IS-20 revision 1: new CSN requirements for spent nuclear fuel containers

Key data

Holders of nuclear installations with temporary spent fuel storage systems have an immediate obligation: to verify whether their containers and management systems comply with the new technical and safety requirements set by the Nuclear Safety Council (CSN). Instruction IS-20, revision 1, of February 23, 2026, updates the previous regulatory framework by adapting it to the most recent international standards in nuclear safety.

This revision is not a minor change in form: it requires operators to actively evaluate whether their installations and container designs remain compliant, and to take action if they do not. The regulation has direct implications for both operating plants and installations undergoing decommissioning.

What does this regulation establish?

Instruction IS-20 revision 1 sets the technical and safety requirements applicable to containers used for temporary storage of spent nuclear fuel. Its scope covers both already operational systems and those planned to be put into operation.

The central elements regulated by this instruction are:

• Technical requirements applicable to the design of temporary storage containers for irradiated fuel.
• Safety requirements that must be met by management systems associated with such containers.
• Adaptation of Spanish regulations to the most recent international standards in nuclear safety.
• Application to nuclear installations that operate or plan to operate dry storage systems for irradiated fuel.
• Implications for the management of the nuclear fuel cycle and the closure of plants undergoing decommissioning.

This instruction replaces and updates the previous version of IS-20, which means that operators already adapted to the previous version must verify whether the changes introduced in this revision require additional modifications to their installations or documentation.

Economic and operational impact

The impact of this instruction materializes in two distinct areas:

Operational level: Operators must conduct a technical review of their dry storage systems. If current container designs do not conform to the new requirements, adaptations may be necessary that could involve engineering modifications, updating of technical documentation, and review of maintenance and monitoring programs.

Regulatory and sanctioning level: Non-compliance with the requirements set by IS-20 revision 1 may result in two direct consequences according to the instruction itself:

• Initiation of sanctioning procedures by the CSN.
• Suspension of storage authorizations, which would halt critical spent fuel management operations.

For installations undergoing decommissioning, compliance with this instruction is especially critical, as temporary storage of irradiated fuel is an unavoidable phase in the orderly closure of a nuclear plant.

Who does it affect?

• Holders of nuclear installations that operate dry storage systems for irradiated fuel.
• Operators of nuclear installations that plan to put temporary spent fuel storage systems into operation.
• Nuclear installations undergoing decommissioning that manage irradiated fuel pending transfer or final disposal.
• Technical and regulatory compliance teams of companies holding nuclear licenses in Spain, responsible for verifying compliance with CSN requirements.
• Suppliers and manufacturers of nuclear fuel storage containers that supply Spanish installations, insofar as their designs must conform to the new requirements.

Practical example

A Spanish nuclear plant operating an individualized temporary storage facility (ATI) with dry storage containers for irradiated fuel must, as of February 23, 2026, verify that the design of those containers and associated management procedures comply with the updated requirements of IS-20 revision 1.

If during this review it is detected that some aspect of the design or management systems does not conform to the new international standards incorporated by this revision, the installation must document the deviations, propose an adaptation plan, and communicate it to the CSN. Failure to do so exposes the installation to a sanctioning procedure or, in the worst case scenario, to suspension of the storage authorization, which would block spent fuel management and directly affect the operation or decommissioning process of the plant.

What should companies do now?

1. Review the container inventory: Identify all temporary storage containers for irradiated fuel in operation or in planning and verify whether their designs are documented in accordance with the requirements of IS-20 revision 1.
2. Evaluate technical compliance: Compare the technical and safety requirements of the new instruction with the current specifications of the containers and associated management systems. Identify possible deviations.
3. Update management systems: If deviations are identified, initiate the design modification or management procedure processes necessary to achieve compliance.
4. Document and communicate with the CSN: Maintain an updated record of adaptation actions undertaken and, if applicable, communicate to the CSN the compliance status or adaptation plans.
5. Review the impact on the decommissioning plan: For installations undergoing closure, evaluate how the new requirements affect the schedule and procedures for managing irradiated fuel.
6. Consult with the technical-legal team: Given that non-compliance may result in sanctions or suspension of authorizations, it is recommended that the regulatory compliance area and technical team work jointly on the compliance assessment.

Official source