produce Pu and long-life Minor Actinides (MA, such as 237 Np, 241 Am, and 242 Cm), whereas Pu and MA produced in the Th/U fuel cycle are one order of magnitude less than those in the U/Pu fuel cycle. Therefore, the Th/U fuel cycle is beneficial to reduce the long-life nuclear waste and prevent nuclear proliferation. The feasibility and reliability of the physical Nuclear Fusion -One Noble Goal and a Variety of Scientific and Technological Challenges design for the subcritical blanket based on thorium depend on the accuracy of 232 Th nuclear data and calculational tool. It is essential to carry out the fusion neutronics experiments for validating the evaluated 232 Th nuclear data and studying the breeding properties.
A small number of fusion neutronics experiments on thorium were carried out, and there exist essential differences between the calculations and experiments [3][4][5]. The 232 Th fission rate with fast neutrons was determined by detecting the gamma rays emitted from 140 Ba and 140 La, and the calculated-to-experimental ratio was 0.9 based on ENDF/B-IV [4]. The thorium fission reaction rate in a metallic sphere setup was determined by absolute measurement of the gamma-emission from 143 Ce, the experimental uncertainty was 5.2%, and the calculation to experiment ratio was 1.17 employing ENDF/B-IV [5].
The integral fusion neutronics benchmark experiments for macroscopic thorium assemblies with a D-T fusion neutron source were carried out at Institute of Nuclear Physics and Chemistry (INPC) [6][7][8][9][10][11][12][13][14][15][16][17]. The method for measuring integral 232 Th reaction rate and its application in an experimental assembly were developed and investigated [6][7][8]. In this chapter, the progress in the fusion neutronics experiments for thorium assemblies is described. The overview of main results is presented. The thorium assemblies with a D-T fusion neutron source consist of a polyethylene shell, depleted uranium shell, and thorium oxide cylinder. The 232 Th reaction rates in the assemblies and leakage neutron spectra are measured separately. The benchmark experiments on fuel and neutron breeding properties derived from the 232 Th reaction rates in representative thorium assemblies are carried out and analyzed. The breeding properties are valuable to the breeding ratio in the conceptual design of subcritical blanket based on the Th/U fuel cycle. The experimental results are simulated by using the MC code with different evaluated data. The ratios of calculation to experimental values are analyzed.

Methods
The fusion neutronics experiments contain the measurements of the 232 Th(n,γ), 232 Th(n, f), and 232 Th(n,2n) reaction rates, and the neutron spectra for thorium assemblies with a D-T fusion neutron source.

232 Th reaction rates
The experimental method of activation of γ-ray off-line measurement of 232 Th reaction rates is used. The activation γ-rays are measured by using an HPGe γ spectrometer.
The 232 Th capture reaction rate (THCR) indicates the fuel breeding, that is, the production rate of fissile 233 U ( 233 Pa decay). THCR can be deduced by measuring 311.98 keV γ rays emitted from 233 Pa [6,7]. The reaction process is as follows: The 232 Th fission (with threshold of 0.7 MeV) reaction rate (THFR) indicates energy amplification and neutron breeding. The fission fragment yield correction method is used [8]. THCR can be deduced by measuring 151.16 keV γ rays emitted from the decay of 85m Kr from 232 Th (n, f) reaction. The reaction process is as follows: DOI: http://dx.doi.org/10.5772/intechopen.81582 The 232 Th(n,2n) 231 Th (with threshold of 6.5 MeV) reaction rate (THNR) indicates neutron breeding. THNR is obtained from measuring 84.2 keV γ rays emitted from 231 Th [9]. The reaction process is as follows: The 232 Th reaction rates are deduced from the measured activity and corrections, which include detection efficiency of the HPGe γ spectrometer, cited value of branching ratio, D-T neutron yield during irradiation, self-absorption of gamma rays in the foils, 85m Kr yield only for THFR, etc. The 232 Th reaction rates are normalized to one source neutron and one 232 Th atom.

Breeding properties
The breeding ratio in the conceptual design of subcritical blanket is more than one [1]. The experiment on breeding properties of thorium is used to support the design [17]. The breeding properties are relevant to the reaction type, cross section, and neutron spectrum. The breeding properties contain the fuel breeding and neutron breeding. The fuel breeding is derived from the reaction rate ratio of 232 Th capture to fission, and neutron breeding from the 232 Th(n,2n) and fission reaction rates. The different neutron spectra are constructed by using the macroscopic assemblies in which the material is relevant to that of the conceptual design. The breeding properties under different assemblies are obtained and analyzed from the measured 232 Th reaction rates.

Neutron spectra
The neutron spectra leaking from the ThO 2 cylinders of different thickness are measured by the proton recoil method and the liquid scintillator [16]. The n-γ pulse shape discrimination is based on the cross-zero method. The spectra are resolved by using iterative method, and their range is from 0.5 to 16 MeV.

Assemblies
The experimental assemblies are composed of polyethylene shell, depleted uranium shell, and ThO 2 cylinder with a D-T fusion neutron source and thorium samples.

Polyethylene shell
One can assume the elastic scattering cross sections of H and C, which are widely used as standard cross sections [18] to be reliable. The polyethylene (PE) shell is adopted for checking the method of measuring the 232 Th reaction rates. The inner radius (IR) and the outer radius (OR) of the PE shell are 80 and 230 mm [11], respectively. Five slices of ThO 2 (concentration > 99.95%) foils are put in the radial channel at 0° to the incident D + beam, as shown in Figure 1. The mass and size of foils are about 4.2 g and ϕ30 × 1 mm, respectively.

Nuclear Fusion -One Noble Goal and a Variety of Scientific and Technological Challenges
A D-T fusion neutron source is located in the center of the shell. The 14 MeV neutrons are produced by a neutron generator at INPC. The energy of D + beam bombarding a T-Ti target is 225 keV. An Au-Si surface barrier semiconductor detector is at an angle of 178.2° to the incident D + beam in the drift tube and used to measure the absolute yield by counting associated α particles [19,20]. D-T neutron yield is about 3 × 10 10 /s.

Depleted uranium shell
In the conceptual design of a subcritical blanket based on thorium, the neutrons from the U reaction process are used to maintain the Th/U fuel cycle. The depleted uranium (DU) shell is adopted for studying Th reaction. The IR/OR of the DU shell is 131/300 mm [12]. Six slices of ThO 2 samples are put in the radial channel at 90° to the incident D + beam, as shown in Figure 2. ThO 2 samples are foils made from ThO 2 powder filling a plexiglass box with IR/OR of 9/9.5 mm. The mass of ThO 2 powder is about 0.45 g, and the thickness is about 0.7 mm. The D-T neutron source is located in the center of the shell.

ThO 2 /DU cylinders
The thorium oxide (ThO 2 ) cylindrical assembly with the thickness of 150 mm is produced and consists of three ThO 2 cylinders with the thickness of 50 mm and the   diameter of 300 mm. The ThO 2 cylinders are made by pressing ThO 2 powder using PEO (CH 2 CH 2 O) as the binder and their densities are 4.25-5.59 g/cm 3 [9,10]. The structure of the ThO 2 cylinders as benchmark is simple. To change neutron spectra in ThO 2 cylinders, the latter can be combined with DU cylinders. The combination of two ThO 2 cylinders and one DU cylinders is shown in Figure 3. Three slices of the ThO 2 samples are put in axial channel of the assembly. The front surface of the assembly is 113 mm from the center of a tritium target.

ThO 2 powder cylinder
Based on thorium oxide powder, the ThO 2 assembly is produced, as shown in Figure 4 [13][14][15]. ThO 2 powder fills a stainless steel/aluminum cylinder container with IR/OR of 93.4/96.2 mm. The height of the ThO 2 cylinder is 168.9 mm and the density 1.5 g/cm 3 . Five pieces of ThO 2 foils are put at 0° to the incident D + beam and fixed using holders consisting of aluminum plate and stainless steel. The mass and size of ThO 2 foils are about 5.0 g and ϕ30 × 1 mm, respectively. The distance between the tritium target center and the front end of the cylinder is 78.8 mm.

Neutron spectra in three assemblies
The neutron spectra in PE, DU, and ThO 2 assemblies are simulated by using the MCNP4B code [21] with ENDF/B-VII.0 [22], in which the S (α, β) thermal scattering model in PE is considered. The angular dependences of the source neutron  energy and intensity are calculated by "DROSG-2000" code [23]. The neutron spectra at foils with different distances d to the neutron source in three assemblies are relatively compared, as shown in Figure 5. The ordinate is a normalized neutron fraction, that is, the proportion of the neutron number in each energy segment to the one in the whole energy range [11,13]. The results show that the differences of the fractions are very obvious, especially in the low-energy region.

232 Th reaction rates in PE shell
The PE shell assembly for measuring 232 Th reaction rates is shown in Figure 1. THCR is deduced from measuring 311.98 keV γ rays emitted from 233 Pa (its half-life is 26.967 days, it is obtained from 233 Th decay). THFR is deduced from measuring 151.16 keV γ rays emitted from 85m Kr decay (its half-life is 4.48 hour), which is one of the fragments of 232 Th(n,f) reaction, and using the fragment yield correction method. THNR is deduced from measuring 84.2 keV γ rays emitted from 231 Th (its half-life is 25.52 hour).
The experiment is simulated by using the MCNP code with evaluated nuclear data from different libraries, including ENDF/B-VII.0, ENDF/B-VII.1 [24] and JENDL-4.0 [25]. The model is completely consistent with the structure of the  assembly; it takes into account the target chamber and experimental hall. The calculated statistical uncertainty is less than 1%. The ranges of C/E with ENDF/B-VII.0 are 0.96-1.02 for THCR, 0.95-0.97 for THFR, and 0.89-0.91 for THNR. The results show that the experiment and calculation for THCR and THFR are well consistent within the range of experimental uncertainties, respectively. It is shown that the γ-ray off-line method is feasible for determining the 232 Th reaction rates.
The distributions of 232 Th reaction rates obtained from the experiments and calculations with ENDF/B-VII.0 are shown in Figure 6. The reaction rate ratio of 232 Th capture to fission gives fissile production rate in unit of fuel burn-up [12]. The relative ratios measured are about 10.76-20.17 with the increase of radius in PE shell.
The ratios of calculation to experimental values (C/E) are analyzed. The C/E ratios of 232 Th reaction rates are shown in Figure 7, and the 232 Th(n,f) reaction results for different evaluated nuclear data are shown in Ref. [11]. The calculations with ENDF/B-VII.0 and ENDF/B-VII.1 for THNR underestimate the experimental values. Meanwhile, large differences still exist in the 232 Th(n,2n) 231 Th cross sections among different evaluated data [26]. Fractions with different energies in the PE shell are calculated by using ENDF/B-VII.0, and neutrons of energy more than 6.5 MeV account for 33-48% in the whole energy range, as shown in Figure 5. Since the neutron spectra in the PE shell are reliable, it is suggested that 232 Th(n,2n) reaction cross sections should be studied further.

232 Th reaction rates in DU shell
The DU shell assembly for measuring 232 Th reaction rates is shown in Figure 2.
The 232 Th reaction rates are measured by the same method as described above.
The experimental uncertainties are 3.1% for THCR, 5.3-5.5% for THFR [6,8], and 6.8% for THNR in DU shell.   The C/E ratios of 232 Th reaction rates with different evaluated data are shown in Figure 9. The calculations for THNR overestimate the experiments. Meanwhile, large differences still exist in C/E of THNR. The range of C/E with ENDF/B-VII.0 is 1.07-1.12. Fractions with different energies in DU shell are calculated by using ENDF/B-VII.0, and neutrons of energy more than 6.5 MeV account for 4-9% in the whole energy range, as shown in Figure 5. Since U(n,f) cross sections are standard in the wide energy range, it is suggested that U inelastic cross sections and 232 Th(n,2n) reaction cross sections should be studied further.

232 Th fission and (n,2n) reaction rates in ThO 2 cylinder
The ThO 2 assembly for measuring 232 Th reaction rates in three ThO 2 cylinders with the thickness of 150 mm (without DU cylinder) is shown in Figure 3. The 232 Th fission and (n,2n) reaction rates are measured by the same method as described above.
The 232 Th reaction rates are calculated by using MCNP code with ENDF/B-VII.0. The ranges of C/E are 0.77-0.91 for THFR, and 0.92-1.0 [12] for THNR, respectively. The results show that the calculations generally underestimate the experiments for THFR. The PEO influence on THFR is described below. The distributions of 232 Th reaction rates by the experiments and calculations are shown in Figure 10.

232 Th fission rates in ThO 2 /DU cylinders
Experimental and simulative studies of THFR are carried out on three sets of ThO 2 /DU cylinder assemblies to validate the evaluated thorium fission cross section and code [9,10]. The size of each ThO 2 cylinder and DU cylinder is ϕ300 × 50 mm. The ThO 2 cylinders with PEO contents of 7.28, 1.1, and 0.55% are named as number 1, number 2, and number 3, respectively. The DU cylinder is named as number 4. Three sets of cylinder assemblies are combined with different cylinders, and named as "3 + 2 + 1," "4 + 2 + 1" (as shown in Figure 3) and "3 + 4 + 2 + 1" assembly, respectively.  THFR in the axial direction of the assemblies is obtained by using the activation method as described above, with experimental uncertainties about 5.6-5.9%.
THFRs are calculated by using MCNP code with ENDF/B-VII.0 and ENDF/B-VII.1. The calculations are 5-21% smaller than experimental ones, while the calculations with ENDF/B-VII.0 show better agreement with experimental ones. C/E distributions in the three assemblies are presented in Figure 11. The influence of the PEO in the ThO 2 cylinders is also evaluated by MCNP simulation employing ENDF/B-VII.0. The results show that the PEO influence on THFR under the measured level is negligible.
In order to gain more experimental results, it is necessary to design a new integral experiment employing thorium transport medium in which the ingredient is single and precisely known, and to determine THFR based on more kinds of fission  products, as described below. The stage results could provide reference for the evaluation of neutron-induced thorium fission cross section, and the conceptual design margin of the subcritical blanket.

232 Th reaction rates in ThO 2 powder cylinder
The ThO 2 power cylinder assembly for measuring 232 Th reaction rates is shown in Figure 4. The 232 Th reaction rates are measured by the same method as described above.
The experiment is simulated by using the MCNP code with different evaluated data [10,11]. The C/E ratio of 232 Th reaction rates with ENDF/B-VII.0 are shown in The calculations for THNR underestimate the experiments. Fractions with different energies in ThO 2 powder cylinder are calculated by using ENDF/B-VII.0, and neutrons of energy more than 6.5 MeV account for 62-72% in the whole energy range, which is the largest among the assemblies, as shown in Figure 5. The suggestion described above is that 232 Th(n,2n) reaction cross sections should be studied further.

232 Th fission rate based on 135 I in ThO 2 powder cylinder
The ThO 2 power cylinder assembly for developing the activation method of measuring THFR is shown in Figure 4. THFR in the axial direction of the cylinder is determined by measuring the 1260.409 keV gamma emitted from 232 Th fission product 135 I, with experimental uncertainties of 6.2% [14].  Fusion Neutronics Experiments for Thorium Assemblies DOI: http://dx.doi.org /10.5772/intechopen.81582 and the data obtained in this work could provide reference for the validation of thorium fission parameters. The C/E ratio of 232 Th fission rates based on different evaluated data is presented in the [14].

Fuel breeding
The primary conversion rate is one of the important parameters in the conceptual design of subcritical blanket. The relative reaction rate ratio of 232 Th capture to fission as the fissile production rate indicates fuel breeding in the fuel burn-up unit [12]. The ratios of 232 Th capture to fission measured in PE shell, DU shell, and ThO 2 powder cylinder are obtained.
The ratios are about 10.76-20.17 with the increase in radius of the PE shell. It is demonstrated that the fuel breeding efficiency under the neutron spectra in the PE shell is quite high.
The ratios are about 6.71-12.23 with the increase in radius of the DU shell. It is demonstrated that the fuel breeding efficiency under the neutron spectra in DU shell is high.
The ratios are only about 0.11-0.19 with the increase in radius of the ThO 2 powder cylinder. It is demonstrated that the fuel breeding efficiency under the neutron spectra in ThO 2 powder cylinder is low.
The results show that the ratios are relevant to neutron spectra in the assemblies. The ratios in the three assemblies are compared and shown in Figure 13.

Neutron breeding
The bred neutrons from 232 Th(n,2n) and 232 Th(n,f) react with thorium or relevant nuclides to maintain the Th/U fuel cycle. THNRs in three assemblies, that is, under different neutron spectra, are compared and shown in Figure 14. The results show that the 232 Th(n,2n) reaction rates are relevant to the fraction of high-energy neutrons in the assemblies as described above, and the decreasing trend of THNR with the increase in distance to the neutron source are similar for three assemblies. Since 230 Th half-life (7.54 × 10 4 years) is very long, measurement of 232 Th(n,3n) 230 Th (with threshold of 11.6 MeV) reaction rate by the activation method is very difficult. The 232 Th(n,4n) reaction has high threshold 19 MeV and is not involved in this work.
The prompt neutron and delayed neutron yields from 232 Th(n,f) reaction are about 3.7 and 0.0265 per fission at 14.1 MeV [28], respectively. THFRs in three assemblies, that is, under different neutron spectra, are compared and shown in Figure 15. From Figures 14 and 15, THNRs are higher than THFRs in the three assemblies.

Leakage neutron spectra
Three assemblies consist of the ThO 2 cylinders with thicknesses of 50, 100, and 150 mm (without DU cylinder), respectively, as shown in Figure 3. The front   surface of the assembly is 0.22 m from the center of a T-Ti target. The leakage neutron spectra are measured by using a 50.8 mm diameter and 50.8 mm length BC501A liquid scintillator coupled to a 50.8 mm diameter 9807B photomultiplier [16]. The distance from the detector to the neutron source is 10.75 m. The detector is at a 0° to the incident D + beam and arranged in shielding room. The influence of background neutrons is negligible.
The leakage neutron spectra from the three assemblies are measured. The spectra are normalized to one source neutron and unit area. The experimental uncertainties are 9.7% for 0.5-1 MeV, 6.7% for 1-3 MeV, and 6.3% for 3-16 MeV. The experiments are calculated by using MCNP code with ENDF/B-VII.0. The results show that the experiments and calculations are generally consistent within the range of experimental uncertainties, and the spectra (<5 MeV) should be analyzed further, as shown in Figure 16.

Conclusions
To validate 232 Th nuclear data, the fusion neutronics experiments for the three kinds of thorium assemblies with a D-T neutron source have been carried out. The two spherical assemblies based on the DU and PE shells, and the cylindrical assemblies based on ThO 2 have been designed and established. The assembly materials are referable to the conceptual design of subcritical blanket of a hybrid reactor. The 232 Th(n,γ), 232 Th(n,f), and 232 Th(n,2n) reaction rates in the assemblies are measured by the foil activation technique. The results show that the developed activation approach can work well for the experiments, and the 232 Th reaction rates are relevant to neutron spectra in assemblies. The reaction rate ratios of 232 Th capture to fission are obtained. The fuel and neutron breeding properties under different neutron spectra are compared and analyzed. The leakage neutron spectra from ThO 2 cylinders are measured. The experimental results are compared to the numerical results calculated by using the MCNP code with different evaluated data. The results show that the experiments are benefit to validate Th nuclear data and support the conceptual design of subcritical blanket with thorium in a hybrid reactor. Furthermore, it should be beneficial to measure relevant 232 Th excitation curve at white neutron source of China Spallation Neutron Source (CSNS) [29] for verifying 232 Th nuclear data.