Operation Mosaic: How a secret 70yr-old British nuclear test sent radioactive cloud across Australian mainland

It has been 70 years since Britain conducted nuclear tests on the Montebello Islands off the north-west coast of Western Australia.

The small archipelago of 174 small islands is located 1400 km northwest of Perth, 120 km from Dampier and at the northern tip of Western Australia’s iconic Ningaloo Reef.

Between 1952 and 1956 the site was the subject of three nuclear tests carried out by the British Government.

One of the last was so large that it was destroyed by World War II. There are disputed reports that it was 81 kilotons larger than the bomb dropped on Hiroshima in World War II.

The 1980s Royal Commission into Britain’s nuclear testing in Australia revealed the true scale of the tests and the secrecy surrounding the radioactive fallout from the explosion, which was recorded as far away as Queensland.

But to date, there is still much unknown about the impact of the testing on the environment, public health and Aboriginal communities living on the nearby mainland.

Seventy years ago, the United States and the Soviet Union were in the grip of the Cold War.

The USSR had conducted its first nuclear test, and the US and UK were eager to keep up with the arms race.

America stopped sharing information about its nuclear technology with Britain after World War II; This was a major blow to the UK, and the UK subsequently developed its own nuclear program to accommodate this.

When the United Kingdom detonated its first atomic bomb on the Western Australian islands as part of Operation Hurricane on 3 October 1952, the Yanks were already one step ahead.

Four weeks later, the United States detonated the hydrogen bomb.

This situation made England try to launch a new program to keep up with technology.

A series of operations were carried out at Emu Field and Maralinga in South Australia, but the fields were unsuitable and the British decided to return to the Montebello Islands.

Operation Mosaic began in June 1955, when an agreement was reached between the then British and Australian prime ministers Sir Anthony Eden and Robert Menzies.

Menzies agreed that thermonuclear tests could be carried out at Montebello, provided the bombs did not exceed 60 kilotons of TNT and did not pose a danger to people or animals.

This was a British-led military operation that resulted in Australia establishing the first Atomic Weapons Testing Safety Committee to monitor the operation on behalf of the government.

He had the authority to veto the opening fire if any security criteria were not met.

However, the tests were rushed so that they could be carried out under suitable meteorological conditions.

Royal Australian Navy ships HMAS Warrego, Karangi, Fremantle and Junee assisted the operation with anchoring, survey, marking, shipping and logistical support.

The Royal Australian Air Force supported Mosaic with transport, signals, air traffic control, security patrols and cloud sampling operations.

The first test carried out at Mosaic was known as G1, a turret-mounted bomb, and was detonated at around 11.50am on 16 May 1956.

The explosion caused an energy of 15 kilotons and a cloud that rose to 21,000 feet, much higher than the estimated altitude of 7,000 feet.

At 6 a.m. the next day, radioactivity was detected on the mainland from Onslow to Broome, approximately 1200 kilometers north of the Montebello Islands.

Radioactivity continued for the next 24 hours but was “well below” the public safety level set by the Committee of Safety.

Mosaic G2 was also a tower-mounted bomb and was detonated on Alpha Island at around 10.14am on 16 June 1956 and was much larger, producing a yield of around 60 kilotons, making it the largest device detonated in Australia.

However, the mystery regarding the actual size of the bomb remains; some estimates suggest the bomb may have measured up to 98 or 120 kilotons.

Top secret British Government documents relating to the G2 explosion remain classified to this day.

Evidence given to the Royal Commission in the 1980s revealed that the cloud rose 47,000 feet, 10,000 feet higher than predicted; it also covered parts of the mainland as far as Broome, approximately 1400 km north of the test site.

Low levels of radioactivity were detected on the mainland, but according to the Safety Committee this did not pose any danger to people, animals or property.

However, the Commission later found that allegations had emerged that the main cloud had “passed over the mainland” and that “something had gone radically wrong with the testing”.

“The Mosaic explosion that really caused controversy was the G2, a much larger weapon,” Commissioner Jim McClelland said in his report.

“The fallout on the mainland after G2 was again greater than expected and occurred in places where there was no fallout

was predicted.”

The Royal Commission heard mainland fallout was detected on the undercarriage of a plane flying from Onslow to Pearce, near Perth.

A ground survey using a Geiger counter found pieces of radioactive contamination the size of a tennis court in the back of a Holden van near Onslow.

Fallout from the explosion was detected in Port Headland and Derby 24 hours after the bomb detonated, with particles rising from the top of the cloud’s body and from the base of the cloud up to 30,000 feet.

Marble Bar miner Stewart Stubbs testified before the commission that he measured the radioactive rain with Geiger counters, which put them off scale.

Sheep in Northern and Central Australia also recorded higher levels of iodine-131 in their thyroids after testing.

Radioactive rain was also detected on the Queensland coast.

The Safety Committee was criticized by the commissioner for failing to recognize the presence of Aboriginal communities on the mainland near the Montebello Islands and their extra vulnerability to the impact of nuclear fallout.

“It was a gross oversight that the issue of acceptable dose levels for Aboriginal people was recognized as an issue at Maralinga but was ignored in setting the efficacy criteria for the Mosaic tests,” Judge McClelland said.

It found that the theoretical predictions for both Mosaic tests were incorrect and that the Security Committee had acted irresponsibly and grossly misled the Minister of Supply about the security of the mainland.

West Australian author Paul Grace investigated nuclear testing on the Montebello Islands and wrote a book about Operation Hurricane.

His grandfather was an RAAF pilot who was ordered to fly around looking for fallout from the cloud after the first bomb exploded in 1952.

He said one of the most controversial aspects of the series to date has been the debate around the exact size of the Mosaic G2.

“Initially they said it was 60 kilotons, and even at 60 kilotons, this is by far the largest in Australian history and much larger, probably about four times larger than Hiroshima,” he said.

“During the Royal Commission, Australian lawyers revealed the official report by two British scientists who worked on the test, which actually measured the test at 98 kilotonnes, which is obviously much larger again.”

Mr Grace said this was put to William Penney, the chief operating scientist at the Royal Commission, but the equipment measuring the reading was unreliable and they used more reliable methods.

“The royal commissioners believed him and were happy to do so as long as they made no reference to the 98 kiloton report in their report,” he said.

Researcher Liz Tynan is about to publish her third book on the subject, her new book focusing solely on Operation Mosaic, titled Nuclear Archipelago.

His research found that the British had turned away from fission weapons, which relied on splitting the atom, in favor of hydrogen bombs by 1956, and that although Mosaic was not a hydrogen weapon, it used technology known as enhanced fission.

“What they wanted was a really powerful weapon that had similar power to a hydrogen weapon, but used slightly different technology,” he said.

“They were using some form of lithium, adding it to a fission weapon, and the idea was that it would greatly increase the explosive output.”

Ms Tynan said GI had used a lead tamper, or lead coating, around the explosive core, but she believed G2 had used a uranium tamper but could not access all British data on it.

“There are some things they still keep top secret, and this is one of them, but it appears that the uranium tampering increased the explosive yield of the weapon, and then the question is: How much did it increase the weapon?” he said.

“That’s where we still have a bit of a debate.”