Abstract

To proceed with the decommissioning of the Fukushima Daiichi Nuclear Power Station, analyses of unexpected fuel debris criticality accidents are needed. Supercritical transient analyses have been conducted for fuel debris using the Multiregion Integral Kinetic (MIK) code, which can take the space dependence of fuel debris into account. In those analyses, reactivity is assumed as stepwise insertion because the MIK code does not include delayed neutron effects, which might be negligible. However, reactivity insertion may not always be stepwise. Therefore, it is important to clarify an applicable range of the MIK code for nonstepwise insertion, such as ramp reactivity insertion. To show that kinetics codes without delayed neutron effects could be applied for a supercritical transient induced by ramp reactivity insertion, we established a method to clarify its applicable range. An analysis using the point reactor kinetics model was introduced as a pre-analysis to clarify this range in the case of ramp reactivity insertion in terms of the contribution of delayed neutrons. We applied the methodology to a simple cylindrical fuel debris system and successfully demonstrated a supercritical transient analysis for ramp reactivity insertion using the MIK code.

摘要

为了进行福岛第一核电站的退役，需要对意外的燃料碎片临界事故进行分析。已经使用多区域积分动力学（MIK）代码对燃料残渣进行了超临界瞬态分析，该代码可以考虑燃料残渣的空间依赖性。在那些分析中，将反应性假定为逐步插入，因为MIK码不包括延迟的中子效应，这可以忽略不计。但是，反应性插入可能并不总是分步进行的。因此，重要的是要阐明用于非逐步插入的MIK代码的适用范围，例如斜坡反应性插入。为了显示没有延迟中子效应的动力学代码可以应用于由斜坡反应性插入诱导的超临界瞬变，我们建立了一种方法来阐明其适用范围。引入了使用点反应器动力学模型的分析作为预分析，以根据延迟中子的贡献在斜坡反应性插入的情况下阐明该范围。我们将方法学应用于简单的圆柱状燃料碎片系统，并成功演示了使用MIK代码进行斜坡反应性插入的超临界瞬态分析。