Investigation on radioisotopes evolution in the fuel of Lead-Bismuth eutectic (LBE) cooled SPARK-NC core

Abstract

SPARK-NC, a 10 MW(e) lead–bismuth eutectic (LBE) cooled fast reactor design, exhibits promising characteristics like inherent gamma shielding, natural circulation, and a high boiling point. Following detailed neutronic studies, a thorough investigation of nuclear safety necessitates a detailed analysis of the core radionuclide inventory. This information is particularly crucial for source term calculations, which play a vital role in assessing the potential radiological consequences. This study establishes the life-cycle inventory of SPARK-NC using two independent computational systems: ORIGEN2.2 and NECP-SARAX. ORIGEN2.2, equipped with a reactor-specific library generated by NECP-MCX, is used for average whole-core inventory analysis. NECP-SARAX, on the other hand, explicitly considers core heterogeneity in terms of enrichment, specific power, and burn-up. This work presents the radionuclide inventories and the relative calculation differences observed between the codes. Actinides like uranium and curium display minimal code dependence, while plutonium isotopes exhibit a maximum relative difference of 8 %. Fission products generally agree within 5 %, except for I-131, which shows a discrepancy of around 10 %. The activity of I-131 and Cs-137 are estimated to be approximately 1 × 10¹⁶ Bq and 3 × 10¹⁵ Bq, respectively. Additionally, the photon source strength is 10¹⁷/s at 1 MeV, dropping to 10¹⁶/s below 6 MeV. Fission products and actinides contribute a decay heat of 0.65 MW. Assembly-wise analysis reveals a direct proportionality between radionuclide inventory and peaking factor, with the average assembly inventory being roughly 25 % lower than the peak assembly inventory. Rare earth elements (Ce, Sm, Pm, Pr, Nd, La, Y) exhibit a maximum mass of approximately 8.5 kg with a 3 % relative difference between the codes. Conversely, halogens (I, Br) have a minimum mass of around 0.2 kg with a 13 % relative difference. These findings, alongside the quantification of radionuclides, provide valuable insights into the code selection for future analyses of SPARK-NC and similar reactor systems.