## Facts

- Ireland and the United Kingdom are States Parties to the United Nations Convention on the Law of the Sea (UNCLOS).
- The dispute concerned the MOX (mixed oxide) fuel fabrication plant at Sellafield (United Kingdom), the international movement of radioactive materials associated with it, and alleged risks of radioactive pollution to the marine environment of the Irish Sea (a semi-enclosed sea).
- On 25 October 2001, Ireland instituted arbitral proceedings against the United Kingdom under UNCLOS Part XV and Annex VII by filing a notification and statement of claim in “the dispute concerning the MOX plant, international movements of radioactive materials, and the protection of the marine environment of the Irish Sea.”
- On 25 October 2001, Ireland also sought provisional measures from the United Kingdom pending constitution of the Annex VII arbitral tribunal.
- On 9 November 2001, before the Annex VII tribunal was constituted, Ireland submitted to the International Tribunal for the Law of the Sea (ITLOS) a request for provisional measures under UNCLOS article 290(5).
- ITLOS fixed hearings for 19–20 November 2001; both parties appeared, and Ireland appointed a judge ad hoc (Mr. Alberto Székely).
- Ireland sought measures that would, in substance, suspend the authorisation/commissioning/operation of the MOX plant and restrict movements of nuclear material, together with disclosure of information and monitoring data about environmental risks.
- The United Kingdom objected that the conditions for provisional measures were not met, questioned jurisdiction at this stage, and argued that suspending a domestically authorised facility was not justified on the record and would go beyond what provisional measures should do.
- ITLOS issued its Order on 3 December 2001, acting only on provisional measures pending constitution and action of the Annex VII arbitral tribunal.

## Issues

1. Whether ITLOS had authority under UNCLOS article 290(5) to prescribe provisional measures pending constitution of an Annex VII arbitral tribunal.
2. Whether, for purposes of article 290(5), the future Annex VII tribunal would prima facie have jurisdiction over Ireland’s claims under UNCLOS.
3. Whether the circumstances showed urgency and a risk of irreparable prejudice to rights claimed under UNCLOS or of serious harm to the marine environment before the Annex VII tribunal could act.
4. Whether provisional measures should include suspension of the MOX plant’s commissioning/operation and related transports, or whether more limited measures were appropriate without prejudging the merits.
5. Whether the parties should be ordered to refrain from conduct that could aggravate or extend the dispute or make its resolution more difficult.

## Decision

- ITLOS held that it could act under UNCLOS article 290(5) because the dispute had been submitted to Annex VII arbitration and the arbitral tribunal had not yet been constituted.
- ITLOS found that, on a prima facie basis, the Annex VII arbitral tribunal would have jurisdiction over the dispute.
- ITLOS determined that provisional measures were warranted to protect rights at issue under UNCLOS and to address risks to the marine environment pending the arbitral tribunal’s decision.
- ITLOS did not order suspension of the authorisation, commissioning, or operation of the MOX plant, and did not grant the shutdown-type relief requested by Ireland.
- Instead, ITLOS ordered Ireland and the United Kingdom to cooperate and to consult, including by exchanging information relevant to risks of pollution of the marine environment connected with the MOX plant and associated movements of radioactive materials, and by monitoring those risks.
- ITLOS ordered both parties to ensure that no action was taken that might aggravate or extend the dispute or make it more difficult to resolve.
- The measures were prescribed to apply pending a decision by the Annex VII arbitral tribunal.

## Legal Principles

- Under UNCLOS article 290(5), ITLOS may prescribe provisional measures pending constitution of an Annex VII tribunal if ITLOS considers prima facie that the tribunal to be constituted would have jurisdiction.
- At the provisional-measures stage, ITLOS does not decide the merits; it assesses only prima facie jurisdiction and whether the circumstances call for temporary measures.
- Provisional measures must be appropriate to preserve the rights of the parties in dispute and may be directed to preventing serious harm to the marine environment.
- Urgency requires a real possibility of irreparable prejudice to the rights claimed or serious harm occurring before the tribunal deciding the merits can act.
- In disputes concerning marine environmental protection, procedural duties under UNCLOS—such as cooperation, consultation, and exchange of information—are capable of protection through provisional measures.
- Provisional measures should be framed so as not to prejudge disputed questions of fact or law; measures with effects comparable to final relief (such as ordering a plant shutdown) require strong justification at the provisional stage.
- A non-aggravation obligation is a standard provisional measure: parties must refrain from acts that could aggravate or extend the dispute or hinder its resolution.

## Conclusion

In MOX Plant (Ireland v. United Kingdom), ITLOS (Case No. 10) ruled under UNCLOS article 290(5) that the future Annex VII arbitral tribunal would prima facie have jurisdiction and that interim measures were needed given the alleged risk of marine pollution, but it refused to order suspension of the MOX plant; instead, it required the parties to cooperate, consult, exchange information and monitoring data, and avoid actions that could aggravate or extend the dispute, pending the Annex VII tribunal’s decision.


MOX Plant (Ir. v. U.K.), Provisional Measures, Order of 3 December 2001, ITLOS Rep. 2001, at 95.