SIXEP at 40
Source: gov.uk (2025-06-17)

A workhorse of the Sellafield site is celebrating 40 years of reducing our environmental impact.

Sellafield’s Site Ion Exchange Effluent Plant (SIXEP) reached a historic milestone this month as it clocked up four decades of safe, dependable operations.

Best described as the kidney of Sellafield, the plant plays a vital role in receiving and filtering contaminated water from a range of nuclear buildings and processes.

Water used to store spent nuclear fuel and effluents from legacy waste stores and clean-up operations is piped into the plant and filtered to remove radioactive ions like caesium and strontium.

As a result, SIXEP has had a huge positive impact on our environmental performance at Sellafield, removing 99.9% of the radioactive ions from the water so it can be safely discharged to sea.

Learn more about SIXEP and the ion exchange process (https://www.gov.uk/government/case-studies/the-sellafield-site-ion-exchange-effluent-plant-sixep)

Originally constructed with a 30-year lifespan, teams operate the plant 24 hours per day, 365 days per year and have kept it operational with minimal down time since it first came online in May 1985.

They’ve processed more than 30 million cubic metres of water—enough to supply every household in the UK for three days – and removed over 130,000 TBq of radioactivity.

They’ve also had to adapt to changing priorities and regulations at the site over the years but have risen to every challenge to meet demand and keep the site safe and productive.

Roddy Miller, Sellafield Ltd’s chief operating officer, said:

For four decades SIXEP has been a constant at Sellafield, quietly doing its job and making a big difference in protecting the environment and supporting operations across the site.

This anniversary is a chance to reflect on everything that’s been achieved in that time, not just by the plant but by the people behind it.

I want to congratulate and thank everyone who has contributed to the safe and successful operation of SIXEP in the past and today. I know its future is in good hands.

The SIXEP operations team

Mark Wareing, programme manager at the NDA, has been with the NDA since its inception and has been closely involved in SIXEP at Sellafield during that time. He said:

Since the inception of the NDA 20 years ago, SIXEP performance has been central to NDA’s mission in supporting electricity generation and addressing some of the UK’s and Europe’s most significant nuclear clean-up challenges.

The performance of the plant over the last 40 years of operations is a testament to the engineers who designed the facility and the dedication, expertise and ingenuity of the people who operate and maintain it.

I have dedicated a large portion of my career to supporting the remediation of the legacy ponds and silos at Sellafield and, without SIXEP, the great progress that has been delivered to date could not have been achieved.

Demand for SIXEP is set to continue until the 2060’s as the site’s clean-up mission and fuel storage programmes progress, so we’re constructing the SIXEP Continuity Plant (SCP) to ensure it can keep doing its vital work.

SCP will integrate seamlessly with the existing plant to provide new sand bed filters and ion exchange vessels, as well as additional storage and waste management capabilities.

It’s one of our major construction projects at Sellafield being delivered by the Programme and Project Partners and is on course to be operational in 2029.

The SCP project team recently celebrated a significant delivery milestone as they completed the manufacture of 14 bespoke pump and valve modules that will form the inner workings of the plant.

This was supported by an extensive supply chain including West Cumberland Engineering Ltd who completed the final module manufacturing, intricate connecting pipework, and welds.

Once commissioned, these key components will work together to treat effluent from legacy buildings across the site including the First-Generation Magnox Storage Pond and Magnox Swarf Storage Silo.

This will help us to maintain our high standards of environmental safety and operational efficiency for many years to come.

Further reading

SIXEP at 40: the unsung environmental hero at Sellafield – blog post (https://nda.blog.gov.uk/sixep-at-40-the-unsung-environmental-hero-at-sellafield/)

SIXEP case study (https://www.gov.uk/government/case-studies/the-sellafield-site-ion-exchange-effluent-plant-sixep)

SIXEP Continuity Plant case study (https://www.gov.uk/government/case-studies/sixep-continuity-plant)