JAP]Rf-Conf 2004-012 III11111 1111111111 11111 11111 11111 11IN JP0450771 7.2 Recent tritium breeding experiments with lithium titanate mock-ups irradiated with DT neutrons Axel Klix", Yury Verz iloVb , Kentaro Ochiai6, Takeo Nishitan b, Akito Takahashi' ' Department of Nuclear Engineering, Osaka nVuersity, Sulta, Japan bFusion Neutronics Source, Japan Atomic Energy Research Institute, Tokaimura, Japan Experiments with breeding blanket mock-ups composed of layers of beryllium, ferritic steel F82H and 6Li enriched lithium titanate, Li2TiO3 are currently under investigation at Fusion Neutronics Source (FNS) of JAERI. Pellets of Li2TiO3 were used as detectors for the tritium production rate inside the tritium breeding layer. A method for direct measurement of low concentrations of tritium in Li2TiO3 was developed. After irradiation, the pellets were dissolved in concentrated hydrochloric acid and the tritium activity in the sample solution was measured by liquid scintillation counting. This method was applied to several mock-up experiments and the experiments analyzed with three-dimensional Monte Carloc calculations. In some cases the tritium production rate was found to be overestimated by the calculations. KEYWORDS: FNS, D-T neutrons, breeding blanket, lithium titanate, tritium I Introduction 100 r -So rce: FENIS_ Tritium is a fuel for fusion reactors based on the E T Deuterium-Tritium (DT) fusion reaction. It must be pro- 1: 10 duced artificially in a lithium compound in the breeding .0 Q) T :7 -7 7-7, blanket which surrounds the fusion zone. A sufficiently U) _ i high tritium production performance is an important de- U3 f sign parameter for the breeding blanket. To maximize the tritium production rate (TPR), the lithium in the .2 blanket is usually enriched in 6Li in current conceptual re _T- designs since the reaction cross section for the tritium 0.1 production is so high for low energy neutrons compared 6 T TFP7 F= of 7 7 T -T T1 FF. to that Li which is a threshold reaction, see Figure Li (nnaT L h 1. Therefore, the blanket designs contain also neutron evil 1evil ii".i moderating, and for enhancing the tritium production, 1E-6 1E-4 0.01 1 100 neutron-multiplying materials. A primary candidate for Neutron energy, MeV moderation and multiplication is beryllium. But other materials in the blanket, such as structural aterials (steel) and impurities may reduce the TPR by neutron Figure 1: Cross section of tritium production reactions in capture reactions such as (n,-) and (n, c.p. 2). lithium Mock-up experiments with blanket materials are there- fore conducted which allow to study the effects on the steel F82H, lithium titanate enriched in 6Li, and beryl- TPR experimentally and also to validate the computer lium. They were irradiated with the DT neutron source code and nuclear cross section data which are essential of the 80 degree beam line. tools for the R&D work. Recently, mock-up experiments with candidate materials 2 Local TPR measurement for the future DEMO fusion reactor have been conducted at Fusion Neutronics Source (FNS) of JAERI. The breed- A method has been developed which allows to mea- in,11 assemblies consisted of layers of low activation ferritic sure tritium directly in lithium titanate by means of small 'Corresponding author. ak1ixWnshp.tokai.jaeri.go.jp 2charged particle - 209 - JAERI-Conf 2004-012 position and experimental conditions. Liquid scintillation counting (LSC) techniques allow to measure such low tritium activities, but it is necessary to dissolve the pellet and prepare a solution which is compatible with the -applied liquid scintillator. For the presented method, concentrated hydrochloric acid was used as solvent for the pellets. A typical pellet with a mass of 034 g was dissolved in 48 g of HCI at room temperature in a teflon vial. After several days a clear yellow solution was obtained. The solution was neutral- ized with a lwt%NaOH solution. This NaOH concen- tration was chosen because a stronger NaOH solution caused too much heat release upon addition to the acidic lithium titanate solution. During the neutralization, a white deposit appeared which was taken into account when calculating the TPR from the measurement. From the clear part of the neu- tralized solution, I - 2 ml were taken and added to the Figure 2 A block of lithium titanate and a pellet detector liquid scintillator (Ultima Gold XR, Packard Bioscience) in a standard 20 ml PE vial. The finished scintillation pellet detectors which can be inserted into holes in blocks cocktail was counted in an Aloka-5100 beta spectrometer. of lithium titanate used to build the breeding layers, Fig- Figure 3 shows the entire procedure schematically. ure 2 It is expected that the application of TPR detec- tors made of the same material as the breeding layers will 4 Cocktail performance reduce uncertainties in the measured TPR caused by in- homogeneities especially in the 6Li concentration. This is When adding the sample solution to the scintillator, important because in such a thermal-type blanket (most the counting efficiency decreases due to quenching. But at the same time, the decays per time in the vial will of the TPR from thermal and epithermal neutrons) the increase since more tritium is now in the cocktail. A thermal neutron spectrum and hence the TPR change performance parameter of such a cocktail is the figure substantially within short distances due to the presence of merit (FOM) defined as the product of efficiency and of strong neutron absorbers 6Li). incorporated sample mass. Figures 4 and show the ef- ficiency and FOM which was obtained for cocktails pre- 3 Sample processing technique pared from lithium titanate powder of natural isotopic The source neutron yield of the DT source is on the composition. Scintillation cocktails with a sample load order of 1016 neutrons of 14 MeV energy into 4 during (solution volume/ cocktail volume) higher than 20% were each of the experiments. The accumulated tritium ac- unstable and decayed into two phases. Such two-phase tivity in the pellets is 20 200 Bq depending on their cocktails cannot be used for counting. Pellet lis NaOH solution W wo weeks from clear part C1 ntinato P Photomultiplier Figure 3 The pellet detectors are dissolved and liquid scintillation counting is applied. - 210 JAERI-Conf 2004-012 47 1 E 1 ....................... ..... ....... ................. .......................... ....... .q 46 45 > .. ...... ........ ... ...> . ............. >.. ...... 4 4 ................ .......... ............ ....... ....... .. ............... ...... -.-------------................. .......... ..... ...... .0co ......... ...... ....................... 4 3 ............ ........ .... .......... ...... .... ........ ...... 1E -1 .................................... >, 42 .. ........ -------.................. ............................................... .... 4 1 -------.......... ............. ..... ......... ..... FE 0 4 0 1E -2 .... ........................................................... ................ ............ co Pellet P29 . ------------ - --- . ..r ......... 3 9 ry t L % "L i ................ :::m:-. .. ....... ........................ I................... ...... ----------- 38 1E-3 I 1 . I I 1000 37 0 5 0 1500 0.05 0.10 0.15 0.20 Gamma energy [keVI Load [Sample volume cocktail volume] Figure 6 Gamma lines of scandium isotopes in one pellet Figure 4 Efficiency of the scintillation cocktail. after irradiation with DT neutrons. 1.2X 03 To vacuum pump CD .OX,03 FECD E 8.Oxl 0-4 Cocktail unstable E M Li d 6.Oxl O' nitrogeEl n Flask 1 0 LL.4.Oxl 04 D 0.05 0.10 0.15 O.O 0.25 0.30 Load [Sample volume Gocktail volume] To vacuum pump Figure 5: Figure of Merit (FOM) of the scintillation cocktail. The maximum of the FOM is reached when the cocktail becomes unstable, an optimum mixture is then a sample load just below 20%. Liquid nitrogen Hot 5 Activated impurities water Flask I Flask 2 Activated impurities in the sample solution may in- crease the counting rate for the sample and add uncer- Dewar tainty to the measurement. A pellet with 40% 6 Li enrichment was analyzed with a high purity germanium detector after irradiation with 14 MeV neutrons from the Rotating Target of FNS and Figure 7 Cryo distillation apparatus. First, the salty solu- a waiting time of a few days. Gamma lines could be tion is frozen in Flask 1, after evacuation Flask I is heated identified only for three scandium isotopes from (nc.p.) and the water collected in cooled Flask 2 211 - JAERI-Conf 2004-012 4.5 a negligible influence on the measured TPR value with 40 6Li enrichment the salty solution. The TPR in the diagrams is normal- ized to the specific activation of Nb foils attached to each 4.0 - pellet during irradiation, instead of the usual sour ce neu- Cr tron yield. 3.5 6 Application in mock-up experiments C 0 A typical set-up for the initial series of experiments with one breeding layer 40% and 95% enrichment) is 2 3.0 shown in Figure 10 A more detailed description of the CL Li TO solution E 2 3 experiments can be found in Ref. [1]. 2 Liji distillation 03 2.51 1 2 C:, Pellet number FNS 80 deg. line CD TiT target 300 Figure 8: Comparison of samples from the salty solution and 200 ------ ----- ---------- cryodistilled samples. C) Li'CO 3.0 3 95 % 6 Li enrichment Li TO solution 2 3' O 0- Li2TiO3 distillation Aluminum frame I 2.5 Steel dek V 'O 16 F82H sheet(t 1.0) 0 C.) 2.0 F82H sheet :3 (t 1.6) P ............... CL .. ............ ................................... E .................. ............... .............. 1.51 Pellet 1 2 3 4 5 1 Pellet 6 Pellet number ..........Ox ..................... F Beryllium Figure 9 Comparison of samples from the salty solution and cryodistilled samples. Pellet (t =2 3ded by BC Unit: mm reactions of titanium. They emitt also beta particles, but the energies of these betas are mostly outside of the Figure 10: Cross sectional view at the assemblies with one counting window of the tritium measurement. breeding layer. To estimate the influence of those impurities, pellets of 40% and 95% 6 Li enrichment each were irradiated and a The assembly consisted of sheets of F82H steel, solution was prepared as described above.