Experimental Verification and Analysis of Neutron Streaming Effect through Void Holes for Control Rod Insertion in HTTR.

Abstract

Control rod columns of the High Temperature Engineering Test Reactor (HTTR) core have each three cylindrical void holes for insertion of the control rods. The reactivity worths of the simulated void holes were measured in the Very High Temperature Reactor Critical Assembly (VHTRC) by the pulsed neutron method with the purpose of evaluating the neutron streaming effect on the reactivity worths of the void holes. The reactivity worths were determined using the revised King-Simmons' formula. The measured results are compared with the calculated ones based on the nuclear data of ENDF /B-IV, using the Benoist's anisotropic neutron diffusion coefficients. It is concluded that the reactivity worths are enlarged up to about twice as large as that obtained using the conventional isotropic neutron diffusion coefficients, and it is also found that the neutron streaming effect that is defined as the relative fraction components exceeding the reactivity worths obtained by the conventional isotropic neutron diffusion coefficients are likely overestimated about 11 and 32% with the void holes being in the core and reflector regions, respectively. Then, the reactivity decrease due to the neutron streaming effect is able to be evaluated around 1 %Llk for the initial critical core of the HTTR.