On-line monitoring of activity buildup through gamma spectroscopy

On-line monitoring of activity buildup has been pioneered at the Forsmark BWRs by Ullberg and co-workers, in part within the framework of the WACOL programme. The details are as follows.

Dose-rate measurements and gamma scans are performed each refuelling outage in Forsmark 1–3 and in most other BWRs. Interpretation of the results is made difficult by the low frequency of measurement. In particular, it is often impossible to answer questions such as:

• what caused an increase/decrease of the dose rate?

• when did the increase/decrease occur?

• is the measured increase/decrease real, or is it an artefact?

Problems like these would be well addressed by line gamma scans, but for a long time on-line gamma scanning in BWRs was deemed not to be possible because of high background levels from short-lived activity in the reactor water, such as N¹⁶ with t_½ = 7.1 s.

In 1996 it was found by G. Granath at Ringhals that very good data could be obtained for Co⁶⁰ on a pipe in the RWCU system at some distance from the reactor. The transport time of the reactor water to the measuring point was 1 min, corresponding to eight half-lives for N¹⁶.

FIG. 26. Results of in-core ECP monitoring at the Duane Arnold BWR before and after application of NMCA treatment (after Hettiarachi et al.).

FIG. 27. Location of the OLA measurement system.

Useful data were also obtained for a number of other important nuclides, such as Co⁵⁸. The same type of on-line activity measurement was implemented in Forsmark 2 in 1998 using a measurement system called OLA (On Line Activity), which consists of a detector, associated electronics and a PC for data collection. The detector is situated in a passage in the reactor building, outside a shielded cell containing a heat exchanger and piping for the RWCU system. The detector monitors the activity on a vertical pipe in the cell through a hole drilled in the concrete wall, see Fig. 27. The pipe contains hot reactor water and the concrete wall acts as radiation shield. In the hole in the wall, there is a collimator and a background sample.

The OLA equipment collects one spectrum every four hours. The only required maintenance is replenishment of liquid nitrogen once a week. However, a considerable amount of work is necessary to process the raw data and to evaluate it. Some results obtained to date are as follows.

To reduce the risk of enhanced spacer shadow corrosion of the fuel cladding, iron has been injected into the feedwater of Forsmark 2 since November 1998. OLA has been used to study the effect of the modified reactor water chemistry on pipe surface activity. Iron injection caused similar, minor reductions of the levels of Co⁵⁸ and Co⁶⁰ and a major reduction of the level of Sb¹²⁴ in the reactor water. In the middle of April 1999, NaOH injection (to be able to determine carry over by Na²⁴ measurement) caused a temporary reduction of all three activated corrosion products.

Fig. 28 shows a significant reduction of the surface activity of Co⁵⁸ and minor reduction of Co⁶⁰. (The figure shows untreated data for Co⁶⁰ and a moving average over five data points for Co⁵⁸.) Both nuclides are, of course, chemically identical. Therefore, it is reasonable to attribute the different behaviour to the different half lives, 71 days and 5.3 years, respectively.

0

98-10-01 98-10-31 98-11-30 98-12-31 99-01-30 99-03-02 99-04-01 99-05-02 99-06-01 0

FIG. 28. Surface activity of Co⁶⁰ (upper curve) and Co⁵⁸ (lower curve) on RWCU pipe in Forsmark 2 since start of iron injection.

From previous experience, it is known that the residence time of Co⁶⁰ and Co⁵⁸ on system surfaces in Forsmark 2 is long. It is consequently reasonable to assume that the shortest time constant of the system is given by the half life of Co⁵⁸. An exponential function with the Co⁵⁸ decay rate in the exponent is seen to fit the Co⁵⁸ data well. From this fit we may predict that Co⁵⁸ will eventually settle at a new level of approx. 45% of the original. The expected reduction of the surface activity is consequently greater than the reduction of the Co⁵⁸ level in the reactor water. This indicates a lower deposition tendency of Co⁵⁸ on system surfaces in Forsmark 2 when iron is injected. The decreasing trend seems to have been temporarily interrupted by the chemistry transient caused by NaOH injection in April 1999. From the chemical identity, we may predict that Co⁶⁰ is also decreasing towards approx. 50% of the level in November 1998. The surface dose rate of the pipe will follow Co⁶⁰.

The experience at Forsmark enables the following conclusions and recommendations. Activity buildup in nuclear plants must be investigated at a high level of detail. This makes simulation in the laboratory extremely difficult. The data obtained demonstrate that subtle factors such as the balance of corrosion products, may have a profound effect on surface activity. The effect may also differ radically from one activated corrosion product to another. Direct study in-plant is therefore the preferred approach. To be able to relate cause and effect, it is necessary to have plant data of sufficient quality and frequency. It is obvious that the on-line data of Fig.

29 are an enormous improvement over the "once-a-year data". It is noteworthy that the significant advance described here did not require development of a single new component.

Only judicious application of existing technology is involved.

A significant amount of manual work is currently required for processing of the raw data and for subsequent evaluation. Automatic evaluation is still some way off, due to limited knowledge, but processing of the raw data should be automated as far possible by further development of the software.

FIG. 29. Schematic diagram of technology in use for real-time monitoring of SCC behaviour in commercial BWRs (after Gordon & Miller).