ORNL recently published a report titled Safeguards for the Lithium Fluoride Thorium Reactor: A Preliminary Nuclear Material Control and Accounting Assessment. A summary of the report can be found below.

This report was developed for Flibe Energy, Inc. The design of a LFTR (liquid fluoride thorium reactor) presents challenges for both the NRC’s and the IAEA’s safeguards. Domestic safeguards, as defined by the NRC, are material control and accounting (MC&A) and physical protection. This is to protect nuclear facilities from theft and diversion.

LFTR is a reactor two-fluid molten salt breeder reactor, which uses U-233 and Th-232 as the fuel and blanket salts, respectively. As a breeder reactor, LFTR relies on thorium neutron capture to create Pa-233, which decays into U-233. A portion of the blanket salt is continually processed in an electrolytic
separator to remove Pa-233 and U-233 from the blanket salt, which are sent to a decay tank to allow time for Pa-233 to decay. LFTR design has two fluorinators, one to remove uranium from the decay tank, and the other to remove uranium from a portion of the fuel salt as the first step of removing fission products. The cleaned uranium then combines with FLiBe salt and reenters the reactor vessel. Another separator is used to remove the fission products from the residual salt of the second fluorinator.

Due to the use of the thorium fuel cycle, LFTRs generate nearly-pure U-233, which is vital to the reactor’s operation. However, precautions must be taken to protect separated U-233 from theft and diversion. Because this uranium is produced in the decay tanks, rather than the reactor itself, the decay tanks must also follow strict nuclear safeguards. Continuous online processing of the salts in both the reactor vessel and the fluorinators make it difficult to measure and identify the location and composition of any nuclear material in the reactor at any given time. However, these challenges allow us to reexamine and optimize the MC&A system, leading to better application of safeguards in the future. Additionally, as Flibe Energy plans to launch a demonstration-scale LFTR, evaluating these nuclear safeguards will prepare them to build a commercial-scale nuclear reactor.

Below we will list several challenges posed by the reactor design, and the suggested MC&A improvements in bulleted lists.

 

Challenge 1: Quantification of decay tank nuclear material

• Explore the use of smaller decay salt inventories.
• Account for decay salt inventories using a custom nondestructive assay (NDA) system.
• Investigate tradeoffs between factors such as geometry, weighing precision, and self-shielding.
• Install tamper indicating devices.
• Use of gamma-ray spectroscopy for isotopic composition.
• Use of neutron counting techniques for mass quantification.
• Quantify the mass of uranium and protactinium transfer using an electrolytic cell.

Challenge 2: Accounting for continually processed radioactive nuclear material

• Account for salt by processing in batches. (This may help with additional shielding and reduction of high temperatures and corrosivity on instrumentation.)
• Use individually sealed containers of decay salt that can disconnect from the filling pipe.

Challenge 3: Decay tank inventory is not self-protecting

• Expand the model to account for decay salt composition, which will verify the expected self-protection of the decay inventory.
• Add separated fission products, which will increase gamma activity in the tank and deter potential theft. However, this may make measurement more difficult.
• Combine the decay inventory with some of the fuel salt to reduce the purity of the uranium vector.
• Increase U-232 content by adding Th-230 to increase self-protection.

Challenge 4: Potential access to fissile materials in decay tank

• Remove a dedicated decay salt inventory. This could be done by redirecting blanket salt directly to the decay fluorinator. This has disadvantages and may conflict with the aforementioned suggestions.

Challenge 5: Large uranium output from fluorinator

• Eliminate the decay fluorinator entirely by redirecting decaying protactinium to outside the reactor vessel.
• Route output of fuel fluorinator to input of decay fluorinator, thus reducing purity of the uranium vector.
• Remove physical access to the UF6 stream.