Michael Fallon has rejected a call for "greater transparency" from the government over the UK's nuclear deterrent, as he faced a grilling from MPs in the House of Commons on 23 January.

The evasive defence secretary also dodged numerous questions relating to an alleged cover-up of a failed Trident missile test in June.

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"There are very few things that we cannot discuss openly in parliament, but the security of our nuclear deterrent is certainly one of them," Fallon said.

But the top Conservative did reveal that the submarine involved in the operation returned to Trident's operation cycle and confirmed that Theresa May knew about the test.

"The details of the demonstration and shakedown operation I'm not going to discuss publicly on the floor of this House," Fallon told MPs.

"All I can do is repeat that HMS Vengeance has successfully been certified again to re-join the operation cycle.

"I've made it very clear that the previous prime minister and this prime minister were of course informed about the maintenance of the nuclear deterrent and the outcome of the test and the successful return of HMS Vengeance to the operational cycle."

The statement, in response to an urgent question from Labour's shadow defence minister Kevan Jones, comes after The Sunday Times claimed the unarmed missile veered off towards the Florida mainland.

Jones repeatedly refused to answer questions about the missile, while CNN News claimed the missile was diverted into the sea as part of a self-destruct mechanism.

The incident, under David Cameron's watch as prime minister, happened just weeks before a House of Commons vote on the renewal of the nuclear weapons system.

What is Trident?

Trident operates a continuous at-sea nuclear deterrent. That means one of the programme's four nuclear submarines, which are based in the Faslane area of Scotland and operated by the Royal Navy, is always on patrol.

These Vanguard-class submarines are around 491ft in length, or over twice the size of two Boeing 747s, and powered by steam. A nuclear reactor inside the underwater vessels boils sea water, the steam from which is then used to propel them.

The four submarines – HMS Vanguard, HMS Vengeance, HMS Victorious and HMS Vigilant – are capable of carrying 16 Trident II D5 ballistic missiles, produced by the American arms manufacturer Lockheed Martin, each armed with up to eight nuclear warheads. As it stands, each submarine only carries three Trident missiles.

Their power and precision is stunning. Each missile is 44ft long, 83 inches in diameter, capable of exceeding speeds of over 13,000mph, and can hit targets up to 7,000 miles away, accurate to within a few feet.

The payload – or, just how powerful the destructive force of each warhead is – isn't known publicly. But, according to a parliamentary research paper, it is thought to be around 100 kilotons.

To put that in context, the atomic bomb Little Boy, dropped on the Japanese city of Hiroshima during the Second World War, had an energy yield of around 15 kilotons, killing as many as 80,000 people instantly. Therefore, a single warhead in a Trident II D5 missile has the destructive power equivalent to over six Hiroshimas at once.

The current fleet of submarines will come to the end of their working life in the 2030s. Because of the length of time it takes to build a new fleet, the issue of renewal is timely now and a decision must be made in 2016. The Trident missiles have already had their lives extended until the 2040s, when they will expire and be decommissioned.

But the cost of doing this – renewing and upgrading the nuclear deterrent infrastructure – is enormous. Estimates put the total cost, when the new submarine fleet comes into action in the 2030s, at £100bn ($147bn, €138bn). In 2013/14 alone, the NHS England budget was £95.6bn.