Hast Operations
The Challenge
Sellafields High Level Waste Plants (HLWP) process Highly Active Liquors (HAL) arising from reprocessing plants. Liquors are evaporated and stored prior to processing into a vitrified matrix in Waste Vitrification Plant (WVP). The Regulator requires a reduction in HAL stocks to buffer stock levels by 2015.
The objectives of this project were to support the achievement of the key goal and to:
- Document current understanding of Highly Active Storage Tank (HAST) operating strategy (along with cooling element failure response rules) into one verified document.
- Develop an operational model to validate and further develop these rules.
- Assess the risk of cooling element failures to reprocessing and WVP programmes.
- Develop failure response strategies and measure the associated improvements.
- Identify opportunities for further risk mitigation activities.
The Solution
The SOA team developed an assumptions document defining current operating rules over time. From this, a probabilistic model was developed and validated. The model was then used to:
- Understand the effect of cooling element failures on the likelihood of emptying the Hasps to buffer volumes within the required time.
- Understand the effect of increasing failure rates on HAST emptying rates.
- Establish the effect of specified asset failures and how they could be mitigated.
- Define optimum values for process variables and ensure timely completion to the programme.
- Identify the benefits of implementing engineering projects designed to mitigate operational risks.
The existing model scope includes Evaporators B and C, old and new side HAST tanks and WVP. The model is being developed to include Thorp, Magnox Reprocessing, the evaporators and WVP in greater detail. This will enable the impact of the performance on and of the upstream and downstream plants to be evaluated more effectively.
Fig 1: Typical Model Results
The Benefit
- A set of validated operational rules to support effective future operations.
- A probabilistic assessment of potential operational risks.
- A tool that provides the capability to help test and prioritise actions to mitigate these risks.
- Greater confidence in safe and effective HAL management.
- Greater Regulator, Shareholder and Stakeholder confidence in the ability to meet HAL and Oxide stock limits while minimising disruption to reprocessing and vitrification.
