US President Joe Biden wants to lock in friendly ties between Japan and South Korea at a summit on Friday, but their readiness to shelve grievances will be tested when Tokyo begins pumping water from its wrecked Fukushima nuclear plant into the sea.

Japan already delayed the release to avoid stirring up political opposition in South Korea before President Yoon Suk Yeol joins Japanese Prime Minister Fumio Kishida for a meeting with Biden at the Camp David retreat on Friday, four officials in Japan and South Korea told Reuters.

The dumping of radioactive water may happen days after the summit, which the US is billing as a “historical” trilateral meeting that will deliver a “bold counter” to regional rival China. That puts less domestic political pressure on Yoon, said one of the officials, who asked not to be identified because of the sensitivity of the issue. Washington needs its Asian allies to work together because they see the military power balance in East Asia, including around Taiwan, shifting in China’s favour. Beijing is also deepening cooperation with Russia in the region, and their mutual ally North Korea is accelerating its missile programme, according to a Reuters report.

“The trilateral is a significant move in the geopolitical landscape like AUKUS was for the defence landscape,” US Ambassador to Japan Rahm Emanuel said at a briefing on Aug.11, referring to the 2021 defence pact between the United States, Australia and Britain.

Even if Fukushima fades as an issue, the risk of bad blood remains real. As relations soured in 2019, for example, Moon nearly scrapped a critical intelligence-sharing deal with Japan, reversing the decision at the last minute under US pressure. By accepting an International Atomic Energy Agency (IAEA) report last month that greenlit Japan’s Fukushima water release, Yoon could encourage fresh dissent that China will try to amplify, analysts say.

“There is certainly some possibility that Yoon will come under pressure over this, particularly if there is data that shows that the water is more dangerous than we otherwise thought,” said Christopher Johnstone, a former East Asia director of Biden’s National Security Council who is now with the Centre for Strategic and International Studies, Reuters adds.

Japan says it will remove most radioactive elements from the water except for tritium, a hydrogen isotope that must be diluted because it is difficult to filter. Seoul expects Tokyo to share information, allow access to its experts, and immediately halt any dangerous releases, a Yoon administration source said, adding that those steps will help facilitate security cooperation. A Japanese foreign ministry spokesperson said Tokyo would continue to work closely with Seoul. US Secretary of State Antony Blinken on Tuesday expressed satisfaction with Japan’s plans. Protests against the release have so far been muted, at least by South Korean standards, with just a few hundred people gathering in Seoul on Saturday to demand a halt. The political fallout for Yoon has also been limited; his approval rating has recovered after dipping when Seoul endorsed the IAEA findings. Nonetheless, a Gallup poll in late June showed that 78% of South Koreans worry about potential contamination of the ocean and seafood. Some analysts say China may see an opportunity to exert influence.

Beijing has banned seafood imports from 10 Japanese prefectures, including Fukushima, and is testing produce from elsewhere.

In July, Chinese foreign ministry spokesman Wang Wenbin said Japan had shown selfishness and arrogance, and had not fully consulted the international community about the water release. The IAEA has criticised what it says is a fabricated document a South Korean media site used in June to claim Tokyo had unduly influenced the organisation.

In July the state-backed Chinese newspaper Global Times, which has published stories questioning the safety of Japanese produce and casting doubt on the IAEA’s findings, picked up the report.