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Tritium Production Rate Measurement in Li/V-alloy Assembly with 14 MeV Neutron Irradiation

Neutronics investigations for the Li/Vanadium alloy (V-alloy) blanket system without Be neutron multiplier have been performed as one of alternatives in the design activity of the helical type reactor FFHR2. To examine the accuracy of the neutronics calculations for the combination of Li and V-alloy, tritium production rates in a Li/V-alloy assembly with a 14 MeV neutron irradiation have been measured at the Fusion Neutronics Source (FNS) facility of JAEA. The Li/V-alloy assembly was constructed with small solid Li blocks and V-4Cr-4Ti vanadium alloy blocks as shown in Fig. 1. The dimensions of the assembly were ~46 x 51 x 51 cm. A V-alloy layer of ~23 x 24 x 5cm was placed in the assembly at the position of ~15 cm from the front surface. On the central axis of the assembly, Li enriched (Li: 95.5 %) and Li enriched (Li: 99.94%) Li2CO3 pellets of 13 mm� x 1.9 mm were installed for a tritium production rate measurement. The generation rate of 14 MeV neutrons at the tritium target was ~8 x 10 n/s and the total irradiation time was ~13 hours. After the irradiation, the Li2CO3 pellets were dissolved in a HNO3 + CH3COOH solution and liquid scintillator was added. Tritium activity in the solution was measured with a liquid scintillation counter . The maximum measurement error was 8 %. The measured tritium production rates were compared with those calculated using the MCNP5 code and JENDL-3.3 library. Figure 2 shows the distribution of the tritium production rates measured with the Li2CO3 pellets. It is noted that more than 99 % of tritium was produced by the Li(n,n�)T reaction (Threshold energy: 2.8 MeV) in the Li enriched pellets. In the Li enriched pellets, the contribution of Li to the tritium production was < 5 % and most of tritium was produced by the Li(n,�)T reaction with low energy neutrons. The distribution for the Li enriched pellets shows the enhancement of Li(n,�)T reaction around the V-alloy layer which is considered because of the elastic and inelastic scattering and (n,2n) reaction in the V-alloy layer. Comparison of the measured and calculated tritium production rates is shown in Fig. 3. The calculated/experimental (C/E) values for the Li enriched pellets are 1.01-1.04 and those for the Li enriched pellets are 1.02-1.08, respectively. The distribution shows a tendency of slight overestimation for the tritium production rates by the Li(n,�)T reaction around the V-alloy layer. The present result indicates that the measured and calculated tritium production rates in the Li/V-alloy assembly are consistent within 8 %. The impact of the present result to the neutronics investigations of the Li/Valloy blanket is under study.