Senior Principal Mechanical Engineer

Assystem; West of England; Hybrid; GBP 90,000 - 120,000; Full time

Benefits offered by this job

Hybrid work

Holiday bonuses

Ongoing training

Internal promotion

Job summary

Assystem is seeking a Senior Principal Mechanical Engineer to lead design across fusion and modular nuclear projects in the UK.

You will guide multidisciplinary teams, conduct risk analyses, and develop technical specifications for cooling water systems and related interfaces. The role requires SC clearance eligibility and willingness to travel.

Qualifications

• Proven ability to lead cross‑disciplinary design teams within regulated nuclear projects.
• Experience with safety‑critical design processes and risk assessment methods.
• Strong communication and stakeholder management skills.
• Familiarity with regulatory standards and project lifecycle.
• Ability to travel to UK offices and customer sites as needed.
• Professional accreditation and active SC clearance eligibility.

Responsibilities

• Lead design development for personnel airlocks, main equipment access hatch, fuel transfer channel, piping penetrations, electrical penetrations and specialist doors.
• Perform DFMEA to identify failures early and drive mitigations in design.
• Conduct HAZID to document risks across the system.
• Maintain Human Factors checklists ensuring ergonomics in design decisions.
• Update and manage the project’s Risk Register and mitigation actions.
• Produce design packages aligned with project specs, standards and operations.
• Coordinate interfaces between engineering teams, suppliers and stakeholders.
• Create technical specifications for procurement, fabrication and installation.
• Collaborate with cross‑functional teams including internal staff and secondees.

Skills

Multidisciplinary team leadership

DFMEA (Design Failure Mode and Effects

HAZID (Hazard Identification)

Human factors

Risk register management

Interface management

Technical specifications

Systems engineering

Subcontractor management

Security clearance eligibility

Education

Chartered Engineer (CEng)

Mechanical Engineering degree (BEng/MEng)

Regulatory knowledge (ONR/EA/HSE)

Job description

Today, Assystem is among the top three independent nuclear engineering firms worldwide. With over 60 years of experience in highly regulated sectors, the group supports public and industrial stakeholders in the execution of complex and strategic infrastructure projects, subject to high safety and security requirements.

Assystem mobilizes 8,000 experts in 13 countries and intervenes across the entire project lifecycle, in engineering, project management and digital solutions.

Today, Assystem is among the top three independent nuclear engineering firms worldwide. With over 60 years of experience in highly regulated sectors, the group supports public and industrial stakeholders in the execution of complex and strategic infrastructure projects, subject to high safety and security requirements.

Assystem mobilizes 8,000 experts in 13 countries and intervenes across the entire project lifecycle, in engineering, project management and digital solutions.

Job Description

Join the Fusion and Modular Nuclear Business Unit! Assystem has an exciting opportunity for a Senior Principal MechanicalEngineer to join our expanding Fusion and Modular Nuclear (FMN) business unit. As leaders in the design and development of new technologies, you will have the opportunity to work across multiple cutting‑edge projectsdeveloping technical solutions making net‑zero power generation a reality.

Your mission:

• Lead and support the development of designs for the following systems:
• Personnel Airlocks, Main Equipment Access Hatch, Fuel Transfer Channel, Piping Penetrations, Electrical Penetrations, Specialist Nuclear Doors
• Conduct DFMEA (Design Failure Mode and Effects Analysis) to identify and mitigate potential failure modes early in the design process.
• Perform HAZID (Hazard Identification Study) to assess and document risks across the system.
• Complete and maintain Human Factors Checklists to ensure ergonomics and usability are considered in design decisions.
• Update and manage the project’s Risk Register to track, evaluate, and mitigate emerging risks.
• Design Package Production in Line with Requirements – Oversee the creation of comprehensive design packages, ensuring alignment with project specifications, technical standards, and operational needs.
• Management of Interfaces – Coordinate effectively between engineering teams, suppliers, and stakeholders to ensure seamless integration of cooling water systems with other reactor components.
• Preparation of Technical Specifications – Develop clear, detailed technical specifications to guide procurement, fabrication, and installation of key cooling water system components.
• Collaborate with cross‑functional teams, including both new and existing FTEs and secondees across engineering and project functions.

Why Join the Community of Switchers?

• Over 55 years in nuclear engineering.
• Ranked among the top 3 largest nuclear engineering companies.
• Attractive social benefits, including an engaging Employee Committee and holiday bonuses.
• Ongoing training with numerous career opportunities.
• 70% of our managers are promoted internally!
• Hybrid work: 1-2 days remote per week.

Qualifications

• Multi-Disciplinary Team Leadership – Skilled in managing and coordinating diverse engineering teams.
• Steam Cycle Modelling Oversight – Ability to define modelling outcomes and ensure accurate implementation.
• Project Budget & Lifecycle Planning – Experienced in budgeting and planning for full design lifecycles.
• Cooling Water Equipment Expertise – Strong track record in designing and optimizing cooling water systems.
• Regulatory & Compliance Knowledge – Well‑versed in ONR, Environment Agency, and HSE regulations.
• Requirements Capture Experience – Skilled in gathering and defining technical requirements.
• Systems Engineering Awareness – Strong understanding of integrated design principles.
• Subcontractor Management – Experienced in overseeing and coordinating subcontractors.
• Chartered Engineer (CEng) – Professional accreditation in a relevant discipline.
• Security Clearance Eligibility – Capable of obtaining SC clearance for secure projects.

Additional Information

• Excellent communication skills, written and oral
• Strong leadership skills
• Focus on delivering quality output to time and budget
• Act with integrity and instils this in others
• Has an appreciation of wider technical issues beyond own discipline – Understands interdisciplinary challenges and their impact on projects.
• Ability to work with a high degree of independence, using their own initiative and self-motivation – Proactive and capable of managing tasks autonomously.
• Confident in a multi-party, multi-national environment – Comfortable working with global teams and diverse stakeholders.
• Ability to travel to Assystem offices and customer sites within the UK
• To undertake work packages supporting major UK and international Nuclear Clients – Contributes to high-profile nuclear projects with global impact.

Due to the nature of work to be undertaken applicants will be required to meet certain residency criteria in order to attain a minimum level of UK security clearance if not already security cleared to a minimum SC level.

We are committed to equal treatment of candidates and promote, as well as foster all forms of diversity within our company. We believe that bringing together people with different backgrounds and perspectives is essential for creating innovative and impactful solutions. Skills, talent, and our people’s ability to dare are the only things that matter! Bring your unique contributions and help us shape the future.