New cask for China

This summer, Spain’s Equipos Nucleares (Ensa) shipped its first ENUN 24P dual-purpose metal cask to China. The cask has been designed to transport spent nuclear fuel from the Ling Ao and Daya Bay plants in southeast China, to the Lanzhou storage facility, almost 4000km away, an effort which involves transport through different regions and climate conditions and both road and rail.

Ensa, which won the contract to develop the cask in 2013, adapted its existing cask to meet Chinese requirements. This involved modifying the design of the ENUN cask, including the trunnion design, to meet the requirements of China’s transportation routes, as required by the customer. The new cask, which was recently granted a licence from the Spanish nuclear regulator, is Ensa’s first licensed for high burnup spent fuel (up to 57GWd/tU).

It also meets all the requirements of a Type B(U) package as per international transport regulation IAEA No. SSR-6.

When Ensa was awarded the contract, there was not much documentation on how to justify casks for high burnup fuel (over 45GWd/tU). The burnup level affects fuel temperature, radioactivity and physical makeup. In addition, when spent fuel is placed in a dry storage system and the water is removed, the temperature of the fuel increases and the properties of the cladding can change.

As a result, the fuel cladding may become less ductile, according to the US Nuclear Regulatory Commission, which in 2015 issued a draft version of a guide outlining licensing approaches that it said would provide reasonable assurance for meeting regulatory requirements for the transport and storage of high burnup spent fuel.

Designing for Daya Bay

Since 2003, used fuel from Daya Bay has been transported to the Lanzhou interim wet storage facility in Gansu province, central China.

Initially, Ensa and NAC International manufactured two STC spent fuel transport casks for CNNC Everclean. The STC cask is a stainless steel and lead cask system, designed to transport up to 26 pressurised water reactor (PWR) fuel assemblies with a maximum burnup of 45GWd/tU. However, by 2013, the plant operator had begun investigating transport of higher burnup spent fuel, in order to improve economics. By the end of that year, Ensa had been awarded a contract to supply an ENUN 24P cask, ancillary equipment and training services to the operating personnel at Daya Bay and Ling Ao. The company was also required to, obtain a Certificate of Compliance for transport of high burnup spent fuel from the Spanish (CSN) and Chinese (NNSA) nuclear regulatory authorities.

The ENUN 24P cask is an evolution of the ENUN 32P concept developed by Ensa for spent fuel management at Ascó, Almaraz, Trillo and Vandellós. ENUN 32P allows two different basket configurations to support transport and storage of up to 32 fuel assemblies, including both KWU (16x16) and Westinghouse (17x17) fuel.

The ENUN 24P cask was originally designed to hold 32 fuel assemblies (AFA 2G, AFA 3G and AFA 3GAA fuel) however, the criticality control evaluation was initially performed giving credit to the burnup of the spent fuel to the actinides, according to ISG-8, Rev. 3. But the Chinese authorities and the customer did not initially allow the application of that burnup credit methodology, so Ensa decided to include a flux trap concept in the basket layout to increase the gap between adjacent fuel cell positions, and assure the spent fuel remained in subcritical condition – considering it as fresh fuel for the criticality evaluations. As a result, the maximum capacity of the cask had to be reduced to 24 fuel positions, which in turn allowed the inner cavity diameter to be reduced from 1866mm to 1669mm.

Another change related to the shielding. Storing fuel after just three years’ cooling means that high gamma and neutron flux, and elevated decay heat, had both to be taken into account. In addition the Chinese client had specified that dose rates on the package surface had to be conservatively reduced to 2mSv/h. To put this in context, IAEA SSR-6 allows maximum dose rates of 10mSv/h on the package surface. To meet these stringent requirements, Ensa’s engineers had to increase the gamma and neutron shielding, but also ensure the weight of the filled cask was below 130t, due to the lifting capacity of the cranes in the fuel buildings.

Five changes were proposed in the design stage, including:

• Increasing the thickness of the aluminum profiles in the basket to reinforce the gamma shielding capabilities and enhance the heat rejection capacity.
• Welding a set of stainless steel plates to the outside of the basket to provide gamma shielding closer to the fuel, without adding significantly to weight.
• Tripling the thickness of the outer shell to improve gamma and neutron shielding.
• Increasing the inner volume of aluminium fins to allow more neutron shield resin and improved shielding.
• Adding discs of neutron shielding material at the centre of the impact limiters, to reduce the contribution of the neutron source in the axial dose rates.

These features were proposed during the pre-licensing phase, but later in the design phase additional changes were made to the cask dimensions and anti-corrosion coatings, to meet client requirements. Due to the tight timeframe Ensa said that fabrication activities had to be carried out in parallel with the detailed design evaluation.

New dimensions

During the bidding phase, the maximum cask diameter was set at 3400mm, but this was later reduced to 3300mm due to restrictions along the transport route. In order to meet these new requirements, Ensa had to reduce the diameter of the casks’ impact limiters.

These were designed in collaboration with the US Sandia National Laboratories, and they are made from polyurethane foam and aluminum honeycomb blocks enveloped by a stainless-steel cover. They were tested using a mockup of an ENUN 32P cask, and passed 9m free-drop and 1m puncture-impact tests, meeting the requirements of IAEA SSR-6.

The first set of drop tests performed on the cask with reduced diameter impact limiters (3300mm) resulted in accelerations over the acceleration design limit in the centre of the package. However, a slight modification to the geometry of the internal structure of the impact limiters meant more honeycomb absorbing material could be used, bringing the maximum acceleration of the cask within design limits. Ensa also came up with a new concept for the trunnions by drilling an inner cavity in the trunnion block, which avoided depletion of the energy absorbing material during all the drop tests.

Some modifications were needed to the ancillary equipment, particularly the lifting yoke and transport skid, for fuel loading and unloading operations. Ensa developed a pneumatic actuator system and a double blade system to reinforce the structural behaviour of the lifting yoke. Ensa said Chinese personnel will be trained on use of the new system.

Delivery

Ensa delivered the first cask to China in May 2017 for cold testing, and the company is currently working with URC to support design validation required by the China Nuclear Regulatory Authority. This will eventually enable the first commercial transport of high-burnup spent fuel in China.

In October, Ensa announced that the cask had been certified by the Spanish Ministry of Energy, Tourism and Digital Agenda, making it the first spent fuel cask licensed for the transport of high-burnup fuel in Spain. The licence was granted following a favourable report from the Spanish Nuclear Safety Council (CSN), and as a result of evaluations that were carried out to confirm that the design complies with IAEA Type B(U) packaging requirements.

“Since Ensa is deeply involved in spent fuel management operations in Spain, we can use these methodologies to transport spent fuel in this country, subject to regulatory approval, as well as in other European and overseas countries where Ensa is actively participating in tendering processes for new nuclear spent fuel casks,” said Alejandro Palacio, design and licensing engineer, spent fuel casks, at Ensa. “Since we have strictly followed the US regulations and standards it should stand us in good stead [for export projects].”