AN ASSESSMENT OF A 2500 MWe MOLTEN CHLORIDE SALT FAST REACTOR
Part I. Homogeneous and Quasi-homogeneous Reactors Section 111. Reactors Fueled with Molten-salt Solutions
EDITED BY :
DR R.C. ASHER, AERE
DR. G. LONG, AERE
DR. H.A.C. MCKAY, AERE
DR. D.L. REED
Technical Assessments and Studies Division
As a result of some initial studies in 1964 and 1965, it was, concluded that the line of investigation of most interest to the UK on molten salt reactors and which would also be complementary to the US investigations would be on a fast reactor version . A preliminary study of a fast system using the U233/Th cycle and fluoride salts did not indicate encouraging results and it was therefore decided that a Pu/U238 cycle would be examined . This involved the use of chloride salts and work on salt chemistry began in 1965 which in 1970 was extended to include additional materials aspects . The assessment and fluid flow study work was carried out mainly in 1971 and 1972 . This report presents an overall summary . It is in the nature of a survey report as the limited effort available has meant that the depth to which questions could be investigated has been restricted .
The initial question which is bound to be asked is why consider a fluid fuel, with all the implications of a highly active circuit If it is considered that the ultimate refinement of the solid fuel system will appear as some form of fast reactor, then to make further progress beyond this, some basic change in concept has to be made, and it seems clear that the most fundamental would be to escape from the trammels of fuel fabrication with fine tolerances, expensive active transport and also from central processing if suitable on-line methods can be developed to treat the fuel directly .
A basic safety feature, in principle, is that a fluid fuel can be disposed of from the reactor to ever-safe containers, while good negative temperature coefficients should limit excursions and potentially offer a self-regulating system . The current feasibility studies have sought to demonstrate how far these potentialities might be realised and to give a preliminary appraisal of the economic and performance prospects. The table below summarises the possible advantages and disadvantages which have become apparent as the investigations have been in progress, and it is hoped this will form a useful reference against which to judge the success achieved to date .
Advantages and disadvantages of Molten Salt Fast Reactors (MSFRs)
- No loss of neutrons occurs in fuel cladding or structural material in core.
- Continuous addition or removal of fuel is possible under power .
- Simple control of power may be achievable by coolant conditions and power demand. It seems possible to avoid control rods or equivalent systems .
- Strong negative temperature coefficient gives basic safety .
- High heat capacity of fuel restricts temperature rise on loss of normal cooling. A dump system with independent cooling system can be used for decay heat removal.
- Reactor vessel is small . Prefabrication of the vessel at works should be possible, thus reducing construction time and interest charges . In the event of a fault in the vessel fuel may be removed and the vessel size is such that its replacement can be contemplated .
- Building layout is compact due to the small size of the reactor vessel and primary circuit and absence of elaborate fuel handling route .
- The system can basically be at low pressure apart from pumping pressures .
- Fuel handling by pumps and pipework should be a simpler operation than solid fuel handling .
- The system is suited to on-site close coupled fuel processing .
- A substantial proportion of the fission products can be continuously removed by a close coupled process .
- Savings in fuel element fabrication and development costs seem possible
Potential for high temperatures and higher cycle efficiency is good.
- Higher fuel inventory than in LMFBRs is required to give heat transfer and transport .
- Molten salt fuel has relatively poor heat transfer characteristics compared with sodium.
- During prolonged shutdowns. The temperatures involved prevent access to plant .
- The high melting point of the fuel salt also imposes an additional constraint on heat exchanger systems in order to prevent salt freezing on heat transfer surfaces if low return salt temperatures are employed, especially if high heat transfer rates apply on the coolant side .
- Presence of fission products in the fuel salt – a high standard of plant reliability and leak detection on remote maintenance is required
- Limitation of choice of materials of construction due to corrosion and high temperature .