Some of the technical challenges in advancing towards a fusion reactor can be categorised as follows:

  1. Plasma confinement and heating
  2. Engineering feasibility and system reliability
  3. Tritium fuel cycle
  4. Energy extraction
  5. Cost

Further issues of a different nature include:

  1. Radioactivity
  2. Safety
  3. Environmental impact

These items will be examined below and on other pages on the site.

Plasma confinement and heating

Plasma confinement and heating are issues which have been developed in numerous experimental machines over the past 70 years. A detailed coverage can be found in the book by L.J. Reinders: The Fairy Tale of Nuclear Fusion, Springer, May 2021 and briefly on this website: History of Nuclear Fusion Projects

With ITER, it can be hoped that the remaining problems with plasma confinement, in particular the "disruptions" where the plasma is lost for, as yet, poorly unknown reasons will be solved and the design parameters for the system achieved. However, much is needed to go beyond ITER to an actual reactor.

Engineering feasibility and system reliability

A nuclear fusion reactor is an immensely complicated machine with numerous different new and unique systems. The neutron flux will be enormous, on a level never previously dealt with and it will require new materials to withstand the damage from the radiation. Although progress has been made over the decades of R&D in the field extensive testing in a realistic environment is still required and no design has as yet has been proven to be viable. There are even doubts with components in the ITER design even though the machine is actually in constructed, specifically with the use of beryllium: Further delays at ITER are certain, but their duration isn’t clear: Physics Today: Vol 75, No 5 .

Tritium fuel cycle

The supply of tritium fuel is perhaps the most difficult problem for fusion reactors, perhaps even the show-stopper. Surprisingly, fusion advocates say that the fuel is abundant and an advantage of fusion, e. g. the ITER website says: "Fusion fuels are widely available and nearly inexhaustible". In reality the opposite is the case for tritium.

A single fusion reactor would consume about 50kg/year of tritium but, currently, only about 0.5kg/year is produced worldwide. In all the oceans of the world there is about 30kg of tritium which is produced naturally but decays with a half-life of 12 years.

Current designs for fusion reactors involve breeder systems to regenerate tritium from the neutrons emitted in the D-T fusion reactions. The designs for breeder blankets contain a mixture of elements with different functionality. One element is lithium which when hit by a neutron makes tritium. Another element in needed, e. g. beryllium, to multiply the neutrons so more than 1 tritium atom is produced per incident neutron. The snag with present breeder designs is this number is only 1.15 which is too low to provide a sustainable supply.

The figure below indicates the complexity of a closed tritium fuel cycle in a fusion reactor:

The tritium which enters the reactor exits in three different ways:

  1. Part is burnt 
  2. Part is re-cycled for rapid reuse
  3. Part is absorbed by the inner walls of the reactor and must be re-cycled later

The efficiencies and timescales involved in the rapid recycling(IFC) and breeder circuits (OFC), together with inevitable losses, mean attaining sufficiency in tritium is very difficult and maybe impossible. This issue is described in technical detail in:  Abdou et al. Nuclear Fusion 61 (2021) which states:

"We focus in particular on components, issues and R & D necessary to satisfy three ‘principal requirements’: (1) achieving tritium self-sufficiency within the fusion system, (2) providing a tritium inventory for the initial start-up of a fusion facility, and (3) managing the safety and biological hazards of tritium. A primary conclusion is that the physics and technology state-of-the-art will not enable DEMO and future power plants to satisfy these principal requirements".

A further massive problem with the tritium supply is the initial start-up inventory, equally addressed in Abdou et al. The current world stock of tritium is about 18kg which will increase until the CANDU fission reactors reach the end of their lives. From then on, the radioactive tritium will decay away at a rate of 5%/year. When the ITER project starts operation it will take a major fraction of the world stock. The figure below shows an estimated evolution of the world stock of tritium.

According to this estimate, around 2050 when the construction of a DEMO fusion reactor might start, world tritium supplies could be less than 5kg. The Abdou et al paper gives estimates of required start-up inventories for a reactor.

It seems difficult for multiple fusion reactors to start operations without sufficient start-up supplies of tritium and this might require specific new fission reactors to be built to provide part of the tritium supply.

An excellent treatment of the supply issue of tritium, as well as lithium and other rare minerals for use in a fusion reactor, is given in the book of L.J. Reinders: Economic and Sustainability Aspects of Nuclear Fusion .

A non-technical presentation of the tritium supply problem, with a dramatic conclusion, can be found on the website of Steven Krivit at: The Fuel for Nuclear Fusion Doesn’t Exist .