Fuel Cycle

The HTMR-100 Nuclear Power Plant will utilize a simple, cost effective and effective fuel cycle, yet with a high level of proliferation resistance. In the OTTO (Once-Through-Then-Out) cycle the fuel spheres only move once through the core and reach their final burn-up in one pass. In this way, high helium temperatures can be obtained using relatively low fuel temperatures. For the OTTO cycle, the energy production occurs mainly in the upper part of the core. Burnable poison in the fuel help to flatten the flux profile, however the difference between the fuel and the gas temperature at the exit of the core remains small. The OTTO fuel-handling system is recognized as proven technology based on the experience with the AVR, THTR and other HTR development programs.

Thorium–uranium fuel cycle has many advantages over the conventional uranium fuel cycle. Thorium has a greater abundance (3 to 4 times more abundant than uranium), thorium has superior physical and nuclear properties, the thorium fuel cycle has better resistance to nuclear weapons proliferation as the plutonium and actinide production are an order of magnitude less than conventional uranium fuels.
The inclusion of thorium as fertile material in the fuel mix is a fundamental requirement from STL, both to ensure the beneficiation of their thorium assets and to minimize the production of plutonium and minor actinides which are the biggest burden from the nuclear waste viewpoint.

What makes thorium so attractive as fuel source in Pebble Bed Reactors ?

• When a neutron interacts with a fissile atom it either fissions or is captured and transmuted
• When ²³³U captures a neutron it either fissions or becomes ²³⁴U
• Comparison of fissile isotopes - In the HTR neutron spectrum the probability to fission is:
–      88% for ²³³U
–      76% for ²³⁵U
–      41% for ²³⁹Pu
–      59% for ²⁴¹Pu

Rationale for considering thorium as fuel source
• The current utilization of all nuclear reactors is around 66,500 tons/a
• Uranium leaves us with around 65 – 70 years' supply based on "proven reserves" of 4.7 million tons, i.e. uncertain long term security of supply
• There is 3-4 times more Th in the earth's crust than U
• In thermal reactors the Th energy content is 8 times higher than that of U
• Depending on reactor type (thermal, epithermal, or fast) the efficiency may vary from 60 – 200%. In HTRs we assume 100% efficiency.
• Based on proven reserves, one may conclude that there is a supply of uranium for approximately 70 years. Using thorium this could increase to 2240 years for conventional reactors and 4500 years for HTRs.
• Thorium mixed (MOX) fuels could be used for the incineration of weapons grade plutonium or civilian plutonium.
• Thorium dioxide (ThO₂) is chemically more stable and has a higher radiation resistance than uranium dioxide (UO2). The fission product release rate for ThO₂ based fuels are one order of magnitude lower, has favourable thermophysical properties because of the higher thermal conductivity and a lower coefficient of thermal expansion compared to converntional UO₂.
• ThO₂ has the highest melting point of all oxides and does not oxidize unlike UO₂, which is readily oxidized to U₃O₈ and UO₃. Hence, long term interim storage and permanent disposal in repository of spent ThO₂ based fuel are simpler and safer.