Study on secondary shutdown systems in Tehran research reactor

• A study was undertaken to summarize the techniques for secondary shutdown systems (SSS).
• Neutronic calculation performed for proposed systems as SSS.
• Dumping the heavy water stored in the reflector vessel is capable to shut down reactor.
• Neutronic and transient calculation was done for validating the selected SSS.
• All calculation shown that this system has advantages in safety and neutron economy.

One important safety aspect of any research reactor is the ability to shut down the reactor. Usually, research reactors, currently in operation, have a single shutdown system based on the simultaneous insertion of the all control rods into the reactor core through gravity. Nevertheless, the International Atomic Energy Agency currently recommends use of two shutdown systems which are fully independent from each other to guarantee secure shutdown when one of them fails. This work presents an investigative study into secondary shutdown systems, which will be an important safety component in the research reactor and will provide another alternative way to shut down the reactor emergently. As part of this project, a study was undertaken to summarize the techniques that are currently used at world-wide research reactors for recognizing available techniques to consider in research reactors. Removal of the reflector, removal of the fuels, change in critical shape of reactor core and insertion of neutron absorber between the core and reflector are selected as possible techniques in mentioned function. In the next step, a comparison is performed for these methods from neutronic aspects. Then, chosen method is studied from the transient behavior point of view. Tehran research reactor which is a 5 MW open-pool reactor selected as a case study and all calculations are carried out for it. It has 5 control rods which serve the purpose of both reactivity control and shutdown of reactor under abnormal condition. Results indicated that heavy water as reflector vessel around the reactor core act as an independent shutdown system with sufficient shutdown margin and are very suitable place for irradiation purposes.