Measurement of prompt gamma rays from the reactor core of PHWR during initial criticality experiments

RAPS-5 and 6 is a twin unit reactor of type PHWR of capacity 220 MWe each. Initial reactor core of each unit was loaded with 3496 numbers of safe guarded natural uranium (NTU) fuel bundles and 176 numbers of deeply depleted uranium (DDU) fuel bundles in the set defined pattern. An exhaustive exercise was carried out in co-ordination with RCD, Bhabha Atomic Research Centre to measure the passive gammas originating from reactor core prior to initial criticality and prompt gammas originating from reactor core during various stages of boron removal till first approach to criticality (FAC) of reactor. About seventeen gamma spectra of reactor core of RAPS-6 were acquired to meet the following objectives: Experimental input to assess the gamma fraction of heat by measurement of Core Gamma rays i.e. passive and prompt gamma rays; Experimental input to assess the sources of dose rates by measurement of Gamma ray activities at the FM Vault and Feeder lines; Compilation of base level data, free from interferences by measurements of Virgin Core; Delayed gamma ray activity, free from prompt gamma ray fraction by measurements during shut down period of reactor; Qualitative contribution of passive and prompt gammas in FM Vaults from fresh core to first criticality of reactor besides indication on the effectiveness of reactor core shielding. Initially, the background spectra were acquired in the FM Vault (North) before the end-face of RAPS-6. Spectra were also acquired of the same, at various stages while the boron level was being lowered for making the reactor critical and for 50 minutes during criticality period, when the power level of the reactor was expected to be about 10-10 to 10-11 Full Power (FP). In order to perform the experiment, a complete laboratory based MCA setup was installed every time at the location in spite of difficult environmental conditions due to ongoing startup activities related to FAC. From the acquired spectra, following were observed: Prompt gamma rays from the Calandria of PHWR could be recorded successfully using HPGe at RAPS-6; Prompt gamma rays from the endshield (of Fe, Cr, Ni, Mn and H) were prominent; Prompt gamma ray seen from the Heavy Water was that of Deuterium; Passive gamma rays from N-16 and O-19 were observed; Background gamma ray peaks observed were from the trace amounts of Th and U and their daughter products in the natural decay series present in the concrete of FM vaults, This experiment has provided important information on reactor core behavior that may be used for shielding considerations and by designers for future plants. (author)