(a) General definitions

The upper part of the neutron spectrum is particularly important because of the damage induced in the reactor materials by fast neutrons.

The methods we have seen so far are normally used to calculate ф(Е) also in this upper energy range. Monte Carlo calculations have also been used139’ and they show a reasonable agreement with other transport methods. As the damage is a function of the neutron spectrum, it is unfortunately difficult to express it in one quantity.

One usually speaks of fast neutron dose (or fluence)

or if the flux is constant with time

d = фТ = nvT

dimensionally d is expressed as

Sometimes the area is measured in kilobarn (1 barn = КГ24 cm2) so that the dose can be also measured in neutrons per kilobarn. The real problem consists in defining the fast flux ф (see ref. 40).

A simple definition is

ф = [ ф(Е) dE (8.42)

J E0

where usually E0= 1 MeV, but also lower limits (e. g. 0.18 MeV) have been used. Another definition is related to threshold detectors which are often used to measure the fast flux.

If o-(E) is the detector cross-section, one defines a fission flux ф1,

where o-Ni = 107 mb, calculated according to eqn. (8.44).

(b) Graphite damage

Of primary importance in HTRs is the graphite damage. This can be related to the number of displaced atoms per unit volume.

The carbon displacement rate Cd is given by139’401

Cd=§=f <t>(E)crs(E)p(E)dE

where x = number of displaced atoms per unit volume, o-s(E) = carbon scattering cross-section,

p(E) = number of carbon atom displacements caused by a collision with a neutron of incident energy E.

p(E) is known,<40’41) so that expression (8.46) can be easily calculated. An equivalent graphite fission flux can be defined as

x(E) dE

As many irradiations have been performed in the DIDO reactor at Harwell, this has been used as a standard to define the Equivalent Dido Nickel Flux (EDNF)

Calculated results give1′

= 1260 x КГ24.

V ФNi/ DIDO

As the number of displaced carbon atoms per unit volume is (at a given temperature) related to changes in electrical resistivity, the relative calibration of various irradiation facilities is usually based on measurements of resistivity changes in standardized reference graphite samples.<43)