The British government has called Ireland's case to prevent the commissioning of the MOX plant next month "an ill-founded application" which is "part of a wider war against Sellafield".
Lord Goldsmith, the British Attorney General, said the British government would face "catastrophic losses" if Ireland's "scheme" for an injunction was granted.
"The tribunal is again confronted by a spectre of danger not anything approaching a real risk of serious harm to the Irish Sea," he said at the second day of a hearing at the International Tribunal for the Law of the Sea in Hamburg.
The tribunal, a court which applies the 1982 Convention on the Law of the Sea, has been asked by the Government to examine whether the operation of the MOX plant next month would infringe Ireland's rights under the convention.
If the tribunal agrees to grant "provisional measures" to halt the opening of the MOX plant, the case will go before a full tribunal.
Britain argued yesterday that the tribunal had no jurisdiction over the case, and that it should be dealt with by the European Court of Justice and the OSPAR tribunal, which rules on the OSPAR convention on maritime issues in the north-west Atlantic.
It also argued that the MOX plant, producing ceramic-coated plutonium fuel pellets, would increase safety at Sellafield by reducing shipments of toxic plutonium-oxide through the Irish Sea.
Any shipments are governed by international guidelines drafted by international bodies such as IAEA, of which Ireland is a member, said Lord Goldsmith.
Fuel pellets produced at MOX would first be shipped in October 2002, and in special flasks to reduce risks, he said.
"The risks of transporting highly-radioactive material ... are 'very small'," said Lord Goldsmith, quoting an IAEA report. He said precautions at Sellafield against terrorist attacks, which Ireland claims are inadequate, were in fact "amply robust to cope with any credible threat".
Mr Daniel Bethlehem said Ireland was seeking "some flexible notion of the law ... to make life difficult for those with whom they take issue, even if their claims have no substance".
In his rebuttal, Mr Eoin Fitzsimons, for Ireland, produced a report from a 1985 Select Committee of the House of Commons. It said that a quarter of a tonne of plutonium from "huge volumes of liquid waste from the Sellafield pipeline ... [had made] the Irish Sea the most radioactive sea in the world".
Mr Phillipe Sands, for Ireland, said the United Kingdom believed it "has the right to continue polluting the Irish Sea as it has done for 40 years".
"No account can be taken of Ireland's interests once the MOX plant begins to spew out its radioactive pollution after December 20th," he said.
Mr Sands asked the tribunal to recognise that Ireland had rights under the 1982 convention, and that these rights would be prejudiced by the opening of the MOX plant.
The 21-judge tribunal will return its judgment on December 3rd.
