Burn-up is usually expressed in megawatt-days per tonne (MWd/t) of fuel, referred to the thermal power the fuel has produced in the reactor.

In high-temperature reactors where the fuel is intimately mixed with a part of the moderator this unit can give rise to some confusion. It is then necessary to specify it as megawatt-days per tonne of heavy metal, excluding in this way the weight of carbon and other light elements present in the fuel.

Because of the high burn-up attained by HTR fuel, sometimes the unit megawatt — days per kilogramme (or gigawatt-days per tonne) is used. Other units often used in HTR calculations are FIFA (Fissions per Initial Fissile Atom) and FIMA (Fissions per Initial Metal Atom), sometime expressed in percent. The burn-up measured in MWd/t or in fima differs only by a constant factor. Using the rough approximation that the fission of 1 g of fuel per day corresponds to 1 MW, fima 1 = 105 MWd/t (or more exactly fima 1 = 0.93 x 105 MWd/t).

The relation between fima and fifa depends upon the ratio of fissile to fertile concentration in the fresh fuel. In order to calculate the relation between the energy produced and the number of fissions one has to consider the following distribution of
the fission energy:

Kinetic energy of fission fragments Kinetic energy of fission neutrons

Prompt у Delayed у

P

Neutrinos

to these values one should add the y-rays produced by n, у reactions in various reactor materials.

Except for the neutrinos one can consider that all this energy is converted into thermal energy within reactor core, reflector and shielding. While the kinetic energy of the fission fragments is completely converted into thermal energy within the coated particles, other items, like the kinetic energy of the fission neutrons, are more uniformly spread over the core. One can in general consider that 92% of the power is produced in the fuel and 8% in the moderator. From the above data follows that 1 joule = 3.2 x 1010 fissions (this quantity is weakly dependent on core size).