The Fukushima Event: The Outline and the Technological Background

Abstract

The paper deals with the evaluation of the Fukushima-Daiichi Nuclear Power Plant (NPP) accident in Units 1 to 4: an attempt is made to discuss the scenario within a technological framework, considering precursory documented regulations and predictable system performance. An outline is given at first of the NPP layout and of the sequence of major events. Then, plausible time evolutions of relevant quantities in the different Units, is inferred based on results from the application of numerical codes. Scenarios happening in the primary circuit and containment (three Units involved) are distinguished from scenarios in spent fuel pool (four Units involved). Radiological releases to the environment and doses are approximately estimated. The event is originated by a natural catastrophe with almost simultaneous occurrence of earthquake and tsunami. These caused heavy destruction in a region in Japan much wider than the land around the NPP which was affected by the nuclear contamination. Key outcome from the work is the demonstration of strength for nuclear technology; looking at the past, misleading Probabilistic Safety Assessment (PSA) data and inadequacy in licensing processes have been found. Looking into the future keywords are Emergency Rescue Team (ERT), Enhanced Human Performance (EHP), and Robotics in Nuclear Safety and Security (RNSS).

The Best Estimate Plus Uncertainty approach in nuclear reactor safety and licensing: Brief history and the elements after licensing

“…Five el e ments con sti tute B5: ALARA, BEPU, and IA al ready con nected in sec tion 2.2 above, plus the ex tended safety mar gins (E-SM) and the emer gency res cue team (ERT). Then, the der i va tion of the set of E-SM sig nals, ex pected in the or der of ten thou sand, shall be sup ported by the ex e cu tion of BEPU-FSAR and ERT is an ob vi ous out come from the les son learned from the Fukushima event, [44].…”

Introduction of an Additional Safety Barrier for Nuclear Power Reactors

“…The additional barrier B5 is constituted by a combination of the following elements, which have a heterogeneous nature and role: the As Low As Reasonably Achievable (ALARA) principle, the Independent Assessment (IA) requirement, the Best Estimate Plus Uncertainty (BEPU) approach, the Extended Safety Margin (E-SM) concept and the Emergency Rescue Team (ERT), now a virtual reality. ALARA, IA, BEPU and E-SM are discussed in [11] with more details given in [2], [12] and [13] and ERT is introduced in [14]. The elements are shortly discussed in sections 3.1 and 3.2, distinguishing between 'software' and 'hardware' and their combined role is outlined in section 3.3.…”

“…The ERT consists of a group of highly trained and specialized rescuers, [14], who owns suitable machinery and equipment (helicopters, Diesel Generators, DG, etc.) and the access to each nuclear reactor installed within an assigned geographic region.…”

Insights on Accident Information and System Operations during Fukushima Events

“…Much research has been conducted to distill lessons learned from the accident. D' Auria et al [11] reviewed the accident within a technological framework of nuclear reactor safety and provided relevant findings, observations, and recommendations. Hirano et al [12] reviewed the behavior of reactors at Units 1, 2, and 3, identifying major safety issues such as design basis tsunami, accident management measures, and regulatory treatment of beyond design basis events (BDBEs), among others.…”