Decommissioning developments

Louise Davis

Nuclear safety fears fuel robot wars, reports Boris Sedacca

In February 2017, Tokyo Electric Power (Tepco) revealed that the radiation level in the containment vessel of reactor 2 at the stricken Fukushima No. 1 power plant reached a maximum of 530 sieverts per hour, the highest since the triple core meltdown in March 2011 when it was struck by a Tsunami. Here, we look into the development of robotics for nuclear decommissioning and waste retrieval.

The radiation level at Fukushima far exceeds the previous high of 73 sieverts per hour at the reactor. This will compound the difficulty of removing the fuel debris to decommission at the plant. The Japanese government and Tepco hope to locate the fuel and start removing it in 2021.

According to Tepco, nuclear fuel in the primary containment vessel (PCV) was exposed to the air and melted from the impact of March 2011 earthquake and tsunami.

As a result of the accident analysis, it was found that a portion of melted nuclear fuel might have fallen inside the pedestal.

To remove fuel debris, it is necessary to investigate the PCV and clarify the conditions of debris and surrounding structures.

Onet Technologies has been actively involved in providing remotely controlled dismantling solutions for the Fukushima plant since 2013 through the development of innovative processes. 

Laser-cutting technology

The remotely operated laser-cutting technology is especially suited to cutting very thick materials in a hazardous area. It allows for remote operation while offering accurate position tolerance for cutting heterogeneous layers of materials; moreover, it generates fewer aerosols than most other available techniques.

Damien Roulet, international project manager at Onet, says: “We work across the entire spectrum of nuclear power, including the building of new installations, reactors, laboratories and facilities.

“We carry out maintenance operations for EDF in France on nuclear power stations, but we also operate internationally in the field of nuclear decommissioning, including Fukushima, where we deploy robotic laser-cutting technology.”

The French Alternative Energies and Atomic Energy Commission (known as the CEA) nuclear energy division provides government and industry with the expertise and innovation for nuclear energy facilities. It has also acquired considerable experience in nuclear clean-up and dismantling operations and related R&D, through a number of large-scale decommissioning projects.

Implemented by Onet Technologies as a world first in December 2015, the CEA’s patented technology has demonstrated its full potential in the ongoing project to dismantle MAR200 dissolvers in the spent-fuel reprocessing facility at the CEA Marcoule site in France. The dismantling process was also nominated in the WNE Awards and received the 2016 SFEN award for technological innovation.

Roulet adds: “We have been actively involved with CEA since 2014 in a project to remove melted fuel debris from damaged reactors at the Fukushima Daiichi plant.

“August 2016 marked the end of initial phase focusing on simulant manufacturing and cutting performance, with the second project phase focusing on deep-water cutting and characterisation of resulting aerosol emissions, and more widely, on secondary releases from debris. They can be aerosols or gasses, both in air and underwater.”

Fuel debris

Removing fuel debris from the reactor cores is a vital step in the decommissioning programme. The use of remotely operated laser-cutting technology requires in-depth knowledge spanning a number of fields - a task handled by laboratories at the CEA and the French Institute for Radiological Protection and Nuclear Safety (IRSN).

CEA studies fuel debris resulting from a meltdown and manufactures non-radioactive simulants used to test cutting technology for the project, while the IRSN works on the composition of aerosols resulting from the cutting process.

Roulet continues: “We have demonstrated underwater laser cutting in water up to 5m deep, and demonstrated non-emerging cut, or deep gouging. We also have the first data ever acquired on dust and fume generation during laser cutting on simulated fuel debris, including chemical composition, particle size, gas and radioactivity.”

Onet has previously participated in the design, construction, commissioning and operation of a plutonium contaminated waste treatment facility at Pégase, a decommissioned experimental reactor located on the site of the CEA in Cadarache. After its shutdown, the reactor’s buildings had been used to store 2,714 drums of plutonium contaminated waste.

When the nuclear safety authorities asked the CEA to remove the drums from this temporary storage, they were not in a suitable condition to be transported or stocked elsewhere. In a consortium with Robatel and Millenium, Onet Technologies was awarded the task of reconditioning the drums on the Pégase site.

The mission called for the design of a dedicated facility that would be able to cope with tight space constraints as volume available within Pégase is extremely limited. Led by Onet Technologies, the group designed, built, qualified and operated the facility. In 2014, the last of these non-standard drums was reconditioned.

Sellafield MSSS

At Sellafield in the UK, a Magnox Swarf Storage Silo (MSSS) built in the 1960s provided a particular challenge for Ansaldo Nuclear Engineering Services (NES). The silo comprised 22 compartments of 6m x 6m by 15m deep with a limited space envelope and strict floor loading limits dictated by a building crane capacity of 55 tonnes.

There was 500m3 of waste per silo and access was via 1.7m2 aperture at the top. Compartments 1 to 12 contained sludge and some miscellaneous beta & gamma waste (MBGW), while compartment 15 contained charge tubes and swarf bins, and the remaining vaults contained sludge and some MBGW. Ansaldo NES designed, procured, manufactured, tested and commissioned a mobile silo emptying plant (SEP) for Sellafield, running on rails at the top of the compartments.

Ansaldo Nucleare acquired Nuclear Engineering Services in April 2014 and rebranded as Ansaldo NES Ltd in May 2014. This acquisition was an essential step for the Ansaldo Group in its nuclear market growth strategy after the Fukushima accident. The UK market was identified as the opportunity to achieve Ansaldo Nuclear’s aim for a multi-country presence in the decommissioning and waste management market.

Andrea Basso, technical director at Ansaldo NES, says: “We have the most complicated mobile plant to retrieve waste at Sellafield. It has been quite a long time in the making because the original idea started around 1994/5.

“The machine was almost completed in 1996/7 but there were three or four safety case changes in the meantime. In the nuclear industry, you design against a safety case that tells you practically what you can and cannot do – in this particular case, whether or not you can retrieve the waste. 

“Our initial design was to retrieve waste from the central point but then they established that if that was done, then all the waste around the crater would collapse and might cause an explosion.

“At this point we needed to change the whole philosophy of the retrieval system. We introduced a platter retrieval system to a maximum of 1.5m, after which time you need to level in the waste and retrieve again, and so the levelling and retrieval cycle repeats itself to avoid creating a massive crater in the middle.”

The machine weighs 360 tonnes and was assembled in modules for two reasons. One was the space available to bring the modules inside the building at Sellafield. The other issue was weight, because the crane is limited to 50 tonnes maximum lift capacity.

Basso explains: “The machine has been rebuilt on top of the silo and is now starting ‘inactive commissioning’, where you check that the machine power and everything else works before touching any radioactive waste. When inactive commissioning is completed, we will hopefully have the authorisation to use the machine for retrieving the waste.

“Part of the waste may have some spent magnox fuel fragments. The complicated nature of the machine is such that there is no other equivalent like it in the world.”

Short-term nuclear power demand 

Short-term demand for nuclear power is unlikely to abate – for example, France has a long-term target of 50% of electricity generation from nuclear power, down from the present day figure of 75%.

A number of nuclear power stations have recently shut down for maintenance, which took 9GW of nuclear capacity offline out of a total of 63GW, according to the International Energy Agency (IEA).

France’s electricity generation is highly dependent on nuclear, which accounts for 75% of its power.

Renewables make up about a quarter of electricity capacity, the majority of which comes from hydro.

But with water levels in reservoirs at their lowest levels in 10 years, hydro’s contribution to total generation is also at its lowest in a decade.

Other renewables contribute about 7% of electricity generation, but limited daylight hours in the winter mean restricted generation from solar PV.

Conventional generation, which includes gas, coal and oil, makes up only about 6% of total generation, or roughly one tenth of the country’s nuclear generation in normal times.

France has a significantly low-carbon electricity mix, owing to the key role of nuclear energy.

However, much of its nuclear fleet is reaching the end of its lifetime.

Against this background, France has started an ambitious energy transition: it is a world leader in designing a governance framework with a national low-carbon strategy, carbon budgets, a carbon price trajectory and plans for energy investment.

France plans to reduce the share of nuclear to 50% in the electricity mix by 2025. Although some nuclear reactors may continue long-term operation under safe conditions, maintaining security of supply and a low-carbon footprint while reducing nuclear energy will require investments in renewable energy and efficiency.

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