Novel dissolution technology to accelerate decommissioning

Costain have worked closely with Magnox Ltd to provide a fit for purpose solution using nitric acid, addressing key challenges without introducing undue complexity. Specific services provided on the project include:

• Research & development management
• Concept and front end engineering design
• Detailed design and engineering
• Engineering schedule and design substantiation
• Procurement
• Construction
• Inactive and active commissioning.

Key Challenges

During dissolution of the FED, a significant quantity of heat, hydrogen and NOx is liberated. It is important that excess heat can be removed during operation and that the reaction can be terminated safely under fault conditions. 

Costain designed, procured and installed a novel reaction vessel system which allows the FED to be contacted by acid flowing upwards through a perforated basket. In the event of a fault, the reaction is quickly terminated by draining the acid from the basket to separate the acid from the FED.

To maintain safe temperature levels, acid is allowed to overflow from the basket into a lower chamber. From here it is pumped through an external heat exchanger for cooling before being returned to the reaction basket. This straightforward design ensures that heat can be removed whilst maintaining a closed system.

An abatement system was also required to deal with the significant quantity of NOx given off during reaction. Costain reviewed best available technology and developed the detailed design for a hydrogen peroxide / nitric acid scrubber system to reduce NOx emissions to specified levels.

Key safety features were incorporated into the design to minimize any risks associated with hydrogen:

•  Reaction termination by draining acid in response to high hydrogen and temperature.
• An emergency back-up system to maintain ventilation in the event of electrical failure. Costain designed an air motor with independent air supply to drive the ventilation fan.
• An independent SIL rated shutdown system designed in accordance with IEC 61508 / 11

Remote Handling

A remote handling system was required to retrieve the FED from 60kg storage containers, load the reaction vessel and remove the remaining intermediate level waste from the reactor basket following dissolution.

Costain designed the plant layout around a single semi-automatic FED Retrieval Crane, allowing it to perform multiple remote handling operations throughout the facility, thus maximizing value. Any remaining remote handling operations that could not be carried out by the Retrieval Crane were incorporated into the Basic Process Control System (BPCS), designed in-house by our experienced C&I engineering team.

Successful project delivery

FED dissolution in nitric acid had not been demonstrated at full scale. It was vital that flexibility existed in the design whilst avoiding complexity. Our collaborative approach allowed development of a design which addresses all associated risks and its success represents a vital step forward in Magnox accelerated decommissioning strategy.