In the conversion of fuel in the Tehran Research Reactor core from Highly-
Enriched Uraniutn to Low-Enriched Uranium fuel neutronics analysis, thermal
hydraulic calculations and kinetic performance of the core have to be studied.
In this study, static and dynamic core performance for HEU and LEU fuels
were investigated. In static conditions, two groups of neutron flux distributions
in axial direction were obtained by solving diffusion equations. In transient
cases, the slow and fast transient phenomena, small reactivity and large reactivity
insertion effect, were investigated. To find the temperature feedback effect
on core reactivity, such as coolant drainage tnalfunction, the effective tnultiplication
factor was calculated for two different temperatures, namely 293? K and
373? K, using the cotnputer code WIMS-D/4. From here, the average temperature
coefficient of reactivity was obtained. It should be pointed out that the
coefficient dependence on temperature is approximately a linear function. In
fast transient study, for HEU fuel $1.510.5 reactivity and for LEU fuel $1.3510.5
reactivity were considered. In slow transient study, a reactivity equivalent to
$0.0085/s for HEU fuel and $0.0034/s and $0.00851~ in the case of LEU fuel
was introduced into the core. The result of calculations show that in fast transient
in the case of LEU fuel, reactor power drops sharply before the scram time
set is reached. It seems as if the overall performance of the LEU core is comparable
with that of the HEU fuelled core