報告書番号

U99004

タイトル（和文）

非線形集中定数モデルによる多液面高速増殖炉のマノメータ振動評価

タイトル（英文）

Analysis of Mass Oscillation of a Multiple Free-Surface FBR with a Non-linear Lumped Mass Model

概要 （図表や脚注は「報告書全文」に掲載しております）

トップエントリー方式高速増殖実証炉の一次系に関して、多液面系のマノメータ振動特性を分析するために、マノメータ振動の非線形性とカバーガスの影響に着目したモデルを構築した。この解析モデルは、水と空気を用いた1/8縮尺試験（日本原子力発電が実施）と解析結果の比較により検証した。想定した多液面高速炉体系での数理的な検討と数値解析により、ポンプ流量・配管圧損係数が一定であれば不安定性やカオスの発生はないことが判明した。一方、ポンプ流量が変動する場合は強制振動が発生するが、その振幅は配管圧損とガス系の存在により大幅に抑制される。さらに、マノメータ振動と連動して配管圧損係数が変化するモデルにより、容器内流動の不安定性によって配管圧損係数が変化する場合には、多液面高速炉体系で自励振動が発生する可能性があることを示した。ただし、この場合も液位の振幅はガス系の存在により顕著に抑制される。

概要 （英文）

The primary circuit of a Japanese FBR (fast breeder reactor) is composed of a reactor vessel, three pump vessels and three IHX (intermediate heat exchanger) vessels, which are connected with top-entry piping system. Since the sodium has several free surfaces contacting argon cover gas in each vessel, the primary circuit forms a multiple free-surface system. The behavior of sodium elevation in each vessel is not straightforward, because the cover gas volumes in the vessels are connected with gas tubes, consequently enhancing the complexity of the system. In the present study, a lumped mass model was developed in order to examine the effect of the non-linearity and cover gas on the oscillatory behavior. The non-linearity of the system was incorporated into the basic equation by using the Lagrange equation, while gas dynamics equations were derived, assuming state of gas obeys polytropic law. The nonlinear model was verified with a 1/8-scale model test conducted by JAPCO, where water and air were used as simulant fluids for sodium and argon gas. Comparison between the numerical results of the mathematical model and the experimental results of the reduced-scale test was made for both reactor trip event and gas-line rupture event. The quantitative agreement of temporal change of water level justified the validity of the present model. The results of eigen-value analysis for the basic equations show that there are two eigen modes for manometer mass oscillation. One is the mode where all IHX sodium levels oscillate in in-phase motion, while the other is the mode where the sodium level in a reactor vessel stays still. The periods of both modes strongly depend on existence of cover gas and the period decreases as the pressure increases. The present model was applied to several events presumable in an FBR to demonstrate the applicability of the code. For example, the computed results show that the mass oscillation behavior is very sensitive to the state of cover gas lines. The numerical results also show that the non-linear effect on the mass oscillation is negligible for a configuration of a typical FBR system. Furthermore, it was also shown that the system is stable at the equilibrium point if pump flow rate and pressure loss coefficient are constant in time. On the other hand, if pump flow rate varies in time, a forced oscillation appears, though the amplitude of the manometer oscillation is drastically suppressed by the existence of pressure loss of hot legs and cover gas system. Finally pressure loss coefficient of hot legs was varied in accordance with sodium level velocity in an IHX vessel. Under this condition, self-induced oscillation appears in the system, which indicates the possibility of self-induced mass oscillation in a multiple free-surface FBR. This is most likely to occur when flow in a vessel is unstable to produce temporal variation of pressure loss coefficient of hot legs, though the amplitude of the self-induced oscillation is drastically suppressed by the existence of cover gas system.

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