Page 190 - Radiochemistry and nuclear chemistry

174 Radiochemistry and Nuclear Chemistry

+ --+--+-+ +-+-+-+
INTERSTITIAL -- 4" -- 4" "- [~~...... CATION
.T. + - + .T. 4"__ 4" ~. 4"__ .7. 4"__ VACANCY
+ -
-
+ -+-+ + -+ -+

CATION VACANCY ANION VACANCY
FIG. 7.3. Irradiated NaCI type crystal showing negative and positive ion vacancies.

the dislocations lead to an energy storage in the material (the Wigner effect), which can
become quite significant. For reactor graphite at 30~ this frequently reaches values as high
as 2000 kJ/kg for fluencies of 2 x 1021 n/cm 2. At room temperature the interstitial atoms
return to their normal positions very slowly (annealing), but this rate is quite temperature
dependent. If the elimination of the interstitial atoms occurs too rapidly, the release of the
Wigner energy can cause the material to heat to the ignition point. This was the origin of
a fire which occurred in a graphite moderated reactor in England in 1957, in which conside-
rable mounts of radioactive fission products were rdeasext into the environment.
Inorganic substances exposed to high fluencies of neutron and "),-radiation in nuclear
reactors are found to experience decomposition. Thus:

KNO 3 .... ~ KNO 2 + t,~O 2

The notation .... -~ symbolizes a radiation induced reaction. At high fluxes the oxygen
pressure in the KNO 3 causes the crystal to shatter. However, some crystals are remarkably
stable (although they may become colored), e.g. LigSO 4, K2SO 4, KCrO 4, and CaCO 3.
Theory has not been developed sufficiently to allow quantitative calculation of the
radiation sensitivity of compounds. Usually covalent binary compounds are highly radiation

FIG. 7.4. Zircaloy canned UO 2 fuel after exposure to very high power (high dose and heat)
in test reactor. The pellets have fractured and the canning has been penetrated. (SKI report)