News of the accident became public outside the Soviet Union on 28th April, more than 48 hours after the accident early on the 26th. The Soviet Council of Ministers issued a brief and vague acknowledgement of the accident via the state-run TASS news agency, after monitoring sites in Sweden and other neighbouring Scandinavian countries detected significantly elevated levels of atmospheric radiation.
The first news of the accident in the New York Times (usually a good bellwether for chronology of public knowledge) is in the 29th April 1986 edition, reporting that statement:
"An accident has occurred at the Chernobyl nuclear power plan as one of the reactors was damaged. Measures are being taken to eliminate the consequences of the accident. Aid is being given to those affected. A Government commission has been set up."
US intelligence only became aware of a brewing incident on the 28th, around the same time that Sweden was realising that something was afoot.
Here's a series of partially-declassified memos and correspondence from the CIA's Deputy Director of Scientific and Weapons Research and the Deputy Director of Intelligence, prepared for the Director of Central Intelligence in late July 1986, apparently as part of a collective realisation that the Agency had been caught rather flat-footed.
In one memo, the Deputy Director of S&WR writes:
- Event Reporting: Our first indication of the disaster came [REDACTED] on Monday, 28 April, which indicated a possible leak of radioactive material at a Swedish facility. Later Swedish authorities determined that the radioactivity was coming from the east. Based on these reports, the Air Force was asked to use weather data to attempt to determine the source of the radioactivity. Spurred by the Swedish announcement and international concern the Soviets announced that there had been an accident at the Chernobyl reactor. This announcement was monitored by [the CIA's Foreign Broadcast Information Service] at about 1200 EDT on 28 April and promptly reported. This was the first report giving the location of the event.
(The redaction in the first line there is only a few words, but is likely obscuring discussion of 'sources and methods' of intelligence collection, which intelligence agencies almost invariably withhold from freedom of information or archival releases.)
The Reagan administration confirmed to reporters in early May that they'd been caught off guard; once alerted, the intelligence community quickly adopted a more proactive approach, with the National Security Agency, National Reconnaissance Office, National Geospatial-Intelligence Agency and the CIA's National Photographic Interpretation Centre all devoting significant resources into monitoring the situation on the ground.
This is from Jeffrey Richelson's The Wizards of Langley: Inside the CIA's Directorate of Science and Technology:
Once alerted to the disaster, the intelligence community responded by turning its full set of resources on the Kiev area. A VORTEX signals intelligence satellite sucked up all military and relevant civilian communications within several hundred miles of Chernobyl. Due to launch failures in August 1985 and earlier in April, the U.S. space imagery capability consisted of a lone KH-11 [the 'KEY HOLE' surveillance satellite], 5506. It was reprogrammed to obtain photography of the nuclear reactor at the first opportunity. Its last visit had been almost two weeks before, and the first chance for 5506 to provide imagery came on Monday afternoon [29th April].
Definitely worth having a look at the CIA's FOIA releases relating to Chernobyl — some interesting stuff in there. The NARA records search isn't working for me right now, but that would also be a good place to look for contemporaneous US government documents and images relating to the disaster. Obviously, given the relative recency, a lot of information from that period has yet to be declassified; we don't, for example, have access to the President's Daily Brief beyond 1977 — I bet the PDBs for late April/early May 1986 would make for interesting reading...