報告書番号

L06011

タイトル（和文）

ナトリウム冷却高速炉4S概念と流量喪失事象解析

タイトル（英文）

The concept of the sodium cooled small fast reactor 4S and the analyses of the loss of flow events

概要 （図表や脚注は「報告書全文」に掲載しております）

当所では、反射体燃料補償の採用により、頻繁のメンテナンスを必要とする制御棒を廃し、燃料無交換で長期の炉心寿命をもつ小型高速炉（４Ｓ）の開発を（株）東芝と共同で実施している。電気出力1万ｋＷの４Ｓ炉心は、30年の炉心寿命中負の反応度係数を保持するという特徴を有している。この炉心を用いることで、高い核拡散抵抗性と安全性を併せもつ原子炉概念の成立が期待できる。崩壊熱除去系に自然循環を利用したシステムを採用した高速炉概念を構築し、そのシステムのもつ熱流動上の特徴を把握するとともに、安全裕度を確認するため、当所が開発しているプラント動特性解析コードCERESを用いた評価を実施した。崩壊熱除去系や炉心反応度特性に着目し、炉停止の有無をパラメータとした２つの流量喪失事象の過渡解析を実施した。その結果、遮蔽体部の熱容量効果が過渡特性に強く影響を与えること、崩壊熱除去系の除熱量は十分であること、スクラム失敗時にも負の反応度特性により炉心ピーク温度は抑えられ、両ケースとも判断条件を十分に満たすことを確認した。これらにより、流量喪失時の４S炉の安全裕度を確認することができた。

概要 （英文）

CRIEPI has been developing the 4S reactor (Super Safe, Small and Simple reactor) for application in dispersed energy supply and multipurpose use, in conjunction with Toshiba Corporation. The 4S is sodium cooled fast reactor and their electrical output has two options of 10MWe and 50MWe. In this paper, 10MWe 4S (4S-10M) was proposed. 4S-10M has some unique features. It employs a burn-up control system with annular reflector in place of the control rod that requires the frequent maintenance service. The core life time of the 4S-10M is 30 years and the fuel transport is not required during core life time. All temperature feedback coefficients are negative during core life time. In the latest design for 4S-10M, a pool and tall type reactor design was selected to reduce the construction cost. Two types of decay heat removal system (Reactor Vessel Auxiliary Cooling System; RVACS, Intermediate Reactor Auxiliary Cooling System; IRACS) using natural convection power were adopted.

It is necessary to confirm that these two heat removal system can operate appropriately. The transition analyses were executed by the CERES code to evaluate the design feasibility and the thermal hydraulic characteristics of the 4S-10M. CERES is a multi-dimensional plant dynamics simulation code for liquid metal reactors developed by the CRIEPI. CERES can perform simulations ranging from forced circulation (full/partial power operation) to natural circulation. Components (pumps, IHXs, SGs, pipings, etc.) of the reactor are modeled as one-dimensional. Multi-dimensional plena are connected to such components.

Two loss-of-flow accident sequences are considered. In the first case, it is assumed that the primary and the secondary pump were stopped by the total station black out. The reactor shut down system was assumed to be success. This sequence is referred to as the protected loss-of-flow accident (PLOF). In the second case, it is assumed that the reactor shut down systems fail to operate and the loss of forced flow proceeds at full power. This sequence is referred to as an unprotected loss-of-flow accident (ULOF).

The following became clear by the CERES analysis. In the case of PLOF, the temperature of the fuel and coolant and the cladding CDF value satisfy the criteria sufficiently. Sufficient heat removal by the RVACS and the IRACS was confirmed. The thermal capacity of the shielding surrounding the core was understood to have a major influence on the transient performance of the 4S-10M. In the case of ULOF, core power was reduced by the effect of the negative feedback, and the temperature of every part and the cladding CDF value satisfy the criteria sufficiently. The design validity of 4S in the view point of the event of the loss of flow was confirmed. And the thermal hydraulics characteristics became clear by analytical evaluations.

報告者

キーワード