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The Use of Rod-Based Guaranteed Shutdown State in CANDU Reactors

Abstract of the technical paper presented at:
The 2015 International Congress on Advances in Nuclear Power Plants (ICAPP 2015)
Nice, France,
May 3–6, 2015

Prepared by:
Dumitru Serghiuta and John Tholammakkil
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada


A guaranteed shutdown state (GSS) for a CANDU reactor is one in which the reactor will remain in a stable sub-critical state independent of reactivity perturbations caused by any possible changes in core configuration, core properties or process system failures. The usual procedure incorporated in the CANDU reactor design and specified in the operating procedures for entry into GSS is to over-poison the moderator (typically with Gadolinium nitrate solution). This is called over-poisoned guaranteed shutdown state (OPGSS). Other GSS options are: “moderator dump” GSS at Pickering A units and “moderator drain” at Bruce B and Darlington stations.

Utilities have proposed employing an additional guaranteed shutdown state for currently operating CANDU reactors, rod-based guaranteed shutdown state (RBGSS), using all solid rods, i.e., SDS1, MCA and adjusters, for unplanned and short duration shutdowns. The designer has also considered this option and included it as a new feature in the Enhanced CANDU-6 design.

The use of RBGSS in operating CANDU reactors provides significant safety benefits, such as reduction of doses to the workers and amount of low-level radioactive material, and economical benefits, such as shorter time to reach full power level after an unplanned maintenance shutdown. There have been few operating events, some initiated by unexpected failures which led to Gadolinium precipitation, which challenged the reliability of the OPGSS.
Key high-level regulatory challenges related to the use of RBGSS in existing CANDU reactors are:

  • Separation between process and safety systems, due to the use of components of reactor regulating system, such as adjuster rods and mechanical control absorbers
  • Defence in depth: SDS1 design and application of nuclear criticality double-contingency principle
  • Monitoring and protection: long-term GSS requires installation of start-up instrumentation for monitoring and safety protection
  • Availability of shutdown systems: industry good practice has been that one shutdown system should be available (poised) while in GSS
  • Sub-criticality criterion for nuclear criticality analysis
  • Operating procedures and operator training needs to prevent human error 

The paper discusses the regulatory requirements for GSS and CNSC staff expectations related to information in support of an application for regulatory approval of RBGSS as an additional GSS in existing CANDU reactors.

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