5.1 This practice is useful for the determination of the average energy per disintegration of the isotopic mixture found in the reactor-coolant system of a nuclear reactor (1).5 The value is used to calculate a site-specific activity limit for the reactor coolant system, generally identified as:

The activity of the reactor coolant system is routinely measured, then compared to the value of A_limiting. If the reactor coolant activity value is less than A_limiting then the 2-h radiation dose, measured at the plant boundary, will not exceed an appropriately small fraction of the Code of Federal Regulations, Title 10, part 100 dose guidelines. It is important to note that the measurement of the reactor coolant system radioactivity is determined at a set frequency by use of gamma spectrometry only. Thus, the radionuclides that go into the calculation of and subsequently A_limiting are only those that are measured using gamma spectrometry.

5.2 In calculating , the energy dissipated by beta particles (negatrons and positrons) and photons from nuclear decay of beta-gamma emitters includes the energy released in the form of extra-nuclear transitions such as X-rays, Auger electrons, and conversion electrons. However, not all radionuclides present in a sample are included in the calculation of .

5.3 Individual nuclear reactor technical specifications vary and each nuclear operator must be aware of limitations affecting plant operation. Typically, iodine radionuclides with half-lives of less than 10 min (except those in equilibrium with the parent) and those radionuclides identified using gamma spectrometry with less than 95 % confidence level are not included in the calculation. However, technical requirements specify that the reported activity must account for at least 95 % of the activity after excluding radioiodines and short-lived radionuclides. There are individual bases for each exclusion.

5.3.1 Radioiodines are typically excluded from the calculation of because United States commercial nuclear reactors are required to operate under a more conservative restriction of 1 μCi (37 kBq) per gram dose equivalent ¹³¹I (DEI) in the reactor coolant.

5.3.2 Beta-only-emitting radio isotopes (for example, ⁹⁰Sr or ⁶³Ni) and alpha emitting radioisotopes (for example, ²⁴¹Am or ²³⁹Pu) which comprise a small fraction of the activity, are not included in the E-bar calculation. These isotopes are not routinely analyzed for in the reactor coolant and, thus, their inclusion in the E-bar calculation is not representative of what is used to assess the 10 CFR 100 dose limits. Tritium, also a beta-only emitter, should not be included in the calculation. Tritium has the largest activity concentration in the reactor coolant system but the lowest beta particle energy. Thus, its dose contribution is always negligible. However, its inclusion in the E-bar calculation would raise the value of A_limiting, yielding a non-conservative value for dose assessment.

5.3.3 Excluding radionuclides with half-lives less than 10 min, except those in equilibrium with the parent, has several bases.

5.3.3.1 The first basis considers the nuclear characteristics of a typical reactor coolant. The radionuclides in a typical reactor coolant have half-lives of less than 4 min or have half-lives greater than 14 min. This natural separation provides a distinct window for choosing a 10-min half-life cutoff.

5.3.3.2 The second consideration is the predictable time delay, approximately 30 min, which occurs between the release of the radioactivity from the reactor coolant to its release to the environment and transport to the site boundary. In this time, the short-lived radionuclides have undergone the decay associated with several half-lives and are no longer considered a significant contributor to .

5.3.3.3 A final practical basis is the difficulty associated with identifying short-lived radionuclides in a sample that requires some significant time, relative to 10 min, to collect, transport, and analyze.

5.3.4 The value of E-bar is usually calculated once every 6 months. However, any time a significant increase in the activity of the reactor coolant occurs, the value of E-bar should be reassessed to ensure compliance with 10 CFR 100. Such reassessment should be done any time there is a significant fuel defect that would alter the value and affect A_limiting. The two possible causes to reassess the value of would be:

(1) A significant fuel defect has occurred where the noble gas activity has increased.

(2) A significant corrosion product increase has occurred.

For the case of a fuel defect, the plant staff may need to include new radionuclides not normally used in the calculation of such as ²³⁹U and ²³⁹Np.

1.1 This practice applies to the calculation of the average energy per disintegration ( ) for a mixture of radionuclides in reactor coolant water.

1.2 The microcurie (µCi) is the standard unit of measurement for this standard. The values given in parentheses are mathematical conversions to SI units, which are provided for information only and are not considered standard.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.