Fukushima Daiichi Nuclear Power Plant

Background

Three of the nuclear reactors at the Fukushima nuclear power plant underwent meltdown following the earthquake and tsunami in 2011. Hydrogen explosions occurred in Units 1 and 3, whilst Unit 2 underwent over pressurisation of the primary containment resulting in a release of radioactive contaminated steam which settled throughout the operations floor.

Objectives

The client wanted to better understand the complex radiological environment that has existed within the reactor building following the release of radioactive steam. This information would then be used to develop an effective and efficient strategy for recovery and clean-up operations.

Challenges

Following the core meltdown and steam release, the distribution of radioactive material was unknown and man access to the operations floor was impossible due to the high dose rates. However, gaining access was a high priority so that fuel can be safely retrieved from the fuel pond and moved to an alternative storage location.

Solution

REACT worked closely with its sister company CREATEC and the Japanese contractor to develop a deployment plan for the N-Visage™ Gamma Imager that involved wireless communication and use of a stairclimbing ROV. The N-Visage™ Gamma Imager was deployed into the operations floor of Unit 2 from above the reactor building via purposely made penetrations in the roof via an extendable mast. Using the N-Visage™ Predictive Gamma Modelling software, REACT were able to produce a comprehensive N-Visage™ 3D Activity Distribution Model of the contaminated operations floor in Unit 2. The N-Visage™ Gamma Imager was also deployed elsewhere in Unit 2 and the other stricken reactor buildings via an ROV, producing a number of 360° N-Visage™ Gamma Images.

Outcome

The N-Visage™ survey successfully revealed the widespread distribution of radioactivity and identified the prevailing dose levels within the building. A number of hotspots were identified and the relevant dose contribution from each of these was modelled using the N-Visage™ Predictive Gamma Modelling software and shown as N-Visage™ Dose Planes.

The N-Visage™ survey confirmed that man access to the cell would require a substantial reduction in the prevailing dose rates. The natural decay of the radioisotopes present would not permit man entry for many years so some form of intervention would be required. The results of our survey have been used to develop a targeted and effective strategy for shielding and decontamination and will form a cornerstone of any future operations in Unit 2 and the wider Fukushima nuclear power plant.