IRR-1 Core Optimization for Neutron Radiography

Introduction

One of the utilizations of the Israeli Research Reactor 1 (IRR1, at Soreq NRC) is neutron radiography. We have developed a method that predicts the effective flux at the IRR1 neutron camera based on the computed flux at few locations at the core. Using our core calculation code, we could optimize the core configuration to maximize the flux at the imaging plane of the neutron camera (NC). We report our optimization efforts and some of the results.

Methods

We used our technique of stacked layers Monte-Carlo incident neutron (SALAMI) calculation, which locates the sites from which most of the ballistic neutrons that arrive to the NC originate. We use Monte-Carlo core calculation (coupled with historic fuel burn-up) to calculate the flux at these locations and deduce the scattering rate. The thermal flux at the NC is readily extracted and at a relatively high accuracy, allowing fast calculations of different core configurations.

Within the restrictions imposed by the reactor operational considerations (such as availability of fuel assemblies and mobility of control rods) we calculated core rearrangements. We aimed to increase the NC efficiency by shifting the `weight` of the fuel towards it, producing more NC neutrons per MW reactor power. The configurations that were found had to conform also to safety and other limitations, such as control rod reactivity, and we verified these using MCNP calculations.

Results and conclusions

We have found a reactor fuel configuration that shows a computed 40% increase of NC flux over the operating core configuration. This substantial change will increase the IRR1 efficiency and lifespan, without breaching its predetermined operational limits. This result demonstrates how clear performance requirements and proper calculation tools lead to extension of the core yield. This work may be valuable also to other neutron radiography facilities, such as accelerator-based neutron sources.