NR is used to denote that the reporting of radiation exposure monitoring was not required as stated in Regulatory Guide 8.7, page 8.7-2, section 1.1. NR can be seen as the most ambiguous of the dose codes in that it can be used in two distinct circumstances: when monitoring has not been provided to an individual, or when monitoring was performed, but the results were not required to be reported to the NRC.

Monitoring is not required if a prospective evaluation shows that the individual's dose is not likely to exceed 10% of the annual dose limits (10 CFR 20.1502). A study of the ambient dose rates and the individual's activities in the work area could reveal that the dose levels are expected to be below the 10% threshold. The individual's radiation exposure history may reveal that the individual has not exceeded the monitoring requirement threshold if, in addition, it is not anticipated that the radiological conditions will change over the course of the monitoring year. The analysis to determine monitoring requirements may be performed for an individual or a group of similarly exposed individuals.

The use of NR has several drawbacks. The main problem is the ambiguity of the dose record. By recording NR on the dose record, a future review of the records would not be able to determine whether the individual was actually not monitored at all during this time period, or whether monitoring was provided and the results were simply below the reporting requirement threshold. An uncertainty would need to be applied to the dose record reflecting that the dose could have been as much as 10% of the annual dose limits.

This may have a significant impact on the collective dose analysis performed using the REIRS data. In the past, individual whole body doses below 500 millirem have contributed to a large component of the collective dose for each facility. In 1993, 82% of the monitored personnel receiving measurable dose at nuclear power facilities received whole body doses less than 500 millirem. A sampling of the termination data submitted for workers during 1992 and 1993 indicates that the collective dose for these individuals contributed as much as 15% of the collective dose at these facilities. If the total effective dose equivalent (TEDE) to each of these monitored individuals was reported as NR, a significant amount of the total collective dose to the occupationally exposed work force would go unreported.

However, a threshold below which monitoring is not required is not new to the revised Part 20. Under the provisions of the old Part 20, monitoring was required only when the expected dose exceeded 25% of any applicable quarterly limit. The practice of most licensees has consistently been to monitor and report all doses. This may be due to legal liability concerns. However, there is no reason to expect that reactor licensees will change this practice with the implementation of the revised Part 20. For these reasons, it would be prudent for licensees to continue to record and report the results of monitoring whenever they are performed. This practice would document that monitoring was performed and would record the actual results of monitoring.

Another potential difficulty is for individuals who were not expected to receive exposure above the 10% threshold who subsequently exceed their expected dose. Licensees would then need to calculate the dose received during any time periods where monitoring was not provided. Extensive review of facility survey data, radiation work permits, and time/motion studies may need to be performed to assess this unmonitored dose. See section 1.1 of Regulatory Guide 8.7.

ND is to be used when monitoring has been performed, but the results are below the lower limit of detection (LLD) for the dosimetry as determined by a dosimetry processor accredited through the National Voluntary Laboratory Accreditation Program (NVLAP). By definition, zero results are not detectable and should be recorded as ND on the NRC Forms 4 and 5. Regulatory Guide 8.7, Rev. 1 instructs that zero dose recorded on Form 5 submittals be interpreted as ND for submission to the NRC and subsequent entry into the REIRS data base. Licensees that do not submit in accordance with this guidance will be notified of this interpretation and will be asked to submit information in accordance with this interpretation in future reports.

The definition of ND is perhaps less ambiguous than NR but also poses some problems. The definition of ND is dependent upon the detection limit of the dosimetry device. One licensees may use a different type of dosimeter than another, and even may use different dosimetry for different activities performed by the same individual at a facility. The regulations do not require the licensee to submit the lower limit of detection (LLD) with the dose record. The REIRS data base will make no distinction or evaluation of data submitted at low dose levels. It will be the responsibility of the licensee and the NVLAP accredited dosimetry processor to determine the LLD. It is the responsibility of the licensee to report the appropriate doses on the Form 5.

The dose code NC has been the most misunderstood and misapplied dose code of the three considered here. Ironically, NC has the most unambiguous definition with the most strict interpretation by the NRC. The purpose of this code is to allow licensees to submit raw data whenever recorded without the requirement for the calculation of organ doses when it is not possible for the limit to any organ to have been exceeded. NC may be entered for the committed dose equivalent (CDE) when the committed effective dose equivalent (CEDE) has been determined to be less than 1 rem, and therefore the calculation of CDE to the maximally exposure organ is not needed. If the CDE is reported to be NC, then the total organ dose equivalent (TODE) for that record is also NC. NC is only to be used for recording the CDE and the TODE. NC is not used for recording the CEDE or any other dose value.

It is noted that there is an apparent error in the instructions printed on the back of the Form 5 in Regulatory Guide 8.7 under the instructions for item 15 for CEDE. NC is not an appropriate entry for the CEDE. The text in section 2.2 is correct and accurately defines the use of NC.

Three assumptions can be made when a dose value is reported as NC. First, monitoring is required (not NR), therefore monitoring was performed. Second, it is not ND, so there are known intakes and the results of monitoring were a positive numerical value above some minimum detection limit. Finally, the CEDE, calculated using DAC-hrs times 2.5 mrem/DAC-HR, is less than 1 rem.

If there is an NC entry on a NRC Form 5, there must be intake entries on the Form 5 for this individual, since the individual must have experienced intakes. Therefore, if the code NC is used, the following must be true:

• NC is only used for CDE and TODE

• CEDE is greater than MDA and less than 1 rem

• There must be intake entries recorded on the Form 5

The use of the dose codes on the NRC Form 5, and the requirement for summation of the external and internal dose equivalents, leads to the necessity of defining the rules of addition" for these codes. By definition:

                TEDE = DDE + CEDE
                TODE = DDE + CDE

If dose codes are used for DDE, CEDE, or CDE, these codes will need to be summed to determine the TEDE and TODE. No rules of addition are provided in Regulatory Guide 8.7 where these dose codes are presented. The following represents a matrix of the rules of addition for the NR and ND dose codes.

                ND + numeric = numeric
                NR + numeric = numeric
                NR + ND = ND

The above equations show that the numeric value takes precedence over the dose code. The dose codes are treated as zero in this manner. For ND, this is certainly reasonable in that the possible values represented by the dose code must be very small to be below the detection limit of the NVLAP dosimetry device. Most often, ND is used to represent a value of 0" returned by the dosimetry processor. ND should only be used when monitoring was provided and the measured dose was below the detection limit. Therefore, ND takes precedence over NR in the summation shown above. The resultant ND retains the additional piece of information that monitoring was provided for one component of the dose, and the measured value was below the LLD.

NR represents a larger uncertainty. The individual may have been monitored, but was not anticipated to exceed 10% of any applicable limit and in fact remained under the 10% threshold and therefore was not required to be reported. Or, the individual may not have been monitored at all due to a prospective analysis that the individual was not likely to exceed 10% of the dose limit for a specific part of the body, such as the extremities. An entry of NR for the SDE-ME could be anything from zero to 5 rem. However, since NR could also be used when monitoring was not provided, it has been determined that the NR would be treated as "null" in summation and no assumptions would be made as to whether it denotes dose below the 10% threshold, or that monitoring was not provided.

The uncertainty associated with the use of NR is another argument in favor of the position that actual dose values should be recorded whenever possible. ND should be used to denote when dose values are below the detection limit, and NR would, by default, be used when monitoring was not provided.

It is important to understand, however, that by encouraging the use of NR in only those circumstances that monitoring was not provided does not re-define NR to mean "not monitored." The NRC will continue to define NR as not being required to report.

The rules of addition for NC are straightforward and are based on the fact that if the CDE has not been calculated, the resultant TODE is unknown. This could be stated by NC plus anything equals NC."

                CDE   DDE         TODE 
                NC +  numeric=    NC
                NC +  NR    =     NC
                NC +  ND    =     NC

In the above equations, NC takes precedence over the other dose codes, and even over a numeric value. This may seem contradictory to the position taken for the NR dose code. While it is true that the numeric uncertainty associated with NC is similar to NR, there is an important distinction. When NC is used, it is known that intakes have occurred and that there has been some committed dose to a maximally exposed organ. According to Regulatory Guide 8.7, page 8.7-3, Section 2.2, NC is only used when the CEDE has been calculated, but is less than 1 rem. In such a case NC may be entered to save the licensee the additional effort of calculating the CDE since the non-stochastic organ dose limit could not be reached if the CEDE is below 1 rem.

NC indicates that dose has been accrued since the CEDE is greater than zero but less than one rem. The intakes are recorded on Form 5 even though the CDE is not calculated. Other intakes may have occurred for this individual at other licensees during the monitoring year that would need to be included in the calculation of CDE for the year. Since the intakes contributing to the CEDE and CDE are recorded, the CDE and TODE could be calculated at a later date using an appropriate internal dosimetry calculational model.

If CDE is NC, TODE must also be NC. Data retrieved from REIRS will not include the notation NC. When submitted by licensees, the dose will be calculated using the dose conversion factors in EPA report number 11 and entered into the data base. If a dose history is requested, it will include numerical values for any NC entries submitted by licensees

In summary, ND means very low or zero measured dose. ND is mathematically taken to be zero in summation with other values. NR does not allow for assumption of dose since monitoring may not have been provided or monitoring was provided, and was below the threshold. It is taken as zero for the purpose of calculating collective doses but is known to involve significant uncertainty and would not be useful for epidemiological purposes. NC is only used when internal monitoring has been provided, intakes have occurred, and the CEDE has been calculated and found to be less than 1 rem. NC saves the licensee the effort of calculating the CDE.

Table 1 shows a matrix summarizing the rules of addition for the dose codes. The results most commonly misinterpreted are shown in bolded italics.

Table 1. Addition Matrix

+	NR	ND	Numeric
NR	NR	ND	Numeric
ND	ND	ND	Numeric
NC	NC	NC	NC
Numeric	Numeric	Numeric	Numeric

It should be noted that these interpretations of the dose codes and the rules of summation will allow for future re-interpretation. The REIRS data base will record the specific entries of dose values or dose codes for DDE, CEDE, and CDE so that if it is decided in the future that the rules of summation are to be re- interpreted, the source data will exist to accommodate re-calculation. What is of more immediate importance is that the dose codes are used consistently by NRC licensees so that subsequent analysis of data will be consistent with the intended use at the time the information was recorded.