In the international neutron libraries, the behavior with the energy of the neutron cross sections of hydrogen in light water depends on the thermal scattering laws tabulated in terms of S (α, β). For the Joint Evaluated Fission and Fusion library (JEFF), Mattes and Keinert have established thermal scattering laws by using the LEAPR module of the NJOY code. However, uncertainties on the corresponding S (α, β) were never reported. Such missing information was recently calculated with the nuclear data code CONRAD by determining the covariances between the model parameters involved in LEAPR. The obtained uncertainties were propagated to reactivity coefficients calculated for critical assemblies operating in "cold" conditions (temperature below 80 • C) and for PWR in "hot" operating conditions (300 • C). For the integral benchmarks investigated in this work, we found that the uncertainty on the calculated k e f f , due to the S (α, β) uncertainties, is close to ±130 pcm at room temperature and ±50 pcm at 300 • C.

Abstract. The contribution of the thermal scattering law of hydrogen in light water to isothermal temperature reactivity coefficients for UOX and MOX lattices was studied in the frame of the MISTRAL critical experiments carried out in the zero power reactor EOLE of CEA Cadarache (France). The interpretation of the core residual reactivity measured between 6°C to 80°C (by step of 5°C) was performed with the Monte-Carlo code TRIPOLI4 ® . The nuclear data from the JEFF-3.1.1 library were used in the calculations. Three different thermal scattering laws of hydrogen in light water were tested in order to evaluate their impact on the MISTRAL calculations. The thermal scattering laws of interest were firstly those recommended in JEFF-3.1.1 and ENDF/B-VII.1 and also that recently produced at the atomic center of Bariloche (CAB, Argentina) with molecular dynamic simulations. The present work indicates that the calculation-to-experimental bias is À0.4 ± 0.3 pcm/°C in the UOX core and À1.0 ± 0.3 pcm/°C in the MOX cores, when the JEFF-3.1.1 library is used. An improvement is observed over the whole temperature range with the CAB model. The calculation-to-experimental bias vanishes for the UOX core (À0.02 pcm/°C) and becomes close to À0.7 pcm/°C for the MOX cores. The magnitude of these bias have to be connected to the typical value of the temperature reactivity coefficient that ranges from À5 pcm/°C at Begining Of Cycle (BOC) up to À50 pcm/°C at End Of Cycle (EOC), in PWR conditions.

The thermal scattering law (TSL) of 1H in H2O describes the interaction of the neutron with the hydrogen bound to light water. No recommended procedure exists for computing covariances of TSLs available in the international evaluated nuclear data libraries. This work presents an analytic methodology to produce such a covariance matrix-associated to the water model developed at the Atomic Center of Bariloche (Centro Atomico Bariloche, CAB, Argentina). This model is called as CAB model, it calculates the TSL of hydrogen bound to light water from molecular dynamic simulations. The performance of the obtained covariance matrix has been quantified on integral calculations at “cold” reactor conditions between 20 and 80∘ C. For UOX fuel, the uncertainty on the calculated reactivity ranges from ±71 to ±155 pcm. For MOX fuel, it ranges from ±110 to ±203 pcm.