Standard Test Method for Nondestructive Assay of Plutonium, Tritium and ²⁴¹Am by Calorimetric Assay

Importancia y uso:

5.1 This test method is useful for determining the quantity of one or more isotopes present in a container like in the context of process monitoring, safeguard and inventory of nuclear materials.

5.1.1 Calorimetry is considered to be an established NDA method for the measurement of tritium. It is applied for different physical and chemical forms of tritium (gaseous, adsorbed, tritiated water, organic).

5.1.2 Calorimetry assay is applied to a wide variety of Pu-bearing solids including metals, alloys, oxides, fluorides, mixed Pu-U oxides, mixed oxide fuel pins, waste, and scrap, for example, ash, ash heels, salts, crucibles, and graphite scarfings) (2, 3). This test method has been routinely used at U.S. and European facilities for Pu process measurements and nuclear material accountability since the mid 1960’s (2-9).

5.1.3 Pu-bearing materials are measured in calorimeter containers with diameters ranging from 0.025 m to 0.63 m and heights ranging from 0.076 m to 1.38 m.

5.2 Calorimetry assay requires additional non-destructive isotopic distribution measurements by gamma-ray spectrometry or destructive analysis techniques when it is applied to blends of radioisotopes like it is the case for the assay of Pu.

5.2.1 Gamma-ray spectrometry typically is used to determine the Pu isotopic composition and ²⁴¹Am to Pu ratio (see Test Method C1030). However, isotopic information from mass spectrometry and alpha counting measurements may be used instead (see Test Method C697).

5.3 Calorimeter measurement times range from 20 min (10) for smaller, temperature-conditioned containers up to 72 h (11) for larger containers and items with long thermal time constants.

5.3.1 Four parameters of the item and the item packaging affect measurement time. These four parameters are density, mass, thermal conductivity, and change in temperature. The measurement well of passive calorimeters will also affect measurement time because it too will need to come to the new equilibrium temperature.

5.3.2 Calorimeters operated in power compensation mode maintain a constant measurement well temperature and have no additional effect on measurement time.

5.3.3 Measurement times may be reduced by using equilibrium prediction techniques, by temperature preconditioning of the item to be measured, by operating the calorimeter using the power compensation technique, or by optimization of the item container (low thermal mass and high thermal conductivity) and packaging.

5.4 The packaging conditions and matrix of the item cannot change the heat output of the material, but they are usually the determining factor for measurement time as mentioned in 5.3.1.

5.4.1 Regardless of the packaging conditions and matrix, the heat output of a container is directly proportional to its radioisotope content if (1) the container is designed with thick enough walls to absorb the entire energy deposited by the isotope’s decay—which is to be expected in the case of alpha and beta decay—and (2) there is no other heat source in the container.

5.4.2 This test method is largely independent of the elemental distribution of the nuclear materials in the matrix.

5.5 The thermal power measurement is traceable to the international measurement systems (the SI) through electrical standards used to directly calibrate the calorimeters or to calibrate secondary ²³⁸Pu heat standards.

5.5.1 Physical standards representative of the materials being assayed are not required for the test method.

5.5.2 Heat-flow calorimetry has been used to prepare secondary standards for neutron and gamma-ray assay systems (7-9, 12-14).

Subcomité:

C26.10

Referida por:

C1490-14R23, C1030-10R18, C1207-10R18, C1592_C1592M-21, C1133_C1133M-10R18, C1500-08R17, C1931-23

Volúmen:

12.01

Número ICS:

27.120.30 (Fissile materials and nuclear fuel technology)

Palabras clave:

accountability; americium; calorimetry; inventory; nondestructive assay; plutonium; quantification; tritium;