The HFIR Gamma Irradiation Facility is an experimental facility designed to irradiate materials with gamma radiation from spent fuel elements. The facility chamber is stainless steel and is made of 0.065-thick tubing to maximize the internal dimensions of the chamber. This allows for the largest samples possible that can still fit inside the cadmium post of the spent fuel loading station positions. The interior chamber is approximately 3.75 inches inside diameter and accommodates samples up to 25 inches long.
There are two configurations for the chamber assembly, with the only difference being the plugs. The uninstrumented configuration has a top plug that is used for installation of the samples, to support the inert gas lines, and maintain a leak tight environment while under water. The instrumented configuration has a chamber extension above the chamber and an “umbilical” to permit inert gas lines, electrical cables, and instrumentation cables for an instrumented experiment to connect with heater controls and instrumentation testing equipment in the experiment room.
An inert gas control panel in the experiment room is required to provide inert gas flow and pressure relief to the chamber. Inert gas pressure is maintained at approximately 15 psig to ensure that any leakage from the chamber would be from the chamber to the pool and not water in leakage.
Samples in the chamber may be supported from the bottom of the chamber or from the plug (uninstrumented configuration only).
Radiation Dose Rates and Accumulated Doses
Characterization of the inside surface of the chamber has been performed, and gamma dose rates at this location have been confirmed. Gamma dose rates up to 1.8E+08 can be provided. Selection of an appropriate spent fuel element can provide essentially any required dose rate.
Because of secondary reactions within sample and holder materials in the chamber, we have created neutronic models to estimate the actual dose rates to the samples in different holders and at different locations within the chamber. The peak dose rates are near the vertical center of the chamber and at the horizontal centerline of the chamber. There is a near symmetrical distribution of the dose rate from top to bottom of the chamber.
HFIR personnel are available to assist in the design of sample holders to achieve the required accumulated doses and dose rates. Temperature of the samples from the required dose rate can be estimated.
Recently performed irradiations have shown that temperatures from the gamma heating can be very high, exceeding 500°F in fresh spent fuel elements. Location of the samples near the chamber wall or holder design to transfer heat to the chamber wall can be used to lower the sample temperature. Selection of a more decayed spent fuel element with a lower dose rate may be necessary if temperature limits are a concern.
The minimum temperatures maintained are around 100°F (clean pool water temperature). Temperatures that must be maintained constant or above the temperature achieved by gamma heating can be provided in the instrumented configuration with the use of a internal heater rod that the samples can be attached to.
Typical inert gases used are argon or helium. If the sample must be maintained in a vacuum or other non inert gases, the sample must be contained in a separate pressure retaining enclosure to be placed in the chamber.
Most materials can be irradiated with the following exceptions:
Explosives or materials that could credibly detonate during irradiation or if contacted by water.
Pressurized sample containers which in the failed state could lead to sudden overpressure (>100 psig) of the gamma irradiation chamber.
Sample material in sealed containers that could become pressurized by radiolysis of the sample material, off gassing, phase changes, or other means and that could potentially result in sudden overpressure (>100 psig) of the gamma irradiation chamber.
Materials in a quantity that pose a credible corrosive threat to the HFIR fuel, pool liner, or reactor components in pool storage.
Sample materials must be verified or certified before installation.