The Experimental Breeder Reactor Ii Inherent Safety Demonstration

On April 3, 1986 two milestone tests were conducted in Experimental Breeder Reactor-2 (EBR-II). The first test was a loss of flow without scram and the second was a loss of heat sink without scram. Both tests were initiated from 100% power and in both tests the reactor was shut down by natural processes, principally thermal expansion, without automatic scram, operator intervention or the help of special in-core devices. The temperature transients during the tests were mild, as predicted, and there was no damage to the core or reactor plant structures. In a general sense, therefore, the tests plus supporting analysis demonstrated the feasibility of inherent passive shutdown for undercooling accidents in metal-fueled LMRs. The results provide a technical basis for future experiments in EBR-II to demonstrate inherent safety for overpower accidents and provide data for validation of computer codes used for design and safety analysis of inherently safe reactor plants.

Two tests of great importance to the design of future commercial nuclear power plants were carried out in the Experimental Breeder Reactor-II on April 3, 1986. These tests, (viewed by about 60 visitors, including 13 foreign LMR specialists) were a loss of flow without scram and a loss of heat sink without scram, both from 100% initial power. In these tests, inherent feedback shut the reactor down without damage to the fuel or other reactor components. This resulted primarily from advantageous characteristics of the metal driver fuel used in EBR-II. Work is currently underway at EBR-II to develop a control strategy that promotes inherent safety characteristics, including survivability of transient overpower accidents. In parallel, work is underway at EBR-II on the development of state-of-the-art plant diagnostic techniques.

A test program is being conducted to demonstrate that a power producing Liquid Metal Reactor (LMR) can passively remove shutdown heat by natural convection; passively reduce power in response to a loss of reactor flow and passively reduce power in response to a loss of the balance of plant heat sink. Measurements and pretest predictions confirm that natural convection is a reliable, predictable method of shutdown heat removal and suggest that safety-related pumps or pony motors are not necessary for safe, shutdown heat removal in a LMR. Measurements from tests in which reactor flow and heat rejection to the balance of plant were perturbed show that reactivity feedbacks can passively control power and temperature. This data is a basis for additional tests including a complete loss-of-flow without scram and a complete loss of heat sink without scram.

The Experimental Breeder Reactor II (EBR-II) operated from 1964 to 1994. EBR-II was a sodium-cooled fast reactor operating at 69 MWth producing 19 MWe. Rather than using a loop approach for the coolant, EBR-II used a pool arrangement where the reactor core, primary coolant piping, and primary reactor coolant pumps were contained within the pool of sodium. Also contained within the pool was a heat exchanger where primary coolant, which is radioactive, transferred heat to secondary, nonradioactive, sodium. The nuclear power plant included a sodium boiler building where heat from the secondary sodium generated superheated steam, which was delivered to a turbine/generator for electricity production. EBR-II fuel was metallic uranium alloyed with various metals providing significant performance and safety enhancements over oxide fuel. The most significant EBR-II experiments occurred in April 1986. Relying on inherent physical properties of the reactor, two experiments were performed subjecting the reactor to loss of primary coolant flow without reactor SCRAM and loss of the secondary system heat removal without reactor SCRAM. In both experiments, the reactor experienced no damage. This chapter provides a description of the most important design features of EBR-II along with a summary of the landmark reactor safety experiments.

The initial "Hazard Summary Report - Experimental Breeder Reactor-II (EBR-II)" was published as ANL-5719 in May, 1957. At the time of preparation of ANL-5719 (early 1957), the design of the EBR-II Facility was in progress - construction had not started. This Addendum is limited, for the most part, to changes which have been made in the EBR-II Facility, and to supporting experimental data developed, since the publication of ANL-5719.

The EBR-II reactor consists of an enriched core surrounded on all sides by a fertile blanket of depleted uranium. The fuel elements which comprise the core section of the reactor consist of small diameter cylindrical pin assemblies. The design of the fuel element is influenced by the desire for high thermal performance, high burnup, and simplicity of construction. The fuel pin is a loss fit in a thin-walled tube which provides a clearance annulus between the pin and the tube wall. This annulus is filled with static sodium to provide a heat transfer bond between the fuel and fuel tube. Heat is removed from the fuel element by the primary sodium flowing along the outside of the fuel tube.