Measuring Tritium in Water
Measuring tritium in water and tritium in seawater continuously on a real-time basis is an exceptionally difficult task due to tritium’s low beta emission energy (18.6 keV maximum, 5.6 keV mean) and the shielding properties of water. Traditionally, liquid scintillation counting has been used to determine low level tritium concentration in water, although this method is typically performed by manually taking a sample and analyzing it later.
Overhoff has developed several novel solutions for measuring ultra-low levels of tritium in water on a real-time, continuous basis. Applications include: measuring heavy-water leaks in nuclear power plants (especially of the CANDU type) as well as monitoring tritium concentration in groundwater, drinking water, seawater, and liquid effluents.
The Model 1925 was originally designed in 2004 working in conjunction with AECL and installed at Cernavoda NPP in Romania. The Model 1925 is a real-time, continuous tritium water monitor utilizing an automated liquid scintillation counting system. The primary use of the Model 1925 is to detect the leak of heavy water (tritium) in nuclear power plants, but it can also be used to monitor tritium concentration in groundwater, drinking water, rivers, lakes, or the ocean.
The Model 1925 detects tritium decay via liquid scintillation counting using dual high-quality photomultiplier tubes (PMTs) working in coincidence counting mode. Liquid scintillator and the sample water are automatically mixed at the sample cell and measured. The liquid scintillator is connected to the unit externally and is stored inside a polyurethane drum of 23 liters, which is sufficient for 60 days of continuous 24/7 operation.
The use of highly effective PMTs, a specially designed sampling cell to minimize cosmic radiation and Cherenkov effects, and 1” lead shielding provides for low background noise of only one count per second with a phenomenal counting efficiency of 30%. MDA is 3.7 kBq/L in 9 minutes or less with a 95% confidence level and a resolution of 1.0 kBq/L.
By working in ‘coincidence counting’ mode, the 1925 only counts radiation from actual tritium decay events that hit both PMTs at the same time, rejecting background noise to improve sensitivity and accuracy. Furthermore, pulse shaping and discrimination selects only recognizable tritium decay events, rejecting large amplitude and long-duration gamma effects and system noise.
The Model 1925 is an unparalleled automated liquid scintillation counting system capable of measuring tritium in water or seawater down to 3.7 kBq/L or 100,000 pCi/L in 9 minutes or less with 95% confidence.
The Model 1925-PR is an automated, real-time low-level tritium in water monitor that utilizes a novel electrolysis method to separate the hydrogen (tritium) from oxygen and measure it using an ultra-sensitive gas proportional detector. The main advantage of the 1925-PR is that it does not require liquid scintillation counting cocktail nor does it produce hazardous mixed waste that must be carefully stored, transported, and disposed of.
An electrolytic hydrogen gas separator is used to separate the input water into hydrogen gas (tritium) and oxygen. The oxygen is diluted and safely released while the hydrogen (tritium) is mixed with P-10 gas and pumped through an ultra-sensitive proportional counting detector. By utilizing proportional counting technology, the user only has to acquire a compact standard steel tank of proportional gas (8.5 m3 or 300 ft3), which will last for up to two months of continuous operation and is available from a variety of suppliers.
Measurement via this method achieves same or better low-end sensitivity, without having to deal with LSC fluid and waste. A second, sealed proportional counter eliminates the effects from external gamma fields and 1” lead shielding around the counter tubes further reduces background gamma and improves the statistical signal to noise ratio to enhance sensitivity.
The Model Trimaran-H20 is our most sensitive automated, continuous tritium water monitor. It is based on the 1925-PR but includes a sample enrichment loop to iteratively concentrate the tritium for the lowest MDA possible.
After the input water is electrolytically separated, the oxygen is diluted and safely released while the hydrogen gas containing tritium is cycled iteratively through a proprietary enrichment loop which increases the concentration of tritium (HT and T2). After sufficient enrichment, the tritium sample is mixed and diluted with P-10 gas and then measured by an ultra-sensitive gas proportional counter. The low concentration of hydrogen is dealt with safely through dilution with the P-10 gas and having the instrument enclosures under negative pressure.
The sensitivity of the Trimaran-H20 is 14,000 pCi/L (500 Bq/L) in 8 hours or less, which is below the US EPA’s tritium drinking water limit of 20,000 pCi/L (740 Bq/L).
The Models 1925, 1925-PR, and Trimaran-H20 are all capable of measuring both freshwater and seawater and include an appropriate prefiltration system.
For measuring low to high concentrations of tritium in water, several other real-time, continuous tritium water monitors are also available. No liquid scintillant is required for these systems, allowing you to save money on operating costs.
Model NEX-Tritium: Low Levels
370 kBq/L or 10 µCi/L in 1 hour
37 kBq/L or 1.0 µCi/L in 6 minutes with optional temperature control and cooling
The Model NEX-Tritium utilizes a unique flow-through detector assembly consisting of a crushed bed of our custom-made anthracene scintillation crystals. By using custom crushed scintillation crystals, the input water is intimately mixed with the scintillator material in order to provide better sensitivity in a flow-through, real-time system. The light-tight detector assembly is viewed by a pair of matched photomultiplier tubes (two 5” diameter PMTs, totaling 200cm2 sensitive area) working in coincidence mode. The output from the photomultiplier tubes is analyzed by pulse height and coincidence, thereby permitting the system to eliminate counting most background (noise) counts. Optional: The input water is chilled by a cooling unit and the detection assembly is mounted inside of a refrigeration unit to provide long-term accuracy, stability, and enhanced sensitivity.
Model LIQ-X(H3): Mid to High Industrial Levels
Low- Sensitivity: 1,850 kBq/L or 50 µCi/L in 2 hours
Mid- Sensitivity: 7,400 kBq/L or 200 µCi/L in 2 minutes
High- Sensitivity: 370,000 kBq/L or 10,000 µCi/L in 2 minutes
The Model LIQ-X(H3) is a relatively simple and cost-effective design for measuring industrial levels of tritium in water. It includes a light-tight flow-through detection assembly continuously monitored by a matched pair of photomultiplier tubes working in coincidence mode. The output from the photomultiplier tubes is analyzed by pulse height and coincidence, thereby permitting the system to eliminate counting most background (noise) counts. Includes statistical significance setting (high, low, or not significant) to prevent most false positives or negatives.