Jerry,
There are many variables associated with zinc. One big one would be core duty and core inventory. Higher duty cores use sub-nucleate boiling. The higher your core duty usually means the high crud inventory and subsequent releases. You are a 3-loop plant which is very favorable as core duty is usually lower. During end of cycle deboration periods, the primary system water wants to come to equilibrium so boron is released from the fuel. This will also release the cobalts, manganese, iron, etc and other isotopes with the boron thus resulting with higher RCS activity. When zinc is added even more of these isotopes are released due to displacement. Since cyclic zinc is a player your strategy is important. Typically during the first few cycles of depleted zinc injection you add at a reduced rate and only during the last few months of the cycle. Since that period is also the period the core releases crud in higher duty cores you could see higher RCS than during previous outages. Typically first cycle is limited to 40-50 ppb months or 5 ppb for 8 months. Higher numbers will cause higher releases during pre-outage periods. Even after several cycles of zinc injection, if the injection rate is increased at the end of cycle (say from 5 ppb to 8 ppb) you may see another increase in RCS activity. So in effect the two mechanisms of release (deboration/core crud inventory AND depleted zinc acetate injection rate/ppb months) can results with elevated end of cycle dose rates. Together these two items can be down right nasty. Mitigation attention can be focused on the clean-up during the end of cycle period. * Fresh on-line Letdown bed put in 45 days prior to outage (possible overlay of some type of specialty resin), * Good (120 gpm) on-line letdown flow rates, * Sub-micron CVCS filtration. We prefer smaller sub-micron filters but only if it results with minimal filter changes. You don't want to sacrifice letdown flowrates for filter changes so this needs optimized. We prefer not more than 1 change per 2 week period but zero is best. * Increased EOC boron (20 ppm will see reduced core releases while 10 ppm will sum what stabilize releases) can help alleviate some of the problems. The Brits use 100 ppm EOC boron but US plants will not waste the lost generation. We have seen very good results at 50 ppm but it would take a mid-cycle forced outage before nuclear fuels group will permit such high EOC numbers. The EOC boron will only effect pre-outage and shutdown releases. Forced oxidation will release that 2-5% not released during the deboration period. Additional on-line dose rate monitoring: * We used our in plant dose rate instrumentation for ERO during pre-outage * If we saw dose rate increases on area rad monitors (AR system) we increased the RP dose rate base point survey periodicity in accessible areas for CV / CS systems. * I would also recommend increase chemistry sampling of RCS during 10 weeks of pre-outage for early detection of potential increases with RCS activity. RCS activity increases will be seen before dose rate increases are seen. Additional outage dose rate monitoring: * SI and RH systems will only see increases during shutdown periods when RH is in use (e.g. reactor cool down and/or during forced oxidation). This means initial shutdown surveys may not see increased dose rates in RH/SI until they are put into service. * If higher pre-outage dose rates are seen, during shutdown you may want to pay attention to low flow areas of: * Pressurizer (RY), especially small bore lines or bypass lines to spray valves * Reactor Coolant (RC), especially loop crossovers, bypass, and pressure equalization lines. Dead legs such as the RC warm up lines may also be affected. * RCDT input and transfer to radwaste lines * Floor (RF) Drains (some RE lines tie into RF) * Equipment (RE) Drains * Any other primary system drain piping such as S/G bowl drains. Even if higher RCS pre-outage activity is experienced once forced oxidation clean-up is completed elevated dose rates may be avoided. Items like clean-up duration, flow rates, and resin strategy can help avoid bad outages. ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Barber, Jerry Sent: Saturday, March 02, 2013 2:34 PM To: [email protected] Subject: Powernet: Zn Injection - Initial Increases in Dose Rates We are about to start injecting Zn at Robinson (finally!) which is a 3 loop Westinghouse PWR. In addition to placing more ARMs at locations along CVCS components and piping, we are also looking at any changes to our routine surveillances that should be implemented. For those plants that still have folks around who can remember back when they first started Zn injection: * Did you have any dose rate "surprises"? If so, where? * Did you change the frequency of any routine radiological surveillances? Jerry Barber - RP Supervisor - Field Operations Robinson Nuclear Plant 843-857-1496 Work 843-861-2524 Cell [email protected] ----------------------------------------- ************************************************** This e-mail and any of its attachments may contain Exelon Corporation proprietary information, which is privileged, confidential, or subject to copyright belonging to the Exelon Corporation family of Companies. This e-mail is intended solely for the use of the individual or entity to which it is addressed. If you are not the intended recipient of this e-mail, you are hereby notified that any dissemination, distribution, copying, or action taken in relation to the contents of and attachments to this e-mail is strictly prohibited and may be unlawful. If you have received this e-mail in error, please notify the sender immediately and permanently delete the original and any copy of this e-mail and any printout. Thank You. **************************************************
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