Sodium Reactor Experiment

SRE nuclear power plant in 1958

Sodium Reactor Experiment ( SRE ) is the name of the first commercial nuclear power plant in the United States in the history of nuclear energy . It operated from 1957 to 1964 and was located on the grounds of the Santa Susana Field Laboratory in California. The nuclear power plant represented a feasibility study for reactors cooled with liquid sodium .

history

Nuclear Historical Landmark plaque

In 1954, the United States Atomic Energy Commission announced that it would test various reactor concepts for civil use. To this end, it was planned to build five nuclear power plants with various experimental reactors within five years. One of these power plants was the Sodium Reactor Experiment.

Construction of the power plant began in June 1954. On April 25, 1957, the reactor became critical for the first time , and the first network synchronization took place on July 12 of the same year. Construction and operation were carried out by Atomics International .

Already in July 1959 there was an accident with the release of radioactive gases. After the reactor had been repaired, it was restarted in September 1960 and operated without further incidents until it was finally shut down on February 15, 1964.

The reactor and its auxiliary systems were dismantled by 1981, the building complex was demolished in 1999.

The use of liquid sodium, which was tested in this reactor, did not become widely accepted. The boiling water reactors (tested in the Argonne National Laboratory ) and pressurized water reactors (tested in the Shippingport nuclear power plant ), which were also tested in the plans of the Atomic Energy Commission, were more promising. Only in breeder reactors is liquid sodium still preferably used as a coolant because it hardly moderates neutrons .

construction

Cross section through the reactor of the SRE

A liquid metal-cooled ( sodium ), graphite- moderated reactor was tested in the SRE . In contrast to water, the liquid sodium used as a coolant has a relatively low vapor pressure at the operating temperatures of the reactor , which means that it is largely possible to dispense with the complex design of the power plant systems for high pressure.

In order to prevent the sodium from expanding when it solidifies and thus damaging the system, it must be kept permanently liquid. In power mode, the reactor's thermal output is sufficient. If this is switched off, however, heating elements in the lower area of ​​the reactor pressure vessel take over the heating of the sodium.

The actual core of the reactor sat in the lower area of ​​the reactor pressure vessel made of steel and filled with liquid sodium . This core consisted of 43 fuel elements with seven fuel rods each . The approximately 1.8 m long fuel rods were also made of steel and each contained twelve fuel balls made of 2.78% enriched metallic uranium .

Since the sodium used as a coolant is itself activated by the neutron radiation to which it is exposed, it was necessary to design the nuclear power plant with two separate coolant circuits in order to contain the radioactivity . At full power, the sodium in the primary circuit flowed into the lower area of ​​the reactor vessel at a temperature of approx. 260 ° C and left the upper area at a temperature of approx. 510 ° C. The coolant was circulated by converted centrifugal pumps and, depending on the operating mode, transported to a specific one of the two heat exchangers . In each of these, the sodium in the primary circuit gave part of its heat to the sodium in a secondary circuit. For operation in small power regions, one of these secondary circuits was provided, which released the thermal power generated by the reactor to the atmosphere via air cooling on the roof of the reactor building. In normal operation, however, the heat output was used to generate electricity: In the second secondary circuit, the liquid sodium circulated through a steam generator , in which water was boiled using the heat energy of the sodium . The steam generated in this way drove a conventional steam turbine and generated electricity via a generator, which was fed into the network of the nearby town of Moorpark .

Sketch of a typical fuel assembly from the reactor

The reactor pressure vessel is not completely filled with liquid sodium. In the upper area there is gaseous helium , which is kept under a pressure of approx. 0.2  bar . This allows the coolant to expand due to temperature fluctuations during operation. Helium is well suited for this task because it is not activated by the neutron radiation occurring in the reactor . This gas bubble is connected to a sodium tank and four gas tanks via pipes. If the gas pressure in the reactor changes, the helium flows into one of the four gas tanks, where any radioactive gases are retained. If their activity has declined to a harmless level, they are diluted with ambient air and released into the atmosphere.

Incident in 1959

Molten fuel rod, July 1959

As already noted in the name of the power plant, it was not only used to generate electricity, but mainly as a platform for tests and experiments in order to be able to assess the behavior of these reactors with regard to their suitability for use in energy generation. To do this, the engineers carried out several series of tests, the so-called "runs", during which the reactor was in operation. During this, the behavior of the plant was assessed and suggestions for improvement for the reactor and the power plant were derived, which were implemented between the runs.

During run 8, black deposits were seen on the fuel elements removed from the reactor. It was believed that this was decomposed tetralin , an oil-like liquid. During the next few runs, the engineers observed several unusually high temperatures in the individual fuel assemblies. It was only towards the end of Run 13, when irregular increases in output occurred, that it was noticed that the heat distribution of the reactor was irregular and severely impaired. This has now been attributed to the decomposed tetralin. Tetralin was used in the power station to cool various systems as well as in the seals of pumps and from there it had apparently seeped into the primary circuit. There it decomposed due to the hot sodium and formed small lumps. These hindered the cooling of a total of 13 fuel assemblies, which damaged them. It is very likely that the coolant also partially boiled (boiling point sodium: 883 ° C), which allows conclusions to be drawn about the local temperatures. However, the melting temperature of the metallic uranium used as fuel was not reached, only the fuel rod cladding began to change into a liquid state. The exact date of the damage is unknown, but could be narrowed down to the period between July 12 and 26, 1959.

As a result of the damage to the fuel rod cladding, radioactive fission products were released into the primary circuit, which arise during regular operation and are retained in intact fuel rods. Solid fission products were distributed in the liquid sodium of the cooling circuit, gaseous elements mixed with the helium used as protective gas in the upper area of ​​the reactor. This was then moved to the gas tanks to wait for the main xenon-135 contained therein to decay. The gases contained were then mixed with air and released into the atmosphere.

The tetralin remaining in the primary circuit was removed with 11,300 m³ of nitrogen . For this purpose, the 13 damaged and 17 other fuel elements that remained intact were removed from the reactor core in order to impede the flow of nitrogen within the reactor as little as possible. In addition, the sodium level in the reactor was lowered in order to keep the pressure low despite the amounts of nitrogen added to the primary circuit. The helium contained in the reactor pressure vessel as protective gas was fed into the gas tanks to ensure that there were no radioactive gases in the reactor. Then the introduction of nitrogen via the coolant pumps began. After the primary circuit had been cleaned of tetralin, the nitrogen had to be removed from it. For this purpose, a gas mixture of helium and argon was used , which was supposed to replace the nitrogen in the upper part of the reactor pressure vessel. The displaced nitrogen was fed to two of the gas tanks and released into the atmosphere in a controlled manner.

Data of the reactor

Reactor block	Reactor type	net power	gross power	start of building	Network synchronization	Commercialization of essential operation	switching off processing
Sodium Reactor Experiment (SRE)	Liquid metal-cooled, graphite-moderated reactor	20 MW	6.5 MW	1954	04/25/1957	07/12/1957	02/15/1964

Technical specifications	Sodium Reactor Experiment (SRE)
Nuclear fuel	metallic uranium
Enrichment in U 235	2.78%
Nuclear fuel quantity	2273.4 kg
Number of fuel assemblies	43
Number of fuel rods per fuel assembly	7th
Fuel rod length	1.8 m
Fuel rod diameter	19 mm
Number of control rods	4th
Absorber material	B-Ni
Moderator	graphite
Coolant	Na (liquid sodium)

Web links

Commons : Sodium Reactor Experiment  - Collection of Images, Videos and Audio Files

Individual evidence

1. ↑ A History of the Atomic Energy Commission (English, PDF; 2.5 MB).
2. ↑ Symposium on Sodium Reactors Technology (English, PDF; 1.4 MB).
3. ↑ ^{a b c d e f} Investigation of Releases from Santa Susana Sodium Reactor Experiment (English, PDF; 12.26 MB).
4. ↑ ^{a b c} Chemical Behavior of Iodine- 13 1 during SRE Fuel Element Damage in July 1959 (English, PDF; 5.94 MB).
5. ↑ ^{a b c d} An Assessment of Potential Pathways for Release of Gaseous Radioactivity Following Fuel Damage During Run 14 at the Sodium Reactor Experiment (English, PDF; 1.3 MB).
6. ↑ Sodium Reactor Experiment (SRE) Accident (English, PDF; 10.79 kB).