UNMET CHALLENGES TO SUCCESSFULLY MITIGATING SEVERE ACCIDENTS IN MULTI UNIT CANDU REACTORS

摘要

One sees eerie similarities here in Canada to the cozy relationship between regulator and utilities in 'pre-Fukushima' Japan. Such ties are hardly limited to Canada though. The chronic degradation of real commitments to continued improvements in reactor safety systems and a decline in overall safety culture that discourages critical design reviews and willfully ignores well justified, safety critical hardware upgrades, has created alarming conditions that are likely inching us towards another nuclear disaster. Operating CANDU reactors are now close to being obsolete but have barely seen any substantive severe accident related risk reduction upgrades nine years after Fukushima, hoopla in Canada around some minor improvements and premature closure of even otherwise sparse and what were really weak regulatory 'Fukushima Action Items' notwithstanding. With a number of common barriers to fission product releases to environment missing or weak, one would expect the regulator to be extra vigilant in promoting prevention and encouraging delays in onset of core damage. On the contrary, it has only made matters worse by its collusion & obfuscation as long summarized in [1] and even denying the additional burden of age related degradations as in long operating licenses 50% longer than design life at Pickering [2]. Whether the regulatory actions are out of ignorance, inability or intent is debatable but equally disturbing. The multi-unit CANDU stations sport some of the weakest and leakiest containments in the world. With no reactor pressure vessel to isolate the overheating channel and debris, these leaky containments will directly see un-attenuated fission products releases from the fuel. They will trap combustible D_2 gas in interconnected from below inverted cup like crowded reactors vaults to an increased gas explosion potential. The reactor units have high steam and air oxidation potential on both sides of over 10 km of low carbon steel feeder piping with over 1800 m~2 hot surface areas exposed for each of internal steam and external air oxidation and copious amounts of core Zircaloy (> 50,000 kg, twice of that in a BWR of similar power). Combustible gas detection and mitigation systems are designed for Hydrogen (H_2) instead of Deuterium (D_2) gas in these D_2O cooled and D_2O moderated PHWRs. The pressure relief systems in primary cooling and moderating systems are dangerously inadequate, resulting likely in pressure boundary ruptures and early containment bypass, accelerated onset of core damage and vessel failures. Backup diesel generators are located low and close to water as in Fukushima. Spent fuel pools are overcrowded with horizontally stacked fuel bundles akin fish in fish-baskets. Yet, the emphasis has shifted to passing wishful thinking of low off-site releases [3] and convenient half-truths of an early core collapse terminating further core degradation and releases into containment as facts and ignoring [4] known design vulnerabilities that amplify risk actively denying [5] even the basic science on high temperature oxidation of carbon steel [6]. Even more dangerous are the unsubstantiated claims being made of near impossibility of off-site releases of long lived species from these multi-unit reactors by utility management [7] without nay a challenge by the regulators. The life management issues of ageing, elongating, thinning, hydriding, embrittling and deforming CANDU Pressure Tubes is yet to be resolved but these obsolete reactors keep getting ever longer license extensions (e.g. for 10 more years, over 50% beyond original Pickering pressure tube design life -ignoring their own data [8] that suggests that safe operation cannot be guaranteed due to elongation. There are loud, ambiguous references to compliance with un-named IAEA documents and standards. No IAEA document has yet identified or discussed the PHWR design vulnerabilities that may lead to disastrous outcomes and this paper is repeating in forums akin ICONE for the nth time. Of equally great harm to risk reduction are the IAEA team of experts missions (Integrated Regulatory Review Service (1RRS) follow-up missions - for example [9] that issue oversight certifications / seals of approval to the Canadian regulator CNSC without anything resembling a technical evaluation of CANDU design elements that contribute to risk. Many critical vulnerabilities and proposed engineering fixes that can be undertaken to overcome also been highlighted routinely [10] but are groundlessly rejected as in [11] which begs for an international impartial scrutiny in ingrained obdurate industry intransigence against changes and investments into substantive safety improvements and risk reduction. Emergency preparedness by civil authorities has been illogically conditioned for the smallest possible 'Large Release' source term (of e.g. 100 TBq of Cs-137) and available response time for mitigation measures have been exaggerated baselessly. Both acts are irresponsible and dangerous to public and first responder safety. A number of early mitigation measures to externally replenish boiler inventory (a measure common to all PWRs) will not work due to an unusually low, below core, placement of pressurizer that will gradually gravity drain much primary coolant from boiler tubes. So the most important emergency measure to restore core cooling by reflooding boilers to induce natural circulation flows will go to waste. Operators will never know why the core never cooled. Inability of the utilities to accept responsibility for reactor upgrades and inability of the regulatory management to act independently are the signs of impending implosions in our nuclear industry. It is likely because the regulatory body CNSC is critically dependent upon the licensees financially in a 'cost recovery' plan. Not likely, but perhaps if we get lucky, an impending disaster can be avoided by a return to the first principles, and not mere slogans, of 'safety first'. Right now, an unmitigated station blackout in a CANDU multi-unit station will make the Fukushima disaster look like a walk in the park.