As was written, subcriticality of the spent fuel pool is ensured by:
- the design of the spent fuel pool,
- requirements on boric acid diluted in water,
- limiting of stored fuel (e.g. fuel enrichment, assembly burnup)
Today, spent fuel is usually stored in so called high-density racks or in the maximum-density rack (MDR). Using such racks, fuel assemblies can be stored in about one half the volume required for storage in standard racks. Higher storage densities have been achieved without the risk of a nuclear chain reaction by adding neutron absorbing materials (typically boron) in storage racks and baskets, and dissolved in the water itself. These racks incorporate (boron-10) or other neutron-absorbing material to ensure subcriticality. Boron-10 is generally present in the chemical form of boron carbide (B4C) within a metal matrix (e.g., Boral and Metamic (trademark of Metamic, LLC)) or a polymer matrix (e.g., Boraflex (trademark of BISCO), Carborundum, and Tetrabor), although borated stainless steel incorporates the boron-10 atoms into the alloy composition.
In the high-density rack design, the spent fuel storage pool may divided into two separate and distinct regions which, for the purpose of criticality considerations, are considered as separate pools. In Region 2, maximum reactivity fuel is not allowed to load, while it can be loaded in the racks of Region 1 of the pool.
Most conservative approach requires that the multiplication factor, assuming flooding with pure water and infinite geometry, does not exceed 0.95 with a full loading of the maximum anticipated enrichment. To satisfy this design criterion, the assumptions in the criticality evaluation are as below:
- The fuel assemblies have the maximum approved initial enrichment with the highest reactivity in fuel’s lifetime, and without the control rods (burnable poisons may be taken into account).
- In the flooding condition, all soluble poison is assumed to have been lost, specify that the limiting keff of 0.95 (5% subcriticality) be evaluated in the absence of soluble boron.
- The array of the fuel assemblies can be taken as infinite geometry, or with reflective boundary condition.
- The effect of structure material and the fixed neutron absorber can be considered.
- Unless the double contingency principle is taken, the presence of the boron in the moderation should not be considered. This principle shows at least two independent, unlikely and concurrent incidents have to happen to lead a criticality accident.
The water in the spent fuel storage pool normally contains soluble boron, which results in large subcriticality margins under actual operating conditions. We mean that boric acid is dissolved in the coolant. Boric acid (molecular formula: H3BO3), is a white powder that is soluble in water. According to the NRC guidelines, based upon the accident condition in which all soluble poison is assumed to have been lost, specify that the limiting keff of 0.95 (5% subcriticality) be evaluated in the absence of soluble boron. Hence, the design of the spent fuel pool is based on the use of unborated water. Unless the double contingency principle is taken, the presence of the boron (boron credit) in the moderation should not be considered. Nuclear plant owners are facing increasing fuel assembly enrichments, spent-fuel assembly burnup limitations, spent fuel pool storage cell restrictions, and problems with fixed neutron absorber degradation, all of which are challenging their traditional criticality analyses. The credit for soluble boron (boron credit or partial boron credit – PBC) in the spent fuel pool criticality analysis offers a solution to these concerns.
For example, according to NUREG-0800 (9.1.1-4):
“For PWR pools where partial credit for soluble boron is taken, both of the following criteria must be met:
- When the spent fuel storage racks are loaded with fuel of the maximum permissible reactivity and are flooded with full-density unborated water, the maximum K(eff) must be less than 1.0 for all normal and credible abnormal conditions. The K(eff) must include allowance for all relevant uncertainties and tolerances.
- When the spent fuel storage racks are loaded with fuel of the maximum permissible reactivity and are flooded with full-density water borated to a minimum concentration (CB,min, measured in parts per million of boron), the maximum K(eff) must be no greater than 0.95 for all normal conditions. Plant technical specifications must incorporate the CB,min. The K(eff) must include allowance for all relevant uncertainties and tolerances.”
Double Contingency Principle – Double Contingency Approach
“The double contingency approach requires a demonstration that unintended criticality cannot occur unless at least two unlikely, independent, concurrent changes in the conditions originally specified as essential to criticality safety have occurred.”
Source: Nuclear Safety Technical Assessment Guide. NS-TAST-GD-041 Revision 5. ONR, 2016.
The double contingency principle discussed in ANSI/ANS-8.1 allows credit for soluble boron under other abnormal or accident conditions, since only a single accident need be considered at one time. For example, the most severe accident scenario is associated with the movement of fuel within spent fuel pool, and accidental misloading of a fuel assembly in the Region 2 of the spent fuel pool. This could potentially increase the criticality of Region 2. To mitigate these postulated criticality related accidents, boron is dissolved in the pool water.
In criticality licensing of spent fuel pool, taking credit for the decrease in fuel reactivity due to fuel burnup is known as burnup credit. Burnup credit is similar to boron credit. The concept of burnup credit is taking credit for the reduction in reactivity due to irradiation of nuclear fuel when the criticality safety analysis is carried out for the spent fuel. In spent fuel storage, it has generally been required that for criticality analyses that it be assumed that fuel si at its peak reactivity, which is generally the fresh fuel. But in the spent fuel pool, most of fuel assemblies have higher burnup with lower reactivity.
The reduction of reactivity is a combinative effect of:
- the net reduction of fissile nuclides,
- the production of neutron-absorbing nuclides (non-fissile actinides and fission products)
In the high-density rack design, the spent fuel storage pool may divided into two separate and distinct regions which, for the purpose of criticality considerations, are considered as separate pools. There are two kinds of storage racks:
- The storage racks applied to the spent fuel assemblies for which the burnup values must be equal or more than the burnup value credited in the criticality safety calculation. (Region 2)
- The storage racks applied to the spent fuel assemblies assuming non-irradiated, with the maximum reactivity (Region 1).
It should be clear that the burnup credit is not attempt to reduce the safety margins in criticality safety. It is just to reduce the analysis conservatism, in another word, reduce the uncertainties in safety margins by a more accurate safety analysis. The fresh fuel assumption can be very conservative and result in a significant reduction in capacity for a given storage or cask volume. The issue is complicated by ensuring that adequate administrative controls and measurement systems exist to prevent higher reactivity fuel from being placed in storage racks. Regulators also have concerns about verifying the accuracy of being allowed burnup credit.
For example, abnormal conditions should include consideration of fuel assemblies loaded into storage racks not approved for their storage (e.g., fuel not meeting minimum burnup requirements stored in burnup credit storage racks).
Special reference: Introduction of Burn-up Credit in Nuclear Criticality Safety Analysis. Guoshun You, Chunming Zhang, Xinyi Pan. Nuclear and Radiation Safety Center of MEP.