FBR大型炉設計主要目に関する研究 軸非均質燃料被覆管の軸方向温度差に起因する応力の解析評価

Study on the main design parameters of large scale FBR core characteristics; Analysis of AHC fuel cladding stress due to axial temperature difference

菰田 成一*; 金城 勝哉*; 中西 征二; 谷山 洋*

not registered; not registered; Nakanishi, Seiji; not registered

軸非均質炉心(AHC)成立性に関する燃料設計側からの検討課題の一つである,炉心部と内部ブランケット(IB)部の境界部における軸方向温度差に起因する被夜管応力について検討した。FINAS(FiniteElementNonlinearStructuralAnalysisSystem)を用いた,管壁方向と軸方向に関する二次元(軸対称)熱伝導解析により,軸非均質燃料IB境界部の被覆管二次元温度分布を求めた。この温度条件のもとで,再びFINASを用い,最大発熱部被覆管応力を解析評価した。その結果,照射初期に生じる熱膨張差に伴う応力は,クリープにより照射とともに漸減するが,スエリングが現れる照射中期以降では漸増することが分かった。3年寿命末期応力は,照射初期の応力と比較して,その符号が逆であるものの,大きさは同程度となった。また,管壁温度差に起因する均質燃料最大発熱部応力も解析評価し,軸非均質燃料最大発熱部応力評価結果と比較した。その結果,極端な差違はなく,したがって,軸方向温度差に起因する被覆管応力はAHC成立性上の障害にならないとの見通しが得られた。尚,被覆管材としては改良オーステナイト鋼(PNC1520,15Cr-20Ni-2.5Mo-0.25Ti/0.1Nb)を採用することを前提とした。

This work is on the study of fuel cladding stress due to axial temperature difference at the boundaries of internal blanket(IB) sections, which is one of issues in fuel design field connected with the realization of axial heterogeneous core(AHC). Two-dimensional cladding temperature distribution near the boundaries of IB sections of AHC fuel pins is obtained by two-dimensional (axial and radial) thermal conduction analysis using Finite Element Nonlinear Structural Analysis System (FINAS). On the basis of this temperature distribution, cladding stress at the maximum linear heat rate level is evaluated by FINAS. As a result, it is found that cladding stress induced by thermal expansion rate difference at the beginning of irradiation gradually decreases with irradiation time owing to irradiation creep, but gradually increases inversely in and after the middle of irradiation when a swelling appears. The sign of cladding stress at the end of fuel life after 3 years irradiation duration is contrary to the one at the beginning, but the magnitudes of cladding stress are comparable to each other. Is also evaluated cladding stress due to radial temperature difference at the maximum linear heat rate level of homogeneous core(HOC), and the result is compared with the corresponding cladding stress in AHC. It is turned out that the difference of cladding stress between AHC and HOC is not so great. Consequently, it may be expected that fuel cladding stress due to axial temperature difference is not an obstacle to the realization of AHC. It should be mentioned that a cladding material is supposed to be an advanced austenitic steel(PNC1520, 15Cr-20Ni-2.5Mo-0.25Ti/0.1Nb).