Two Fluid Molten Salt Reactors with Graphite

I have remained interested in the two-fluid graphite-moderated molten-salt reactor approach to the LFTR. ORNL two-fluid graphite MSR designs were plagued by what is called the "plumbing problem". In the classic ORNL two-fluid design, liquid salt flows through the reactor core through interlaced graphite tubes or cells. ORNL 4119 which also described the two fluid design stated:

The core of the reactor is made up of 336 cylindrical graphite fuel cells mounted as close together as tolerances permit to form essentially a cylindrical array approximately 8.3 ft in diameter. The graphite cells are extruded cylinders with center holes 1 1/2 in. in diameter, surrounded at 120° angles with three 7/8-in.-diam holes. At the top of each cell there is a graphite cap machined to provide a smooth communication between the four holes of the cell. Figure 9.6 shows the arrangement of one of these cells.

The tube design was slightly different in the ORNL-4528 reactor design. In both reactor core designs, the liquid salt flowed up each tube and when it reached the top it flowed through a channel that made a 180 degree U turn. and flowed back down again. Tubes carrying fertile thorium carrying salts surrounded the the inner set of U-233 carrying fuel salts. There was, however a problem with this design Graphite exposure to high energy neutron radiation over a period of time leads first to graphite shrinkage and then swelling, as energetic neutrons knock carbon atoms from their original location in the graphite lattice. ORNL 4528 states

The effect of neutron irradiation, however, is to first shrink and then swell the graphite to cause an increase in porosity and, we expect, a deterioration in physical properties. The dimensional changes occur slowly, and their effects on the neutronics of the reactor can be accommodated by gradually adjusting the fuel-salt composition, although at a small detriment to the nuclear performance. The radiation damage to the graphite, however, limits the useful life of the reactor core.

The swelling causes components made of graphite to change dimensions. Thus if reactor plumbing components are made of graphite swell because of exposure to high energy radiation, there are multiple consequences. First as the dimensions of graphite tubes increase the amount of fertile and fissionable material they carry is sufficiently altered to effect the performance of the reactor. But the effect of the swelling on plumbing fit is an even more significant problem.

ORNL reactor researchers understood that core graphite swelling limited the time that a graphite core was useful. ORNL-4528 states:

the limited performance of the graphite undoubtedly restricts the design and imposes a maintenance penalty, . . . The major concern was whether mechanical failure of graphite tubes in the reactor core would cause the effective lifetime of the core to be significantly less than the eight years imposed by the effects of irradiation on the graphite.

ORNL-4528 explained

Neutron irradiation produces substantial changes in length of the graphite elements, and the difference in expansion of the graphite and the metal parts of the reactor vessel with temperature changes can also be large. These effects must be accommodated without overstressing the graphite. We propose to accomplish this by making the graphite elements in the form of concentric tubes connected to the reactor vessel at only one end in order to provide freedom for axial expansion and contraction. The fuel salt would flow in and out at the same end of the elements, and the connections would be to tube sheets at the bottom of the reactor vessel to allow the salt to drain completely.

Because of the irradiation effects, the graphite tubes will have to be replaced periodically. . . . . The reactor vessel and intemals will be highly radioactive after a short time at high power, and with the graphite elements brazed to a tube sheet in the bottom of the reactor vessel, individual tubes could not be readily inspected or replaced. We concluded that the most practical way to renew the graphite in the core would be to replace the entire reactor vessel and its contents. Suitable provisions would be required for remotely operated tools and viewing equipment to cut, weld, and inspect joints in the piping system. Provisions for handling and disposing of spent reactor vessels would have to be included in the plant.

Eventually ORNL reactor researchers sought to extend graphite core life by a radical alteration of the design of the MSR. ORNL-4528 proposed to build 4 small reactors rather than one large reactor. The small reactors could taken out of service for core replacement one at a time. Thus the MSBR facility would be assured of 3/4th power almost all of the time. However ORNL reactor designers took the even more radical approach of completely scrapping the two-fluid reactor design, in favor of a one-fluid design. This approah, of course, lead to a new set of problems.

It should be noted that the graphite swelling problem continues to be of interest to nuclear scientist who continue to evaluate new graphite materials for improved resistance to neutron swelling.

Recently Canadian physicist and reactor designer David LeBlanc noted a further problem with the graphite-tube two-fluid design

The "plumbing problems" of interlacing fuel and blanket salts within the core is probably still nearly insurmountable if for no other reason than the shrinking and expanding graphite tends to dramatically change the ratio of the two fluids within the core (and the work on using metal looked even more hopeless). Hopefully we can convince them of the merits of a new simple geometry of tube within tube that only needs one barrier, not thousands. The old ORNL Two Fluid design also has a positive temperature reactivity coefficient for the blanket salt which they rarely mentioned. If your blanket salt is only surrounding a core of fuel salt, it ends up to also have a negative temp coefficient since it acts as a reflector (hotter=less dense=less neutrons reflected back)

David added in another comment which he noted in a communications to me:

I also agree with other posts and the paper in rightly worrying about the safety issues of losing blanket salt (something ORNL didn't really mention much). That is one of the huge benefits of switching from the old idea of interlacing fuel and blankets salts within the core and the new concept of only having the blanket salt outside a small diameter core (and going to a modestly long cylinder or tube if you want to get a healthy total core power). With the blanket only outside the core, it is acting as a very weak reflector of neutrons back into the core, thus if it drains away or gets less dense by heating up then you actually lower reactivity because the core is now losing some of these reflected neutrons. As a point of clarification, this assumes that you don't put any sort of good reflecting material within or outside the blanket zone because you could then also end up with a positive void/temp coefficient for the blanket salt, i.e. you can have graphite in the core but avoid using much in the blanket or at the outer vessel wall.

David's comments would seem to write the obituary for the ORNL version of the two-fluid design, but would leave it open for his own greatly modified two-fluid concept.