Monday, March 19, 2007

Mobile Nuclear Power Stations?

I came across an interesting notion, which is one of "small" mobile nuclear reactors that might be used in stand-alone applications. The US are keen on the idea because such a "power plant" might meet the needs of developing nations without the risk that the by-products (e.g. plutonium) could be used to make weapons - hence the device is intended to be kept sealed, and delivered to a site where it would provide electricity for around 30 years, by which time its fuel would be spent and it could then be removed for replacement or refuelling. It's developers claim that it would be impossible to remove fissile material from the reactor core which would be maintained within a "tamper-proof" cask, protected by a "thicket" of security alarms. The device is known as the Small, Sealed, Transportable, Autonomous Reactor (SSTAR), and would generate power without requiring either refuelling or maintenance. In contrast, conventional nuclear reactors are attended with the potential threat of proliferation because they must be charged periodically with new fuel, which later has to be removed for replenishment: both steps allow an opportunity for fissile material to be diverted to weapons programmes, as is believed to have happened in North Korea and in Iran.

Conventional nuclear power stations produce typically around one Gigawatt of electricity (from around 3 GW of thermal power), which is far more than is necessary for a power plant somewhere in a developing country where there is no extensive national electricity grid to distribute it. In the SSTAR, the nuclear fuel, along with a lead coolant and a steam generator will be sealed within the housing, along with steam-pipes that can be connected to an external steam-turbine for producing electricity. An apparatus to produce 100 Megawatts would be 15 metres high, three metres in diameter and weigh 500 tonnes - O.K., I use the word "Mobile" loosely, but such an entity could be moved! A 10 MW generator would weigh-in more modestly at under 200 tonnes. It is intended that the US will take charge of the delivery and installation of the SSTAR, moving it by ship and by truck to its final intended location. When the fuel ultimately is exhausted, the "old" reactor would be taken-away by the supplier, intended for recycling or disposal. The DoE (Department of Energy) hopes to have a prototype operating by 2015.

There are undoubtedly technical challenges to be overcome before this can become a reality. In conventional reactors, the fission chain-reaction depletes the uranium-235 fuel in the fuel-rods, and so they need to be replaced every few years. For a reactor to run unmolested for 30 years, as is planned for the SSTAR, the sealed reactor would need to be of the fast-breeder type, which uses some of the fast-neutrons to "breed" fresh fuel from uranium-238 into plutonium-239. To extend the lifetime of the reactor, the cylindrical reactor core will be fabricated so that fission is sustained only when it is surrounded by a cylindrical metal mirror that reflects neutrons back into the fuel. Over the course of the operating time for the device, the mirror will move slowly from one end of the core to the other, consuming fuel as it goes, but the engineering physics involved to render the process reliable over such a long time and with the components continually exposed to high doses of radiation will be formidable.

Automated controls will monitor continuously the status of the covert reactor, adjusting its electrical output as necessary, according to Craig Smith of the DoE funded Lawrence Livermore National Laboratory in California. Should "tampering" be detected (somebody taking a hammer and chisel to it?), the automated mechanism would shut down the reactor as it would if malfunctioning is detected. Alerts will be transmitted over secure satellite radio channels to the DoE or to an unspecified international agency placed in charge of overseeing the operation of the SSTARs. It does begin to smack of "Big Brother", though, and would any country really be prepared to hand-over control of its electricity provision to the US? Imagine if the reactor could be shut-down from remote, via the same secure satellite channels?! It is debatable too that every country would care what the US thought of them, and hack the thing open regardless of how loud the alarm was sounding ("ear-plugs" in!), thus getting hold of the plutonium-rich fuel for redirection to a dirty bomb or an actual nuclear device.

I would have more confidence if the SSTARs were to be fuelled using thorium, for the reasons I outlined in my posting last December - "Thorium gets good press over uranium" - which poses far less of a threat in terms of weapons proliferation; although the design might need to be more complex, so as not to over-expose the protactinium-233 to neutrons, converting it to protactinium-234, before it has the chance to decay to uranium-233, which is the desired "breeder-fuel". However, it might be possible to use thorium in a small scale reactor of the liquid fluoride kind, which might be incorporated in such a device rather than the massive accelerator-driven thorium reactors which could not. I think thorium might prove altogether a more convenient fuel than uranium for SSTAR and similar types of small-scale nuclear reactors.