Monday, 27 October 2014

Simple Molten Salt Reactor, by Moltex LLP

Moltex LLP is a small UK engineering design company based in London. On 20 Oct Ian Scott of Moltex presented his SMSR, lasting 15 minutes, at a House of Lords meeting.

Ian was influenced by the very first Molten salt design from 1950, which placed molten fuel inside narrow cylinders. Ian's design has several such cylinders full of fuel inside a tank of coolant. Both coolant and fuel are molten salts. The fuel circulates in these cylinders by convection, as does the coolant in the tank. A 1 GWe reactor will have a tank about 8 metres in diameter. There are no pumps moving molten salts - circulation is all done by convection. The tank will be a nickel alloy, probably Hastelloy. No moderator either, so it's a fast reactor. Ian reckons the reactor will last many decades.

Stated advantages of the SMSR

  • unpressurized
  • the reaction is barely critical
  • no volatile fissile materials will be left in the reactor (gases will bubble out)
  • safe coolant
  • no pumps
  • materials are all standard industrial parts
  • cheap
  • fuel will be made from spent nuclear fuel, SNF, extracted by a "simple single-stage process".

Potential Issues:

  • The primary coolant is sodium chloride. Natural chlorine is a mixture of isotopes: mainly Cl-35, Cl-37. Cl-36 is present as a trace, and is radioactive, half-life = 300k years, undergoing mainly beta decay to Ar-36, S-36. The thermal neutron cross-section of chlorine-35 = 35.5 σa/barns [can't find the fast version, but the thermal spectrum is worryingly high]. It looks like quite a lot of neutrons may be lost to chlorine-35 absorption, producing chlorine-36 which is radioactive. Ian does not believe enough neutrons will be lost to make the reactor too inefficient, but the coolant will become radioactive. A way around this is to use isotopically separated chlorine-37 in the coolant salt. Using chlorine-37 alone, will prevent chlorine-36 forming and Cl-37 is stable against neutron bombardment. Several quite inexpensive routes are available for the separation of Cl-35 / Cl-37. The cost estimate has been done, still giving a very viable project.
  • The fuel tubes are a consumable item with an anticipated 5 year life, functioning in a similar manner to fuel rods and needing periodic replacement.

A thorium breeder?

Ian believes that a converter makes economic sense now. A breeder will have to wait till the future:

There would then be an economic case for developing a nuclear breeder version of the reactor (this exists now in outline), which would operate on the thorium fuel cycle. That outline design is far simpler, safer and cheaper than current designs for sodium cooled fast breeder reactors.
- [Moltex Energy LLP – Written evidence, section 29]

The best introduction to the SMSR may be references: 6, 4 [translated via Google], 2, 1, in that order. Refs. 2, 1, 3 contain all the detail.

  1. Slides
  2. Evidence to House of Lords, pages: "Moltex Energy LLP – Written evidence"
  3. Patent Application WO-2014128457-A1
  4. Blog on Moltex (in French)
  5. Moltex LLP
  6. Next Big Future - UK MSRs

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